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Accepted papers to appear in an upcoming issue

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Phase Retrieval with Fast Convergence Employing Parallel Alternative Projections and Phase Reset for Coherent Communications

Haoshuo Chen, Hanzi Huang, Nicolas Fontaine, and Roland Ryf

DOI: 10.1364/OL.385435 Received 10 Dec 2019; Accepted 21 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: Phase-retrieval (PR) receivers can reconstruct complex-valued signals using only direct detection without the use of any optical carriers.We propose two PR receiver solutions with faster and better convergence.First, we demonstrate a PR receiver based on parallel alternative projections that are produced by propagating the signal through an array of dispersive elements of increasing length followed by direct detection.Fast convergence and high retrieved phase accuracy are achieved using a modified Gerchberg–Saxton (GS) algorithm that uses each projection as an intensity constraint.We experimentally reconstruct a 30-Gbaud QPSK signal after 55-km single-mode fiber transmission using the proposed solutions with a reduced number of iterations.

Infrared photodetector based on GeTe nanofilms with high performance

yiqun zhao, Libin Tang, Shengyi Yang, KAR TENG, and Daniel Lau

DOI: 10.1364/OL.385280 Received 05 Dec 2019; Accepted 20 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: GeTe is an important narrow band gap semiconductor material, which has found application in the fields of thermoelectricity, phase change storage as well as switch. However, it has not been studied for application in the field of photodetector. Here, GeTe thin films were grown by magnetron sputtering and their material structure, optical and electrical properties were compared before and after annealing. High-performance photodetectors with detectivity of ~10^14 Jones at 850 nm light were demonstrated. Thus the novel application of GeTe in optoelectronic devices is reported in this work.

Plasma Mirror focal spot quality for glass andaluminum mirrors for laser pulses up to 20picoseconds

Brandon Edghill, Pierre Foresteier-Colleoni, Jaebum Park, Alexander Rubenchik, Farhat Beg, and Tammy Ma

DOI: 10.1364/OL.385326 Received 16 Dec 2019; Accepted 20 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: High intensity short pulse lasers are being pushed furtheras applications continue to demand higher laser intensities.Uses such as radiography and laser-driven particle accelerationrequire these higher intensities to produce the necessaryx-ray and particle fluxes. Achieving these intensitieshowever is limited by the damage threshold of costlyoptics and the complexity of target chambers. This is evidencedby the Advanced Radiographic Capability (ARC)short pulse laser, at the National Ignition Facility (NIF) atthe Lawrence Livermore National Laboratory, producingfour high energy 1 kJ laser pulses at 30 ps pulse durationbeing limited to an intensity of 10^18 W/cm^2 by the large focalspot size of 100 µm. Due to the setup complexity ofNIF, changing the location of the final focusing parabola inorder to improve the focal spot size is not an option. Thisleads to the possible use of disposable ellipsoidal plasmamirrors (PMs) placed within the chamber, close to the targetin an attempt to refocus the four ARC beams. However,the behavior of plasma mirrors at these relatively longpulse durations (tens of ps) is not well characterized. Theresults from the COMET laser at the Jupiter Laser Facility(JLF) carried out at 0.5 to 20 ps pulse durations on flat mirrorsare presented as a a necessary first step towards focusingcurved mirrors. The findings show defocusing at longerpulse durations and higher intensities, with less degradationwhen using aluminum coated mirrors.

Circular ripple patterns on silicon induced by bubble-diffracted femtosecond laser pulse in liquid

Sergey Romashevskiy, S.I. Ashitkov, and M.B. Agranat

DOI: 10.1364/OL.385672 Received 11 Dec 2019; Accepted 20 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: We report on a new technique of silicon surface nanostructuring in liquid with a pair of Gaussian-shaped femtosecond laser pulses. The bubble, generated in liquid near the molten silicon surface by the first pulse, serves as a dynamic microscale obstacle for spatial-modulation of the intensity profile of the second pulse, following at a certain delay, via scattering processes. As a result, the circular ripple patterns with anomalously high surface-relief modulation, undersurface annular nanocavities and interfacial smoothness are produced at the surface. The possibility of the control over the specific pattern through the laser intensity variation is shown.

Low-cost multiple FBGs interrogation technique for static applications

Dia Darwich, ayman youssef, and haitham zaraket

DOI: 10.1364/OL.386053 Received 16 Dec 2019; Accepted 20 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: In this work, a low-cost interrogation technique for multiple Fiber Bragg Gratings is demonstrated. The design uses the major advantage of the distributed feedback lasers which is tuning their emitted wavelength by current modulation and temperature variation. The sensitivity of this method and the influence of optical losses on the measurement have been carefully studied and an accuracy of 5 pm is achieved.

Silicon microparticles as handles for optical tweezers experiments

Marcio Rocha, Tiago Moura, Ulisses Andrade, and Joaquim Mendes

DOI: 10.1364/OL.383139 Received 14 Nov 2019; Accepted 19 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: By characterizing the behavior of silicon microparticles in a 1,064 nm Gaussian-beam optical tweezers for various different experimental parameters (laser power, focus height, etc), we show that the properties of this semiconductor can be wisely explored to allow the realization of different types of optical micromanipulation experiments using the same bead and the same experimental setup. Among the feasible applications are the exertion of dynamical oscillatory forces on the systems of interest and the extension of conventional optical tweezers to accurately work in the femtoNewton force range.

Effective Rabi dynamics of Rydberg atoms and robust high-fidelity quantum gates with a resonant amplitude-modulation field

Jin-Lei Wu, Shi-Lei Su, Yan Wang, Jie Song, Yan Xia, and Yongyuan Jiang

DOI: 10.1364/OL.386765 Received 24 Dec 2019; Accepted 18 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: With a resonant amplitude-modulation field on two Rydberg atoms, we propose a Rydberg antiblockade (RAB) regime, where the Rabi oscillation between collective ground and excited states is induced. A controlled-Z gate can be yielded through a Rabi cycle. Further, several common issues of implementing the RAB gates are solved to some certain extent. The robustness against the control error and the fidelity are enhanced by using a shaped pulse. The requirement of control precision of the strength of the Rydberg-Rydberg interaction is relaxed. In addition, breaking the RAB restrains the atomic excitation and therefore enhances the gate robustness against the atomic decay.

Tm: YAG ceramic derived multimaterial fiber with high gain per unit length for all-fiber mode-locked fiber laser applications

Guoquan Qian, Wenlong Wang, Guowu Tang, Xianchao Guan, Wei Lin, Qi Qian, Dongdan Chen, Changsheng Yang, Jiantao Liu, Guiyao Zhou, Shanhui Xu, and Zhongmin Yang

DOI: 10.1364/OL.386005 Received 16 Dec 2019; Accepted 18 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: In this work, Tm: YAG (Tm: Y3Al5O12) ceramic derived multimaterial fiber was successfully fabricated by using the molten core method, which has a high gain per unit length of 2.7 dB/cm at 1950 nm. To our knowledge, this is the highest gain per unit length at 2 μm band in similar Tm: YAG derived multimaterial fibers, which is helpful to construct fiber lasers with short laser cavity. A distribute Bragg reflector (DBR) fiber laser operating at 1950 nm was built based on a 10-cm-long as-drawn fiber. The achieved 1950 nm laser, which has a maximum output power of ~240 mW and a slope efficiency of 16.5 %, was pumped by a self-developed 1610 nm fiber laser. What is more, an all-fiber integrated passively mode-locked fiber laser based on the 10-cm-long as-drawn fiber was realized. The mode-locked laser operated at 1950 nm with duration of ~ 380 ps, and the repetition rate is 26.45 MHz. The results described here showing that the Tm: YAG ceramic derived multimaterial fiber with high gain per unit length has promising applications in 2 μm all-fiber ultrafast fiber lasers.

Resonant broadband unidirectional light scattering based on genetic algorithm

Menghao Liu, Yaxin Xie, Tianhua Feng, and Yi Xu

DOI: 10.1364/OL.381431 Received 31 Oct 2019; Accepted 18 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: The spectrum overlapping of the radiative power between magnetic and electric dipole moments in nanoparticles can be used to realize unidirectional light scattering, which is promising for various kinds of applications. Nevertheless, it is still challenge to achieve such overlapping in a broadband manner. Herein, we propose that the combination of genetic algorithm, the Maxwell's equations and electromagnetic multipole expansion can be used to design a nanoparticle which supports resonant broadband forward light scattering. Microwave experiments are performed to demonstrate our numerical results. The propose method is quite general and it can be straightforwardly generalized to design functional unidirectional scatters.

Ultra-wideband far-infrared absorber based on anisotropically-etched doped silicon

Xiaolong You, Aditi Upadhyay, Yongzhi Cheng, Madhu Bhaskaran, Sharath Sriram, Christophe Fumeaux, and Withawat Withayachumnankul

DOI: 10.1364/OL.382458 Received 06 Nov 2019; Accepted 18 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: Far-infrared absorbers exhibiting wideband performance are in great demand in numerous applications, including imaging, detection, and wireless communications. Here, a non-resonant far-infrared absorber with an ultra-wideband operation is proposed. This absorber is in the form of inverted pyramid cavities etched into moderately doped silicon. By means of a wet-etching technique, the crystallinity of silicon restricts the formation of the cavities to a particular shape, in an angle that favors impedance matching between lossy silicon and free space. Far-infrared waves incident on this absorber experience multiple reflections on the slanted lossy silicon side walls, and being dissipated towards the cavity bottom. The simulation and measurement results confirm that an absorption beyond 90% can be sustained from 1.25 to 5.00 THz. Furthermore, the experiment results suggest that the absorber can operate up to at least 21.00 THz with a specular reflection less than 10% and negligible transmission.

Evidencing the nonlinearity independence of resolution in femtosecond laser ablation

Mario Garcia Lechuga, Olivier Uteza, Nicolas Sanner, and David Grojo

DOI: 10.1364/OL.382610 Received 05 Nov 2019; Accepted 17 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: To overcome the resolution limits in laser processing technologies, it is highly attractive to translate concepts used in advanced optical microscopy. In this prospect, the nonlinear nature of absorption in dielectrics with femtosecond lasers is recurrently taken as a direct advantage in an analogous way to excitation in multiphoton microscopy. However, we establish that no direct benefit in resolution can be expected when laser ablation is the observable. We explore widely different non-linear regimes using ultrashort pulses at various wavelengths (1550 and 515 nm) and target materials of different bandgaps (3.8 eV to 8.8 eV). We find in the experiments that the shapes of all ablation features correspond to a one-to-one mapping of the beam contours at a strict threshold-intensity. The nonlinearity-independent response shows that the incorporation of extreme UV should provide a direct route to the nanoscale resolutions routinely achieved in lithography.

Improvement of liquid crystal tunable lenses with weakly conductive layers using multi-frequency driving

Jeroen Beeckman, Kristiaan Neyts, Tom Vanackere, Tom Vandekerckhove, Elke Claeys, and John George

DOI: 10.1364/OL.383443 Received 20 Nov 2019; Accepted 17 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: A common technique to realize the gradient electric field profile that is required in liquid crystal tunable lenses is the use of a weakly conductive layer. Thanks to this layer, an applied voltage with a certain frequency allows to obtain a refractive index profile that is required for the lens operation. Due to the limited degrees of freedom however, it is not possible to avoid aberrations in a weakly conductive layer based tunable lens for a continuously tunable focal length. In this work, we discuss the use of additional higher frequency components in the voltage signal to reduce the lens aberrations drastically.

Imaging trapped quantum gases by off-axis holography

Jasper Smits, Allard Mosk, and Peter van der Straten

DOI: 10.1364/OL.384120 Received 21 Nov 2019; Accepted 16 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: We present a dispersive imaging method for trapped quantum gases based on digital off-axis holography. Both phase delay and intensity of the probe field are determined from the same image. Due to the heterodyne gain inherent to the holographic method it is possible to retrieve the phase delay induced by the atoms at probe beam doses two orders of magnitude lower than phase-contrast imaging methods. Using the full field of the probe beam we numerically correct for image defocusing.

Continuous-wave terahertz self-referencing digital holography based on Fresnel’s mirrors

Dayong Wang, Yaya Zhang, Lu Rong, Duoxuan Ma, Jie Zhao, and Yunxin Wang

DOI: 10.1364/OL.385943 Received 13 Dec 2019; Accepted 16 Jan 2020; Posted 16 Jan 2020  View: PDF

Abstract: Continuous-wave terahertz digital holography (TDH) is a booming full-field phase-contrast imaging method validated in both in-line and Mach-Zehnder off-axis geometries. In this letter, a self-referencing TDH approach is proposed based on the Fresnel’s mirrors, by which the object wavefront is split and reflected. Two beams interfere with each other to form an off-axis hologram. The proposed recording configuration is immune from the superposed twin-image and has higher temporal stability than Mach-Zehnder interferometer. To evaluate the phase-contrast imaging performance, different types of samples are measured.

Generation of mid-infrared femtosecond vortex beam from an optical parametric oscillator

Hui Tong, Guoqiang Xie, Zhen Qiao, Zhipeng Qin, Peng Yuan, Jingui Ma, and Liejia Qian

DOI: 10.1364/OL.388096 Received 13 Jan 2020; Accepted 16 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: Mid-infrared femtosecond vortex beams generated by optical parametric oscillators (OPO) are reported for the first time. Order tunable femtosecond Hermite-Gauss (HG) beams from 1st to 6th order are produced from a synchronously pumped OPO, and then are converted into the corresponding 1st-6th order femtosecond vortex beams by a cylindrical lens mode converter. By slightly tuning the cavity length, the wavelength of the vortex beam can be continuously tunable in the range from nm to 82 nm, and the pulse duration can be changeable from ~400 fs to ~1.1 ps. The work provides a flexible and reliable way to generate mid-infrared femtosecond vortex beams, and is of special significance for expanding the wavelength range of femtosecond vortex beams and their application fields.

Optical Refrigeration in a Silica-based fiber at Atmospheric Pressure

Jennifer Knall, Pierre-Baptiste Vigneron, Magnus Engholm, Peter Dragic, Nanjie Yu, John Ballato, Martin Bernier, and Michel Digonnet

DOI: 10.1364/OL.384658 Received 05 Dec 2019; Accepted 15 Jan 2020; Posted 16 Jan 2020  View: PDF

Abstract: For the first time, to the best of our knowledge, optical cooling is reported in a silica gain medium. The sample is a fiber with a core 21-µm in diameter selectively doped with 2.06 wt% Yb3+ and co-doped with Al2O3 and F- to increase the critical quenching concentration by a factor of 16 over the largest reported values for Yb-doped silica. Using a custom slow-light FBG sensor with mK resolution, temperature changes up to -50 mK were measured with 0.33 W/m of absorbed pump power per unit length at 1040 nm. The measured dependencies of the temperature change on pump power and pump wavelength are in excellent agreement with predictions from an existing model and reflect the fiber’s ground-breaking quality for high-power radiation-balanced fiber lasers.

On the actual spatial resolution of Brillouin Imaging

Silvia Caponi, Daniele Fioretto, and Maurizio Mattarelli

DOI: 10.1364/OL.385072 Received 05 Dec 2019; Accepted 15 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: Brillouin imaging is an emerging optical elastography technique able to generate maps of the mechanical properties at microscale, with great potential in biophysical and biomedical fields. A key parameter is the spatial resolution, which is usually identified with that of the confocal microscope coupled to the Brillouin interferometer. Conversely, here we demonstrate that the mean free path of acoustic phonons is a key parameter and that it can dominate the lateral resolution in high numerical aperture confocal setups. Surprisingly, the resolution of elastography maps can even deteriorate when decreasing the scattering volume.

Relation between the localization length and level repulsion in two-dimensional optical Anderson localization

Sushil Mujumdar and Sandip Mondal

DOI: 10.1364/OL.383748 Received 20 Nov 2019; Accepted 15 Jan 2020; Posted 15 Jan 2020  View: PDF

Abstract: We report on the relation between the localization length and level-spacing characteristics of two-dimensional optical localizing systems. Using the tight-binding model over a wide range of disorder, we compute spectro-spatial features of Anderson localized modes. The spectra allow us to estimate the level-spacing statistics while the localization length ξ is computed from the eigenvectors. We use a hybrid interpolating function to fit the level spacing distribution, whose repulsion exponent β varies continuously between 0 and 1, with the former representing Poissonian statistics and the latter approximating the Wigner-Dyson distribution. We find that the (ξ, β) scatter points occupy a well-defined locus that is well fit by a sigmoidal function. This implies that the localization length of a disordered medium with an arbitrary disorder strength can be estimated by spectral means using the level spacing statistics. This technique is also immune to dissipation since the repulsion exponent is insensitive to level widths, in the limit of weak dissipation.

Sub-volt electro-optical modulator on thin-film lithium niobate and silicon nitride hybrid platform

Abu Naim Rakib Ahmed, Sean Nelan, Shouyuan Shi, Peng Yao, Andrew Mercante, and Dennis Prather

DOI: 10.1364/OL.381892 Received 30 Oct 2019; Accepted 15 Jan 2020; Posted 16 Jan 2020  View: PDF

Abstract: A low voltage operation electro-optic modulator is critical for applications ranging from optical communications to analog photonic links and mm-wave Imaging. This paper reports a hybrid silicon nitride and lithium niobate electro-optic Mach Zehnder modulator that employs 3 dB multimode interference couplers for splitting and combining light. The presented amplitude modulator with an interaction region length of 2.4 cm demonstrates a DC half-wave voltage of only 0.875 V, which corresponds to a modulation efficiency per unit length of 2.1 V.cm. The power extinction ratio of the fabricated device is about 30 dB.

Hybrid femtosecond laser fabrication of a size-tunable microtrap chip with a high-trapping-retention rate

Bing Xu, Shengyun Ji, Deng Pan, Wenjin Hu, Suwan Zhu, Yanlei Hu, Jiawen Li, Dong Wu, Jiaru Chu, and Koji Sugioka

DOI: 10.1364/OL.386095 Received 16 Dec 2019; Accepted 14 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: In this Letter, we propose a new-promising concept of a hybrid femtosecond (fs) laser processing method composed of single-point scanning and holographic light modulation fabrication for manufacturing a tunable-size microtrap chip. The hybrid method not only ensures the key microfluidic devices’ precision, but also greatly improves the fabrication speed. By using a new asymmetry-bracket-shaped microtrap design with a mechanical strain stretching method, real-time size-tunable trapping is obtained and a 100% particle trapping retention is realized ignored the flow fluctuation. Finally, the microtrap array is successfully applied to trap single yeast cells and hold them for ~10 hours without escaping.

Detecting WDM Visible Light Signals by a SingleMulti-Color Photodiode with MIMO Processing

Alessandro Messa, Giulio Cossu, Marco Presi, Stefan Schidl, Kerstin Schneider-Hornstein, Horst Zimmermann, and Ernesto Ciaramella

DOI: 10.1364/OL.385641 Received 10 Dec 2019; Accepted 14 Jan 2020; Posted 15 Jan 2020  View: PDF

Abstract: For the first time, we experimentally demonstrate that MIMO processing allows using a single photodiode to detect simultaneously an RGB (red, green and blue) VLC signals. The photodiode has a triple junction and, when it is illuminated by a WDM signal, the junctions produce inherently three photocurrents that are unusable to detect any of the WDM signals. However, by means of linear MIMO processing, we are able to recover the transmitted signals exactly. Bit error ratio measurements confirm the effectiveness of the proposed solution. This opens a new scenario for practical WDM-VLC systems.

Optical frequency comb based on nonlinear spectral broadening of a phase modulated comb source driven by dual offset locked carriers

Nagarjun KP, shiva vikram bhagavatula, Roopa Prakash, Ajay Singh, Shankar Kumar Selvaraja, and V R Supradeepa

DOI: 10.1364/OL.381319 Received 25 Oct 2019; Accepted 14 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: We demonstrate a versatile technique to generate a broadband optical frequency comb source in the C-band. This is accomplished by nonlinear spectral broadening of a phase modulated comb source driven by dual frequency offset locked carriers. The locking is achieved by setting up a heterodyne optical frequency locked loop to lock two phase-modulated electro-optic 25GHz frequency combs sourced from individual seed carriers offset by 100GHz, to within 6.7MHz of each other. We realize spectral broadening in highly nonlinear fiber after suitable amplification to obtain an equalized, nonlinearly broadened frequency comb. We obtain ~86 lines in a 20dB band spanning over 2THz.

Temporal and Spectral Coding over Amplified Spontaneous Emission (ASE) for Secure Optical Coherent Communications

Yetian Huang, Haoshuo Chen, Hanzi Huang, Nicolas Fontaine, Roland Ryf, Yingxiong Song, and Min wang

DOI: 10.1364/OL.384732 Received 02 Dec 2019; Accepted 14 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: We demonstrate secure optical coherent communications employing low-coherence matched detection based on the randomness of amplified spontaneous emission (ASE) noise. Two-level physical-layer optical encryption is achieved through temporal and spectral coding over a broadband ASE source. ASE-carried signal and unmodulated carrier are polarization multiplexed, transmitted over a same single-mode fiber (SMF) and separated with the aid of polarization tracking before matched detection at the receiving side. The impact of chromatic dispersion (CD) on low-coherence matched detection system is analyzed and experimentally investigated. We experimentally realize optically-coded 20-Gbaud QPSK and 8-PSK signals transmission over a 43-km SMF span with a maximum line rate of 60 Gbits/s.

Uniform focusing with extended depth range and increased working distance for optical coherence tomography by an ultrathin monolith fiber probe

Jianrong Qiu, Tao Han, Zhiyi liu, Jia Meng, and Zhihua Ding

DOI: 10.1364/OL.383428 Received 18 Nov 2019; Accepted 13 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: It’s difficult to maintain high transverse resolution over an increased depth range using miniature probes for optical coherence tomography (OCT) due to the rapid divergence of light and the space limitation. To solve this problem, we introduce a fiber-based filter in the proposed probe to manipulate its output beam. Significant mode interference is exploited to enhance the depth of focus (DOF) and the mode phase difference is tuned to achieve a uniform axial intensity within the DOF. The magnified mode interference field instead of the diffracted one is adopted as the final pupil filter in the probe to increase its working distance (WD). The probe is fabricated with a diameter of 125 µm and a total length of 2.6 mm for its distal fiber optics. Compared to the conventional probe with similar minimal lateral resolution of better than 4.4 µm, the proposed probe achieves two times of DOF gain and 1.7 times of WD. Improvements in performance of the probe are demonstrated by OCT imaging using a fresh lemon and human skin. With merits of enhanced imaging quality, easy fabrication, flexibility and robustness, the proposed probe poses great potential for important applications, especially for endoscopic imaging of human internal organs in vivo.

Attosecond all-optical control and visualization of quantum interference between degenerate magnetic states by circularly polarized pulses

Chuancun Shu, Yu Guo, KaiJun Yuan, Daoyi Dong, and Andre Bandrauk

DOI: 10.1364/OL.386879 Received 26 Dec 2019; Accepted 13 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Controlling coherence and interference of quantum states is one of the central goals in quantum science. Different from energetically discrete quantum states, however, it remains a demanding task to visualize coherent properties of degenerate states (e.g., magnetic sublevels). It becomes further inaccessible in the absence of an external perturbation (e.g., Zeeman effect). Here, we present a theoretical analysis of all-optical control of degenerate magnetic states in the molecular hydrogen ion, H$_2^+ $, by using two time-delayed co- and counter-rotating circularly-polarized-attosecond XUV pulses. We perform accurate simulations to examine this model by solving the three-dimensional time-dependent Schr\"{o}dinger equation. A counterintuitive phenomenon of quantum interference between degenerate magnetic sublevels appears in the time-dependent electronic probability density, which is observable by using X-ray-induced transient angular and energy-resolved photoelectron spectra. This work provides an insight into quantum interference of electron dynamics inside molecules at the quantum degeneracy level.

2.9 µm lasing from Ho3+/Pr3+ co-doped AlF3 based glass fiber pumped by a 1150 nm laser

Pengfei Wang, Shunbin Wang, Jiquan Zhang, Shijie Jia, Gilberto Brambilla, and Niannian Xu

DOI: 10.1364/OL.384216 Received 28 Nov 2019; Accepted 13 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Ho3+/Pr3+ co-doped AlF3 based glass fibers were fabricated by using a rod-in-tube method based on the matrix glass composition of AlF3 - BaF2 - CaF2 - YF3 - SrF2 - MgF2 - LiF - ZrF4 - PbF2. Under the pump of a 1150 nm Raman fiber laser, 2.9 µm laser was observed in a 19 cm long Ho3+/Pr3+ co-doped AlF3 based glass fiber with an output power of 157 mW and a slope efficiency of 10.4%. Ho3+/Pr3+ co-doped AlF3 based glasses were fabricated to investigate the deactivation effects of Pr3+ ions on the Ho3+: 5I7 level. Our results showed that the Ho3+/Pr3+ co-doped AlF3 based glass fibers are potential gain media for ~ 2.9 µm lasers.

Efficient fabrication of high-aspect-ration AFM tip by one-step exposure of long focal depth holographic femtosecond axilens beam

Deng Pan, Shunli Liu, Shengyun Ji, Ze Cai, Jiawen Li, Yaopin Hou, Weijie zhang, Shengying Fan, Rui Li, Yanlei Hu, Wulin Zhu, Dong Wu, and Jiaru Chu

DOI: 10.1364/OL.384249 Received 26 Nov 2019; Accepted 13 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: In this letter, we demonstrate a laser fabrication strategy that using the long focal depth femtosecond axilens laser beam to manufacture the HAR micropillars and AFM probes by one step exposure. The long depth of focus (LDOF) is generated by modulating laser beam to focused at different positons. By adjusting the exposure height of the morphology of HAR micropillars can be tuned flexibly and the micropillar with ultra-high aspect ratio (diameter of 1.5um, height of 102um, AR=70) can be fabricated within 10ms which is a great challenge for other processing methods to obtain such a HAR microstructure in such a short time. In addition, the HAR micropillar is fabricated onto a cantilever to form the AFM probe. The homemade probe shows fine imaging quality. This method greatly improves the processing efficiency while ensuring the fabrication resolution which provides a powerful method for processing HAR microstructures.

All-optical neuromorphic XOR operation with inhibitory dynamics of a single photonic spiking neuron based on VCSEL-SA

Shuiying Xiang, Zhenxing Ren, Yahui Zhang, Ziwei Song, and Yue Hao

DOI: 10.1364/OL.383942 Received 21 Nov 2019; Accepted 13 Jan 2020; Posted 15 Jan 2020  View: PDF

Abstract: We propose a simple hardware architecture for solving XOR task by using a single photonic spiking neuron based on VCSEL with embedded saturable absorber subject to dual-polarized pulsed optical injection. We model the inhibitory photonic spiking neuron by extending the Yamada model and spin flip model to incorporate the two polarization-resolved modes and the saturable absorber. It is shown that, by carefully adjusting the temporal difference according to the inhibitory window, the XOR operation can be realized in a single photonic spiking neuron, which is interesting and valuable for the photonic neuromorphic computing and information processing.

A 400 m Rolling Shutter based Optical Camera Communications Link

Elizabeth Eso, Shivani Teli, Navid Bani Hassan, Stanislav Vitek, Zabih Ghassemlooy, and Stanislav Zvanovec

DOI: 10.1364/OL.385423 Received 06 Dec 2019; Accepted 12 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: In this paper, we develop a novel technique to increase the link span (Ls) of a rolling shutter (RS) based optical camera communications (OCC) by reducing the spatial bandwidth of the camera in the out of focus regions. We demonstrate a 400 m line-of-sight RS-based OCC link, which is to date, the longest Ls reported in these systems and developed a detection method to extract the information out of the video frames, successfully. The proposed system relaxes the condition of a large surface area for the transmitter light source. Consequently, we show that at 400 m Ls and exposure times of 100-800 µs a data rate of 450 bps is achieved successfully.

4 Mb/s under 3-m transmission distance using quantum dot light-emitting diode and NRZ-OOK modulation

Hua Xiao, Rui Wang, Gui He, Zhijian Lv, Zhaojun Liu, and Kai Wang

DOI: 10.1364/OL.386175 Received 24 Dec 2019; Accepted 12 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: We realize signal transmission with a miniature light source fabricated by a 4-mm2 red-emissive CdSe/ZnS QLED in visible light communication (VLC). The light emitted from the 60°-designed QLED transmit in free space with data rate of 4 Mb/s at 3-m transmission distance by using a simple modulation scheme of none-return-to-zero on-off keying. The maximum data rate of 2.5 Mb/s with BER under the forward-error-correction (FEC) limit is achieved within the optical angles of ± 20°. The influences caused by voltage, distance, and optical angle of emitting light are taken into consideration during communication. The performance of the QLED based light source is excellent among all solution-processed devices both in efficiency, luminance, bandwidth, transmission speed, and distance. Additionally, this is the first time to realize QLED communication, and our results should be instructive for further application of QLEDs in VLC.

A Realistic phase screen model for forward multiple-scattering media

Mu Qiao and Xin Yuan

DOI: 10.1364/OL.383923 Received 21 Nov 2019; Accepted 12 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Existing random phase screen (RPS) models fail to incorporate ballistic light. In this letter, we redesign the angular spectrum of the screen by means of Monte-Carlo simulation based on an assumption that a single screen should represent all the scattering events a photon experiences between two adjacent screens. Three examples demonstrate that the proposed model exhibits more realistic optical properties than conventional RPS models in terms of attenuation of ballistic light, evolution of beam profile and angular memory effect. The proposed model also provides the flexibility to balance the computing accuracy, speed and memory usage by tuning the screen spacing.

Continuous wave operation of DFB laser diodes based on GaN using 10th order laterally coupled surface gratings

Ji-Hye Kang, Hans Wenzel, Erik Freier, Veit Hoffmann, Olaf Brox, Jörg Fricke, Luca Sulmoni, Mathias Matalla, Christoph Stölmacker, M Kneissl, Markus Weyers, and Sven Einfeldt

DOI: 10.1364/OL.385002 Received 04 Dec 2019; Accepted 11 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: Single longitudinal mode continuous wave operation of distributed-feedback (DFB) laser diodes based on GaN is demonstrated using laterally coupled 10th-order surface Bragg gratings. The gratings consist of V-shaped grooves alongside a 1.5 μm wide p-contact stripe fabricated by using electron-beam lithography and plasma etching. By varying the period of the Bragg grating, the lasing wavelength could be adjusted between 404.8 nm and 408.5 nm. The feasibility of this device concept was confirmed by mode-hop free operation up to an optical output power of 90 mW, a low temperature sensitivity of the lasing wavelength and a Gaussian lateral far field distribution.

Real-time monitoring of temperature using pulsed laser diode based photoacoustic system

Paul Upputuri, Dhiman Das, muneesh maheshwari, Yang Yaowen, and Manojit Pramanik

DOI: 10.1364/OL.386173 Received 17 Dec 2019; Accepted 11 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Tissue local temperature information is necessary for guiding energy-based medical treatments. In cancer treatments like thermal therapy, heating is applied to local tissue to kill the tumor cells. These techniques require a temperature monitoring device with high sensitivity. In this work, we demonstrate a pulsed laser diode (PLD) based photoacoustic temperature sensing (PATS) system for monitoring tissue temperature in real-time. The system takes advantage of high repetition rate (7000 Hz), near-infrared wavelength (803 nm), and relatively high energy 1.42 mJ/pulse laser. The system is capable of providing local temperature information at high temporal resolution 1 millisecond and high sensitivity 0.26oC. The temperature data measured with PLD-PATS system are compared with the data provided by the commercial fiber Bragg grating (FBG) sensor. The proposed system will find applications in radiofrequency ablation, photothermal therapy, and focused ultrasound, etc. used for cancer treatments.

High-resolution, large dynamic range multimode interferometer sensor based on a suspended core microstructured optical fiber

Ping Shum, Yu Zheng, Yiyang Luo, Yanan Zhang, Wenjun Ni, Guanghui Wang, Zhifang Wu, Xuan Dinh, Jean-Louis Auguste, and Georges Humbert

DOI: 10.1364/OL.386296 Received 18 Dec 2019; Accepted 11 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: The performance of sensors, including optical fiber sensors, is commonly limited by the trade-off between a large dynamic range and a high resolution. Here, in order to optimize both, we propose an inline multimode interferometer sensor based on a suspended core microstructured optical fiber. Due to the existence of multiple pairs of mode interferences, the transmission spectrum of the interferometer consists of dense fringes modulated by a lower envelope. Since these mode interferences take place in the uniform material with the same length, the dense fringes and the lower envelope have an identical sensing response without crosstalk. Hence, the sensor integrates the large dynamic range of the lower envelope and the high resolution of the dense fringes. Strain sensing performance is investigated to validate the characteristic of the large dynamic range and high resolution of the proposed sensor. The dynamic range, theoretically 0-9200 µε, is 12 times larger than the dense fringes and the resolution is 17.5 times higher than the lower envelope.

High efficiency Al/Sc-based multilayer coatings in the EUV wavelength range above 40 nanometers

Jennifer Rebellato, Regina Soufli, Evgueni Meltchakov, Eric Gullikson, DE ROSSI Sébastien, and Franck Delmotte

DOI: 10.1364/OL.384734 Received 29 Nov 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: In this work, we have developed new and highly efficient periodic multilayer mirrors Al/Sc, Al/Sc/SiC and Mo/Al/Sc with optimized reflectance at wavelengths between 40 nm and 65 nm. We have reached record values in measured peak reflectance: 57.5% at 44.7 nm and 46.5% at 51 nm, with Al/Sc/SiC at near-normal incidence. Furthermore, we have achieved the largest reported bandwidth with Mo/Al/Sc at 57 nm and the narrowest bandwidth with Al/Sc at 60 nm wavelength. These new and promising results demonstrate that Al/Sc-based multilayer coatings are excellent candidates for future generations of EUV instruments for solar physics, EUV lasers and attosecond science, in a wavelength range that hasn’t been fully explored.

Wide area quantitative phase microscopy by spatial phase scanning digital holography

Myung Kim

DOI: 10.1364/OL.385322 Received 06 Dec 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: A new technique of digital holographic microscopy is introduced for large area quantitative phase microscopy. Dubbed spatial phase scanning (SPS) digital holography, the object specimen in an interferometer is scanned across the tilted reference phase field, while acquiring camera frames at regular intervals. Both the large area scan and phase shift acquisition are achieved in one sweep, using a simple optomechanical system. The technique can be useful in diverse applications such as fast scans of blood smear, cell and tissue cultures, and microelectronic surface profiles.

A light field camera study of near ground turbulence anisotropy and observation of small outer-scales

Chensheng Wu, Daniel Paulson, John Rzasa, and Christopher Davis

DOI: 10.1364/OL.386444 Received 19 Dec 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Understanding turbulence effects on laser beam propagation is critical to the emerging design, study, and test of many long range free space optical (FSO) communication and directed energy (DE) systems. Conventional studies make the prevalent assumption of isotropic turbulence, while more recent results suggest anisotropic turbulence for atmospheric channels within a few meters elevation above the ground. As countless FSO systems have been and continue to be deployed in such channels, analysis of anisotropic modelings has become one of the fastest growing areas in FSO research. This in turn motivates new tools which can distinguish anisotropic characteristics to improve both modeling accuracy and physical interpretations. Wavefront sensors such as Shack-Hartmann sensors, interferometers, and plenoptic sensors have been devised and used in experiments, however they all require rigid alignments that lack resilience against temperature gradient build-up and beam wander. We find that by using a light field camera (LFC) which extracts perturbation of individual light rays, the wave structure function of turbulence can be retrieved with high reliability. Furthermore, we find through experiments that the outer scales of near ground turbulence tend to be a magnitude smaller than conventional theoretical assumptions, which agrees with new findings by others but is absent in current theoretical modelings. As a result, we believe that the LFC is an ideal candidate in the frontier of turbulence research, which is both commercially available and easy to adapt to turbulence experiments.

Photonic generation of background-free frequency-doubled phase-coded microwave pulses with immunity to periodic power fading

Wu Zhang, Qinggui Tan, Xiaojun Li, and Di Wang

DOI: 10.1364/OL.386138 Received 16 Dec 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: A novel photonic approach for the generation of background-free frequency-doubled phase-coded microwave pulses with immunity to periodic power fading is proposed. By switching between the carrier suppressed double sideband (CS-DSB) modulation with reverse quadrature phase difference and carrier suppressed single sideband (CS-SSB) modulation, background-free phase-coded microwave pulses with frequency doubling of the LO signal can be obtained. Thanks to the CS-DSB/CS-SSB modulation, the generated microwave pulses can be directly transmitted via single mode fiber (SMF) without the influence of the periodic power fading. Since no filter is utilized, the proposed scheme owns a wide frequency tunable range. An experiment is conducted to verify the proposed approach. Two 1-Gbit/s phase-coded microwave pulses with center frequencies of 6 GHz and 12 GHz are generated and successfully transmitted through a 25-km SMF. The pulse compression ratio (PCR) and main-to-sidelobe ratio (MSR) after the transmission are measured as 13 and 8.84 dB, respectively.

All-fiber Nonlinear Optical Wavelength Conversion System from the C-Band to the Mid-Infrared

Imtiaz Alamgir, François St-Hilaire, and Martin Rochette

DOI: 10.1364/OL.386272 Received 18 Dec 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: We demonstrate an all-fiber wavelength conversion system from the C-band to the wavelength range of 2.30-2.64 µm of the mid-infrared. A series of nonlinear processes are used to perform this spectral shift in excess of 80 THz: From optical pulses in the C-band, self-phase modulation spectral broadening and offset filtering generate probe pulses in the L-band. In parallel to this, Raman-induced soliton self-frequency shift converts pulses from the C-band into pump pulses in the thulium band. The resulting synchronized probe and pump pulses interact via degenerate four-wave mixing to produce wavelength-converted idler pulses in the mid-infrared. Silica fiber is used for nonlinear processes at wavelengths up to the thulium band whereas chalcogenide glass is used for nonlinear processes at wavelengths longer than the thulium band. This system is a major step towards the development of compact MIR optical sources generated from widespread pump lasers of the C-band.

Real-time High-speed Three-dimensional Surface Imaging using CoaXPress-Interfaced Band-Limited Illumination Profilometry

Jinyang Liang, Cheng Jiang, Patrick Kilcullen, Xianglei Liu, Alan Boate, Jeremy Gribben, and Tsuneyuki Ozaki

DOI: 10.1364/OL.378939 Received 25 Sep 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: High-speed three-dimensional (3D) surface imaging by structured-light profilometry is currently driven by industrial needs, medical applications, and entertainment. However, the limited speeds in fringe pattern projection, image acquisition, and data transmission have strained the existing methods from reaching kilohertz (kHz)-level acquisition, processing, and display of 3D information during the occurrence of dynamic events (i.e., in real time). To overcome these limitations, we have developed CoaXPress-interfaced band-limited illumination profilometry (CI-BLIP), which enables real-time 3D surface information reconstruction at 1 kHz. We have demonstrated the system’s performance by imaging various static and fast-moving 3D objects in real time. We have also applied this system in fluid mechanics by imaging dynamics of a flag, which allowed observation of the wave propagation, gravity-induced phase mismatch, and asymmetric flapping motion. We expect CI-BLIP to find diverse scientific and industrial applications.

Indocyanine green provides absorption and spectral contrast for optical coherence tomography at 840 nm in vivo

Conrad Merkle, Marco Augustin, Danielle Harper, and Bernhard Baumann

DOI: 10.1364/OL.380051 Received 10 Oct 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: In recent years, there has been growing interest in the application of exogenous contrast agents to supplement the traditional strengths of optical coherence tomography (OCT) and provide additional biological information. Here we present how indocyanine green, a common fluorescent contrast agent approved by the United States Food and Drug Administration, can provide absorption and spectral contrast for OCT imaging in the mouse eye in vivo. We further demonstrate high stability of spectral contrast measurements for the long term monitoring of contrast agents in spite of fluctuations in intensity.

Wavelength- and dispersion-tunable ultrafast holmium-doped fiber laser with dual-color operation

Maria Pawliszewska, Anna Dużyńska, Mariusz Zdrojek, and Jaroslaw Sotor

DOI: 10.1364/OL.383788 Received 20 Nov 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: We present a versatile ultrafast holmium-doped fiber laser with an intracavity Martinez compressor. The compressor enables continuous dispersion control, spectral filtering, and dual-color operation of the laser. Mode-locking is supported for net cavity dispersion values ranging from highly anomalous (-1.42 ps²) to net normal (0.3 ps²), and wavelength tuning of the optical solitons is obtained in a 2021-2096 nm span. Dual-color pulsed operation of the laser is reached by implementing a mechanical bandstop filter within the compressor. The repetition rate offset of the two emitted frequency combs can be tuned in a 3-8 kHz range by adjusting the net cavity dispersion, or by changing the beam block diameter. We show that a relatively simple fiber resonator integrated with a Martinez compressor can serve as a highly tunable laser source.

Photovoltaic splitting of water microdroplets on a y-cut LiNbO₃:Fe crystal coated with oil-infused hydrophobic insulating layers

Xiong Zhang, Kaifang Gao, Zuoxuan Gao, Zhitao Zan, Lihong shi, Xiaohu Liu, Mengtong Wang, Hongjian Chen, and Wenbo Yan

DOI: 10.1364/OL.385212 Received 06 Dec 2019; Accepted 09 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: We demonstrate a successful photovoltaic splitting of water microdroplets on a y-cut LiNbO₃:Fe substrate coated with an oil-infused hydrophobic layer. The temporal evolution of the microdroplet contact angle upon a central illumination and the distinct behaviors of two sub-droplets during a following boundary illumination reveal that both electrowetting and electroosmotic effects induced by the dipolar photovoltaic potential on the substrate contribute to the water micro-droplet splitting. The reciprocal relationship between the splitting time and the illumination intensity verifies the inherent photovoltaic nature of the water microdroplet splitting. The splitting time is found to be linearly dependent on the initial microdroplet size. These points are quite important to the practicalization of LN-based microfluidic chips in the biological field.

Precise length definition of active GaAs based optoelectronic devices for low loss silicon photonics integration

Heidi Tuorila, Jukka Viheriala, Nouman Zia, Matteo Cherchi, Mikko Harjanne, Riku isoaho, Timo Aalto, and Mircea Guina

DOI: 10.1364/OL.382109 Received 08 Nov 2019; Accepted 09 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: The length variation associated with standard cleaving of III-V optoelectronic chips is a major source of loss in the integration with the micron-scale silicon-on-insulator waveguides. To this end, a new approach for precise definition of the III-V chip length is reported. The method employs lithography and wet etching of cleave marks outside the active III-V waveguides. The marks follow a specific crystallographic orientation and are used to initiate and guide the cleaving process. Besides minimizing the air gap between the butt-coupled III-V and Si waveguides and hence minimizing the coupling losses, the use of precisely defined length significantly improves the integration yield owing to the increased length uniformity. We apply this technique to defining the lengths of GaAs-based semiconductor optical amplifiers and demonstrate length control with an accuracy better than 250 nm per facet. This variation is more than one order of magnitude smaller than with the traditional cleaving methods, resulting in improvement of coupling by several dBs.

Visible light generation in cladding of optical fibers carrying near-infrared continuous-wave lasers due to Cherenkov-phase matched harmonic conversion

Santosh Aparanji, Arun S, V Balaswamy, and V R Supradeepa

DOI: 10.1364/OL.384581 Received 28 Nov 2019; Accepted 09 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: In this work, we report and analyze the cause of the surprising observation of visible light generation in the cladding of silica-based continuous-wave (CW), near-infrared (NIR) fiber lasers. We observe a visible rainbow of hues in a cascaded Raman fiber laser, which we attribute to second and third harmonic conversion of the different wavelength components propagating in the core of the fiber. The light in the cladding of the fiber is explained to be through Cherenkov-type phase matching, and a mathematical analysis is presented to estimate the power of the harmonic light generated. We then extend this theory to visible light generation in other types of fiber lasers. Specifically, we analyze the case of a CW supercontinuum generated in standard telecom fibers, and verify our theoretical predictions with experimental results through visible spectra collected.

Observation of PT-symmetry in a fiber ring laser

Jianping Yao, Zhiqiang Fan, Wefeng Zhang, and Qi Qiu

DOI: 10.1364/OL.381106 Received 22 Oct 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: A wavelength-tunable single-mode laser with a sub-kHz linewidth based on parity-time (PT)-symmetry is proposed and experimentally demonstrated. The proposed PT-symmetric laser is implemented based on a hybrid use of an optical fiber loop and a thermally tunable integrated microdisk resonator (MDR). The MDR, implemented based on silicon on insulator (SOI), is operating with the optical fiber loop to form two mutually coupled cavities with an identical geometry. By controlling two light waves passing through the two cavities with one having a gain coefficient and the other a loss coefficient, but with an identical magnitude, a PT-symmetric laser is implemented. Thanks to an ultra-narrow passband of the cavity due to PT-symmetry, single-longitudinal mode lasing is achieved. The tuning of the wavelength is implemented by thermally tuning the MDR. The proposed PT-symmetric laser is demonstrated experimentally. Single-longitudinal-mode lasing at a wavelength of around 1555 nm with a sub-kHz linewidth of 433 Hz is implemented. The lasing wavelength is continuously tunable from 1555.135 to 1555.887 nm with a tuning slope of 75.24 pm/℃.

Wavelength-tunable Focusing via Fresnel Zone Microsphere

Yan Zhou, Rong Ji, Jinghua Teng, and Minghui Hong

DOI: 10.1364/OL.382872 Received 11 Nov 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: In this paper, a novel structural configuration on a transparent microsphere is proposed to engineer the focusing light field. By patterning a hybrid diffractive Fresnel zone plate structure on a partially milled microsphere using focused ion beam (FIB), wavelength-dependent switching between mono-focal and multi-focal functionalities can be achieved. Generation of on-axis tri-foci and mono-focus light fields under high numerical aperture (NA > 0.67) conditions at two working wavelengths (405 nm and 808 nm) have been demonstrated both numerically and experimentally.

744-nm wavelength conversion of PAM-4 signal using an AlGaAsOI nanowaveguide

Deming Kong, Minhao Pu, Yong Liu, Yi Zheng, Elizaveta Semenova, Kresten Yvind, Leif Oxenløwe, Michael Galili, and Hao Hu

DOI: 10.1364/OL.383085 Received 14 Nov 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: Exploring new frequency bands for optical transmission is essential to overcome the capacity crunch. The 2-μm band is becoming a research spotlight due to available broadband thulium-doped fiber amplifiers as well as low-latency, low-loss hollow-core fibers. Yet most of the 2-μm band devices designed for optical communication are still in their infancy. In this letter, we propose wavelength conversion based on four-wave mixing in a highly nonlinear AlGaAsOI nanowaveguide to bridge the 2-μm band and the conventional bands. Thanks to the strong light confinement of the AlGaAsOI nanowaveguide, high order phase match is enabled by dispersion engineering to achieve a large synergetic conversion bandwidth with high conversion efficiency. Simulation results show a possible conversion bandwidth over an octave. An AlGaAsOI nanowaveguide with 3-mm length and a nominal crosssection dimension of 320 nm × 680 nm is used for the wavelength conversion of a 10 Gbit/s NRZ-OOK signal and a 10 Gbit/s Nyquist-shaped PAM-4 signal. A conversion efficiency of -28 dB is achieved using a 17.5- dBm continuous-wave pump in the C band, with 744 nm conversion from 1999.65 nm to 1255.35 nm.

Tunable ultra-flat optical comb enabled reconfigurable and efficient coherent channelized receiver

Huan Huang, ruofan wang, Chongfu Zhang, Yong Chen, Haifeng Yang, and Kun Qiu

DOI: 10.1364/OL.385458 Received 09 Dec 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: Due to the limitation of analog-to-digital/digital-to-analog converters, optical channelized receivers are thought to be a promising approach for receiving broadband microwave signals. Herein, based on two advanced tunable ultra-flat optical frequency comb generators (T-FOCGs), a coherent channelized receiver with high channelized efficiency and reconfigurability is proposed. In the T-FOCG, the number of 1-dB comb lines increases with the gain, but the optical power of these 1-dB comb lines has almost the constant variance. In the proposed scheme, one optical carrier can support four sub-channels. Meanwhile, the number and bandwidth of sub-channels, as well as the bandwidth and center frequency of an original broadband signal are all tunable. We verify the feasibility of the coherent channelized receiver by channelizing a 4 GHz signal with a 20 GHz center frequency into four 1 GHz sub-channels, and the reconfigurability is demonstrated by channelizing a 10 GHz signal with frequencies from 18 GHz to 28 GHz into five 2 GHz sub-channels. Moreover, the performance of the coherent channelized receiver for signal reception is discussed based on 625 Mbit/s 64-QAM signals. The error-vector magnitudes (EVM) curves of the directly received and the channelized QAM signal at different amounts of beat noise are compared.

Acoustic Phonon Sideband Dynamics During Polaron Formation in a Single Quantum Dot

Daniel Wigger, Vage Karakhanyan, Christian Schneider, Martin Kamp, Sven Höfling, Paweł Machnikowski, Tilmann Kuhn, and Jacek Kasprzak

DOI: 10.1364/OL.385602 Received 12 Dec 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: When an electron-hole pair is optically excited in a semiconductorquantum dot the host crystal lattice needsto adapt to the presence of the generated charge distribution. Therefore the coupledexciton-phonon system has to establish a new equilibrium, which isreached in the form of a quasiparticle called polaron. Especially,when the exciton is abruptly generated on a timescale faster thanthe typical lattice dynamics, the lattice displacement cannot followadiabatically. Consequently, a rich dynamics on the picosecondtimescale of the coupled system is expected. In this study wecombine simulations and measurements of the ultrafast, coherent,nonlinear optical response, obtained by four-wave mixing spectroscopy, to resolve theformation of this polaron. By detecting and investigating the phononsidebands in the four-wave mixing spectra for varying pulse delaysand different temperatures we have access to the influence ofphonon emission and absorption processes which finally result in theemission of an acoustic wave packet out from the quantum dot.

Thermal-induced luminescence enhancement of BAC-P in bismuth-doped phosphogermanosilicate fibers

Zhao Qiancheng, Qun Hao, Yanhua Luo, and GangDing Peng

DOI: 10.1364/OL.386293 Received 18 Dec 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: The thermal quenching effect has been systematically investigated in Bi-doped phosphogermanosilicate fiber with varying thermal conditions. For the first time, the activation of phosphor-related bismuth active center (BAC-P) is achieved by thermal quenching at 400 °C with a heating time of 10 min, evidenced by the enhanced luminescence of BAC-P (~1.3 times) at 1300 nm. The experimental results reveal that a relatively low heating temperature with prolonged heating time stimulates the growth of BAC-P, whereas higher operating temperatures (≥500 °C) result in irreversible destruction of BAC-P. The underlying mechanism for the thermally-stimulated process of BAC-P is also analyzed and discussed.

Generation of (3, 1) vector signals based on optical carrier suppression without pre-coding

jiangnan xiao, Xingxing Feng, Wen Zhou, Bo Liu, Xu Dong, Chuang Zhao, Jiangli Zuo, Jiao Zhang, and Li Zhao

DOI: 10.1364/OL.382624 Received 11 Nov 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: We experimentally and numerically demonstrated the generation of a (3,1) vector signal by a single Mach-Zehnder modulator (MZM) without pre-coding. The MZM is driven by the (3,1) modulated signal after photoelectric conversion by the ‘square law’ of photo detector (PD). Although the phase changes, the corresponding constellation distribution is consistent with that of the regular signal. Our proposed scheme effectively avoids the pre-coding process with a simple architecture. The bit-error-ratio (BER) results indicate that the (3,1) signal has a better BER performance than the pre-coded quadrature phase shift keying (QPSK) vector signal, and both are below 3.8×10-3 after 25 km optical fiber transmission.

Asymmetric direct detection of twin-SSB signals

Xueyang Li, MINGYUE ZHU, Zhenping Xing, Maurice O'Sullivan, and David Plant

DOI: 10.1364/OL.383029 Received 13 Nov 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: We propose the asymmetric direct-detection (ADD) of twin-SSB signals based on a simple receiver front-end composed of one optical filter and two photodiodes. ADD exploits the photocurrent difference between a filtered and unfiltered signal pair to reconstruct and linearize the received twin-SSB signal with a high electrical spectral efficiency (ESE). We evaluate the performance impact of the critical system parameters on ADD and demonstrate 1-Gb/s net rate 16-QAM twin-SSB transmission with 6.03 b/s/Hz ESE over 80 km standard single-mode fiber (SSMF) below the 1×10-2 hard-decision forward error correction (HD-FEC) threshold. We also found that the BER performance of ADD is robust against the relative center wavelength drifting of the optical filter.

Transfer printing of AlGaAs-on-SOI micro-diskresonators for selective mode coupling and low-powernon-linear processes

John McPhillimy, Stuart May, Charalambos Klitis, Benoit Guilhabert, Martin D. Dawson, Marc Sorel, and Michael Strain

DOI: 10.1364/OL.384962 Received 03 Dec 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: The transfer printing of aluminium gallium arsenide(AlGaAs) micro-disk resonators onto a silicon-on-insulator(SOI) waveguide platform is demonstrated.The integrated resonators exhibit loaded Q-factorsreaching 4x10⁴, and the vertical assembly approach allowsselective coupling to different spatial mode families.The hybrid platform’s non-linearity is characterizedby four-wave mixing (FWM) with a measured nonlinearcoefficient of γ=325 (Wm)¹, with the devicesdemonstrating minimal two-photon absorption (TPA)and free-carrier absorption (FCA) losses that are inherentto SOI at telecommunications wavelengths.

Compact plasmonic fiber tip for ultra-sensitive and fast humidity and human breath monitoring

bobo du, Dexing Yang, Yinlan Ruan, Peipei Jia, and Heike Ebendorff-Heidepriem

DOI: 10.1364/OL.381085 Received 30 Oct 2019; Accepted 09 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: We demonstrate the first plasmonic fiber tip for relative humidity detection by integrating a gold nanomembrane onto the end-face of a multimode optical fiber via a flexible and high-efficiency transfer method. Fast water condensation/evaporation is responsible for the ultrahigh performance of the fiber tip in response to RH. An ultrahigh sensitivity of 279 pm/%RH (relative humidity) is obtained in the range of 11%~92%RH. Taking advantages of the ultra-fast dynamics (response and recovery times of 156 ms and 277 ms), the plasmonic fiber tip offers an excellent capability towards detection of human breaths at varied frequencies and depths. The compact, easy-fabrication, and fast-dynamics plasmonic platform has versatile potential for practical applications, including environmental, healthcare monitoring, as well as biochemical sensing.

Silicon-based high-responsivity GeSn short-wave infrared heterojunction phototransistors with a floating base

Guo-En Chang, Wei Ting Hung, Devesh Barshilia, RIKMANTRA BASU, and Henry Cheng

DOI: 10.1364/OL.383171 Received 15 Nov 2019; Accepted 09 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: We demonstrate silicon-based p-n-p floating-base GeSn heterojunction phototransistors with enhanced optical responsivity for efficient short-wave infrared (SWIR) photodetection. The narrow-bandgap GeSn active layer sandwiched between the p-Ge collector and n-Ge base effectively extends the photodetection range in the SWIR range and the internal gain amplifies the optical response by a factor of more than three at a low driving voltage of 0.4 V compared to that of a reference GeSn p-i-n photodetector. We anticipate our findings will be leveraged to realize CMOS-compatible, sensitive, low driving voltage SWIR photodetectors in a wide range of applications.

Flexible Spectrum Sharing of Two Asynchronous QAM Signals using Power Division Multiplexing

Peicheng Liao, Kaiheng Zou, Huibin Zhou, Ahmad Fallahpour, Nanzhe Hu, Yinwen Cao, Ahmed Almaiman, Fatemeh Alishahi, Changjing Bao, Moshe Tur, and Alan Willner

DOI: 10.1364/OL.383931 Received 20 Nov 2019; Accepted 09 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: We numerically and experimentally report the flexible spectrum sharing of two asynchronous quadrature amplitude modulated (QAM) signals using power division multiplexing. We show that a hybrid QAM signal is generated when two QAM signals with different power are superposed. By exploiting successive interference cancellation, a 20 Gbaud “strong” signal combined with a 9- or 4-Gbaud “weak” can both be recovered sequentially with bit-error rate (BER) performance below the forward error correction (FEC) threshold. In addition, we show the dependence of system performance on the power ratio between the strong and weak signals. These two signals can contain different baudrates, pulse shapes and modulation formats.

Upconversion pumping of a 2.3-µm Tm3+:KY3F10 laser with a 1064-nm ytterbium fiber laser

Alphan Sennaroglu, Yagiz Morova, Mauro Tonelli, and Valentin Petrov

DOI: 10.1364/OL.384284 Received 26 Nov 2019; Accepted 09 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: We report efficient lasing of the isotropic Tm3+:KY3F10 crystal near 2.3 µm via upconversion pumping with a 1064-nm ytterbium (Yb) fiber laser as the pump source. When pumped at 1064 nm, an x-cavity Tm3+:KY3F10 laser operated at the free running wavelength of 44 nm. Lasing was obtained with output couplers having transmission in the range of 1-3%, generating as high as 124 mW of continuous-wave (cw) output power with 604 mW of absorbed pump power. Broadly tunable cw lasing could be obtained in the 2268- 73 nm wavelength range. Analysis of the experimental power efficiency data shows that nearly all of the absorbed pump photons were converted to 2.3-µm laser output after accounting for the quantum defect of the laser transition and resonator losses. We foresee that it should be possible to achieve even higher lasing efficiency with respect to the incident pump power by using longer crystals to absorb more of the pump power and by using pump sources that are tuned to the peak of the excited-state absorption resonance

Experimental investigation on temporal contrast of pre-pulses by post-pulses in a petawatt laser facility

Hiromitsu Kiriyama, Yasuhiro Miyasaka, Akito Sagisaka, Koichi Ogura, Mamiko Nishiuchi, Alexander Pirozhkov, Yuji Fukuda, masaki kando, and Kiminori Kondo

DOI: 10.1364/OL.384759 Received 11 Dec 2019; Accepted 09 Jan 2020; Posted 15 Jan 2020  View: PDF

Abstract: We experimentally explore the generation of pre-pulses by post-pulses, created through internal reflection in the optical components, by the non-linear process associated with the B-integral in the laser chain of the petawatt facility J-KAREN-P. At large time delay between the main and the post-pulses, we have found that the pre-pulses are not generated from their counterpart post-pulses at an identical time difference before the main pulse and the temporal shapes of the pre-pulses are greatly distorted asymmetrically. We have also observed the peak intensities of the pre-pulses are drastically suppressed compared to the expected value at small time delay. We briefly describe the origins of the pre-pulses generated by the post-pulses and demonstrate the removal of the pre-pulses by switching to optical components with a small wedge angle at our petawatt laser facility.

A compact ultrastable photonic microwave oscillator

Michele Giunta, Jialiang Yu, Maurice Lessing, Marc Fischer, Matthias Lezius, Xiaopeng Xie, GIORGIO SANTARELLI, Yann Le Coq, and Ronald Holzwarth

DOI: 10.1364/OL.385503 Received 09 Dec 2019; Accepted 09 Jan 2020; Posted 15 Jan 2020  View: PDF

Abstract: Frequency comb synthesized microwaves have been so far realized with tabletop systems, operated in well-controlled environments. Here we demonstrate state-of-the-art ultrastable microwave synthesis with a compact rack-mountable apparatus. We present absolute phase noise characterization of a 12 GHz signal using an ultrastable laser at ~194 THz and an Er:fiber comb divider, obtaining -83 dBc/Hz at 1 Hz and < -166 dBc/Hz for offsets greater than 5 kHz. Employing semiconductor coating mirrors for the same type of transportable optical frequency reference, we show that -105 dBc/Hz at 1 Hz is supported by demonstrating residual noise limit of division and detection process of -115 dBc/Hz at 1 Hz. This level of fidelity paves the way for the deployment of ultrastable photonic microwave oscillators and for operating transportable optical clocks.

A miniature Fabry-Perot interferometer based on a movable microsphere reflector

Ke Tian, Jibo Yu, Xin Wang, Haiyan Zhao, Dejun Liu, Elfed Lewis, Gerald Farrell, and Pengfei Wang

DOI: 10.1364/OL.385222 Received 05 Dec 2019; Accepted 08 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: We propose and demonstrate a miniature Fabry-Perot interferometer (FPI) based on a movable microsphere reflector. The movable microsphere acts as a good reflector, with the reflections occurring at the spliced single-mode fiber/hollow-core fiber interface and the surface of a microsphere, resulting in two-beam interference. The silica microsphere is formed at the tip of a half tapered optical fiber, and its diameter can be reduced to miniaturize the FPI. The movable microsphere interferometer exhibits a highly linear response to external displacement change, and a high displacement sensitivity of 11.9 pm/nm with a nanoscale resolution of 1.7 nm is achieved. Wide-range displacement can also be measured by monitoring the changes in free spectral range (FSR) of the reflection spectrum. This miniaturized FPI may therefore find use in applications in nano-displacement measurement fields, and the concept of a movable microsphere reflector is of great significance for the miniaturization of micro-photonic devices.

Theoretical formalism for off-normal angular coupling into selective and parity-dependent modes in a planar waveguide

Brett Carnio and Abdulhakem Elezzabi

DOI: 10.1364/OL.381569 Received 31 Oct 2019; Accepted 07 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: A theoretical formalism is presented to describe coupling of an electromagnetic field into the modes of a planar waveguide, where the electromagnetic field has a non-uniform transverse profile and is incident at an arbitrary angle. This formalism is essential to phase-matched frequency-conversion waveguides based on nonlinear optical phenomena, which can rely on coupling the excitation field into selective higher-order waveguide modes of either even or odd parity. The theoretical approach is used to investigate coupling of a Gaussian electromagnetic field into a LiNbO3 planar waveguide, where the calculations are shown to be in excellent agreement with finite-different time-domain simulations. Emphasis is placed on exciting the waveguiding at an off-normal incident angle, since this allows for coupling of the electromagnetic field into the odd-ordered waveguide modes, which cannot be excited at normal-incidence.

Tunable Stokes laser based on the cascaded stimulated polariton scattering and stimulated Raman scattering in RbTiOPO4 crystal

Feilong Gao, Xingyu Zhang, zhenhua cong, Zhaojun Liu, Xiaohan Chen, Zengguang Qin, Peng Wang, and Jinjin Xu

DOI: 10.1364/OL.383885 Received 21 Nov 2019; Accepted 07 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: The stimulated polariton scattering (SPS) and the stimulated Raman scattering (SRS) in RbTiOPO4 (RTP) crystal is combined in an intracavity-pumped Stokes parametric oscillator (SPO) to extend the tunable Stokes laser spectral range. The pumping laser wavelength is 1064 nm from a diode-end-pumped acousto-optically Q-switched Nd:YAG laser. By the SPS process in the SPO, the SPS-Stokes wave can be discontinuously tuned in the range of 1075.7-1076.0 nm, 1077.7-1080.4 nm, 1081.8-1082.2nm, and 1084.8-1087.8 nm, respectively. By the following SRS process in the same RTP crystal, the laser wavelength is further shifted in the range of 1107.7-1108.1 nm, 1109.0-1112.7 nm, 1114.3-1115.1 nm, and 1117.8-1121.1 nm, respectively. The peak wavelength is 1118.8 nm at which the average output power is maximal. The obtained maximum output power of the SRS-Stokes wave is 970 mW. It is obtained when the diode power is 7.9 W and the pulse repetition frequency (PRF) is 10 kHz.

Spatial differential operation and edge detection based on geometric spin Hall effect of light

Shanshan He, Junxiao Zhou, Shizhen Chen, Weixing Shu, Hailu Luo, and Shuangchun Wen

DOI: 10.1364/OL.386224 Received 17 Dec 2019; Accepted 07 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: Unlike the conventional spin Hall effect of light (SHEL) originating from the light-matter interaction, the spin-dependent splitting in the geometric SHEL is purely geometric effect and independent from the properties of matter. Here, it is shown that the geometric SHEL is not only of fundamental theoretical interest in understanding the spin-orbit interaction of light, but also sheds light on important technological applications. This Letter describes the theoretical foundation and experimental realization of optical differential operation and edge detection based on the geometric SHEL.

Continuous scanning of a dissipative Kerr-microresonator soliton comb for broadband, high resolution spectroscopy

Naoya Kuse, Tomohiro Tetsumoto, Gabriele Navickaite, Michael Geiselmann, and Martin Fermann

DOI: 10.1364/OL.383036 Received 12 Nov 2019; Accepted 07 Jan 2020; Posted 08 Jan 2020  View: PDF

Abstract: Dissipative Kerr-microresonator soliton combs (hereafter called soliton combs) are promising to realize chip scale integration of full soliton comb systems providing high precision, broad spectral coverage and a coherent link to the micro/mm/THz domain with diverse applications coming on line all the time. However, the large soliton comb spacing hampers some applications. For example, for spectroscopic applications, there are simply not enough comb lines available to sufficiently cover almost any relevant absorption features. Here, we overcome this limitation by scanning the comb mode spacing by employing PDH locking and a microheater on the microresonator, showing continuous scanning of the soliton comb modes across nearly the full FSR of the microresonator without losing soliton operation, while spectral features with a bandwidth of as small of 5 MHz are resolved.

Chalcogenide microfiber-assisted silica microfiber for ultrasound detection

huibo fan, Liang Chen, and Xiaoyi Bao

DOI: 10.1364/OL.383238 Received 14 Nov 2019; Accepted 07 Jan 2020; Posted 08 Jan 2020  View: PDF

Abstract: An ultra-compact chalcogenides (ChGs) microfiber-assisted silica microfiber is fabricated to detect ultrasound wave with high signal-to-noise ratio (SNR) over broadband frequency response based on a piezoelectric transducer (PZT) centered at 3.7 MHz as ultrasound source. ChGs (As2Se3) microfiber with a diameter of 2 μm is adhered to the surface of silica microfiber with a diameter of 5 μm via Van der Waals force. Owing to high refractive index and small Young’s modulus of ChGs material, transmission spectrum with high contrast are produced based on multi-mode interference. SNR response could be up to 70 dB, especially at low frequency due to the soft ChGs microfiber as sensing unit to magnify the ultrasound signal, along with SNR over 12 dB at ultrasound of 31.2 MHz. As a comparison, silica taper with the same size as ChGs microfiber is also placed on the silica microfiber with diameter of 5 μm to detect ultrasound signal over 18 kHz to 9.4 MHz with SNR lower than that based on ChGs microfiber up to 38.8 dB, showing the high sensitivity of ChGs microfiber.

Asymmetric Chiroptical Effect from Chiral Medium Filled Golden Slit Grating on Substrate

Ming Yong, Chen Feiliang, Xuannan Wu, Lin Pang, fuhua Gao, and Yidong Hou

DOI: 10.1364/OL.384773 Received 03 Dec 2019; Accepted 06 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: In this work, we report a giant and robust asymmetric chiroptical effect (ACOE) in the Chiral medium filled golden slit Grating on glass Substrate (CMGSG-GS). This ACOE comes from the influence of interface asymmetry on the electromagnetic cross-coupling in CMGSG-GS, and is inherently different with that reported in the Faraday medium and the planar anisotropic chiral metamaterials. Both of the polarization eigenstate and the transmission matrix are highly-depended on the metal structure used in CMGSG-GS. The polarization eigenstates of CMGSG-GS are two co-rotating elliptical states with ellipticity of nearly 0, and keeps mainly unchanged for opposite directions. The transmission matrices of opposite directions are normal matrices, which do not show any symmetric law although the geometry of CMGSG-GS owns a high rotational symmetry. The reported ACOE gives a measureable physical parameter to reveal the events happening at interface.

A two-photon microscope using a fiber based approach for supercontinuum generation and light delivery to a small footprint optical head

Youbo Zhao, Gopi Maguluri, R. Daniel Ferguson, Haohua Tu, Kush Paul, Stephen Boppart, Daniel Llano, and Nicusor Iftimia

DOI: 10.1364/OL.381571 Received 28 Oct 2019; Accepted 06 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: We report a low-cost, portable two-photon excitation fluorescence microscopy imager that uses a fiber-based approach for both femtosecond supercontinuum (SC) generation and light delivery to the optical head. The SC generation is based on a tapered polarization-maintaining photonic crystal fiber that uses pre-chirped femtosecond narrowband pulses to generate a coherent SC spectrum with a bandwidth of ~ 300 nm. Using this approach, high-power, near-transform-limited, wavelength-selectable SC pulses are generated and directly delivered to the imaging optical head. Preliminary testing of this imager on brain slices is presented, demonstrating high signal-to-noise ratio, sub-cellular imaging capabilities to a depth of approximately 200 um. These results demonstrate the suitability of the technology for ex vivo and potentially in vivo cellular-level biomedical imaging applications.

Dual-side View Optical Coherence Tomography for Thickness Measurement on Opaque Materials

Wu Qian, Wang Xiwen, Linbo Liu, and Jianhua Mo

DOI: 10.1364/OL.384337 Received 28 Nov 2019; Accepted 05 Jan 2020; Posted 06 Jan 2020  View: PDF

Abstract: Optical coherence tomography (OCT), as an optical interferometric imaging technique, has found wide applications in various fields. In principle, OCT is well suited for imaging layered structures and thus one of the typical applications is thickness measurement. However, due to the limited imaging depth resulting from light attenuation, thickness measurement by OCT is limited to non-opaque materials. In this study, we developed a novel dual-side view OCT (DSV-OCT) system for thickness measurement on opaque materials. The dual-side view was achieved on conventional swept source OCT platform by creating two symmetrical sampling arms. This allows to image both sides of the material simultaneously and produce the surface contours of the two sides in a single C-scan. Finally, the thickness of the opaque material can be calculated from the two surface contours above. We demonstrated that our DSV-OCT technique can measure the thickness of opaque material with an accuracy of about 3 μm.

Hartmann-Shack wavefront reconstruction with bitmap image processing

Vitalii Bezzubik, Nickolai Belashenkov, Oleg Soloviev, Vladimir Vasilyev, and Gleb Vdovin

DOI: 10.1364/OL.383464 Received 18 Nov 2019; Accepted 04 Jan 2020; Posted 06 Jan 2020  View: PDF

Abstract: We report on an algorithm and its implementation to reconstruct the wavefront as a continuous functionfrom a bitmap image of a Hartmann-Shack pattern. The approachworks with arbitrary raster geometry and does not requireexplicit spot definition and phase unwrapping. The system matrix, defining the coefficients of wavefront decomposition in the system of basis functions, is obtained as a result of a series of convolutions and thresholding operations on the reference and sample images.

Sparse hand-held probe for optoacoustic ultrasound volumetric imaging: an experimental proof of concept study

Mohammad Azizian Kalkhoran, Francois Varray, ALESSANDRO SAVOIA, and Didier VRAY

DOI: 10.1364/OL.384002 Received 21 Nov 2019; Accepted 04 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: We present an experimental proof of concept study on the performance of sparse segmented annular array for optoacoustic imaging. A capacitive micromachined ultrasonic transducer (CMUT) was equipped with a negatively focused acoustic lens and scanned in an annular fashion to exploit the performance of sparse array geometry proposed in our recent numerical studies [1,2]. A dedicated water tank was made using a 3D printer for light delivery and mounting the sample. A phantom experiment has been carried out to showcase the possibility of full-field optoacoustic ultrasound (OPUS) imaging and confirm the earlier numerical results. This proof of concept opens the door towards a prototype of OPUS imaging for the (pre-) clinical studies.

Fully waterproof two-axis galvanometer scanner for enhanced wide-field optical-resolution photoacoustic microscopy

Jaeyul Lee, Sangyeob Han, Daewoon Seong, Junsoo lee, Sungjo Park, Ruchire Wijesinghe, Man Sik Jeon, and jeehyun kim

DOI: 10.1364/OL.380032 Received 08 Nov 2019; Accepted 04 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: Large field-of-view and fast scanning of photoacoustic microscopy relatively have been difficult to obtain due to the water-drowned structure of the system for the transmission of ultrasonic signals. It has been eagerly studied to achieve a waterproof scanner for dynamic biological applications with the high-resolution and high signal-to-noise ratio. This letter reports a novel waterproof galvanometer scanner-based photoacoustic microscopy system with successfully attainable 9.0 mm × 14.5 mm scan-region, amplitude-scan rate of 40 kHz, and spatial resolution of 4.9 μm. The in vivo characterization of mouse brain in intact-skull microvascular visualization demonstrated its capability in biomedical imaging and anticipating to be an effective technique for various preclinical and clinical studies.

Illumination modulation for the reflective and fluorescent separation

Ying Fu, Yunhao Zou, Liheng Bian, Yuxiang Guo, and Hua Huang

DOI: 10.1364/OL.384624 Received 02 Dec 2019; Accepted 03 Jan 2020; Posted 16 Jan 2020  View: PDF

Abstract: Fluorescence commonly exists in various living things and minerals, and has been widely applied in fluorescence probe, drug tracing, and image relighting, to name a few. In this letter, we present a novel illumination modulation method for the reflective and fluorescent separation by using only a spectral image. Specifically, we present an illumination modulation system using off-the-shelf devices to generate high-frequency illumination that is desirable in reflective-fluorescent separation task. Besides, we exploit the total variation regularization scheme to account for spectral-spatial correlation, which makes our method robust to noise. Experiments on both simulated and real data verify the practicality and effectiveness of our method.

Mid-infrared electro-optic dual-comb spectroscopy with feed-forward frequency stepping

XINYI REN, Hu Dai, Detian Li, Kun Huang, MENGYUN HU, Tianjian Lv, Ming Yan, and Heping Zeng

DOI: 10.1364/OL.385464 Received 10 Dec 2019; Accepted 03 Jan 2020; Posted 06 Jan 2020  View: PDF

Abstract: In this work, we utilize an acoustic-optic frequency shifter, in a feed-forward manner, for automatic interpolation of dual-comb spectroscopy, where frequency tuning can be achieved at 5.45 THz/s with step size precisely locked to the line spacing (54.5 MHz) of a referenced optical comb, without complicated electronics or control programs. Our dual-comb spectrometer involves two near-infrared electro-optic combs at 25 GHz line spacings, nonlinearly converted into the mid-infrared region, revealing fundamental absorption lines of methane gas at 54.5 MHz resolution within a spectral range from 88.04 to 89.04 THz. The method and the system may find useful in many applications including gas sensing.

Graphene mode-locked operation of Tm3+:YLiF4 and Tm3+:KY3F10 lasers near 2.3 μm

Abdullah Muti, Ferda Canbaz, Mauro Tonelli, Ji Bae, Fabian Rotermund, Valentin Petrov, and Alphan Sennaroglu

DOI: 10.1364/OL.385629 Received 16 Dec 2019; Accepted 03 Jan 2020; Posted 06 Jan 2020  View: PDF

Abstract: We report, for the first time to our knowledge, experimental demonstration of graphene mode-locked operation of Tm3+:YLiF4 (YLF) and Tm3+:KY3F10 (KYF) lasers near 2.3 μm. To scale up the intra-cavity pulse energy, the cavity was extended and double end pumping was employed with a continuous-wave, tunable Ti3+:sapphire laser delivering up to 1 W near 780 nm. The extended Tm3+:KYF laser cavity was purged with dry nitrogen to eliminate pulsing instabilities due to atmospheric absorption lines but this was not needed in the case of the Tm3+:YLF laser. Once initiated by graphene, stable uninterrupted mode-locked operation could be maintained with both lasers. With the extended cavity Tm3+:YLF laser, 921-fs pulses were generated at a repetition rate of 17.2 MHz at 04 nm. 739-fs pulses were obtained at the repetition rate of 54 MHz from the Tm3+:KYF laser at 40 nm. The corresponding pulse energy and peak power were 2.4 nJ and 2.6 kW for the Tm3+:YLF laser, and 1.2 nJ and 1.6 kW for the Tm3+:KYF laser. We foresee that it should be possible to generate shorter pulses at higher pump levels.

Characteristic Mueller matrices for direct assessment of breaking of symmetries

Hui Ma, Pengcheng Li, Honghui He, and Tariq Aziz

DOI: 10.1364/OL.375543 Received 16 Aug 2019; Accepted 03 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: Mueller polarimetry is a powerful tool in the analysis of microstructural information of optical samples. However, the relation between individual Mueller matrix elements and physical properties of the sample is not explicit.Various matrix decomposition algorithms corresponding to specific optical models were proposed to extract physical information from the measured Mueller matrix, but we still lack a prior assessment method to decide which model is appropriate.In this letter, we propose a set of characteristic Mueller matrices, which allow us to obtain general information about breaking of rotation, mirror and reciprocal symmetry properties in the sample by the direct inspection of several Mueller matrix elements.By further analysis the possible origin of symmetry breaking, we can estimate the type of anisotropies and their mixing status in the measured optical sample, thus help experimenters to decide appropriate Mueller matrix decomposition methods. Our theory is verified with Monte Carlo simulation of polarized light scattering in isotropic or anisotropic medium containing different configurations of spherical and cylindrical scatterers.

A novel fiber endface illumination diffuser for endo-cavity photoacoustic imaging

Quing Zhu, Hongbo Luo, and Guang Yang

DOI: 10.1364/OL.379844 Received 07 Oct 2019; Accepted 02 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: The laser illumination delivery method is important in designing probes that achieve high imaging quality and deep tissue penetration. Here, we present a novel fiber diffuser tip using microspheres dispersed within an ultraviolet (UV) adhesive to scatter light. This diffuser keeps the skin surface fluence under the maximum permissible exposure (MPE), while enabling higher laser energy injection to enhance the photoacoustic (PA) signal generated from the tissue. We compare the light diffusion effects of different microsphere materials, sizes, and concentrations, and find 10 um silica microspheres provide the best light scattering with minimal 5% output energy loss. With Zemax simulation and experimental validation, we show this fiber diffuser tip is a useful tool for endo-cavity photoacoustic imaging.

Chirped anti-resonant reflecting optical waveguide for the distributed sensing of pressure

Ran Gao, Qi Zhang, Xin Xiangjun, Qinghua Tian, and Feng Tian

DOI: 10.1364/OL.382716 Received 08 Nov 2019; Accepted 02 Jan 2020; Posted 06 Jan 2020  View: PDF

Abstract: A chirped anti-resonant reflecting optical waveguide (ARROW) for the simultaneous measurement of pressure intensity and spatial localization has been proposed and experimentally demonstrated. A section of the hollow-core fiber was etched to fabricate a gradient cladding, which forms a novel chirped ARROW. The etched chirped ARROW shows a chirped spectral characteristic due to the gradient thickness of the cladding, which generates a multiple-resonance condition. Besides, an in-line Mach-Zehnder interferometer is also formed with the core mode and higher-order modes. The pressure intensity and spatial localization can be detected by interrogating the wavelength shift of the in-line Mach-Zehnder interferometer and the chirped ARROW, respectively. The experimental results show that the pressure sensitivity of −4.42nm/MPa can be achieved. Meanwhile, multi-point pressure detection can be realized by interrogating wavelength peaks corresponding to different resonance conditions. The proposed fiber optic sensor can be used for multi-point pressure detection in the fields of security, structure monitoring, and oil exploration, etc.

Theory of the quasi-steady-state self-focusing of partially coherent light pulses in nonlinear media

HUAN WANG, Xiaoling Ji, YU DENG, Xiaoqing Li, and Hong Yu

DOI: 10.1364/OL.379902 Received 09 Oct 2019; Accepted 02 Jan 2020; Posted 06 Jan 2020  View: PDF

Abstract: The quasi-steady-state (QSS) self-focusing of partially coherent light pulses (PCLPs) in nonlinear media is studied. The analytical formulae of the QSS self-focusing of PCLPs in nonlinear media (e.g., the beam width, the spatial coherence width and the focal length, etc.) are presented. The effect of spatial coherence on the focal length and the focus moving is investigated in detail. In particular, it is found find that a PCLP has more advantages to avoid the optical damage of materials than a fully coherent light pulse (FCLP).

Stimulated Four-Wave Mixing in Linearly Uncoupled Resonators

Kang Tan, Matteo Menotti, Zachary Vernon, John Sipe, Marco Liscidini, and Blair Morrison

DOI: 10.1364/OL.381563 Received 28 Oct 2019; Accepted 02 Jan 2020; Posted 06 Jan 2020  View: PDF

Abstract: We experimentally demonstrate stimulated four-wave mixing in two linearly uncoupled integrated Si3N4 micro-resonators. In our structure the resonance combs of each resonator can be tuned independently, with the energy transfer from one resonator to the other occurring in the presence of a nonlinear interaction. This method allows flexible and efficient on-chip control of the nonlinear interaction, and is readily applicable to other third-order nonlinear phenomena.

Particle trapping and beaming using a 3D nanotip excited with plasmonic vortex

Kai Liu, Nicolò Maccaferri, Yuefeng Shen, Xueyun Li, remo proietti zaccaria, Xuejin Zhang, Yuri Gorodetski, and Denis Garoli

DOI: 10.1364/OL.384899 Received 03 Dec 2019; Accepted 02 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: Recent advances in nanotechnology have prompted the need for tools to accurately and non-invasively manipulate individual nano-objects. Among the possible strategies, optical forces have been widely used to enable nano-optical tweezers capable of trapping or moving a specimen with unprecedented accuracy. Here, we propose an architecture consisting of a nanotip excited with a plasmonic vortex enabling effective dynamic control of nanoparticles in three dimensions. The structure illuminated by a beam with angular momentum can generate an optical field which can be used to manipulate single dielectric nanoparticles. We demonstrate that it’s possible to stably trap or push the particle from specific points, thus enabling a new platform for nanoparticle manipulation.

High-energy 2-µm pulsed vortex beam excitation from a Q-switched Tm:LuYAG laser

Ying Chen, Manman Ding, jianlei wang, Li Wang, Qiyao Liu, Yongguang Zhao, Ying LIU, Deyuan SHEN, zhengping wang, Xin-guang Xu, and Valentin Petrov

DOI: 10.1364/OL.384201 Received 25 Nov 2019; Accepted 01 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: We report on the first direct generation of pulsed vortex beams from a Q-switched 2-µm Tm:LuYAG laser. High-energy first-order Laguerre-Gaussian (LG0,1) pulsed laser beams with well-defined handedness are selectively excited through spatially matched pump gain distribution and asymmetric cavity loss without using any intracavity handedness-selective optical elements. Pulse energy of 1.48 mJ for the LG0,+1 mode and 1.51 mJ for the LG0,–1 mode is respectively achieved at a repetition rate of 500 Hz. The pulsed laser beams with helical wave front are potentially useful for studying OAM transformation dynamics, generation of mid-IR vortex beams, and nanostructuring of organic materials.

480-nm distributed-feedback InGaN laser diode for 10.5-Gbit/s visible-light communication

Jorge Holguin Lerma, Meiwei Kong, Omar Alkhazragi, Xiaobin Sun, Tien Khee Ng, and Boon Ooi

DOI: 10.1364/OL.385954 Received 13 Dec 2019; Accepted 30 Dec 2019; Posted 06 Jan 2020  View: PDF

Abstract: In this letter, we demonstrate a novel distributed-feedback (DFB) InGaN-based laser diode with narrow-linewidth emission at ~480 nm (sky blue) and its application to high-speed visible-light communication (VLC). A significant side-mode suppression ratio (SMSR) of 42.4 dB, an optical power of ~14 mW, and a resolution-limited linewidth of ~34 pm were obtained under continuous-wave operation. A 5-Gbit/s VLC link was realized using non-return-to-zero on-off keying (NRZ-OOK) modulation, whereas a high-speed 10.5-Gbit/s VLC data rate was achieved by using a spectral-efficient 16-quadrature-amplitude-modulation orthogonal frequency-division multiplexing (16-QAM-OFDM) scheme. The reported high-performance sky-blue DFB-laser is promising in enabling unexplored dense wavelength-division multiplexing (DWDM) schemes in VLC, narrow-line filtered systems, and other applications where single-frequency lasers are essential such as atomic clocks, high-resolution sensors, and spectroscopy. Single-frequency emitters at the sky-blue wavelength range will further benefit applications in the low-path-loss window of underwater media as well as those operating at the H-beta Fraunhofer line at ~486 nm.

Dark solitons in the exploding pulsation of the bright dissipative soliton in ultrafast fiber lasers

Xuewen Shu, Du Yueqing, and Mengmeng Han

DOI: 10.1364/OL.381293 Received 23 Oct 2019; Accepted 30 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: Soliton explosion is an extremely pulsating behavior of the bright dissipative soliton in ultrafast lasers. By numerical simulation, we find the dark solitons can coexist with the bright soliton during the exploding process. The collapsed temporal structure of the exploding soliton is induced by the dark solitons. We reveal the birthing, evolving and decaying of the dark solitons inside the bright dissipative soliton. The time-frequency analysis of the exploding soliton helps us to better understand the temporal and spectral structures of the exploding soliton, which might be useful for real-time spectroscopy of the coexisted dark and bright solitons during the soliton explosion.

Nanoheater-tuned whispering gallery mode lasing in liquid-filled hollow microcavities

Hanyang Li, Xiaolei Hao, Yanzeng Li, li Xu, bojian shi, and Lu Liu

DOI: 10.1364/OL.383024 Received 13 Nov 2019; Accepted 30 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: An all-optical tunable whispering gallery mode (WGM) lasing from the liquid-filled hollow glass microsphere (LFHGM) is proposed and experimentally verified. The LFHGM-based microlaser is prepared by injecting NaNdF4/dye co-doped liquid into the HGM, and WGM resonance is obtained under excitation of a 532 nm pulse laser. Since the high-efficiency absorption of the 793 nm continuous wave laser by NaNdF4 nanocrystals can result in photothermal effect-induced effective refractive index change of the microcavity, a secondary 793 nm laser is irradiated into the LFHGM to excite the NaNdF4 dispersed in the liquid core, thereby realizing shift of resonant frequencies. The influence of doping concentration of NaNdF4 nanocrystals on tuning range and sensitivity over the power intensity range of 0-1.68 W/mm2 is investigated experimentally, obtaining maximum values of 4.95 nm and 2.95 nm/(W∙mm-2). The ability to generate stable lasing in a LFHGM cavity highlights the practical application of the microscale lasers in future all-optical networks.

Multi-Soliton Formation in Femtosecond Degenerate Optical Parametric Oscillators

Chengxiao Ning and Zhaowei Zhang

DOI: 10.1364/OL.383974 Received 21 Nov 2019; Accepted 30 Dec 2019; Posted 06 Jan 2020  View: PDF

Abstract: Multi-soliton formation is observed theoretically and experimentally in femtosecond degenerate optical parametric oscillators (OPOs). We show that when the pulse energy of the pump source of a femtosecond OPO is increased to a certain level, the existing soliton will move towards the leading edge of pump pulses due to the enhanced nonlinear-acceleration, and the un-depleted pump energy will support the generation of more solitons. We have successfully observed the generation of double-soliton and triple-soliton in experimental demonstrations, with the measured pulse characteristics agreeing well with the theoretical ones. The generation of multi-soliton sources may stimulate many novel applications, including data storage, telecommunication and all-optical signal processing.

Mode insensitive switch for on‐chip interconnectmode division multiplexing systems

Guowu Zhang, Hassan Mojaver, Alok Das, and Odile Liboiron-Ladouceur

DOI: 10.1364/OL.384771 Received 03 Dec 2019; Accepted 30 Dec 2019; Posted 08 Jan 2020  View: PDF

Abstract: A mode insensitive switch is proposed and experimentally demonstrated on a silicon on insulator (SOI) platform using balanced MZI structure with a mode insensitive phase shifter for on-chip mode-division multiplexing (MDM) interconnects. Switching the first three quasi-transverse electric (TE) modes consuming less than 40 mW power is demonstrated. The whole system exhibits approximately -2 dB, -3.7 dB and -5.2 dB insertion loss (IL) for the TE0, TE1 and TE2 modes at 1550 nm respectively. The corresponding crosstalk is less than -8.6 dB (-9 dB), -8 dB (-10.3 dB), and -10 dB (-10.3 dB) within the wavelength range of 40 nm (1535 nm – 1575nm) for the cross (bar) states, respectively. The extinction ratios (ER) for the cross (bar) states are 20.1 dB (19.5 dB), 22.8 dB (33.7 dB), and 15.4 dB (18.1 dB) for the TE0, TE1 and TE2 modes at 1550 nm, respectively. The payload transmission is also conducted using non-return-to-zero PRBS-31 data signals at 10 Gb/s for single mode transmission and three simultaneous mode transmissions. For all the scenarios, open eyes are observed.

In situ observation of slow and tunnelling light at the cutoff wavelength of an optical fiber

Misha Sumetsky and Yong Yang

DOI: 10.1364/OL.384514 Received 27 Nov 2019; Accepted 30 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: Slow waves and tunneling waves can meet at the cutoff wavelengths and/or transmission band edges of optical and quantum mechanical waveguides. The experimental investigation of this phenomenon, previously performed using various optical microstructures, is challenged by fabrication imperfections and material losses. Here, we demonstrate this phenomenon in situ for whispering gallery modes slowly propagating along a standard optical fiber, which possesses the record uniformity and exceptionally small transmission losses. Slow axial propagation dramatically increases the longitudinal wavelength of light and allows us to measure nanosecond-long tunneling times along tunable potential barriers having the width of hundreds of microns. This demonstration paves a simple and versatile way to investigate and employ the interplaying slow and tunneling light.

An Ultra-Compact Hybrid Plasmonic Mode Convertor Based on Unidirectional Eigenmode Expansion

ruixuan chen, Zhiping Zhou, Bowen Bai, and Fenghe Yang

DOI: 10.1364/OL.383092 Received 15 Nov 2019; Accepted 29 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: In this letter, an ultra-compact hybrid plasmonic mode convertor (HPMC) is demonstrated based on a hybrid plasmonic slot waveguide (HPSW) structure. Benefit from the unidirectional eigenmode expansion approach, a mode-interference based TE00-to-TM01 mode convertor is realized for the first time with a record compact footprint of the device is only 2.33×7 μm2. At the wavelength of 1550 nm, the insertion loss is below 2.34 dB, and the extinction ratio is 25.6 dB with mode conversion purity as high as 94.6%. The extinction ratio is over 15.5 dB for the entire C-band with bandwidth of extinction ratio above 10 dB larger than 110 nm. The transmissivity of the cross talk TE10 and TE02 at 1550 nm is -16.1 dB and -22.7 dB, respectively.

Universal digital filtering for denoising volumetric retinal OCT and OCT angiography in 3D shearlet domain

Jianlong Yang, Yan Hu, Liyang Fang, Jun Cheng, and Jiang Liu

DOI: 10.1364/OL.383701 Received 18 Nov 2019; Accepted 27 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: Retinal optical coherence tomography (OCT) and OCT angiography (OCTA) suffer from the degeneration of image quality due to speckle noise and bulk-motion noise, respectively. Because the cross-sectional retina has distinctive features in OCT and OCTA B-scans, existing digital filters that can denoise OCT efficiently are unable to handle the bulk-motion noise in OCTA. In this Letter, we propose a universal digital filtering approach that is capable of minimizing both types of noise. Considering the retinal capillaries in OCTA are hard to differentiate in B-scans while having distinct curvilinear structures in 3D volumes, we decompose the volumetric OCT and OCTA data with 3D shearlets thus efficiently separate the retinal tissues and vessels with the noise in this transform domain. Compared with wavelets and curvelets, the shearlets provide better representation of the layer edges in OCT and the vasculature in OCTA. Qualitative and quantitative results show the proposed method outperforms the state-of-the-art OCT and OCTA denoising methods. Besides, the superiority of 3D denoising is demonstrated by comparing the 3D shearlet filtering with its 2D counterpart.

Pancharatnam-Berry geometric phase memory basedon spontaneous parametric down-conversion

Wen-Rong Qi, Rui Liu, Ling-Jun Kong, Zhou-Xiang Wang, Shuang-Yin Huang, Chenghou Tu, Yongnan Li, and Hui-Tian Wang

DOI: 10.1364/OL.384363 Received 27 Nov 2019; Accepted 27 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: Phase memory is an effect that the interaction between a coherent pump beam and a nonlinear crystal generates photon pairs via the spontaneous parametric down-conversion process, then the down-converted photons (signal and idler) can carry the phase information of the pump beam. There have been many researches on the phase memory of dynamic phase so far, however, there is no report on the phase memory of non-dynamic phase. Here we acquire a Pancharatnam-Berry (PB) geometric phase in a physical system when light travels along a trajectory in polarization-state space. Induced coherence occurs in a cascaded scheme composed of two nonlinear crystals, when the idler photons in both crystals are aligned to be indistinguishable. A NOON (N = 2) state is established when blocking the two idler photons. We explore the PB geometric phase memory of the NOON state and the induced coherence. We find that the first-order interference of the two-photon state or signal photons can be controlled by introducing the PB geometric phase to the pump light. This may facilitate to precisely control the phase of the down-converted photons.

Real-time Terahertz Wave Channeling via Multifunctional Metagratings: A Sparse Array of All-Graphene Scatterers

Sahar Behroozinia, Hamid Rajabalipanah, and Ali Abdolali

DOI: 10.1364/OL.383001 Received 14 Nov 2019; Accepted 27 Dec 2019; Posted 06 Jan 2020  View: PDF

Abstract: Acquiring full control over a large number of diffraction orders can be strongly attractive in the case of realizing multifunctional devices such as multichannel reflectors. Recently, the concept of metagrating has been introduced which enables obtaining the desired diffraction pattern through a sparse periodic array of engineered scatterers. In this letter, for the first time, a tunable all-graphene multichannel meta-reflector is proposed for operating at terahertz (THz) frequencies. In the super cell level, the designed metagrating is composed of three graphene ribbons of different controllable chemical potentials which can be regarded as a five-channel THz meta-reflector. Several illustrative examples have been presented in which by choosing proper distribution of DC voltages feeding the ribbons, our design can realize different intriguing functionalities such as anomalous reflection, retro-reflection, and three-channel power splitting within a single shared aperture and with high efficiency. This work paves the way toward designing highly-efficient and tunable THz multichannel meta-reflectors with many potential applications in photonics and optoelectronics.

Frequency induced rotation of high contrast angular intensity fringes from an uncoated SPP device

Yisa Rumala and Matthew McMahon

DOI: 10.1364/OL.383657 Received 21 Nov 2019; Accepted 27 Dec 2019; Posted 06 Jan 2020  View: PDF

Abstract: Angular intensity fringes are generated by reflecting laser light from an uncoated spiral phase plate (SPP) device. As the laser frequency going into the device is tuned, the fringes rotate. Measured transverse fringe patterns match their theoretical predicted values. They have unity contrast, and their measured intensity varies with laser frequency in a fashion similar to a Fabry-Perot etalon. This effect can be used to enable new miniature devices for angle metrology, imaging, and microscopy.

Parallel structured optical fiber in-line Fabry-Perot interferometers for high temperature sensing

Dongning Wang, Xinlei Cui, and Hua Zhang

DOI: 10.1364/OL.384594 Received 28 Nov 2019; Accepted 26 Dec 2019; Posted 06 Jan 2020  View: PDF

Abstract: We propose and demonstrate parallel structured optical fiber in-line Fabry-Perot interferometers for high temperature sensing. The device consists of three Fabry-Perot cavities in parallel connection, which allows three independent fringe patterns superimposed at its output and as a result, a number of dominant fringe peaks/dips appear, thus enabling unambiguous measurement in a large range. The device is featured with compact size, robust structure and excellent high temperature sustainability, which makes it promising in extreme environment monitoring.

Graphene mode-locked Fe:ZnSe laser

Fedor Potemkin, Andrey Pushkin, Ekaterina Migal, Shigeki Tokita, and Yuriy Korostelin

DOI: 10.1364/OL.384300 Received 27 Nov 2019; Accepted 26 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: We report, for the first time to our knowledge, mode-locked Fe:ZnSe laser. Passive mode-locking was implemented using graphene as a saturable absorber. The laser operates at a repetition frequency of 100 MHz with 415 mW output power pumped by fiber 7-W Er:ZBLAN laser. The designed laser cavity supports broadband generation in mid-IR with spectrum spanning 42 nm which corresponds to Fourier-transform limited pulse of about 670 fs. As well, we observe pulsed nanosecond oscillation under continuous-wave pumping and strong amplitude modulation caused by Kerr self-focusing. The present study fills the gap in operating regimes of Fe:ZnSe lasers and paves the way for the development of powerful ultrafast high-repetition-rate mid-IR sources for the most advanced fields of science.

Soliton Maxwell demons and long-tailed statistics in fluctuating optical fields

Feifei Xin, Fabrizio Di Mei, ludovica falsi, Davide Pierangeli, Aharon Agranat, and Eugenio Del Re

DOI: 10.1364/OL.383895 Received 20 Nov 2019; Accepted 26 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: We demonstrate experimentally in biased photorefractive crystals that collisions between random-amplitude optical spatial solitons produce long-tailed statistics from input Gaussian fluctuations. The effect is mediated by Raman nonlocal corrections to Kerr self-focusing that turn soliton-soliton interaction into a Maxwell demon for the output wave amplitude.

Optical parametric oscillator pumped by a 100-kHz burst-mode Yb-doped fiber laser

Keisuke Nagashima, yoshihiro ochi, and Ryuji Itakura

DOI: 10.1364/OL.383397 Received 18 Nov 2019; Accepted 26 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: We demonstrate an optical parametric oscillator that is pumped at a repetition rate of 100 kHz by a burst-mode Yb-doped fiber laser. Pulse energies of 1.5 μJ were generated with five 4.8-μJ pump pulses. Pulse-to-pulse fluctuations could be suppressed even when only five pump pulses were used. The measured pulse length was 190 fs, which was considerably shorter than the 350-fs pump pulse length. The burst-mode operation is an easy and powerful way to increase the pulse energies of optical parametric oscillators pumped with femtosecond pulses.

A random vibration-driven continuous wave CRDS system for calibration-free gas concentration measurement

Zhenyuan Song, Lijun Xu, Heng Xie, and Zhang Cao

DOI: 10.1364/OL.382697 Received 13 Nov 2019; Accepted 26 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: Random vibrations were employed to pick up each monochromatic component in a continuous wave CRDS system using a bichromatic laser source. Light frequencies were selected within flat portions of an absorption profile to suppress the jitter in laser frequency during measurements. Interference effect caused by cavity length variations was suppressed by optimizing the initial fit point for each ringdown transients. Difference in exponential decay rates of two frequencies determined the gas mole fraction, and no calibration of empty cavity losses was necessary. Experiments on varying humidity were conducted, and the results agreed with the readings of a commercial hygrometer.

Generation of Hofstadter’s Butterfly Spectrum Using Circular Arrays of Microring Resonators

Tyler Zimmerling and Vien Van

DOI: 10.1364/OL.384552 Received 04 Dec 2019; Accepted 26 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: Hofstadter's butterfly spectrum, which characterizes the energy bands of electrons in a two-dimensional lattice under a perpendicular magnetic field, has been emulated and experimentally characterized in periodic bandgap structures at microwave and acoustic frequencies. However, measurement of the complete spectrum at optical frequencies has yet to be demonstrated. Here, we propose a simple topological photonic structure based on a circular array of microrings with periodic resonant frequency detunings which can be implemented on an integrated optics platform. We show that this ring-of-rings structure exactly emulates the Harper equation and propose an experimental approach for measuring Hofstadter's butterfly spectrum at optical frequencies.

A seeded OPO light source for precision spectroscopy

Zi-Tan Zhang, Yan Tan, Jin Wang, Cunfeng Cheng, Yu Sun, An-Wen Liu, and Shui-Ming HU

DOI: 10.1364/OL.384582 Received 29 Nov 2019; Accepted 26 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: Precision spectroscopy of fundamental bands of molecules in the mid-infrared (MIR) region is of great interest in applications of trace detection and testing fundamental physics, where high-power and narrow-linewidth MIR lasers are needed. By using a frequency-stabilized near-infrared laser as a seed of the signal light of a continuous-wave optical parametric oscillator (OPO), we established a broadly tunable MIR light source which has an output power of several hundred milliwatts and a linewidth of a few tens kilohertz. The MIR laser frequency drift was reduced to below 1 kHz by using an optical frequency comb to stabilize the frequency of the 1064 nm pumping laser. The performance of the light source was investigated and tested by measuring the saturated absorption spectroscopy of a few molecular transitions at 3.3 μm.

Single-frequency chirally-coupled-core all-fiber amplifier with 100 W in a linearly-polarized TEM₀₀-mode

Sven Hochheim, Michael Steinke, Peter Wessels, Omar de Varona Ortega, Joona Koponen, Tyson Lowder, Steffen Novotny, Joerg Neumann, and Dietmar Kracht

DOI: 10.1364/OL.379002 Received 26 Sep 2019; Accepted 26 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: Large mode area fibers have become indispensable to address the power requirements of laser sources in gravitational wave detectors. Besides high power capabilities, the system must provide excellent beam quality and polarization. In this paper we present the characterization of a monolithic high-power fiber amplifier at 1064nm built using an Yb-doped chirally-coupled-corefiber, which achieves an output power of 100W in a linearly-polarized TEM₀₀-mode in an all-fiber setup.

Far Infrared Properties of Cyclic Olefin Copolymer

Edward Wollack, Kevin Denis, Alyssa Barlis, Meng-ping Chang, Alexander kutyrev, Kevin Miller, and Peter Nagler

DOI: 10.1364/OL.384430 Received 27 Nov 2019; Accepted 26 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: Cyclic olefin copolymer (COC) is an amorphous thermoplastic with desirable dielectric and mechanical characteristics for optical applications. In particular, its low refractive index, overall mechanical strength, and absence of strong absorption features make it a promising substrate material for far infrared applications which include frequency selective surfaces, scattering filters, and windows. The dielectric properties of selected COC formulations are surveyed from $\approx10-700\,{\rm cm}^{-1}$ ($1000-14\,\mu{\rm m}$) and representative usage as a thin film membrane structure in optical filters is presented.

A novel high-resolution wavemeter using Rayleigh speckle obtained by optical time domain reflectometry

Yangyang Wan, Wang Shuai, Xinyu Fan, Zuyuan He, and Zhaopeng Zhang

DOI: 10.1364/OL.384248 Received 25 Nov 2019; Accepted 25 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: The characteristics of Rayleigh speckle generated from a single mode fiber are related to wavelength of the incident lightwave, which makes it possible to use Rayleigh speckle for spectrum measurement. Based on this feature, we propose a novel wavemeter with high resolution realized using optical time domain reflectometry to obtain Rayleigh speckle. We show that the spectral resolution is inverse to the probe pulse width, with a spectral resolution of 6 fm corresponding to a 2 μs probe pulse. A correlation coefficient method is introduced to further increase the spectral resolution from 6 fm to 0.4 fm. Moreover, it is experimentally demonstrated that the system has the capability to resolve dynamic multi-wavelength signals with 25 μs time resolution. The proposed high-resolution wavemeter shows a good prospect for the cost-effectiveness, compactness and excellent performance of spectral resolution.

3D broadband waveguide cloak and light squeezing in terahertz regime

Shan Zhu, Yanyan Cao, Yangyang Fu, Lei Gao, Xiaochao Li, Huanyang Chen, and yadong xu

DOI: 10.1364/OL.378826 Received 04 Oct 2019; Accepted 24 Dec 2019; Posted 06 Jan 2020  View: PDF

Abstract: In this work, by engineering dielectric layer with gradient thickness in a circular waveguide, we present a simple method of realizing a 3D broadband waveguide cloak at Terahertz regime. It is numerically shown that such proposed device exhibits nearly perfect cloaking performance with a broadband response for transverse-electric polarization, and the working mechanism behind the waveguide cloaking is attributed to dynamic evolution of guided mode. Distinct from all previous cloaks using transformation optics, our proposed cloak scheme only requires isotropic dielectric material and therefore is much easier to implement, which enable more superiorities in potential applications.

Low power charge state depletion nanoscopy of the defect in diamond with a pulsed laser excitation

Dengfeng Li, Bo Du, Xiangdong Chen, Guang-can Guo, and Fang-Wen Sun

DOI: 10.1364/OL.383388 Received 14 Nov 2019; Accepted 24 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: The two-photon charge state conversion has been utilized to improve the spatial resolution of the sensing and imaging with nitrogen vacancy (NV) center in diamond. Here, we studied the charge state conversion of NV center under picosecond pulsed laser excitation. With the same average power, the charge state conversion rate can be improved approximately 24 times by reducing the repetition rate of the laser pulse from 80 MHz to 1 MHz. Subsequently, a pulsed laser with a low repetition rate was applied for the super-resolution charge state depletion microscopy of NV center. The average power of the depletion laser was reduced approximate 5 times. It can decrease the optical heating, which affects the accuracy and sensitivity of sensing. With the assisting of an additional near-infrared laser, a resolution of 12 nm was obtained with 1 mW depletion laser power. Combining with spin manipulation, we expect our results can be used for the developing of a diffraction unlimited NV center sensing.

Autler-Townes splitting and Induced transparency windows in multi-mode microfiber knot

Kai Ma, Yundong Zhang, Huaiyin Su, Guo Yi, Changqiu Yu, and Jinfang Wang

DOI: 10.1364/OL.382460 Received 07 Nov 2019; Accepted 24 Dec 2019; Posted 06 Jan 2020  View: PDF

Abstract: In this paper, Autler-Townes splitting and induced transparency windows are observed in a multi-mode microfiber knot. The microfiber knot is fabricated using tapered single-mode fiber, with the knot position located at the transition area of the tapered fiber. The spectrum, in analogy to Autler-Townes splitting, derives from the mode splitting of two high-order excited modes, which is theoretically explained by the multimode transfer matrix method. Moreover, without adding resonators, two induced transparency windows are realized with the tunable coupling coefficients and phase difference of excited knot modes. The tunable, easy fabricated, compact, and robust microfiber knot has potential applications in optical sensing, filters, slow light, and optical switching.

Generation of a four-component optical cat state

Jacob Hastrup, Jonas Neergaard-Nielsen, and Ulrik Andersen

DOI: 10.1364/OL.383194 Received 15 Nov 2019; Accepted 24 Dec 2019; Posted 24 Dec 2019  View: PDF

Abstract: The four-component cat state represents a particularly useful quantum state for realizing fault-tolerant continuous variable quantum computing. While such encoding has been experimentally generated and employed in the microwave regime, the states have not yet been produced in the optical regime. Here we propose a simple linear optical circuit combined with photon counters for the generation of such optical four-component cat states. This work might pave the way for the first experimental generation of fault-tolerant optical continuous variable quantum codes.

Simultaneous turbulence mitigation and channel demultiplexing for two 100-Gbit/s orbital-angular-momentum-multiplexed beams by adaptive wavefront shaping and diffusing

Runzhou Zhang, Hao Song, Zhe Zhao, Haoqian Song, Jing Du, Cong Liu, Kai Pang, Long Li, Huibin Zhou, Ari Willner, Ahmed Almaiman, Yiyu Zhou, Robert Boyd, Brittany Lynn, Robert Bock, Moshe Tur, and Alan Willner

DOI: 10.1364/OL.383714 Received 20 Nov 2019; Accepted 24 Dec 2019; Posted 24 Dec 2019  View: PDF

Abstract: We experimentally demonstrate simultaneous turbulence effects’ mitigation and channel demultiplexing by adaptive wavefront shaping and diffusing (WSD) light beams in a 200-Gbit/s orbital-angular-momentum (OAM) multiplexed link. Different realizations of two emulated turbulence strengths (the Fried parameter r_0=0.4,1.0 mm) are mitigated. The experimental results show that (i) crosstalk between OAM l=+1 and l=-1 modes can be reduced by >10.0 dB and >5.8 dB, respectively, under the weaker turbulence (r_0=1.0 mm); crosstalk is further improved by >17.7 dB and >19.4 dB, respectively, under most realizations in the stronger turbulence (r_0=0.4 mm), and (ii) optical signal-to-noise ratio (OSNR) penalties for the BER performance are measured to be ~0.7 dB and ~1.6 dB under weaker turbulence, while ~3.2 dB and ~1.8 dB under stronger turbulence for OAM l=+1 and l=-1 mode, respectively.

Single-shot, omni-directional X-ray scattering imaging with a laboratory source and single-photon localization

Erik Dreier, Chantal Silvestre, Jan Kehres, Daniel Turecek, Mohamad Khalil, Jens HEMMINGSEN, Ole Hansen, Jan Jakubek, Robert Feidenhans'l, and Ulrik Olsen

DOI: 10.1364/OL.381420 Received 25 Oct 2019; Accepted 23 Dec 2019; Posted 24 Dec 2019  View: PDF

Abstract: Omni-directional, ultra small angle X-ray scattering imaging provides a method to measure the orientation of micro-structures without having to resolve them. In this letter, we use single-photon localization with the Timepix3 chip to demonstrate the first laboratory-based implementation of single-shot, omni-directional X-ray scattering imaging with the beam-tracking technique. The presented setup is shown to retrieve an accurate scattering orientation and to be an order of magnitude faster than competing approaches. The fast image acquisition makes it the first single-shot technique which may be applied in 3-D scattering imaging on a laboratory-based setup. Furthermore, the Timepix3 chip makes the setup capable of energy resolved X-ray scattering imaging.

C-band 56-Gbit/s OOK system over 100-km dispersion uncompensated link using only receiver-side digital signal processing

Ji Zhou, Haide Wang, Long Liu, Changyuan Yu, Yuanhua Feng, shecheng Gao, Weiping Liu, and Zhaohui Li

DOI: 10.1364/OL.384168 Received 26 Nov 2019; Accepted 22 Dec 2019; Posted 23 Dec 2019  View: PDF

Abstract: In this Letter, we present a record C-band 56-Gbit/s intensity-modulation/direct-detection optical on-off keying (OOK) system over 100-km dispersion uncompensated link using only receiver-side digital signal processing (DSP). The proposed DSP mainly includes adaptive moment estimation (Adam)-based polynomial nonlinear equalizer (PNLE), autoregressive (AR)-based post filter and maximum likelihood sequence estimation (MLSE). Due to square-law detection, chromatic dispersion induces 11 nulls on 28-GHz spectrum of 56-Gbit/s OOK signal after 100-km SSMF transmission. Adam-based PNLE eliminates major linear and nonlinear distortions, but cannot compensate the nulls. After the Adam-based PNLE, AR-based post filter and MLSE further deal with the inter-symbol interference caused by the nulls to improve the system performance. The proposed C-band 56-Gbit/s OOK system shows great potential for future metro networks and data center networks.

Ultra compact Bragg grating devices with broadband selectivity

ang li, Jordan Davis, and Yeshaiahu Fainman

DOI: 10.1364/OL.384688 Received 29 Nov 2019; Accepted 22 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: Bragg gratings are one of the most important building blocks for photonics integrated circuits. They could generate stopbands in the transmission spectra with corresponding passbands in the reflection spectra by introducing periodic perturbations to the waveguide to support Bloch modes. The perturbation is typically realized in a form of corrugation of the waveguide width/sidewalls with certain apodization profile in order to suppress sidelobes and avoid higher order stopbands. However, since such perturbation approach is limited to weak perturbations to avoid intolerable scattering loss, it is difficult to produce ultra-wide stopbands. In this manuscript we report ultra-compact Bragg grating device with strong perturbations by drilling nanoholes in the waveguide core to enable ultra-large stop band with apodization achieved by proper location of nanoholes. With this approach we demonstrate a stopband as wide as 80~nm with sidelobes less than 18 dB and high extinction ratio for a 15 um long device.

Universal self-similar asymptotic behavior of optical bump spreading in random medium atop incoherent background

Chunhao Liang, Xiaofei Li, Sergey Ponomarenko, Zhiheng Xu, Fei Wang, and Yangjian Cai

DOI: 10.1364/OL.385246 Received 06 Dec 2019; Accepted 21 Dec 2019; Posted 23 Dec 2019  View: PDF

Abstract: We demonstrate theoretically that the average spatial intensity profile of any partially coherent optical beam, composed of a finite-power bright intensity bump atop a fluctuating background, evolves into a universal self-similar Gaussian upon long-term propagation in a statistically homogeneous, isotropic linear random medium. The result depends neither on the degree of the background spatial coherence nor on the strength of the medium turbulence. To our knowledge, this is the first demonstration of universal self-similar asymptotics in linear random media.

High acoustic numerical aperture photoacoustic microscopy with improved sensitivity

Mingsheng Li, Jiangbo CHEN, and Lidai Wang

DOI: 10.1364/OL.384691 Received 29 Nov 2019; Accepted 20 Dec 2019; Posted 23 Dec 2019  View: PDF

Abstract: Limited by the numerical aperture of ultrasonic detection, optical resolution photoacoustic microscopy has not achieved the optimal sensitivity. To address this problem, we develop a high acoustic numerical aperture (~0.74) optical-resolution photoacoustic microscopy (HNA-OR-PAM). Via engineering the acoustic lens, we implement the highest acoustic numerical aperture that a spherical concave lens can achieve. The sensitivity of HNA-OR-PAM is improved to around 160% as the state-of-the-art OR-PAM. Without averaging, the new system can image oxygen saturation in vivo with only 10-nJ pulse energy. The improved sensitivity allows us to image weaker absorbers, penetrate deeper, or reduce nonlinear effects induced by high pulse energy. Moreover, the photoacoustic view angle is augmented to 51.8 degrees and makes tilted features more visible. We validate the improved view angle in both phantom study and brain imaging.

Convex silica microlens arrays via femtosecond laser writing

Jian-Guan Hua, Hang Ren, Ao Jia, Zhen-Nan Tian, Lei Wang, Saulius Juodkazis, Qi-Dai Chen, and Hong-Bo Sun

DOI: 10.1364/OL.378606 Received 24 Sep 2019; Accepted 20 Dec 2019; Posted 20 Dec 2019  View: PDF

Abstract: We report fabrication of silica convex microlens arrays (MLAs) with controlled shape, size and curvature by femtosecond laser direct writing. A back-side etching in dye solution was utilised for laser machining high fidelity control of material removal and real-time surface cleaning from ablation debris. Thermal annealing was applied to reduce surface roughness to 3 nm (rms). The good optical performance of the arrays was confirmed by focusing and imaging tests. Complex 3D micro-optical elements over footprint of 100 × 100 μm² were ablated within 1 hour (required for practical applications). Material removal speed of 120 μm³/s (6 × 10⁵ nm³/pulse) was used which is more than order of magnitude higher compared to the back-side etching using a mask projection method. The method is applicable for fabrication of micro-optical components on transparent hard materials.

3D dark-field confocal microscopy for subsurface defects detection

Jian Liu, Liu Jing, Liu Chenguang, and Yuhang Wang

DOI: 10.1364/OL.384487 Received 28 Nov 2019; Accepted 19 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: The three-dimensional (3D) precision measurement of subsurface defects (SSD) remains a long-term, critical, and urgent challenge in advanced manufacturing technology. In this study, we present a 3D dark-field confocal microscopy technique with complementary illumination and detection apertures to detect the SSD in ultraprecise optical components, which are widely employed at laser fusion facilities. Under an annular illumination generated using a pair of axicons, the specular reflected beam from the surface can be blocked by a diaphragm placed in the detection path, while the scattered beam from the SSD can be effectively collected by the detector. We constructed a dark-field confocal microscope that could readily detect the SSD 60 μm beneath the surface in neodymium glass. Furthermore, the 3D volume distributions of the SSD were also reconstructed.

The Origin of Cell Contrast in Offset Aperture Adaptive Optics Ophthalmoscopy

Andres Guevara-Torres, David Williams, and Jesse Schallek

DOI: 10.1364/OL.382589 Received 11 Nov 2019; Accepted 18 Dec 2019; Posted 19 Dec 2019  View: PDF

Abstract: Offset aperture and split detector imaging are variants of adaptive optics scanning ophthalmoscopy recently introduced to improve the image contrast of retinal cells. Unlike conventional confocal scanning ophthalmoscopy, these approaches collect light laterally decentered from the optical axis. A complete explanation of how these methods enhance contrast has not been described. Here, we provide an optical model with supporting in vivo data that shows contrast is generated from spatial variations in refractive index as it is in phase contrast microscopy. A prediction of this model is supported by experimental data that shows contrast is optimized when the detector is placed conjugate with a deeper backscattering screen such as the RPE and choroid, rather than with the layer being imaged as in conventional confocal imaging. This detection strategy provides a substantial improvement in the contrast these new methods can produce.

Mode characteristic manipulation of the random feedback interferometers in Brillouin random fiber laser

Zichao Zhou, Liang Chen, and Xiaoyi Bao

DOI: 10.1364/OL.383124 Received 15 Nov 2019; Accepted 18 Dec 2019; Posted 24 Dec 2019  View: PDF

Abstract: Brillouin random fiber laser (BRFL) suffers high intensity noise which mainly comes from longitudinal mode beating at different mode frequencies. In this paper, we propose and demonstrate that the mode characteristic of BRFL can be manipulated by the distributed random feedback which acts as the longitudinal mode filter. Theoretical model is developed for the first time to analyze the mode characteristics of BRFL with different length of weak FBG array. In experiment, single FBG, weak FBG array (reflection of -40 dB) at various lengths, and Rayleigh scattering fiber are used to provide the random feedback respectively. Both theoretical analysis and experimental results show that single longitudinal mode operation can be realized with the distributed random feedback interferometer, leading to a stable temporal intensity output of the BRFL in time domain.

Extended Imaging Depth of En-face Optical Coherence Tomography Based on Fast Measurement of Reflection Matrix by Widefield Heterodyne Detection

Qiang Yang, Jing Cao, Yusi Miao, Jiang Zhu, and Zhongping Chen

DOI: 10.1364/OL.382898 Received 13 Nov 2019; Accepted 17 Dec 2019; Posted 17 Dec 2019  View: PDF

Abstract: Multiple light scattering in biomedical tissue limits the penetration depth of optical imaging systems such as optical coherence tomography. To increase the imaging depth in scattering media, a computational method based on coherent reflection matrix measurement has been developed by using a low coherence interferometry. The complex reflection matrix is obtained via point-by-point scanning followed by phase shifting method, and then singular value decomposition is used to retrieve the singly back-scattered light. However, the in vivo application of the current reported method is limited due to slow acquisition speed of the matrix. In the present study, wide-field heterodyne-detection method is adopted to speed up the complex matrix measurement at a deep tissue layer. Compared to the phase-shifting method, heterodyne-detection scheme retrieves depth-resolved complex amplitudes faster and is more stable without mechanical movement of the reference mirror. As a result, the matrix measurement speed is increased by more than one order of magnitude.

Time-moduated non-reciprocal metasurface absorber for surface waves

Aobo Li, Yun Bo Li, Jiang Long, Ebrahim Forati, Zhixia Du, and Daniel Sievenpiper

DOI: 10.1364/OL.382865 Received 03 Dec 2019; Accepted 17 Dec 2019; Posted 23 Dec 2019  View: PDF

Abstract: We have investigated a magnet-free, non-reciprocal surface wave absorber based on high impedance surfaces (HIS) using a spatial-temporal modulation approach. By controlling embedded switches with a travelling wave, the HIS metasurface is modulated to break the time and spatial symmetry, which enables surface waves to propagate in one direction, but be absorbed when propagating in the reverse direction. The non-reciprocity has been demonstrated by an EM-circuit co-simulation. We envision that this could be possibly applied in future communication systems that preferably transmit unidirectionally but absorb interference from the reverse direction caused by reflections or other devices.

Fabrication of flexible transparent Ag square-shaped mesh electrode by top-flat nanosecond laser ablation

DONGFENG QI, Xiaohan Yu, Qingwei Zhang, dongwoo paeng, xiang han, Andong Wang, donglin huang, Letian Wang, zifeng zhang, Songyan Chen, Shixun Dai, and Xiang Shen

DOI: 10.1364/OL.384463 Received 02 Dec 2019; Accepted 17 Dec 2019; Posted 20 Dec 2019  View: PDF

Abstract: We report a facile top-flat square nanosecond (ns) laser direct writing (LDW) ablation technique in a thin silver film substrate to fabricate silver square-shaped cell structure of flexible transparent electrodes. The square silver cell structures feature smooth surface morphology, excellent edge definition, mechanical stability, strong adhesion to the substrate, and favorable resistance and transparency. In particular, this strategy enables fabrication of a high square-shaped cell areal density (ablated square cell to the total area) Ag mesh, substantially improving transparency (>85%) without considerably sacrificing conductivity (<5 Ω sq−1 unit of resistance). Consequently, the proposed metallic square-shaped structure shows compatibility with polyethylene naphthalate (PEN) flexible substrate for silver-based wearable electronic devices without any protective layer over the electrodes.

GaN Microdisk with Direct Coupled Waveguide for Unidirectional Whispering Gallery Mode Emission

Chap To, Wai Yuen Fu, Kwai Hei Li, Yuk Cheung, and H. Choi

DOI: 10.1364/OL.381767 Received 29 Oct 2019; Accepted 17 Dec 2019; Posted 20 Dec 2019  View: PDF

Abstract: Microdisks are some of the best optical resonators that be formed on planar wafers, but their emissions are invariably isotropic and thus difficult to be extracted efficiently for practical applications. In this work, a waveguide with a width of 0.16 μm coupled a microdisk with a diameter of 10 μm is fabricated on a 0.77 μm thick GaN thin film containing InGaN/GaN multi-quantum wells. The waveguide is connected directly to the microdisk at the circumference forming a coupling junction, eliminating the need for precision patterning as with evanescent coupling schemes whereby gaps of the order of tens of nanometers between the waveguide and resonator must be maintained. The fabrication was carried out using nanosphere and nanowire lithography. Non-evanescent coupling of whispering-gallery modes (WGMs) to the waveguide from the microdisk is successfully demonstrated.

Discretized superimposed optical fiber long period gratings

Cosimo Trono, Federico Valeri, and Francesco Baldini

DOI: 10.1364/OL.382325 Received 11 Nov 2019; Accepted 15 Dec 2019; Posted 20 Dec 2019  View: PDF

Abstract: A novel technique for the inscription of superimposed long period gratings with arbitrary grating pitches is proposed and experimentally validated. The technique is based on the discretization of an ideal continuous sinusoidal refractive index pattern with a step function. The refractive index variation is induced by means of the irradiation of a photosensitive fiber with a 248 nm UV laser beam. The non-linear relation between the induced refractive index change and the UV fluence was experimentally derived. Two superimposed LPGs with different grating pitches have been realized with the discretization technique; the transmission spectrum was compared with the one of two superimposed LPGs obtained with the traditional square wave refractive index modulation. The validity of the proposed technique was demonstrated by the better spectral characteristics of the discretized superimposed LPGs.

Composite material anti-resonant optical fiber electromodulator with 3.5 dB depth

Adam Lewis, Francesco De Lucia, Walter Belardi, Chung Che Huang, John Hayes, Francesco Poletti, Daniel William Hewak, and Pier Sazio

DOI: 10.1364/OL.382921 Received 14 Nov 2019; Accepted 15 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: The hollow regions of an anti-resonant fiber (ARF) offer an excellent template for the deposition of functional materials such as semiconductors. When the optical properties of such materials can be modified via external stimuli, it offers a method to control the transmission properties of the fiber device. In this letter, we show that integration of a MoS2 film into the ARF voids allows the fiber to act as an electro-optical modulator. We record a maximum modulation depth of 3.5 dB at 744 nm, with an average insertion loss of 7.5 dB.

Delay-based reservoir computing: tackling performance degradation due to system response time

Silvia Ortin and Luis Pesquera

DOI: 10.1364/OL.378410 Received 24 Sep 2019; Accepted 10 Dec 2019; Posted 11 Dec 2019  View: PDF

Abstract: We analyze the degradation of the computational capacity of delay-based reservoir computers due to system response time.We demonstrate that this degradation is reduced when the delay time is greater than the data injection time. Performance improvement is demonstrated on several benchmarking tasks

Superlensing Plano-Convex-Microsphere (PCM) lens for direct laser nano marking and beyond

Bing Yan, Liyang Yue, James Monks, Xibin Yang, daxi xiong, Chunlei Jiang, and Zengbo Wang

DOI: 10.1364/OL.380574 Received 17 Oct 2019; Accepted 07 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: A high-performance all-dielectric lens, formed by integrating a conventional Plano-Convex lens with a high-index Microsphere lens (PCM), was developed for far-field super-resolution applications. The PCM lens features a resolution of ~λ/3.5 in air with a working distance ~2 μm away from the lens. The super-resolution properties were theoretically and experimentally verified, and utilized for direct laser nano writing of arbitrary patterns and nanostructures on various substrates for the first time. This work can be naturally extended to other super-resolution applications including imaging, sensing, trapping and more, with potentials to develop the next-generation low-cost direct laser nano marking machine and super-resolution imaging nanoscope

Spatiotemporal complexity of a phase-conjugate feedback laser system

Tushar Malica, Guillaume Bouchez, D Wolfersberger, and Marc Sciamanna

DOI: 10.1364/OL.383557 Received 20 Nov 2019; Accepted 06 Dec 2019; Posted 09 Dec 2019  View: PDF

Abstract: An 852 nm semiconductor laser is experimentally subjected to phase-conjugate time-delayed feedback achieved through four-wave mixing in a photorefractive (BaTiO₃) crystal. Permutation entropy (PE) is used to uncover distinctive temporal signatures corresponding to the sub-harmonics of the round-trip time and the relaxation oscillations. Complex spatiotemporal outputs with PE mostly upwards of ∼0.85 and chaos bandwidth (BW) up to ∼31 GHz are observed over feedback strengths up to 7%. Low feedback region counterintuitively exhibits spatiotemporal reorganization and the variation in the chaos BW is restricted within a small range of 1.66 GHz; marking the transition between the dynamics driven by the relaxation oscillations and the external cavity round-trip time. The immunity of the chaos BW and the complexity against such spatiotemporal reorganization shows promise as an excellent candidate for secure communication application.

External serrodyne modulation for the suppression of low-frequency noise in quadrature interferometry

Vincent Michaud-Belleau, Mark Charlet, Alex Tourigny-Plante, Jean-Daniel Deschênes, and Jérôme Genest

DOI: 10.1364/OL.376542 Received 04 Sep 2019; Accepted 06 Dec 2019; Posted 09 Dec 2019  View: PDF

Abstract: In interferometry, reaching a high signal-to-noise ratio at low frequencies can be challenging when the additive noise is non-stationary. Although this problem is typically solved by inserting a frequency shifter into one of the arms, in some cases the interferometer cannot or should not be modified in this way. This Letter presents an alternative solution, based on external serrodyne frequency modulation, which is comparable to the typical approach in terms of complexity and performance yet does not require the modification of a passive interferometer. We demonstrate a prototype which achieves frequency shifting at 500 kHz with 89% power efficiency, leading to the wideband suppression of low-frequency additive noise by more than 19 dB. This enables a fully passive measurement of the thermoconductive noise of a 100-meter single-mode fiber.

Mueller-Jones matrices as representing conformal Lorentz transformations

Tiberiu Tudor

DOI: 10.1364/OL.383444 Received 21 Nov 2019; Accepted 05 Dec 2019; Posted 06 Dec 2019  View: PDF

Abstract: It is shown that Mueller-Jones matrices represent conformal Lorentz transformations. Thus the necessary and sufficient condition of a polarization device to be deterministic is to be describable by a conformal Lorentz transformation

Snapshot hyperspectral light field imaging using Image Mapping Spectrometry

Qi Cui, Jongchan Park, R. Theodore Smith, and Gao Liang

DOI: 10.1364/OL.382088 Received 31 Oct 2019; Accepted 23 Nov 2019; Posted 04 Dec 2019  View: PDF

Abstract: In this letter, we present a snapshot hyperspectral light field imaging system using a single camera. By integrating an unfocused light field camera with a snapshot hyperspectral imager, the Image Mapping Spectrometer, we captured a 5D (x,y,u,v,λ) ( x,y, spatial coordinates; u,v, emittance angles; λ, wavelength) datacube in a single camera exposure. The corresponding volumetric image (x,y,z) at each wavelength is then computed through a scale-depth space transform. We demonstrated the snapshot advantage of our system by imaging the spectral volumetric scenes in real time.

Broadband amplitude and frequency demultiplexer for terahertz frequencies using parallel-plate-waveguides technology

Arturo Hernandez, Daniel Mittleman, and Emma Pickwell-MacPherson

Doc ID: 379411 Received 07 Oct 2019; Accepted 17 Nov 2019; Posted 22 Nov 2019  View: PDF

Abstract: In this work we report a novel broadband frequency/polarization demultiplexer based on parallel-plates-waveguides (PPWG) for terahertz (THz) frequencies. The fabrication and experimental validation of this polarization sensitive demultiplexer is demonstrated for the range from 0.2 THz to 1 THz. By adding a second demultiplexer stage, a fifty-fifty amplitude splitter is also demonstrated in the same frequency range. The multiplexer is based on a traveling-wave antenna, exhibiting strong mechanical robustness. This unique device exhibits three splitting mechanisms in the same device; amplitude, polarization and frequency splitting. This is a significant improvement for the next generation of THz passive components for communication purposes.

Attosecond pulse generation from H2+ ions using multicolor beam superposition method

Saeid Ebrahimzadeh, Seyed Mohammad Barzi, morteza lotfollahi, Saeed Sarikhani, and S. N. Tabastabaei

DOI: 10.1364/OL.378494 Received 18 Oct 2019; Accepted 11 Nov 2019; Posted 09 Jan 2020  View: PDF

Abstract: The behavior of the high-order harmonics and output attosecond pulses from hydrogen molecule ions with various internuclear distances which are exposed to high intensity incoming pulses is investigated. The incoming pulses which are spectrally wide yield from superposition of monochromatic beams with constant frequency distance. Our simulations, show that the most intense and shortest attosecond pulses can result from hydrogen molecular ions with internuclear distance of greater than 6 a.u. which are exposed to irradiation of in-tense pulses with frequency width greater than 0.03 a.u.

Dither-free stabilization of a femtosecond doubly-resonant OPO using parasitic sum-frequency mixing

Yuk Shan Cheng, Richard McCracken, and Derryck Reid

Doc ID: 378020 Received 13 Sep 2019; Accepted 09 Nov 2019; Posted 12 Nov 2019  View: PDF

Abstract: Stable operation of a doubly-resonant femtosecond optical parametric oscillator (OPO) requires sub-micron matching of the OPO and pump-laser cavity lengths, which is normally implemented using a dither-locking feedback scheme. Here we show that parasitic sum-frequency-mixing between the pump and resonant pulses of a degenerate femtosecond OPO provides an error signal suitable for actuating the cavity length with the precision needed to maintain oscillation on a single fringe and at maximum output power. Unlike commonly used dither-locking approaches, the method introduces no modulation noise and requires no additional optical components, except for one narrowband filter. The scheme is demonstrated on a Ti:sapphire-pumped sub-40-fs PPKTP OPO, from which data are presented showing a tenfold reduction in relative intensity noise compared with dither locking.

Bismuth plasmonics for extraordinary light absorption in deep sub-wavelength geometries

Imre Ozbay, Amir Ghobadi, Bayram Butun, and GONUL TURHAN-SAYAN

Doc ID: 379670 Received 15 Oct 2019; Accepted 06 Nov 2019; Posted 07 Nov 2019  View: PDF

Abstract: In this letter, we demonstrate an ultra-broadband metamaterial absorber of unrivaled bandwidth (BW) using extraordinary optical response of Bismuth (Bi) metal; a material selected through our analysis methodology. Based on our theoretical model, we investigate the maximum metal-insulator-metal (MIM) cavity BW achievable by any metal with a known n-k data. We show that an ideal metal in such structures should have positive real permittivity part in the near-infrared (NIR) regime. Contrary to noble and lossy metals utilized by most research groups within the field, this requirement is satisfied only by Bi, whose data greatly adheres to the ideal material properties predicted by our analysis. A Bi nano disc based MIM resonator with an absorption above 0.9 in an ultra-broadband range of 800 nm- 90 nm is designed, fabricated, and characterized. To the best of our knowledge, this is the broadest absorption BW reported for a MIM cavity in the near infrared with its upper to lower absorption edge ratio exceeding best contenders by more than 150%. According to the findings of this paper, the use of proper materials and dimensions will lead to realization of deep sub-wavelength efficient optical devices.

Cell-specific three-photon-fluorescence brain imaging: neurons, astrocytes, and gliovascular interfaces

Alexandr Lanin, Matvey Pochechuev, artem chebotarev, ilya kelmanson, Dmitry Bilan, Darya Kotova, Victor Tarabykin, Anatoly Ivanov, Andrey Fedotov, Vsevolod Belousov, and Aleksei Zheltikov

Doc ID: 377494 Received 11 Sep 2019; Accepted 04 Nov 2019; Posted 05 Nov 2019  View: PDF

Abstract: We present brain imaging experiments on rat cortical areas, demonstrating that, when combined with a suitable high-brightness, cell-specific genetically encoded fluorescent marker, three-photon-excited fluorescence (3PEF) enables subcellular-resolution, cell-specific 3D brain imaging modality that is fully compatible and readily integrable with other nonlinear-optical imaging modalities, including two-photon-fluorescence and harmonic-generation microscopy. With laser excitation provided by sub-100-fs, 1.25-μm laser pulses, cell-specific 3PEF from astrocytes and their processes detected in parallel with a three-photon-resonance-enhanced third harmonic from blood vessels is shown to enable a high-contrast 3D imaging of gliovascular interfaces.

Chirp-controlled high-harmonic and attosecond-pulse generation via coherent-wake plasma emission driven by mid-infrared laser pulses

Alexander Mitrofanov, Dmitry Sidorov-Biryukov, Pavel Borisovich, Mikhail Rozhko, Evgeny Stepanov, Anton Shutov, Sergey Ryabchuk, Aleksandr Voronin, Andrey Fedotov, and Aleksei Zheltikov

Doc ID: 374617 Received 05 Aug 2019; Accepted 29 Oct 2019; Posted 29 Oct 2019  View: PDF

Abstract: Coherent-wake plasma emission induced by ultrashort mid-infrared laser pulses on a solid target is shown to give rise to high-brightness, high-order harmonic radiation, offering a promising source of attosecond pulses and a probe for ultrafast subrelativistic plasma dynamics. With 80-fs, 0.2-TW pulses of 3.9-micrometer radiation used as a driver, optical harmonics up to the 34th order are detected, with their spectrum stretching from the mid-infrared to the extreme ultraviolet. The harmonic spectrum is found to be highly sensitive to the chirp of the driver. Particle-in-cell analysis of this effect suggests, in agreement with the generic scenario of coherent-wake emission, that optical harmonics are radiated as trains of extremely short, attosecond ultraviolet pulses with a pulse-to-pulse interval varying over the pulse train. A positive chirp of the driver pulse can partially compensate for this variation in the interpulse separation, allowing harmonics of highest orders to be generated in the plasma emission spectrum.

Electric Field Vector Measurements Via Nanosecond Electric Field Induced Second Harmonic Generation

Tat Loon Chng, Maya Naphade, Benjamin Goldberg, Igor Adamovich, and Svetlana Starikovskaya

Doc ID: 374833 Received 15 Aug 2019; Accepted 19 Oct 2019; Posted 21 Oct 2019  View: PDF

Abstract: Electric field induced second harmonic generation, or E-FISH, has received renewed interest as a non-intrusive tool for probing electric fields in gas discharges and plasmas using ultrashort laser pulses. An important contribution of this work lies in establishing that the E-FISH method works effectively in the nanosecond regime, yielding field sensitivities of about a kV/cm at atmospheric pressure from a 16 ns pulse. This is expected to broaden its applicability within the plasma community, given the wider access to conventional nanosecond laser sources. A Pockels-cell-based pulse-slicing scheme, which may be readily integrated with such nanosecond laser systems, is shown to be a complementary and cost-effective option for improving the time resolution of the electric field measurement. Using this scheme, a time resolution of ~3 ns is achieved, without any detriment to the signal sensitivity. This could prove invaluable for non-equilibrium plasma applications, where time resolution of a few nanoseconds or less is often critical. Finally, we take advantage of the field vector sensitivity of the E-FISH signal to demonstrate simultaneous measurements of both the horizontal and vertical components of the electric field.

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