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Accepted papers to appear in an upcoming issue

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Measuring the spatial deformation of a LCOS spatial light modulator with a self-interference effect

David Marco Castillo, Asticio Vargas, Maria del Mar Sanchez-Lopez, and Ignacio Moreno

DOI: 10.1364/OL.396105 Received 27 Apr 2020; Accepted 10 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: In this work we show a simple technique to characterize the spatial non-uniformity of a liquid-crystal on silicon (LCOS) spatial light modulator. It is based on illuminating the display with a wavelength out of the operation range so there is a significant reflection at the output surface. As a consequence, a Gires-Tournois interferometer is directly created, without any alignment requirement and insensitive to vibrations. The beam reflected at the output surface is the reference beam while the beam reflected at the silicon backplane can be modulated by adjusting the addressed gray level in order to quantitatively derive its deformation. We provide an experimental demonstration using a LCOS-SLM designed to be operative in the near-infrared (NIR) spectral range, but illuminated with visible light.

22.9 W CW Single-frequency laser at 671nm by frequency doubling of Nd:YVO4 laser

Yixuan Zhang, zhimin wang, Zi Han Zhou, Fengfeng Zhang, xuchao liu, Nan Zong, Bo Yong, and Qinjun Peng

DOI: 10.1364/OL.397184 Received 08 May 2020; Accepted 10 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: A stable, 22.9 W, 671 nm single-frequency laser using a type II non-critically phase-matched external-cavity frequency doubling is demonstrated. The output power of the fundamental laser is 32.1W, the corresponding conversion efficiency of frequency doubling from 1342 nm to 671 nm is calculated to be 71%. The M2 factors are measured to be 1.10 and 1.08 in x and y directions, respectively. To the best of our knowledge, 22.9 W is the highest power obtained for a 671 nm single-frequency laser.

Nonreciprocal conventional photon blockade in driven dissipative atom-cavity

Hong-Zhi Shen, W. S. Xue, and X X Yi

DOI: 10.1364/OL.398247 Received 22 May 2020; Accepted 10 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: In this Letter, we propose a scheme to achieve nonreciprocal conventional photon blockade in a nonlinear device consisting of an atom and spinning cavity by manipulating the detuning between atom and cavity. We show that, the single-photon blockade can be generated by driving the spinning resonator from one side, while two-photon blockade or photon-induced tunneling occurs driven by the other side with the same driving strength. This nonreciprocal conventional photon blockade effect originates from the Fizeau-Sagnac drag, which leads to different splitting of the resonance frequencies for the counter-circulating modes. We give four exact optimal solutions for Fizeau-Sagnac shifts to generate nonreciprocal conventional photon blockade with the arbitrary detunings between atom and cavity. These results can deepen our understanding of the underlying mechanism of nonreciprocal conventional photon blockade.

Resource-efficient frequency conversion for quantum networks via cascaded four-wave mixing

Thomas Wright, Charlotte Parry, Oliver Gibson, Robert Francis-Jones, and Peter Mosley

DOI: 10.1364/OL.398408 Received 22 May 2020; Accepted 10 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: We report a resource-efficient scheme in which a single pump laser was used to achieve frequency conversion by Bragg-scattering four-wave mixing in a photonic crystal fiber. We demonstrate bidirectional conversion of coherent light between Sr+ ²P½→²D3/2 emission wavelength at 1092 nm and the telecommunication C band with conversion efficiencies of 4.2 % and 37 % for up- and down-conversion, respectively. We discuss how the scheme may be viably scaled to meet the temporal, spectral and polarisation stability requirements of a hybrid light-matter quantum network.

Laser ellipticity dependent supercontinuum generation by femtosecond laser filamentation in air

Tie-Jun Wang, Na Chen, Zhongbin Zhu, Hao Guo, Yao Liu, Haiyi Sun, Yuxin Leng, Ruxin Li, and FUKANG YIN

DOI: 10.1364/OL.399206 Received 02 Jun 2020; Accepted 10 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: We experimentally investigate the laser polarization effect on the supercontinuum generation through femtosecond laser filamentation in air. By tuning filamenting laser ellipticity from linear polarization to circular polarization, the spectral intensity of the supercontinuum after filamentation gradually increases, while the spectral bandwidth of the supercontinuum continuously decreases. The laser ellipticity dependent spectral intensity modulation of the supercontinuum is stronger at higher filamenting pulse energy. A physical picture based on the laser ellipticity dependent clamped intensity inside filament together with the Kerr nonlinearity and plasma related self-phase modulations is proposed to explain the observation.

All 2D WSe2/MoS2 heterojunction photodiode and its image sensor application

Jongtae Ahn, Ji-Hoon Kang, Min-Chul Park, and Do Kyung Hwang

DOI: 10.1364/OL.399955 Received 12 Jun 2020; Accepted 10 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: Two-dimensional (2D) layered van der Waals atomic crystals exhibit many fascinating properties. In particular, their dangling-bond-free nature enables different 2D materials to be stacked on the top of each other without restraint, thereby forming a heterostructure. In this study, a high-performance all 2D WSe2/MoS2 heterojunction photodiode with a graphene contact as an electrode is demonstrated. It exhibits an excellent electrical performance (ideality factor of 1.2 and rectification ratio of 10^4), a broad spectral photoresponse (from 450 to 980 nm), and a remarkable linearity with a linear dynamic range of 113 dB. Finally, a self-powered single pixel imager is demonstrated as a feasible optoelectronic application.

Quadrant Fourier transform and its application in decoding OAM signals

Chensheng Wu, Daniel Paulson, and Christopher Davis

DOI: 10.1364/OL.400642 Received 18 Jun 2020; Accepted 10 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: We present a new concept of using quadrant Fourier transforms (QFTs) formed by micro-lens arrays (MLAs) to decode complex optical signals based on the optical intensity collected per quadrant area after the MLAs. From a computational optics viewpoint, we show the most promising use of the QFT in low-cost and passive decoding of laser signals carrying optical angular momenta (OAM) that are prevalent in research frontiers of optical communications, computation, and imaging. There are numerous ways of creating, adding and combining OAM states in optical waves, while decoding or demultiplexing approaches often turn out to be complicated or expensive. The simple OAM decoder formed by a pair of identical MLAs, which are concatenated in focal plane and transversely offset by half pitch length, can accomplish the imaging task with four pixels per cell. By sorting the gradient curls of optical wave into local quadrant cells, the decoder analyzes the intensity reallocation that is proportional to the gradients and computes the gradient curls accordingly. The low-cost, compactness and simplicity of the proposed OAM sensor will further promote OAM-based applications, as well as many other applications that exploit the spatial complexity of optical signals.

Gas identify in the high-Q microbubble resonators

Zhong-Di peng, Changqiu Yu, Hongliang Ren, Chang-Ling Zou, Guang-can Guo, and Chunhua Dong

DOI: 10.1364/OL.400381 Received 15 Jun 2020; Accepted 09 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: A new experimental mechanism is reported to realize the identificationof gas by a microcavity sensor. Instead of measuring the change ofenvironment refractive index or absorption, the gas was indirectlydetected and indentified by using the thermo-optics effect of a highquality factor microbubble resonator. When passing gas through themicrobubble, the pressure would induce a geometric deformation andthus an observable frequency shift, and the thermal bistiability responsevaries due to the higher heat dissipation by gas molecules. By thetwo output parameters, we could unambiguously distinguish gas withdifferent molecular weight, e.g. $\mathrm{He}$ , $\mathrm{N_{2}}$and $\mathrm{CO_{2}}$. Our demonstration opens a new avenue of microcavitysensing by using indirect interaction between light and matter, realizingmultiple-parameters sensing approach for particle sizing, gas or solventidentification.

Partially coherent radially polarized circular Airy beam

Tong li, Dongmei Li, Xian Zhang, Kaikai Huag, and Xuanhui Lu

DOI: 10.1364/OL.397993 Received 19 May 2020; Accepted 09 Jul 2020; Posted 09 Jul 2020  View: PDF

Abstract: We propose a new kind of partially coherent vector beam called the partially coherent radially polarized circular Airy beam (PCRPCAB). PCRPCAB inherits autofocusing ability of radially polarized circular Airy beam (RPCAB) and can create an optical potential well at the center of the beam, whose depth can be adjusted by changing the coherent width. We find that, as coherent width decreases, the intensity becomes higher in the dark hollow caused by the polarization singularity, and the singularity of the degree of polarization (DOP) remains along propagation, with its waist controllable by the coherent width. Our results make PCRPCAB a good candidate for capturing the particles with refractive index greater or less than that of the surroundingmedium, and show us an interesting possibility of using the controllable DOP singularity with autofocusing ability to study disordered optical lattice.

Tailoring the gradient and scattering forces for longitudinal sorting of generic size chiral particles

Hongxia Zheng, Xiao Li, Jack Ng, Huajin Chen, and Lin Zhifang

DOI: 10.1364/OL.398216 Received 21 May 2020; Accepted 08 Jul 2020; Posted 09 Jul 2020  View: PDF

Abstract: Based on the concepts of conservative and nonconservative optical forces (COF and NCOF), we analyze the physical mechanism of longitudinal chirality sorting in some simple optical fields. It is demonstrated, both numerically and analytically for particle of arbitrary size, that the sorting relies solely on the NCOF, which switches its direction when particle chirality is reversed. For particles larger than half of the optical wavelength λ, the NCOF far surpasses its counterpart COF, enabling the longitudinal chirality sorting. When the particle is much smaller than λ, however, the COF outweighs the NCOF, destroying the sorting mechanism. A scenario is thus proposed that suppresses the COF and restores the dominance of the NCOF, extending the applicability of the longitudinal chirality sorting to small particles

Controlling amplified spontaneous emission of quantum dots by polymerized nanostructure interfaces

Jinxin Guo, Jialing Jian, Dayong Wang, and Xinping Zhang

DOI: 10.1364/OL.396264 Received 28 Apr 2020; Accepted 08 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: We report a new polymer/colloidal-quantum-dots film with a nanostructured interface, which is fabricated through a template-assisted photopolymerization method, towards the use of amplified spontaneous emission. It is experimentally demonstrated that the amplified spontaneous emission of colloidal quantum dots is able to be manipulated by changing the nanostructured polymeric interface with a weak scattering ability. The dependences of emission wavelength and threshold on the size of nanostructure and CQDs layer thickness are investigated.

Power insensitive surface-normal electroabsorption modulators

Stefano Grillanda, Ting-Chen Hu, David Neilson, and Mark Earnshaw

DOI: 10.1364/OL.400617 Received 19 Jun 2020; Accepted 08 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: Surface normal electroabsorption modulators (SNEAMs) have unique electro-optic modulation properties, however their behavior and performance at high light intensity is affected by thermal nonlinearities that take place in the modulator active volume. Here, we show a novel approach to make SNEAMs insensitive to optical power without the use of power-hungry heaters or feedback control systems. By passively compensating for the thermo-optic dependence of the SNEAM resonant cavity, we obtain an eight-fold reduction in the wavelength shift of the SNEAM response at 4 dBm of input power. Furthermore, we show no appreciable degradation in the SNEAM eye diagram at 25 Gbit/s when the input power is increased up to 2 dBm, that is about 4 times higher than in conventional SNEAMs.

Photoacoustically-assisted material ejection from fused SiO2 following UV nanosecond laser ablation

Tao Lü, Jianli Shao, Jiuling Meng, YONG JIANG, and Rong Qiu

DOI: 10.1364/OL.396261 Received 27 Apr 2020; Accepted 07 Jul 2020; Posted 09 Jul 2020  View: PDF

Abstract: When an ultraviolet nanosecond laser focuses on the rear surface of a fused SiO2 sample through its front surface, two internal shock waves generating from the rear surface propagate toward the front surface and impact a laser-induced micro-crater. Though these shock waves are derived as consequences of different physical processes giving rise to ablation particles, they will induce micro-ejection particles from the existing micro-crater at the front surface if their intensities are higher than the yield strength of the material. Atomistic simulations reveal the mechanism and possibility of shock-induced ejection from the ablated defects. The densification resulting from the ablation pressure is found to reduce the ejecting threshold significantly, which is in good agreement with the experimental observations.

Electric field controlled Near infrared High speed Electro-Optic switching Modulator Integrated with 2D MgO

Ch. N. Rao, Dnyandeo Pawar, Umesh T. Nakate, Radhamanohar Aepuru, XingGao Gui, Ramalinga V Mangalaraja, S. Kale, Eun Kyung Suh, Wenjun Liu, Deliang Zhu, Youming Lu, and Peijiang Cao

DOI: 10.1364/OL.393796 Received 23 Apr 2020; Accepted 07 Jul 2020; Posted 08 Jul 2020  View: PDF

Abstract: Electro-optic effect in 2D MgO nanoflakes synthesized by microwave assisted process is demonstrated using designed optical fiber modulator. The guiding properties of intense core modes excited by the material cavity are modulated by the external electric field. The feasibility of 2D MgO nanoflakes as an effective electro-optic modulator and switching are experimentally verified for the first time. The proposed optical fiber based electro-optic modulator achieves a linear wavelength shift with high sensitivity of 12.87 pm/V (77.22 nm/kV/mm, in electric field). The results show MgO, as a metal oxide 2D material, is a very promising material for electro-optic modulator and switching.

Recording of a curved digital hologram for orthoscopic real image reconstruction

Jung-Ping Liu, Wen-Ting Chen, Hsuan-Hsuan Wen, and Ting-Chung Poon

DOI: 10.1364/OL.398920 Received 29 May 2020; Accepted 07 Jul 2020; Posted 08 Jul 2020  View: PDF

Abstract: A holographic system for recording a curved digital hologram was proposed and demonstrated. In the system, an interference beam without object information is first generated and then used to two-dimensionally scan a three-dimensional object along a cylindrical path. As a result, a complex curved hologram of a real object is recorded for the first time, to be best of our knowledge. The method of digital reconstruction and the properties of the curved digital hologram are then discussed.

Ultraviolet to Mid-Infrared Supercontinuum Generation in Single-Crystalline Aluminum Nitride Waveguides

Juanjuan Lu, Xianwen Liu, Alexander Bruch, Liang Zhang, junxi wang, jianchang Yan, and Hong Tang

DOI: 10.1364/OL.398257 Received 21 May 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: We demonstrate ultra-broadband supercontinuum generation from ultraviolet to mid-infrared wavelengths in single-crystalline aluminum nitride waveguides. Tunable dispersive waves are observed at the mid-infrared regime by precisely controlling the waveguide widths. In addition, ultraviolet light is generated through cascaded second-harmonic generation in the modal phase-matched waveguides. Numerical simulation indicates a high degree of coherence over the full spectrum. Our results establish a reliable path for multiple octave supercontinuum comb generation in single-crystalline aluminum nitride to enable applications including precision frequency metrology and spectroscopy.

Hot-carrier generation from propagating plasmon in an antenna-spacer-mirror nanostructure

Zhiguang Sun and Yurui Fang

DOI: 10.1364/OL.400049 Received 15 Jun 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: By introducing Au nanodisk antennas, we conveniently got hot carriers from propagating surface plasmon polaritons (SPPs) decay in an Au-antennas/TiO2-spacer/Au-mirror (ASM) structure without additional phase-matching process for SPPs generation. The presence of hot carriers from SPPs is distinguished by opposite photocurrents compared with a similar structure without Au mirror. Analyzed by extinction spectra and electrodynamics simulations, reflection between Au-nanodisk layer and Au mirror induces an optical response of Fabry-Pérot (FP) cavity mode, which excites the SPPs on Au mirror and significantly enhances the light harvesting, thus leads a relatively high hot-carrier density from SPPs decay. The peak of incident photon-to-electron conversion efficiencies at different wavelength also well matches the optical response of the structure.

Unpolarized resonant notch filters for the 8-12 μm spectral region

Kyu Lee, Yeong Ko, Neelam Gupta, and Robert Magnusson

DOI: 10.1364/OL.398744 Received 27 May 2020; Accepted 06 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: The long-wave infrared (LWIR) spectral region spanning ~8 to 12 μm is useful for many scientific and industrial applications. As traditional multilayer film components are not straightforwardly realized at these bands, we provide design, fabrication, and testing of polarization independent bandstop filters based on the guided-mode resonance (GMR) effect. Focusing on the zero-contrast grating architecture, we successfully fabricate prototype filters in the Ge-on-ZnSe materials system. Applying mask-based photolithography and dry etching, photoresist patterns form the desired Ge grating structures. The resulting devices exhibit clean transmittance nulls and acceptably-high sidebands. Moreover, we verify polarization independent notch filtering by assembling two identical GMR filters with gratings oriented orthogonally. This approach to realize effective GMR elements will be useful for various fields including photonic and optoelectronic devices operating in the LWIR region.

Ag nanoparticles ink coupled with graphene oxide cellulose paper: A flexible and tunable SERS sensing platform

Pin Lv, Zhao-Di Chen, Zhuochen Ma, Jiangwei Mao, Bing Han, Dongdong Han, and Yonglai Zhang

DOI: 10.1364/OL.400131 Received 11 Jun 2020; Accepted 06 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: Surface-enhanced Raman scattering (SERS) is highly promising for ultra-sensitive detection in a series of applications. Although extensive advances have been achieved in SERS technologies, the preparation of highly efficient SERS substrates still suffers from several limitations, including complex preparation procedures, high cost and instability for long time storage. To address these problems, we report a novel SERS platform that combines graphene oxide (GO) and cellulose composite paper with colloidal silver nanoparticles (Ag NPs) ink. As an efficient substrate, the GO and cellulose composite paper that features hierarchical micro-nanostructures and improved interaction with target molecules can be fabricated on a large scale, and the Ag NPs ink can be well stored, avoiding being oxidized in ambient condition. In this way, our SERS platform not only reduces the cost, but also improved the stability. The sensitivity, reproducibility, and the tunable SERS detection performance were evaluated using R6G as probing molecules. To demonstrate the capability of our SERS platform in practical analysis, the SERS spectra of two WST-8 solutions of different concentrations have been collected. The SERS platform has revealed great potential for practical application of SERS technologies.

Dual-slope imaging in highly scattering media with frequency-domain near-infrared spectroscopy

Giles Blaney, Angelo Sassaroli, and Sergio Fantini

DOI: 10.1364/OL.394829 Received 09 Apr 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: We present theoretical and experimental demonstrations of a novel diffuse optical imaging (DOI) method that is based on the concept of dual slopes (DS) in frequency-domain near-infrared spectroscopy (FD-NIRS). We consider a special array of sources and detectors that collects intensity (I) and phase (ϕ) data with multiple DS sets. We have recently shown that DSϕ reflectance data features a deeper sensitivity with respect to DSI reflectance data. Here, for the first time, we describe a DS imaging approach based on the Moore-Penrose inverse of the sensitivity matrix for multiple DS data sets. Using a circular 8 source/9-detector array that generates 16 DS data sets at source-detector distances in the range 20-40 mm, we show that DSI images are more sensitive to superficial (<5 mm) perturbations, whereas DSϕ images are more sensitive to deeper (>10 mm) perturbations in highly scattering media.

Measurement of the soliton number in guiding mediathrough continuum generation

David Castello-Lurbe, Antonio Carrascosa, Enrique Silvestre, Antonio Diez, Jürgen Van Erps, Nathalie Vermeulen, and Miguel Andrés

DOI: 10.1364/OL.399382 Received 03 Jun 2020; Accepted 05 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: No general approach is available yet to directly measure the ratio between the chromatic dispersion and the nonlinear coefficient, and hence the soliton number for a given optical pulse, in an arbitrary guiding medium. In this work, we solve this problem using continuum generation. We experimentally demonstrate our method in polarization-maintaining and single-mode fibers with positive and negative chromatic dispersion. Moreover, our technique offers new opportunities to determine the chromatic dispersion of fibers or integrated waveguides over a broad spectral range while pumping at a fixed wavelength.

Supercontinuum generation and optical damage of sapphire and YAG at high repetition rates

Robertas Grigutis, Gintaras Tamosauskas, Vytautas Jukna, Alex Risos, and Audrius Dubietis

DOI: 10.1364/OL.397481 Received 12 May 2020; Accepted 05 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: We experimentally investigated supercontinuum generation and evolution of the optical damage in sapphire and YAG crystals with 180 fs, 1035 nm pulses from an amplified Yb:KGW laser with variable repetition rates, in tight and loose focusing conditions. We demonstrate that the extinction of supercontinuum spectrum always correlates with an occurrence of third harmonic diffraction, which thus readily serves as an indication of the onset of in-bulk optical damage. Damage-related structural changes of the nonlinear material are also justified by an increased intensity and large red-shift of crystal luminescence spectrum corresponding to F center emission. The supercontinuum spectrum in sapphire starts shrinking on the time scale of seconds to minutes by varying the focusing condition from thigh to loose at laser repetition rate of 200 kHz, whereas YAG crystal produces stable performance for several hours at least.

High fill factor microlens array fabrication using direct laser writing and its application in wavefront detection

Yi Huang, Yan Liang Qin, Pu Tu, Qi Zhang, Ming Zhao, and Zhen Yu Yang

DOI: 10.1364/OL.398616 Received 26 May 2020; Accepted 05 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: We fabricate 100% fill factor microlens arrays (MLA) using femtosecond laser direct writing. The array consists of periodical hexagonal plano-convex microlens units with a diameter of 9 μm. The focusing efficiency of each microlens is measured to be 92%. Combined with a CCD camera, the microlens array works as a Shack-Hartmann wavefront sensor. We use it to detect wavefronts of both oblique incident plane beams and vortex beams. Experimental results match well with theoretical ones.

Observation of binary phase states of time-multiplexed degenerate optical parametric oscillator pulses generated using a nonlinear fiber Sagnac loop

Hsin Pin Lo, Takahiro Inagaki, Toshimori Honjo, and Hiroki Takesue

DOI: 10.1364/OL.395779 Received 08 May 2020; Accepted 04 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: We generated time-multiplexed degenerate optical parametric oscillator (DOPO) pulses using a nonlinear fiber Sagnac loop as a phase-sensitive amplifier (PSA) where the pump and amplified light in pump-signal-idler degenerate four-wave mixing can be spatially separated. By placing the PSA in a fiber cavity, we successfully generated more than 5,000 time-multiplexed DOPO pulses. We confirmed the bifurcation of pulse phases to 0 or π relative to the pump phase, which makes them useful for representing Ising spins in an Ising model solver based on coherent optical oscillator networks. We also confirmed inherent randomness of the DOPO phases using the NIST random number test.

Coded coherent diffraction imaging with reduced binary modulations and low dynamic-range detection

Meng Li, Liheng Bian, and Jun Zhang

DOI: 10.1364/OL.397882 Received 19 May 2020; Accepted 04 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: The conventional coherent diffraction imaging (CDI) suffers from inherent phase-retrieval ambiguity due to limited intensity-only measurements. Coded illumination with multiple modulations has been introduced to tackle the under-determined challenge, which however slows down imaging speed. In addition, the required high dynamic-range acquisition at the Fourier plane is also time-consuming. To increase imaging speed, we report an accelerated coded CDI method in this letter. It requires only 3 binarized intensity patterns to illuminate the full field, which can be implemented at ~22 kHz using a digital micromirror device. Each diffraction pattern at the Fourier plane is acquired in a single shot without high dynamic-range synthesis, resulting in three intensity-only images corrupted with under-exposed pixels. We develop an adaptive phase retrieval algorithm to adaptively remove the negative influence of under exposure and reconstruct both the object's amplitude and phase. Both simulations and experiments validate that the method enables fast and high-fidelity complex-field imaging.

Resonant dispersive wave emission in hollow capillaryfibres filled with pressure gradients

Christian Brahms, Federico Belli, and John Travers

DOI: 10.1364/OL.398343 Received 22 May 2020; Accepted 03 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: Resonant dispersive wave (RDW) emission in gas-filled hollow waveguides is a powerful technique for the generation of bright few-femtosecond laser pulses from the vacuum ultraviolet to the near infrared. Here we investigate deep-ultraviolet RDW emission in a hollow capillary fibre filled with a longitudinal gas pressure gradient. We obtain broadly similar emission to the constant-pressure case by applying a surprisingly simple scaling rule for the gas pressure and study the energy-dependent dispersive-wave spectrum in detail using simulations. We further find that in addition to enabling dispersion-free delivery to experimental targets, a decreasing gradient also reduces the pulse stretching within the waveguide itself, and that transform-limited pulses with 3 fs duration can be generated by using short waveguides. Our results illuminate the fundamental dynamics underlying this frequency conversion technique and will aid in fully exploiting it for applications in ultrafast science and beyond.

Modulation nonlinearity characterization for rate equation based diode lasers using cross-correlation calculation enabled behavioral modeling

Chenyu Liang, Wenjia Zhang, Jiangbing Du, and Zuyuan He

DOI: 10.1364/OL.399740 Received 19 Jun 2020; Accepted 03 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: In this paper, we investigate and analyze the performance of cross-correlation enabled behavioral modeling in characterizing the modulation nonlinearity for rate equation based diode lasers. Volterra series is utilized for nonlinearity characterization and kernels are calculated using the cross-correlation method. The effectiveness of behavioral modeling for nonlinearity characterization is validated by both simulation and experiment. Large-signal response for 100-Gb/s PAM-4 signals is fitted to the measured one successfully in simulation and 50-Gb/s PAM-4 in the experiment. It is found that the eye skew and amplitude nonlinearity can be emulated as second-order nonlinear distortion. The results can provide guidance for nonlinear distortion mitigation in high-speed optical interconnects.

300 GHz generation based on a Kerr microresonator frequency comb stabilized to a low noise microwave reference

Tomohiro Tetsumoto, Fumiya Ayano, Mark Yeo, Julian Webber, Tadao Nagatsuma, and Antoine Rolland

DOI: 10.1364/OL.398345 Received 22 May 2020; Accepted 03 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: In this letter, we experimentally demonstrate low noise 300 GHz wave generation based on a Kerr microresonator frequency comb operating in soliton regime. The spectral purity of a 10 GHz GPS-disciplined dielectric resonant oscillator is transferred to the 300 GHz repetition rate frequency of the soliton comb through an optoelectronic phase-locked loop. Two adjacent comb lines beat on a uni-travelling carrier photodiode emitting the 300 GHz millimeter-wave signal into a waveguide. In an out-of-loop measurement we have measured the 300 GHz power spectral density of phase noise to be -88 dBc/Hz, -105 dBc/Hz at 10 kHz, 1 MHz Fourier frequency, respectively. The free-running fractional frequency instability at 300 GHz is 1×10^-9 at 1 second averaging time. Stabilized to a GPS signal, we report an in-loop residual instability of 2×10^-15 at 1 second which averages down to < 1×10^-17 at 1000seconds. Such system provides a promising path to the realization of compact, low power consumption millimeter-wave oscillators with low noise performance for out-of-the-lab applications.

PAM-4 transmission up to 160 Gb/s with surface normal electro-absorption modulators

Patrick Iannone, Xi Chen, Stefano Grillanda, Gregory Raybon, Ells burrows, Andrew Adamiecki, Ting-Chen Hu, David Neilson, Nagesh Basavanhally, Yee Low, Rose Kopf, Alaric Tate, and Mark Earnshaw

DOI: 10.1364/OL.399280 Received 10 Jun 2020; Accepted 03 Jul 2020; Posted 08 Jul 2020  View: PDF

Abstract: We report on the demonstration of multi-level modulation in polarization-independent surface-normal electro-absorption modulators (SNEAMs). Four-level pulse amplitude modulation (PAM-4) at a line rate of 44 Gb/s (22 Gbd) is demonstrated on a fully packaged SNEAM with 30-µm active area diameter and 14-GHz electro-optic bandwidth. High capacity PAM-4 transmission, at 112 Gb/s (56 Gbd) and 160 Gb/s (80 Gbd), is demonstrated on an unpackaged SNEAM chip, with 15-µm active area diameter and ultrawide electro-optic bandwidth (>>65 GHz). Transmission over standard single mode fiber using direct detection is investigated for link lengths up to km at 44 Gb/s and 2 km for 112 and 160 Gb/s. Our results represent the highest bit rate achieved on a SNEAM using multi-level modulation.

Pump depletion in parametric down-conversion with low pump energies

Jefferson Florez, Maria Chekhova, and Jeff Lundeen

DOI: 10.1364/OL.394925 Received 21 Apr 2020; Accepted 02 Jul 2020; Posted 02 Jul 2020  View: PDF

Abstract: We report the efficient generation of high-gain parametric down-conversion, including pump depletion, with pump powers as low as 100 μW (energies 0.1 μJ/pulse) and conversion efficiencies up to 33%. In our simple configuration, the pump beam is tightly focused into a bulk periodically poled lithium niobate crystal placed in free space. We also observe a change in the photon number statistics for both the pump and down-converted beams as the pump power increases to reach the depleted pump regime. The experimental results are a clear signature of the interplay between the pump and the down-converted beams in highly efficient parametric down-conversion sources.

Cascaded telecom fiber enabled high order random fiber laser beyond zero-dispersion wavelength

Yang Zhang, Xu Jiangming, Jun Ye, Xiao Ma, Jiaxin Song, Tianfu Yao, and Pu Zhou

DOI: 10.1364/OL.397361 Received 18 May 2020; Accepted 02 Jul 2020; Posted 02 Jul 2020  View: PDF

Abstract: Four-wave mixing induced spectral broadening near the zero-dispersion wavelength (ZDW) of the fiber is a bottleneck factor which limits the further wavelength extending in cascaded random fiber lasers (RFL). In this letter, we successfully suppress the spectral broadening near the ZDW of the fiber in the cascaded RFL by simply combining two kinds of commercial telecom fibers with different ZDWs, G655C fiber with ZDW around 1.52 μm and G652D fiber with ZDW around 1.31 μm. As a result, 8th order Stokes light component at 1721 nm with a maximum output power of 2.1 W and a spectral purity of 96.94% is realized in this telecom fiber based cascaded RFL. This work provides a reference of nonlinear effect management in fiber lasers as well as affords a cost-effective way with great potential of realizing high power widely tunable fiber lasers.

Femtosecond laser direct-written fiber Bragg gratings with high reflectivity and low loss at wavelengths above 4 μm.

Gayathri Bharathan, Toney Fernandez, Martin Ams, Jean-Yves Carrée, Samuel Poulain, Marcel Poulain, and Alex Fuerbach

DOI: 10.1364/OL.399329 Received 04 Jun 2020; Accepted 01 Jul 2020; Posted 02 Jul 2020  View: PDF

Abstract: We report on the fabrication of the first high reflective fiber Bragg gratings for the 4 μm wavelength range. A second order grating with a coupling coefficient (κ) of 0 m-1 losses <0.25 dB/cm and a bandwidth of approximately 3 nm was inscribed into the core of a passive indium fluoride (InF3) fiber using a femtosecond laser. Thermal annealing of this grating at a temperature of 150° for 90 minutes resulted in the enhancement of κ to 275 m-1. This report paves the way to the development of new wavelength stabilized all-fiber mid-infrared lasers beyond 4 μm.

Fundamental quantum limits in ellipsometry

Lukasz Rudnicki, Luis Sanchez-Soto, Gerd Leuchs, and Robert Boyd

DOI: 10.1364/OL.392955 Received 17 Mar 2020; Accepted 01 Jul 2020; Posted 01 Jul 2020  View: PDF

Abstract: We establish the ultimate limits that quantum theory imposes on the accuracy attainable in optical ellipsometry. We show that the standard quantum limit, as usual reached when the incident light is in a coherent state, can be surpassed with the use of appropriate squeezed states and, for tailored beams, even pushed to the ultimate Heisenberg limit.

Quantum random number generator based on room-temperature single-photon emitter in gallium nitride

Qing Luo, Ze-di Cheng, Junkai Fan, Lijuan Tan, Hai-Zhi Song, Guangwei Deng, You WANG, and Qiang Zhou

DOI: 10.1364/OL.396561 Received 30 Apr 2020; Accepted 01 Jul 2020; Posted 01 Jul 2020  View: PDF

Abstract: We experimentally demonstrate a real-time quantum random number generator by using a roomtemperature single-photon emitter from the defect in a commercialgalliumnitridewafer. Thankstothebrightness of our single photon emitter, the raw bit generation rate is about 1.8 MHz, and the unbiased bit generation rate is about 420 kHz after von Neumann’s randomness extraction procedure. Our results show that commercial gallium nitride wafer has great potential forthedevelopmentofintegratedhigh-speedquantum random number generator devices.

Optical topography of rough surfaces using vortex localization of fluorescent markers

Petr Schovánek, Petr Bouchal, and Zdeněk Bouchal

DOI: 10.1364/OL.392072 Received 05 Mar 2020; Accepted 01 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: Measuring rough surfaces is challenging because the proven topographic methods are impaired by adverse effects of diffuse light. In our method, the measured surface is marked by fluorescent nanobeads allowing a complete suppression of diffuse light by bandpass filtering. Light emitted by each fluorescent bead is shaped to a double-helix point spread function used for three-dimensional bead localization on the surface. This non-interferometric measurement of rough surface topography is implemented in a vibration resistant setup. The comparison of our method with vertical scanning interferometry shows that a commercial profiler is surpassed when ground glass surfaces with steep slopes are measured.

Liquid crystal integrated metalens with dynamic focusing property

Shenghang Zhou, ZX Shen, Xinan Li, Shijun Ge, Yanqing Lu, and Wei Hu

DOI: 10.1364/OL.398601 Received 27 May 2020; Accepted 30 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: Metalens is developing fast towards versatile and integrated terahertz (THz) apparatuses, while tunable ones are highly pursued. Here, we propose a strategy that integrates dielectric metasurfaces with liquid crystals (LCs) to realize the dynamic focal spot manipulation. The silicon pillar meta-units of the metasurface are properly selected to generate different phase profiles for two orthogonal linear polarizations, permitting a laterally or axially altered focal spot. After LCs integrated, polarization-multiplexed focusing can be achieved via electrically varying the LC orientations. We demonstrate two metalenses with distinct functions. For the first one, the uniformly aligned LC works as a polarization converter, and further switches the focal length by altering the bias. For the second one, an LC polarization grating is utilized for rear spin-selective beam deflection. Consequently, a THz port selector is presented. This work supplies a promising method towards active THz elements, which may be widely applied in THz sensing, imaging and communication.

Lasing in liquid crystal systems with deformed lying helix

Nikolai Shtykov, Serguei Palto, Artur Geivandov, Boris Umanskii, Ivan Simdyankin, Dmitry Rybakov, Vladimir Artemov, and M. Gorkunov

DOI: 10.1364/OL.394430 Received 08 Apr 2020; Accepted 30 Jun 2020; Posted 01 Jul 2020  View: PDF

Abstract: We report on experimental observation of the lasing effect in novel chiral liquid crystal (CLC) systems with deformed lying helix (DLH). The lasing is observed for both odd- and even-order field-induced stop-bands, which is characteristic exclusively of the DLH state. The DLH state is achieved in special CLC cells with periodic boundary conditions, when the surface alignment is flipped between planar and vertical states. The alignment surfaces are prepared using focused ion-beam lithography. In an electric field such CLC systems undergo an orientational transition, when the initial Grandjean-plane texture with the helix axis perpendicular to the CLC layer is transformed into the DLH state with the helix axis oriented in plane of the layer. Due to field-induced strong deformation the DLH system is characterized by set of photonic stop-bands with fine spectral structure, and namely on these fine-structured sub-bands we have observed the low-threshold lasing effect.

Low power-consumption polymer Mach-Zehnder interferometer thermo-optic switch at 532nm based on triangular waveguide

Baizhu Lin, Jiawen Lv, Yue Cao, Yue Yang, Yongbo Zhang, Ao Zhang, Yi Yunji, Xibin Wang, Fei Wang, and Da-Ming Zhang

DOI: 10.1364/OL.396196 Received 29 Apr 2020; Accepted 30 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: We designed and fabricated a Mach-Zehnder interferometer (MZI) thermo-optic switch (TOS) with an inverted triangular waveguide. The inverted triangular waveguide achieves fundamental mode in a large waveguide dimension which can reduce the coupling loss and increase the extinction ratio. The triangular switch was simulated with higher heating efficiency and lower power consumption than the traditional rectangular waveguide switch. Compared with the traditional rectangular device, the power consumption of the proposed triangular waveguide device is reduced by 60%. Spacing photobleaching was introduced to fabricate the inverted triangular waveguide and adjust the refractive index to minimize the mode number. Our device shows a rise time and fall time of 110 μs and 130 μs, respectively, and switching power as low as 2.2 mW at 532 nm. The proposed single mode waveguide and low power consumption optical switch could have great potential in visible optical communication field such as wavelength division multiplexing (WDM) and mode-division multiplexing (MDM)

High-efficiency Er-doped yttrium gallium garnet laser resonantly pumped by a laser diode at 1.47 µm

Li You, Yongguang Zhao, Dazhi Lu, Fei Liang, Lei Li, Ying Chen, Jian Liu, jianlei wang, Haohai Yu, Huaijin Zhang, and jiyang wang

DOI: 10.1364/OL.401155 Received 24 Jun 2020; Accepted 30 Jun 2020; Posted 02 Jul 2020  View: PDF

Abstract: Spectroscopic and laser properties of Er3+-doped yttrium gallium garnet crystal, Y3Ga5O12 (YGG), are studied. The stimulated emission cross-section is 1.4 × 10‒21 cm2 at 1.65 µm. Continuous-wave (cw) laser resonantly pumped by a laser diode at 1.47 µm is demonstrated, delivering a maximum output power of 3.34 W. Benefiting from the low phonon energy of the YGG host, the corresponding slope efficiency is as high as ~ 42%. This, to the best of our knowledge, is the highest slope efficiency from the laser-diode resonantly pumped Er-lasers at room temperature in the 1.6 μm spectral range.

User-Specific Power Level Allocation Optimization for OFDM-NOMA in a Multi-User RoF system

Jin Shi, You-Wei Chen, J He, Zhihua Zhou, Xinda Yan, Run-Kai Shiu, and Gee-Kung Chang

DOI: 10.1364/OL.399586 Received 05 Jun 2020; Accepted 29 Jun 2020; Posted 02 Jul 2020  View: PDF

Abstract: In this letter, we propose a digital signal processing (DSP) aided technique to optimize the power ratio among users for orthogonal frequency division multiplexing based non-orthogonal multiple access (OFDM-NOMA) in an integrated optical fiber and millimeter wave (mmWave) wireless communication system. In this way, central or distributed unit can leverage the proposed techniques to maintain the uniformity of the signal-to-noise ratios (SNRs) among subcarriers without requiring any channel information feedback. The proposed mechanism can facilitate the power allocation management by treating all subcarriers equally as an independent channel. In other words, multi-carrier based NOMA no longer requires estimating optimal power ratios for each subcarrier in order to realize maximum throughput, and thus the complexity can be significantly reduced. As an illustration of multiple NOMA scenarios in which a near user with 10 km fiber transmission and far user with either longer fiber distance or additional wireless propagation, are experimentally investigated. Experimental results demonstrate that using the conventional OFDM-NOMA without the proposed DSP-aided technique, the optimal power ratios vary rapidly when subcarrier quality index changes due to high-frequency fading in a mmWave radio over fiber (RoF) system. While, by using the proposed techniques including both orthogonal circulant matrix transform (OCT) and discrete Fourier transform (DFT), the optimal power ratios on all effective subcarriers are optimized at the same level and the users’ performance is significantly improved.

Y4-Net: a deep learning solution to one-shot dual-wavelength digital holographic reconstruction

Kaiqiang Wang, Kemao Qian, Jianglei Di, and Jianlin Zhao

DOI: 10.1364/OL.395445 Received 17 Apr 2020; Accepted 29 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: In this Letter, a deep learning solution (Y4-Net) to one-shot dual-wavelength digital holography is proposed to simultaneously reconstruct the complex amplitude information of both wavelengths from a single digital hologram with high efficiency. In the meantime, by using single-wavelength results as network ground truth to train the Y4-Net, the challenging spectral overlapping problem in common-path situations is solved with high accuracy.

Nonlinear p-th root Spectral Magnitude Scaling Beamforming for Clinical Photoacoustic and Ultrasound Imaging

Seonghee Cho, Seungwan Jeon, Wonseok Choi, Ravi Managuli, and Chulhong Kim

DOI: 10.1364/OL.393315 Received 30 Mar 2020; Accepted 29 Jun 2020; Posted 07 Jul 2020  View: PDF

Abstract: A recently introduced nonlinear p-th root delay and sum (NL-p-DAS) beamforming (BF) technique for ultrasound (US) and photoacoustic (PA) imaging achieving better spatial and contrast resolution compared to a conventional delay and sum (DAS) technique. While the method is advantageous for better resolution, it suffers from signal loss, grainier speckles, and at times dark areas in the image mainly due to the interference of non-sinusoidal functions. In this paper, we introduce a modified NL-p-DAS technique called nonlinear p-th root spectral magnitude scaling (NL-p-SMS), which performs the p-th root on the spectral magnitude instead of the temporal amplitude. This technique improves signal quality by improving the internal coherence of the A-line signal. We evaluated NL-p-SMS against NL-p-DAS by comparing full-width-half-maximum of axial and lateral line profiles, contrast ratio (CR), and contrast-to-noise ratio in phantoms with various values of p for both PA and US imaging. We also compared NL-p-DAS and NL-p-SMS in in vivo US and PA imaging. The developed NL-p-SMS improves the CR by a factor of 1.6 when compared to NL-p-DAS. NL-p-SMS shows the axial resolution twice as good as when p = 5 than NL-p-DAS. We believe, with this enhanced performance, our proposed approach would be an advancement compared to the existing nonlinear BF algorithms.

Multiscale high-speed photoacoustic microscopy based on free-space light transmission and MEMS scanning mirror

Chen Zhang, zhao xuan, Song Xu, ningbo chen, ke LI, xinkuang jiang, liangjian Liu, Lidai Wang, Kenneth Kin-Yip Wong, Jun Zou, Chengbo Liu, Liang Song, and Zhicheng Liu

DOI: 10.1364/OL.397733 Received 14 May 2020; Accepted 29 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: Conventional photoacoustic microscopy (PAM) system makes tradeoffs between lateral resolution and imaging depth, which limits its applications in in vivo biological imaging. Here, we present an integrated optical-resolution (OR) and acoustic-resolution (AR) multiscale PAM based on free-space light transmission and fast MEMS scanning. The lateral resolution for OR is 4.9 µm, and AR is 114.5 µm. The maximum imaging depth for OR is 1.2 mm, and AR is 6 mm. The imaging speed can reach 50k Alines per second. The high SNRs and wavelength throughput are achieved by delivering light via free-space, and the high-speed is achieved by a MEMS scanning mirror. The blood vasculature from superficial skin to the deep tissue of a mouse leg was imaged in vivo using two different resolutions to demonstrate the multiscale imaging capability.

Fast Brillouin optical time-domain analysis using frequency-agile and compressed sensing

Qi Chu, Benzhang Wang, henan wang, Dexin Ba, and Yong Kang Dong

DOI: 10.1364/OL.397884 Received 18 May 2020; Accepted 28 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: A fast Brillouin optical time domain analysis (BOTDA) sensor has been proposed and experimentally demonstrated based on the frequency-agile and compressed sensing technique. The proposed scheme employs a data-adaptive sparse base obtained by the principle component analysis algorithm, enabling the sparse representation of Brillouin spectrum. Then, it can be reconstructed successfully with random frequency sampling and orthogonal matching-pursuit algorithm. In the experiment, the Brillouin gain spectrum (BGS) is mapped by the conventional fast BOTDA, where the frequency step and span are 4 MHz and 500 MHz, respectively. By using the compressed sensing technology, the BGS is successfully recovered with 37 random frequency samples, the number of which is only 30% of the full data. With fewer sampling frequencies, the compressed sensing technology is able to improve the sensing performance of the conventional fast BOTDA, including 3.3 times increase in sampling rate or 70% reduction in data storage.

Generation of tunable mid- and far-infrared pulses during gas ionization by a chirped two-color laser field

Alexander Silaev, Alexander Romanov, and Nikolay Vvedenskii

DOI: 10.1364/OL.394979 Received 13 Apr 2020; Accepted 27 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: We propose and investigate a method for generating tunable and phase-controllable mid- and far-infrared pulses in gas ionized by an intense two-color laser field composed of the chirped fundamental and its second harmonic (SH) pulses with the group time delay. The generation frequency equals to the difference between the SH and the doubled fundamental frequencies and is continuously tunable by varying chirp or time delay.The duration of the generated pulses is determined by ionization duration, which is much smaller than the duration of the ionizing field and is controlled by stretching or changing the intensity of the driving pulse. Our quantum-mechanical calculations and analytical description show that this method can provide a wide tuning range spanning from several to more thana hundred THz using femtosecond lasers.

Magneto-refractive Second Harmonic Generation from Plasmonic Antennas in the Mid-infrared

Ilya Razdolski, Gaspar Armelles, Alfonso Cebollada, and Andrei Kirilyuk

DOI: 10.1364/OL.395792 Received 21 Apr 2020; Accepted 27 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: Active modulation of nonlinear-optical response from metallic nanostructures can be realized with an external magnetic field. We report a resonant 20\% magneto-refractive modulation in second harmonic generation (SHG) from spintronic multilayer antennas in the mid-infrared. We discuss mechanisms of this modulation and show that it cannot be explained by an unequal enhancement of the electromagnetic field. Instead, we propose a novel contribution to the nonlinear susceptibility, which relies on the spin-dependent electron mean free path. In contrast to magneto-optics in ferromagnets, our approach allows simultaneous observation of the enhanced SHG and its large modulation.

Passively Q-switched Er:Lu2O3 laser with MXene material Ti4N3Tx (T= O or OH) as a novel saturable absorber

Guoqiang Li, Tao Li, Wenchao Qiao, Tianli Feng, Chenyang Feng, Jia Zhao, Guiqiu Li, and Shengzhi Zhao

DOI: 10.1364/OL.399317 Received 05 Jun 2020; Accepted 27 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: In this paper, we report a titanium nitride (Ti4N3Tx) passive Q-switched Er:Lu2O3 laser. The home-made two-dimensional Ti4N3Tx saturable absorber shows excellent passive Q-switching performance around 3 μm wavelength region. Under the absorbed pump power of 7.4 W, the passive Q-switching laser yields a maximum output power of 0.778 W at a pulse repetition rate of 113.7 kHz, corresponding to a single pulse energy of 6.84 μJ and peak power of 24.57 W.

Second-harmonic computer-generated holographic imaging through monolithic lithium niobate crystal by femtosecond laser micromachining

Bing Zhu, Haigang Liu, Yi'an Liu, Yan Xiongshuo, Yuping Chen, and Xianfeng Chen

DOI: 10.1364/OL.394162 Received 07 Apr 2020; Accepted 26 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: Computer-generated holography technique is a powerful tool for three-dimensional display, beam shaping, and optical encryption. We have realized nonlinear holography in ferroelectric crystals by utilizing the spatial light modulators in our previous works. Here, we demonstrate an improved method to realize the second-harmonic (SH) holographic imaging through a monolithic lithium niobate crystal based on the binary computer-generated holograms (CGHs). The CGH patterns were encoded with the detour phase method and fabricated by femtosecond laser micromachining. By the use of the birefringence phase-matching process in the longitudinal direction, the bright nonlinear holograms can be obtained in the far-field. The realization of SH holography through monolithic crystal opens the wide possibility in the field of high power laser nonlinear holographic imaging.

Using fiber-bending generated speckles for improved working distance and background-rejection in lensless microendoscopy

Ori Katz and Noam Shekel

DOI: 10.1364/OL.395839 Received 08 May 2020; Accepted 26 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: Lensless flexible fiber-bundle based endoscopes allow imaging at depths beyond the reach of conventional microscopes with a minimal footprint. These multicore fibers provide a simple solution for widefield fluorescent imaging when the target is adjacent to the fiber facet. However, they suffer from a very limited working distance and out-of-focus background. Here, we carefully study the dynamic speckle illumination patterns generated by bending a commercial fiber-bundle, and show that they can be exploited to allow extended working distance and background rejection, using a super-resolution fluctuations imaging (SOFI) analysis of multiple frames, without the addition of any optical elements.

Nanophotonic tantala waveguides for efficient broadband supercontinuum generation

Kieran Lamee, David Carlson, Zachary Newman, Su Peng Yu, and Scott Papp

DOI: 10.1364/OL.396950 Received 27 May 2020; Accepted 26 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: We experimentally demonstrate efficient and broadband supercontinuum generation in nonlinear tantala (Ta₂O₅) waveguides using a 1550 nm femtosecond seed laser. With incident pulse energies as low as 100 pJ, we create spectra spanning up to 1.6 octaves across the visible and infrared. Fabricated devices feature propagation losses as low as 10 dB/m, and they can be dispersion engineered through lithographic patterning for specific applications. We show a waveguide design suitable for low-power self referencing of a fiber frequency comb that produces dispersive-wave radiation directly at the second harmonic wavelength of the seed laser. A fiber-connectorized, hermetically sealed module with 2 dB per facet insertion loss and watt-level average-power handling is also described. Highly efficient and fully packaged tantala waveguides may open new possibilities for the integration of nonlinear nanophotonics into systems for precision timing, quantum science, biological imaging, and remote sensing.

Deep-learning-based image reconstruction for compressed ultrafast photography

Yayao Ma, Xiaohua Feng, and Liang Gao

DOI: 10.1364/OL.397717 Received 15 May 2020; Accepted 26 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: Compressed ultrafast photography (CUP) is a computational optical imaging technique that can capture transient dynamics at an unprecedented speed. Currently, the image reconstruction of CUP relies on iterative algorithms, which are time-consuming and often yield non-optimal image quality. To solve this problem, we develop a deep-learning-based method for CUP reconstruction that substantially improves the image quality and reconstruction speed. A key innovation towards efficient DL reconstruction of a large three-dimensional (3D) event datacube (x, y, t) (x, y, spatial coordinate; t, time) is that we decompose the original datacube into massively-parallel two-dimensional (2D) imaging sub-problems, which are much simpler to solve by a deep neural network. We validated our approach on simulated and experimental data.

Optical modulation in hybrid anti-resonant hollow-core fiber infiltrated with vanadium dioxide phase change nanocrystals

Quandong Huang, Indra Ghimire, Jingyi Yang, Nathan fleer, Kin Chiang, Yingying Wang, Shoufei Gao, Pu Wang, Sarbajit BANERJEE, and Ho Wai Lee

DOI: 10.1364/OL.396311 Received 28 Apr 2020; Accepted 26 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: We present a study of optical modulation by the effect of temperature-induced insulator-to-metal phase transition of vanadium dioxide (VO2) nanocrystals deposited in an anti-resonance hollow-core fiber (AR-HCF). We fabricate such a VO2-filled fiber by embedding alkylsilane functionalized VO2 nanocrystals into the air holes of an AR-HCF. With this fiber, we achieve an optical loss modulation of ~60% at a temperature above ~53°C over an ultra-broad spectral range that encompasses the S+C+L band.

Widefield quantitative phase imaging by second-harmonic dispersion interferometry

Fernando Brandi and Frank Wessel

DOI: 10.1364/OL.395097 Received 20 Apr 2020; Accepted 26 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: Widefield optical characterization of transparent samples is of great importance, e.g., for gas flow and plasma diagnostics, and label-free imaging of biological samples.Optically transparent medium however cannot be imaged by technique based on intensity-contrast measurements. Very well known qualitative phase-contrast imaging methodologies are routinely used to overcome this limitation, and also quantitative phase imaging approaches have been developed. Here we report demonstration of a novel widefield quantitative phase imaging technique based on fully common-path second-harmonic dispersion interferometry combined with homodyne dual-channel polarization dependent phase detection. The device is tested in a harsh environment reaching sub-10 mrad harmonic phase dispersion sensitivity and several tens of microns spatial resolution with a very stable and easy to implement optical configuration. The time resolution of the demonstrated device is 600 ps set by the laser pulse time duration.

Symbiotic quadratic solitons mode-locked non-degenerate dispersive optical parametric oscillators

Mingming Nie and Shu-Wei Huang

DOI: 10.1364/OL.398265 Received 21 May 2020; Accepted 26 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: We analytically and numerically unveil the existence condition of symbiotic solitons in doubly resonant non-degenerate optical parametric oscillators. Resonant signal and idler with terahertz comb bandwidth and femtosecond pulse duration in the mid-infrared are attainable through this symbiotic soliton mode-locking technique. Group velocity mismatch between the three interacting waves is the dominant cause of the symbiotic soliton perturbation and their effects are numerically investigated in detail. The principle can be applied to commonly used mid-infrared material platforms, making it a competitive ultrashort pulse and broadband comb source architecture.

Birefringent large mode area anti-resonant hollow core fiber in the 1.9 µm wavelength window

Grzegorz Stepniewski, Dominik Dobrakowski, Dariusz Pysz, Rafal Kasztelanic, Ryszard Buczynski, and Mariusz Klimczak

DOI: 10.1364/OL.398650 Received 27 May 2020; Accepted 26 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: We report on the development and characterization of a birefringent large mode area anti-resonant silica fiber. The fiber structure is composed of six non-touching capillaries. The birefringence results from breaking of the circular symmetry of an air core with increasing of the diameters of two capillaries located across the fiber diameter. We depart from earlier designs of polarizing hollow core fibers, in which coupling of the guided modes is intentionally facilitated with cladding layout. Instead, with help of numerical simulations, we enhance birefringence in our design by varying the capillary wall thickness between the larger- and smaller-diameter capillary sections of the cladding. The fiber has a large elliptical core with semi-axes of ~55 µm and 41 µm in diameter, effective area of the fundamental mode of 1200 µm2, and total outer diameter of 127 µm. The cladding is composed of two pairs of smaller capillaries, which are 18 µm in diameter with 1.66 µm thick walls, and two larger capillaries with 24 µm diameter and 1.14 µm thick walls, located across the diagonal of the fiber. Measured group birefringence over 1820-1920 nm wavelengths is monotonically increasing from 0.4×10-4 to 2.0×10-4, while its phase birefringence is from 0.5×10-6 to 1.1×10-5. Despite this the fiber holds polarization with 12 dB polarization extinction ratio at 1900 nm over 1.5 m long sample.

Experimental Three-State Measurement-Device-Independent Quantum Key Distribution with Uncharacterized Sources

Xingyu zhou, HuaJian Ding, Chun-Hui Zhang, Jian Li, Chunmei Zhang, and Qin Wang

DOI: 10.1364/OL.398993 Received 03 Jun 2020; Accepted 26 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: Measurement-device-independent quantum key distribution (MDI-QKD) is immune to all detector side-channel attacks and has becomes a promising way for remote secret keys sharing. Several proof-of-principal experiments have been demonstrated to show its security and practicality. However, these practical implementations mostly demand, for example, perfect state preparation or completely characterized sources to ensure security, which are difficult to realize with prior art. Here, we investigate a three-state MDI-QKD with uncharacterized source, with simple requirement of that the encoding state are bidimensional, which eliminates security threats from both the source flaws and detection loopholes. As a demonstration, a proof-of-principal experiment over 170 km transmission distance based on Faraday–Michelson interferometers is achieved, representing the longest transmission distance under the same security level on record.

Efficient single-mode 976 nm amplifier based on a 45 micron outer diameter Yb-doped fiber

Leonid Kotov, Valery Temyanko, Svetlana Aleshkina, Mikhail Bubnov, Denis Lipatov, and Mikhail Likhachev

DOI: 10.1364/OL.398251 Received 22 May 2020; Accepted 26 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: In this paper we present a novel single-mode Yb-doped fiber with 14 µm core and 45 µm cladding diameter. A 976 nm all-fiber high-power amplifier was manufactured based on this fiber. 10 mm long fiber taper was used to launch the pump light, and guidance of high NA pump was provided by a glass-air interface. 13 W output power limited only by the available pump power was achieved with 31 % slope efficiency.

Tunable light absorption of graphene using topological interface states

Y. C. Lin, S. H. Chou, and Wen-Jeng Hsueh

DOI: 10.1364/OL.397738 Received 15 May 2020; Accepted 26 Jun 2020; Posted 07 Jul 2020  View: PDF

Abstract: A tunable light absorption of graphene using topological interface states (TIS) is presented. The monolayer graphene is embedded in the interface of asymmetric topological photonic crystals (ATPC). A strong absorption phenomenon occurs by the excitation of TIS. It is found that the absorption spectra are intensively dependent on the chemical potential of graphene and the periodic number of the ATPC. Furthermore, the absorption can be rapidly switched in a slight variation of chemical potential, which is modulated by the applied gate voltage on graphene. This study not only opens up a new approach for enhancing light-monolayer graphene interactions, but also provides for practical applications in high absorption optoelectronic devices. This strong absorption phenomenon is different from those in Fabry-Perot resonators, nano-cavities photonic crystal and traditional topological photonic crystals (TPC).

Polarization-independent narrowband transmittance filters via symmetry-protected modes in high contrast gratings

Michael Barrow and Jamie Phillips

DOI: 10.1364/OL.397779 Received 15 May 2020; Accepted 25 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: We present a high index contrast dielectric grating design for polarization-independent narrowband transmission filtering. A reduced symmetry hexagonal lattice allows coupling to symmetry-protected modes (bound states in the continuum) at normal incidence, enabling high-Q spectral peaks. The peak linewidth is tunable via degree of geometric symmetry reduction. Using diffraction efficiency calculations, we gain further insight into the design and physics of 1D and 2D asymmetric high contrast gratings. The grating design provides filter response that is simultaneously polarization independent and functional at normal incidence, overcoming limitations of 1D asymmetric gratings and 2D symmetric gratings.

Photonic fractional Fourier transformer for chirp radars with ghost target elimination

Guanyu Han, Shangyuan Li, Xiaoxiao Xue, and Xiaoping Zheng

DOI: 10.1364/OL.399386 Received 03 Jun 2020; Accepted 25 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: We for the first time propose a photonic fractional Fourier transformer (PFrFTer), which is used in chirp radars for detecting multiple non-cooperative targets. Based on photonic rotation of the time-frequency plane, the optimal fractional Fourier domain (FrFD) is formed and the received broadband chirp signals are projected on it, where they behave as impulses. Moreover, through manipulation of the FrFT spectrum, the PFrFTer contributes to the cancellation of two ghost target sources so that the ghost targets in multiple targets circumstances are removed. Simulation and experimental results show that the proposed PFrFTer can adapt to multiple non-cooperative targets environments and is immune to ghost targets at the optimal working condition, which agrees well with the theoretical analysis.

Topological Anderson phase in quasi-periodic waveguide lattices

Stefano Longhi

DOI: 10.1364/OL.399742 Received 08 Jun 2020; Accepted 25 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: The topological trivial band of a lattice can be driven into a topological phase by disorder in the system. This so-called topological Anderson phase has been predicted and observed for uncorrelated static disorder, while in the presence of correlated disorder conflicting results are found. Here we consider a Su-Schrieffer-Heeger (SSH) waveguide lattice in the trivial topological phase, and show that quasi-periodic disorder in the coupling constants can drive the lattice into a topological non-trivial phase. A method to detect the emergence of the topological Anderson phase, based on light dynamics at the edge of a quasi-periodic waveguide lattice, is suggested.

A compact strain sensor fabricated by high frequency CO2 laser

Senyu Wang, Yiwei Ma, Tao Geng, cuiting sun, Zhu Hongjia, Xiaoyang Li, Yi Yang, shuo zhang, Weimin Sun, and Libo Yuan

DOI: 10.1364/OL.391593 Received 28 Feb 2020; Accepted 25 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: In this letter, a novel parallel inclined planes long period fiber grating (PIP-LPFG) for strain measurement is proposed. This structure is fabricated by a high frequency CO2 laser, which has polished periodic parallel inclined planes on a single mode fiber (SMF). Refractive index modulation (RIM) over a large area on the surface of SMF significantly shortens the total length of the grating, and the structure of parallel inclined planes efficiently enhance the strain sensitivity of PIP-LPFG. Experimental results show that this LPFG with a miniature length of 3.9 mm has a good repeatability and stability of strain response, which can reach to 116 pm/με in the dynamic range of 0-425 με. Meanwhile, the temperature sensitivity of PIP-LPFG is 54.7 pm/°C in the dynamic range of 30-170 °C.

Passive Optical Phase Noise Cancellation

Liang Hu, Xueyang Tian, Guiling Wu, and Jianping Chen

DOI: 10.1364/OL.393010 Received 18 Mar 2020; Accepted 25 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: We report on the realization of an optical phase noise cancellation technique by embedding the optical phase information into a radio frequency (RF) signal and shifting the optical frequency with the amount phase noise introduced by optical phase perturbations. Neither phase discrimination nor active phase tracking is required due to the open-loop design, mitigating some technical problems, such as the limited compensation speed and the finite phase/timing jitter, in conventional phase noise cancellation. We experimentally demonstrate that this technique maintains the same delay-limited bandwidth and phase noise suppression capability as in conventional techniques, but significantly shortens the response speed and phase recovery time. Passive decoupling optical phase perturbations represents a powerful technique in the domains of optical frequency standard comparisons and clockworks for future optical atomic clocks, which are now under serious investigation for a potential redefinition of the International Time Scale.

1-D imaging of rotation-vibration non-equilibrium from pure rotational ultrafast coherent anti-Stokes Raman scattering

Timothy Chen, Benjamin Goldberg, Brian Patterson, Egemen Kolemen, Yiguang Ju, and Christopher Kliewer

DOI: 10.1364/OL.394122 Received 01 Apr 2020; Accepted 25 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: We present one-dimensional (1-D) imaging of rotation-vibration non-equilibrium measured by two-beam pure rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS). Simultaneous measurements of the spatial distribution of molecular rotation-vibration non-equilibrium are critical for understanding molecular energy transfer in low temperature plasmas and hypersonic flows. However, non-equilibrium CARS thermometry until now was limited to point measurements. The red shift of rotational energy levels by vibrational excitation was used to determine the rotational and vibrational temperatures from 1-D images of the pure rotational spectrum. Vibrational temperatures up to 5500 K were detected in a CH₄/N₂ nanosecond-pulsed pin-to-pin plasma within 2 mm near the cathode. This approach enables study of non-equilibrium systems with 40 μm spatial resolution.

High Numerical Aperture Hartmann Wave Front Sensor for Extreme Ultraviolet Spectral Range

Lu LI, Jayanath Koliyadu, Hannah Donnelly, Domenico Alj, Olivier delmas, Mabel Ruiz Lopez, Ombeline de La Rochefoucauld, Guillaume Dovillaire, Marta Fajardo, Cangtao Zhou, Shuangchen Ruan, Brendan Dromey, Mathew Zepf, and Philippe Zeitoun

DOI: 10.1364/OL.396356 Received 05 May 2020; Accepted 25 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: We present a novel Hartmann wave front sensor for the extreme ultraviolet spectral range with numerical aperture (NA) over 0.15. The sensor has been calibrated directly using extreme ultraviolet (EUV) radiation source based on gas high harmonic generation. The calibration together with simulation results show an accuracy beyond λ/39 root-mean-square (RMS) at 32 nm and suitable for a spectral range from 15 nm to 45 nm. This compact wave front sensor is high-vacuum compatible and designed for in situ operations, allowing widespread applications for up-to-date EUV sources or high NA EUV optics.

Intense mid-infrared emission at 3.9 μm in Ho3+-doped ZBYA glasses for potential use as a fiber laser

Haiyan Zhao, Ruicong Wang, Xin Wang, Shijie Jia, Ya-Xian Fan, Elfed Lewis, Gerald Farrell, Shunbin Wang, and Pengfei Wang

DOI: 10.1364/OL.397653 Received 14 May 2020; Accepted 24 Jun 2020; Posted 24 Jun 2020  View: PDF

Abstract: Intense mid-infrared emission at 3.9 μm in Ho3+-doped ZBYA glasses after direct upper laser level (Ho3+: 5I5) pumping at a wavelength of 888 nm are reported for the first time. Spectroscopic parameters were determined using the Judd-Ofelt theory and obtained using the measured absorption spectrum. The maximum emission cross-section of the Ho3+-doped ZBYA glass used is estimated to be 2.7×10-21 cm2 at 3906 nm. Additionally, fluorescence spectra and lifetimes of ZBYA glasses with different Ho3+ ions doping concentration were also measured. The results provide theoretical and experimental basis for better selection of rare earth doped matrix glasses to achieve a fluorescence output centered on a wavelength of 3.9 μm.

Mirror-based scanning wavefront-folding interferometer for universal coherence measurements

Atri Halder, Henri Partanen, Aleksi Leinonen, Matias Koivurova, Tommi Hakala, Tero Setala, Jari Turunen, and Ari Tapio Friberg

DOI: 10.1364/OL.398704 Received 27 May 2020; Accepted 24 Jun 2020; Posted 24 Jun 2020  View: PDF

Abstract: We demonstrate a modification to the traditional prism-based wavefront-folding interferometer (WFI) that allows the measurement of spatial and temporal coherence, free of distortions and diffraction caused by the prism corners. In our modified system, the two prisms of the conventional system are replaced with six mirrors. The whole system is mounted on a linear XY-translation stage, with an additional linear stage in the horizontal arm. This system enables rapid and exact measurement of the full four dimensional degree of coherence, even for relatively weak signals. The capabilities of our system are demonstrated by measuring the spatial coherence of two inhomogeneous and non-Schell model light sources with distinct characteristics.

Monolithic integration of ultraviolet microdisk lasers into photonic circuits in a III-nitride on silicon platform

Farsane Tabataba-Vakili, Blandine Alloing, Benjamin Damilano, Hassen Souissi, christelle brimont, Laetitia Doyennette, Thierry Guillet, Xavier Checoury, Mustafa El-Kurdi, Sébastien Chenot, Eric Frayssinet, Jean-Yves Duboz, Fabrice Semond, Bruno Gayral, and Philippe Boucaud

DOI: 10.1364/OL.395371 Received 16 Apr 2020; Accepted 24 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: Ultraviolet microdisk lasers are integrated monolithically into photonic circuits using a III-nitride on silicon platform with gallium nitride (GaN) as the main waveguiding layer. The photonic circuits consist of a microdisk and a pulley waveguide terminated by out-coupling gratings. We measure quality factors up to 3500 under continuous-wave excitation. Lasing is observed from 374 nm to 399 nm under pulsed excitation, achieving low threshold energies of 0.14 mJ/cm² per pulse (threshold peak powers of 35 kW/cm²). A large peak to background dynamic of around 200 is observed at the out-coupling grating for small gaps of 50 nm between the disk and waveguide. These devices operate at the limit of what can be achieved with GaN in terms of operation wavelength.

Cascaded Stokes and anti-Stokes laser based on an optical resonator with a self-assembled organic monolayer

Andre Kovach, Arynn Gallegos, Jinghan He, Hyungwoo Choi, and Andrea Armani

DOI: 10.1364/OL.397861 Received 18 May 2020; Accepted 24 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: Due to their high circulating intensities, ultra-high quality factor dielectric whispering-gallery mode resonators have enabled the development of low threshold Raman microlasers. Subsequently, other Raman-related phenomena, such as cascaded stimulated Raman scattering (CSRS) and stimulated anti-Stokes Raman scattering (SARS), were observed. While low threshold frequency conversion and generation have clear applications, CSRS and SARS have been limited by the low Raman gain. In this work, the surface of a silica resonator is modified with an organic monolayer, increasing the Raman gain. Up to four orders of CSRS is observed with sub-mW input power, and the SARS efficiency is improved by three orders of magnitude compared to previous studies with hybrid resonators.

Hybrid III-V-on-SOI optical spot size converter by self-aligned selective undercut dry etching of Si

Yang-Jeng Chen, Rih Chen, Yi-Hsin Fang, Che-Hsin Liang, Wen-Kuan Hsu, Cong-Long Chen, Wei-Chen Lin, Chen-Yu Weng, Yi-jen Chiu, and JyunYe Chu

DOI: 10.1364/OL.396361 Received 01 May 2020; Accepted 24 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: A technology called self-aligned selective undercut dry etching (SSUDE) processing has been demonstrated for fabricating a highly efficient hybrid optical spot size converter (SSC) on a Si-on-insulator (SOI) template. The process was based on a bonded wafer between the upper InP-based multiple quantum well (MQW) heterostructure and the lower silicon on insulator (SOI) substrate. After defining the mask on the upper InP-based ridge waveguide, CF4/O2 dry reactive ion etching (RIE) was then used for selective undercut etching of the Si material from the surrounding materials, forming a vertical waveguide coupler of the optical SSC. The lower waveguide, whose dimension is even smaller than the upper one, can thus, be vertically self-aligned to the top ridge via an independent processing step. A laterally tapered waveguide ranging from 0.3 to 3 μm in width on the upper InP waveguide was fabricated. The phase matching condition of the vertical coupler leads to a length of 45 μm and extracts 88% conversion efficiency. The selective undercut etching processing in III-V/SOI material provides a vertical self-alignment scheme for realizing compact and submicron scale heterogeneous integration in a Si photonics template.

Recovering the appearance of the capillary blood column from under-sampled flow data

Phillip Bedggood and Andrew Metha

DOI: 10.1364/OL.398168 Received 20 May 2020; Accepted 23 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: The moving blood column in retinal capillaries can be resolved using adaptive optics. However, cells are low in contrast and must be tracked over time to be identified robustly. This is typically achieved with a 2D kymograph, depicting intensity along a vessel through time: cells and plasma appear as alternating dark and bright bands. However, the regular spacing of cells makes them hard to differentiate if the sampling rate is too low; an aliased kymograph and spurious cell trajectories are the result. Here we demonstrate a software method to overcome this problem. The method is validated using high frame rate data with known velocity, which is then artificially down-sampled to produce aliasing. The method assumes that individual frames are spatially well-sampled, that capillary velocity is constant over short epochs, and that the blood column is incompressible. This allows the capillary tube data in successive frames to be incrementally shifted by an amount corresponding to the flow velocity, yielding a “stationary” kymograph that faithfully renders the capillary tube contents. Despite requiring knowledge of the flow velocity, this is not a circular exercise; we show that a) optimization of a simple heuristic allows the method to operate “blindly” to find the correct velocity; and b) a recently proposed velocimetry algorithm appears to extend the aliasing limit, accurately recovering the distribution of the moving blood column in the majority of aliased samples identified.

Refractive index spectroscopy and material dispersion in fused silica glass

Yago Lobato and Raul de la Fuente

DOI: 10.1364/OL.395510 Received 21 Apr 2020; Accepted 23 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: This work is aimed to measure material dispersion of fused silica by using a low coherence interferometric method. This method allows to carried out the measurement in a wide spectral range (hundreds of nanometres), quickly and efficiently. By capturing a few interferograms, both the refractive index and the group index of fused silica are determined and material dispersion is modeled using a Sellmeier equation with three resonances. Three different fits are investigated, and, by far, the best fit is the one that uses both the measured refractive and group index to model dispersion. Second order dispersion is also quantified and the zero dispersion wavelength is determined.

Terahertz Phase Retrieval Imaging in Reflection

Nikolai Petrov, Jean-Baptiste Perraud, Adrien Chopard, Jean-Paul Guillet, Olga Smolyanskaya, and Patrick Mounaix

DOI: 10.1364/OL.397935 Received 21 May 2020; Accepted 23 Jun 2020; Posted 23 Jun 2020  View: PDF

Abstract: Terahertz phase retrieval is a promising technique able to assess the complex diffracted wave properties through an iterative processing algorithm. In this Letter, we demonstrate the implementation of this technique in reflection geometry with a continuous wave acquisition system working at 0.287 THz. To ensure a high signal-to-noise ratio in the measured dataset, we proposed a double parallel recording scheme with one detector and two lock-in amplification operating with the complimentary sensitivity setting. The resulting object amplitude distribution shows better resolution than conventional raster-scanning focal plane imaging. A specialized digital interferometric post-processing procedure was applied to obtain a surface heights map from the reconstructed phase distribution in the object's irradiated area.

Nonlinear underwater propagation of picosecond ultraviolet laser beams

Yu-hsin Chen, J Peterson, Luke Johnson, Theodore Jones, B. Hafizi, Alexander Stamm, antonio ting, John Palastro, Michael Helle, and Dmitri Kaganovich

DOI: 10.1364/OL.398153 Received 28 May 2020; Accepted 23 Jun 2020; Posted 24 Jun 2020  View: PDF

Abstract: Meter-scale nonlinear propagation of a picosecond ultraviolet laser beam in water, sufficiently intense to cause stimulated Raman scattering (SRS), nonlinear focusing, pump-Stokes nonlinear coupling, and photoexcitation, was characterized in experiments and simulations. Pump and SRS Stokes pulse energies were measured and pump beam profiles were imaged at propagation distances up to 100 cm, for a range of laser power below and above self-focusing critical power. Simulations with conduction band excitation energy $U_\text{CB} = 9.5~\text{eV}$, effective electron mass $m_\text{eff} = 0.2 m_e$, Kerr nonlinear refractive index $n_2 = 5 \times 10^{-16}~\text{cm}^2/\text{W}$, and index contribution due to SRS susceptibility $n_2^r = 1.7 \times 10^{-16}~\text{cm}^2/\text{W}$ produced the best agreement with experimental data.

Spectral correlation of four wave mixing generated in a photonic crystal fiber pumped by a chirped pulse

Paul Robert, Coralie Fourcade-Dutin, Romain Dauliat, Raphael Jamier, Hector Muñoz, Pere Pérez-Millán, John Dudley, Philippe Roy, Herve Maillotte, and Damien Bigourd

DOI: 10.1364/OL.398614 Received 26 May 2020; Accepted 23 Jun 2020; Posted 24 Jun 2020  View: PDF

Abstract: The spectral correlation of four-wave mixing in a photonic crystal fiber pumped by a chirped pulse has been measured using the dispersive Fourier transform method. From statistical measurements of multiple shot-to-shot spectral measurements, the spectral correlation between the signal and idler photons reveals physical insights into the particular portion of the pump spectrum responsible for generating the FWM

Single-shot two-dimensional spectroscopic magnetomotive optical coherence elastography with graphics processing unit acceleration

Pin Huang, Rishyashring Iyer, Yuan-Zhi Liu, and Stephen Boppart

DOI: 10.1364/OL.397900 Received 19 May 2020; Accepted 23 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: Biomechanical contrast within tissues can be assessed based on the resonant frequency probed by spectroscopic magnetomotive optical coherence elastography (MM-OCE). However, to date, in vivo MM-OCE imaging has not been achieved, mainly due to the constraints on imaging speed. Previously, spatially-resolved spectroscopic contrast was achieved in a “multiple-excitation, multiple-acquisition” manner, where seconds of coil cooling time set between consecutive imaging frames lead to total acquisition times of tens of minutes. Here, we demonstrate an improved data acquisition speed by providing a single chirped force excitation prior to magnetomotion imaging with a BM-scan configuration. In addition, elastogram reconstruction was accelerated by exploiting the parallel computing capability of a graphics processing unit (GPU). The accelerated MM-OCE platform achieved data acquisition in 2.9 sec and post-processing in 0.6 sec for a 2048-frame BM-mode stack. In addition, the elasticity sensing functionality was validated on tissue-mimicking phantoms with high spatial resolution. For the first time, MM-OCE images were acquired from the skin of a living mouse, demonstrating its feasibility for in vivo imaging.

Utilizing Phase Delays of an Integrated Pixel-array Structure to Generate Orbital-Angular-Momentum Beams with Tunable Orders and Broad Bandwidth

Hao Song, Zhe Zhao, Runzhou Zhang, Haoqian Song, Huibin Zhou, Kai Pang, Jing Du, Long Li, Cong Liu, Xinzhou Su, Ahmed Almaiman, Robert Bock, Moshe Tur, and Alan Willner

DOI: 10.1364/OL.396447 Received 04 May 2020; Accepted 22 Jun 2020; Posted 23 Jun 2020  View: PDF

Abstract: We study the relationship between the input phase delays and the output mode orders when using a pixel-array structure fed by multiple single-mode waveguides for tunable orbital-angular-momentum (OAM) beam generation. As an emitter of a free-space OAM beam, the designed structure introduces a transformation function that shapes and coherently combines multiple (e.g., four) equal-amplitude inputs, with the k-th input carrying a phase delay of (k-1)Δφ. The simulation results show that (i) the generated OAM order l is dependent on the relative phase delay Δφ; (ii) the transformation function can be tailored by engineering the structure to support different tunable ranges (e.g., l = {-1}, {-1, +1}, {-1, 0, +1}, or {-2, -1, +1, +2}); and (iii) multiple independent coaxial OAM beams can be generated by simultaneously feeding the structure with multiple independent beams, such that each beam has its own Δφ value for the four inputs. Moreover, there is a trade-off between the tunable range and the mode purity, bandwidth and crosstalk, such that the increase of tunable range leads to (a) decreased mode purity (from 91% to 75% for l = -1), (b) decreased 3-dB bandwidth of emission efficiency (from 285 nm for l = {-1} to 122 nm for l = {-2, -1, +1, +2}), and (c) increased crosstalk within the C-band (from - .7 to -13.2 dB when the tunable range increases from 2 to 4).

Short spatial resolutions retrieval from a long pulse BOTDA trace

Sheng Wang, Zhisheng Yang, Simon Zaslawski, and Luc Thevenaz

DOI: 10.1364/OL.397101 Received 13 May 2020; Accepted 22 Jun 2020; Posted 23 Jun 2020  View: PDF

Abstract: A novel post-processing technique is proposed to extract with a flexible and variable spatial resolution the information from Brillouin optical time-domain analyzers, obtained using a pulse longer than the acoustic settling time. The negative impact of acoustic transient effect is suppressed, enabling a Brillouin response proportional to the spatial resolution and a Brillouin gain spectrum keeping its natural linewidth. This leads to a better overall sensing performance, in particular for sub-metric spatial resolutions, with no compromises on sensing range and measurement time.

Optimized Amplitude Modulation in Femtosecond Stimulated Raman Microscopy

Maxim Lipkin, Jakob Nixdorf, and Peter Gilch

DOI: 10.1364/OL.397589 Received 13 May 2020; Accepted 22 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: In femtosecond stimulated Raman microscopy (FSRM)two laser pulses (Raman pump and probe) interact atthe focus of a scanning microscope. To retrieve the Ramansignature of the sample an amplitude modulationof the pump pulses is necessary. Here, different methodsto achieve this modulation are presented and compared.

All fiber compact bending sensor with high sensitivity based on a multimode fiber embedded chirped-long period grating

shuo zhang, Tao Geng, Huiwen Niu, Xiang li, yunxiang yan, cuiting sun, Sifan Deng, Zemin Wang, Shengjia Wang, Wenlei Yang, Weimin Sun, and Libo Yuan

DOI: 10.1364/OL.396301 Received 28 Apr 2020; Accepted 22 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: We investigated and prepared a chirped long-period grating for high sensitivity bending measurement. The novel fiber structure is composed of the multimode fiber (MMF) with fixed length and gradually longer single-mode fiber using continuous splicing method. The powerful refractive index modulation ability of the MMF ensures the miniaturization of sensor size. The total length of the sensor is 3.45 mm. Chirped period arrangement is adopted for improving the bending sensitivity. Through numerical calculation, the chirp coefficient is determined and the clear resonance peaks are obtained using the wavelength scanning. The experimental results of several samples show that the maximum bending sensitivity is 54.18 nm/m-1. With the advantages of small size and high sensitivity, the sensor is especially suitable for bending sensing with micro-structure.

Ultrafast electron jets generated by laser-matter interaction in wedged-metallic and dielectric targets.

Mario Galletti, Fabrizio Bisesto, Maria Pia Anania, Massimo Ferrario, Riccardo Pompili, Alexandre Poye, and Arie Zigler

DOI: 10.1364/OL.393503 Received 25 Mar 2020; Accepted 21 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: High intensity ultrashort laser pulses interacting with thin solid targets are able to produce energetic ion beams by means of extremely large accelerating fields set by the energetic ejected electrons. The characterization of such electrons is thus important in view of a complete understanding of the acceleration process. Here, we present a complete temporal-resolved characterization of the fastest escaping hot electron component for different target material and thickness,using a temporal diagnostics based on Electro-Optical Sampling with 100~fs temporal resolution. Experimental evidence of scaling laws for ultrafast electron beam parameters have been retrieved with respect to the impinging laser energy (0.4 - 4 J range) and to the target material, and an empirical law determining the beam parameters as a function of the target thickness is presented.

High optical damage threshold on-chip lithium tantalate microdisk resonator

Yan Xiongshuo, Yi'an Liu, Licheng Ge, Bing Zhu, Jiangwei Wu, Yuping Chen, and Xianfeng Chen

DOI: 10.1364/OL.394171 Received 31 Mar 2020; Accepted 21 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: Lithium tantalate (LT) is one of the most attractive optical nonlinear material as it possesses a high optical damage threshold and great UV transparency (0.28-5.5μm). Recently optical grade LT nanoscale film has been developed. Here a high-quality factor (~10^5) LT microdisk resonator based on Lithium tantalate on insulator (LTOI) film was fabricated by utilizing focused ion beam milling. 2 μW output second harmonic (SH) waves are achieved in the LTOI microdisk at about 500 mW input power. The cascaded third-harmonic generation (THG) is also observed in the fabricated device. This work may pave the way of LTOI in integrated photonic chips.

Microsphere-assisted super-resolved Mueller matrix microscopy

Vahid Abbasian and Ali-Reza Moradi

DOI: 10.1364/OL.395735 Received 23 Apr 2020; Accepted 21 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: Mueller matrix microscopy (MMM) is a powerful approach to probe microstructural and optical information of many important specimens (e.g. tissue and bacteria), which otherwise cannot be obtained directly from intensity or spectral images. Achieving high lateral resolution in MMM, similar to other microscopy approaches, remains a challenge. Here, we extend the idea of microsphere (MS) assisted microscopy into MMM toward resolution-enhanced polarimetric imaging. The goal is achieved by insertion of a transparent and polarization-insensitive MS in the working distance of the imaging microscope objective (MO) in the optical train of a MMM system. We experimentally show that a MS close to the sample in MMM may increase the resolution beyond the intrinsic diffraction limit of the system by redirecting the higher spatial frequencies of the sample into the acceptance cone. In order to be a case in point, the experiment is conducted on a standard holographic diffraction grating with 1 μm line-width which is beyond the diffraction limit of a 10x MO. Two-dimensional images of the Mueller matrix and some of the widely-used quantitative polarimetric parameters of the sample are calculated and compared in the two cases, before and after insertion of MS. The proposed arrangement is easy to implement and has the potential to serve as a high-resolution polarimetric microscope for visualizing the polarization characteristics of the microscopic objects.

Multimode nonlinear simulation technique having near-linear scaling with mode number in circular symmetric waveguides

Jesper Laegsgaard

DOI: 10.1364/OL.398412 Received 22 May 2020; Accepted 21 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: An efficient scheme for performing coupled-mode simulations of nonlinear propagation in multimoded waveguides having circular symmetry is presented. Contrary to currently established modal-expansion methods the scheme displays a nearly linear scaling of numerical complexity with mode number, and may enable simulations with hundreds of guided modes.

Single-mode topological valley-Hall lasing at telecommunication wavelength controlled by the degree of asymmetry

wanwoo noh, Hadiseh Nasari, Hwi-Min Kim, Quynh Levan, Zhetao Jia, Chihsin Huang, and Boubacar Kante

DOI: 10.1364/OL.399053 Received 03 Jun 2020; Accepted 21 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: Topology plays a fundamental role in contemporary physics and enables new information processing schemes and wave device physics with built-in robustness. However, the creation of photonic topological phases usually requires complex geometries that limit the prospect for miniaturization and integration, and, dispossess designers of additional degrees of freedom needed to control topological modes on-chip. By controlling the degree of asymmetry (DoA) in a photonic crystal with broken inversion symmetry, we report single-mode lasing of valley-Hall ring cavities at telecommunication wavelength. The DoA governs four photon confinement regimes at the interface of topologically distinct valley-Hall domains, and, evidences an interplay between the width of the topological bandgap and the quality factor of ring-like modes for single-mode operation. Our results open the door to novel optoelectronic devices and systems based on compact topological integrated circuits.

Hybrid protocol for Sending-or-Not-Sending Twin-Field Quantum Key Distribution

Hai XU, Xiao-Long Hu, Xunli Feng, and Xiangbin Wang

DOI: 10.1364/OL.399137 Received 01 Jun 2020; Accepted 21 Jun 2020; Posted 24 Jun 2020  View: PDF

Abstract: We propose a hybrid protocol for Sending-or-Not-Sending (SNS) Twin-Field Quantum Key Distribution (TF-QKD): relaxing the restriction of the signal sources in original SNS protocol and use phase-randomized general sources to replace weak coherent sources (WCSs) as signal sources, while the decoy sources are still WCSs. By adopting the heralded single-photon source (HSPS) into the new protocol as an example, the performance in key rate and secure distance have been much improved.

A High Sensitivity Free-Space Resonant Polarization Sagnac Interferometer

Hyosub Yim and Kyuman Cho

DOI: 10.1364/OL.399984 Received 10 Jun 2020; Accepted 21 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: A new free-space resonant Sagnac interferometer scheme is proposed. This scheme uses the right hand and left-hand circular polarization as normal modes running clockwise and counter clockwise directions in the resonator, respectively. Details about the theoretical analysis on the proposed interferometer scheme and experimental results on feasibility studies are given. Application of this scheme on the rotation sensor is discussed.

Macroscopic Fluorescence Lifetime Topography enhanced via Spatial Frequency Domain Imaging

Jason Smith, ENAGNON AGUÉNOUNON, Sylvain Gioux, and Xavier Intes

DOI: 10.1364/OL.397605 Received 15 May 2020; Accepted 20 Jun 2020; Posted 23 Jun 2020  View: PDF

Abstract: We report on a Macroscopic Fluorescence Lifetime Imaging (MFLI) Topography computational framework based around machine learning with the main goal of retrieving the depth of fluorescent inclusions deeply seated in bio-tissues. This approach leverages the depth-resolved information inherent to time-resolved fluorescence data sets coupled with the retrieval of in situ optical properties as obtained via Spatial Frequency Domain Imaging (SFDI). Specifically, a Siamese network architecture is proposed with optical properties (OPs) and time-resolved fluorescence decays as input followed by simultaneous retrieval of lifetime maps and depth profiles. We validate our approach using comprehensive in silico data sets as well as with a phantom experiment. Overall, our results demonstrate that our approach can retrieve the depth of fluorescence inclusions, especially when coupled with optical properties estimation, with high accuracy. We expect the presented computational approach to find great utility in applications such as optical guided surgery.

Suitable state basis for nonlinear optical mode conversion protocols

Danilo Gomes Pires, José Carlos Rocha, Alcenisio Silva, and Eduardo Fonseca

DOI: 10.1364/OL.394640 Received 07 Apr 2020; Accepted 19 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: By considering parity-defined Laguerre-Gaussian (LG) and Hermite-Gaussian (HG) beams as input modes, we present arguments through experimental and theoretical results in order to affirm that using HG modes as basis are more suitable for optical mode conversion than using LG modes. By analyzing the normalized overlap integral and the generated modes we determine a clear rule for the dominant mode for nonlinear mixing of HG beams, while the same is not possible for LG beams. In addition, examples of optical modal conversion using both HG and LG modes as input beams are demonstrated.

Experimental demonstration of structural robustness of spatially partially coherent fields in turbulence

Abhinandan Bhattacharjee and Anand Jha

DOI: 10.1364/OL.395697 Received 21 Apr 2020; Accepted 19 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: Propagation of structured fields that are spatially perfectly coherent has been studied extensively through both free space and turbulent medium in the context of optical communication. The structure in the intensity profile of such fields is used for encoding information in long-distance optical communication. However, in the presence of turbulence, such structures start to degrade upon propagation, and this structural degradation increases with the increase in turbulence strength. On the other hand, theoretical studies have shown that the structured fields that are spatially partially coherent are less affected by turbulence. However, no such experimental demonstration has been reported in the literature. In this letter, we experimentally demonstrate that in the presence of turbulence the structure in a spatially partially coherent field becomes more robust with the decrease in the spatial coherence length of the field.

CO₂ laser-based side-polishing of silica optical fibers

Siyu Fan and Noel Healy

DOI: 10.1364/OL.397939 Received 18 May 2020; Accepted 19 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: In this Letter, an optical fiber side-polishing process is proposed that is non-contact, versatile and scalable. A CO₂ laser, with carefully selected pulse parameters, is used to remove cladding material from the side of an optical fiber in a controlled manner. The resulting side-polished optical fiber has adiabatic polishing transitions and a flat uniform polished region. The technique provides a pristine polishing surface with an RMS surface roughness of less than 2 nm. Furthermore, in contrast to traditional side-polishing methods, the wear of hard tooling, the associated surface flaws, and issues with residual abrasive particulates are all negated. It is anticipated that this technique will provide a robust platform for the next generation of optical fiber devices that are based on in-fiber light-matter interaction with exotic materials, such as, low-dimensional semi-conductors and topological insulators.

Imaging layer thickness of large-area graphene using reference-aided optical differential reflection technique

Chunguang Hu, Hao Wang, Yongtao Shen, Shuchun HUO, Wanfu Shen, Xiaodong Hu, and Xiaotang Hu

DOI: 10.1364/OL.398196 Received 21 May 2020; Accepted 19 Jun 2020; Posted 19 Jun 2020  View: PDF

Abstract: Transparent layer is critical to enhance optical contrast of graphene on a substrate. However, it turns to a key obstruction for optical methods while being fully covered with large-area graphene. The thickness uncertainty of the transparent layer reduces the analytical accuracy of graphene thickness. Here, we propose a reference-aided differential reflection (DR) method with dual light path. The accurate thickness of transparent layer is obtained by improving the sensitivity of DR spectrum using the designable reference. Hence the analytical accuracy of graphene thickness is guaranteed. For demonstration, a centimeter-scale chemical-vapor-deposition synthesized graphene was measured on a SiO2/Si substrate. The thickness of underlying SiO2 was firstly identified with 1nm resolution by DR spectrum. Then the thickness distribution of graphene was directly deduced from a DR map with submonolayer resolution at a preferred wavelength. The results were confirmed by ellipsometry and atomic force microscopy as well. As a result, this new method provides an extra degree of freedom for optical technique to accurately measure the thickness of large-area 2D materials.

Hertz-level frequency comparisons between diverse color lasers without a frequency comb

Nicolas Bourbeau Hebert, Ashby Hilton, Philip Light, and Andre Luiten

DOI: 10.1364/OL.394572 Received 01 May 2020; Accepted 19 Jun 2020; Posted 19 Jun 2020  View: PDF

Abstract: We present a simple yet powerful technique to measure and stabilize the relative frequency noise between two lasers emitting at vastly different wavelengths. The noise of each laser is extracted simultaneously by a frequency discriminator built around an unstabilized Mach-Zehnder fiber interferometer. Our protocol ensures that the instability of the interferometer is cancelled and yields a direct measure of the relative noise between the lasers. As a demonstration, we measure the noise of a 895 nm diode laser against a reference laser located hundreds of nm away at 1561 nm. We also demonstrate the ability to stabilize the two lasers with a control bandwidth of 100 kHz using a Red Pitaya and reach a sensitivity of 1 Hz^2/Hz limited by detector noise. We independently verify the performance using a commercial frequency comb. This approach stands as a simple and cheap alternative to frequency combs to transport frequency stability across large spectral intervals, or to characterize the noise of arbitrary color sources.

Laser induced damage in coatings for cryogenic Yb:YAG active mirror amplifiers

Hanchen Wang, Alexander Meadows, Elzbieta Jankowska, Emmett Randel, Brendan Reagan, Jorge Rocca, and Carmen Menoni

DOI: 10.1364/OL.399293 Received 02 Jun 2020; Accepted 19 Jun 2020; Posted 06 Jul 2020  View: PDF

Abstract: We report results of a study of the laser induced damage threshold (LIDT) behavior of ion beam sputtered HfO2/SiO2 multilayer coatings on Yb:YAG using 1-on-1 and N-on-1 test protocols. The tests were conducted at ambient, vacuum, and cryogenic conditions using 280 ps pulses at λ = 1030 nm. The 1-on-1 LIDT of antireflection (AR) stacks is found to be only slightly reduced under vacuum and cryogenic conditions, while that of high reflectivity (HR) stacks is insensitive to environmental conditions within the uncertainty of the measurements. Cryogenic N-on-1 tests show the LIDT of the HR coating is almost the same as in the 1-on-1 tests. Conversely, the cryogenic N-on-1 test of the AR coating shows damage at ~13 J/cm2, a fluence lower than the 20.4 J/cm2 of 1-on-1 tests. The AR damage behavior is found to be affected by imperfections at the Yb:YAG surface. These findings show that high surface quality is required to increase energy extraction from active mirror laser amplifiers.

Scaling And Cascading Compact Metamaterials Photonic Waveguide Filter Blocks

Pengfei Xu, Yanfeng Zhang, Shuailong Zhang, Yujie Chen, and Siyuan Yu

DOI: 10.1364/OL.398176 Received 21 May 2020; Accepted 18 Jun 2020; Posted 19 Jun 2020  View: PDF

Abstract: In this work, we reported the design and fabrication of a compact and scalable metamaterials longpass filter with an ultra-small footprint of 5.1×5.1 μm2. In the stopband, light transmission can be blocked and reflected with ~25dB attenuation. In the passband, light can pass through with a low insertion loss around -0.28 dB. Moreover, we presented the transition band shifting (~11.4 nm/1%) and power roll-off enhancement (1.34 dB/nm) features by scaling and cascading the filter block. These results demonstrate the great potential of the metamaterials based waveguide devices for scalable photonic filtering applications.

Material decomposition from a single x-ray projection via single-grid phase contrast imaging

Celebrity Groenendijk, Florian Schaff, Linda Croton, Marcus Kitchen, and Kaye Morgan

DOI: 10.1364/OL.389770 Received 11 Feb 2020; Accepted 18 Jun 2020; Posted 19 Jun 2020  View: PDF

Abstract: This study describes a new approach for material decomposition in x-ray imaging, utilising phase contrast to both increase sensitivity to weakly-attenuating samples and to act as a complementary measurement toattenuation, therefore allowing two overlaid materials to be separated. The measurements are captured using the single-exposure, single-grid x-ray phase contrast imaging technique, with a novel correction that aims to remove propagation-based phase effects seen at sharp edges in the attenuation image. The use of a single-exposure technique means that images could becollected in a high-speed sequence. Results are shown for both a known two-material sample and for a biological specimen.

A two-dimensional encoder with independent in-plane and out-of-plane detection for nanometric measurement

PING WU, Zhiyong Yang, XIANYING WANG, and ZHIGANG ZHANG

DOI: 10.1364/OL.397858 Received 20 May 2020; Accepted 17 Jun 2020; Posted 17 Jun 2020  View: PDF

Abstract: We propose and demonstrate a new optical encoder which can measure in-plane and out-of-plane displacements simultaneously and independently. The symmetrical structure of optical path can eliminate the impact from out-of-plane displacement on the measurement of in-plane displacement. The innovative new geometry also facilitates the multi-reflected diffracted beam to interfere with the same order diffracted beam, so as to eliminate the impact from in-plane displacement on the measurement of out-of-plane displacement. An experimental setup is established to verify the two-dimensional independent measurement. The experiment result coincides with the one measured by two independent interferometers. The output of spectrum analysis shows that the two-dimensional independent encoder can be used for nanometric measurement.

Adaptive-sampling angular spectrum method with full utilization of space-bandwidth product

Hao Zhang, Wenhui Zhang, and Guofan Jin

DOI: 10.1364/OL.393111 Received 18 Mar 2020; Accepted 17 Jun 2020; Posted 18 Jun 2020  View: PDF

Abstract: As convolution-based diffraction calculation methods, Rayleigh-Sommerfeld convolution and angular spectrum method (ASM) usually require zero padding to avoid circular convolution errors. This greatly increases the computational complexity and wastes a large amount of the sampling points. In this Letter, based on the analysis of sampling properties in the convolution process, we propose an adaptive-sampling ASM, which can adjust the sampling parameters according to the propagation distance to avoid circular convolution errors without zero padding. The sampling condition of the transfer function can be adaptively satisfied by rearranging the sampling points in the spatial frequency domain. Therefore, the computational complexity is significantly reduced and all the sampling points are effectively used, which leads to a full utilization of the space-bandwidth product.

Exciton polaritons in mixed-dimensional transition metal dichalcogenides heterostructures

Qing Zhang, Shaohua Dong, Guangtao Cao, and Guangwei Hu

DOI: 10.1364/OL.396626 Received 01 May 2020; Accepted 17 Jun 2020; Posted 18 Jun 2020  View: PDF

Abstract: Transition-metal dichalcogenides (TMDs) have shown the great promise for advanced optoelectronic applications, thanks to their visible or near-infrared and layer-dependent bandgaps. Even more exciting phenomena happen via stacking the TMDs to form the vertical heterostructures, such as the exotic interlayer excitons in atomically rearranged bilayer TMDs as the result of the tunable interlayer hopping of two monolayers. So far, those literature studies focus on either two-dimensional (2D) TMDs or the layered bulky three-dimensional (3D) TMDs. The mixed-dimensional TMDs remain a fundamental yet not fully appreciated curiosity. In this paper, we have theoretically and numerically investigated the exciton polaritons in the hybrid system composed by the nanostructured layered (3D) and monolayer (2D) TMDs. The strong coupling has been observed of the lattice mode in high-index patterned 3D TMDs and exciton from the direct bandgaps of 2D TMDs, with the tunable Rabi splitting by geometrically shaping 3D TMDs. We believe that our mixed-dimensional system with the novel stacks of 2D/3D van der Waals heterostructures may allow for controlling the exciton transport for advanced quantum, polaritonic and optoelectronic devices.

Optical bottle microresonators with axially-uniform eigenmode field distribution

Misha Sumetsky

DOI: 10.1364/OL.394467 Received 03 Apr 2020; Accepted 17 Jun 2020; Posted 17 Jun 2020  View: PDF

Abstract: We show that the fundamental eigenmode of a shallow optical bottle microresonator (also called a SNAP microresonator) can be made exceptionally uniform along its axial length. The introduced microresonator has the effective radius variation resembling the profile of a bat with ears and wings. Remarkably, reduction of the axial size of this microresonator by cutting the wings does not alter the uniformity of its fundamental eigenmode. Being of general interest, our findings pave a way for improving the perceptibility of micro/nanoparticle sensing. These results also suggest a bottle microresonator suitable for accurate assembling of quantum emitters near the maximum of its eigenmode important in cavity quantum electrodynamics.

Single-shot Omnidirectional Full Stokes Polarimetry

Aristide Dogariu and Mahdi Eshaghi

DOI: 10.1364/OL.396528 Received 29 Apr 2020; Accepted 16 Jun 2020; Posted 17 Jun 2020  View: PDF

Abstract: Many active sensing applications benefit from measuring, as fast as possible, the polarization state of target reflections. Traditional polarimetry however relies on (i) the assumption of field transversality and (ii) on knowing the direction of wave propagation. When this is not known, one must regard the field as being three-dimensional, which inherently complicates the polarimetry due to experimental constraints imposed by the planar geometry of detector arrays. We demonstrate a single-shot, full Stokes polarimetry approach that alleviates these limitations. The approach is based on the spatial Fourier analysis of the interference between the unknown wave and controlled reference fields.

Vector Lissajous laser beams

Alexey Porfirev, Svetlana Khonina, and Andrey Ustinov

DOI: 10.1364/OL.398209 Received 21 May 2020; Accepted 16 Jun 2020; Posted 16 Jun 2020  View: PDF

Abstract: We consider a new type of vector beam, vector Lissajous beams (VLBs), which are of double order (p, q) and a generalization of cylindrical vector beams characterized by single order p. The transverse components of VLBs have an angular relationship corresponding to Lissajous curves. A theoretical and numerical analysis of VLBs was performed, showing that the ratio and parity of orders (p, q) affect the properties of different components of the electromagnetic field (whether they will be real, imaginary or complex). In addition, this allows one to engineer the imaginary part of the longitudinal component of the electromagnetic field and control the local spin angular momentum density that is useful for optical tweezers and future spintronics applications.

Cascade recurrent neural network-assisted nonlinear equalization for a 100-Gb/s PAM4 short-reach direct detection system

Zhaopeng Xu, Chuanbowen Sun, Tonghui Ji, Jonathan Manton, and William Shieh

DOI: 10.1364/OL.394048 Received 08 Apr 2020; Accepted 16 Jun 2020; Posted 17 Jun 2020  View: PDF

Abstract: We propose a novel cascade recurrent neural network (RNN)-based nonlinear equalizer for a pulse amplitude modulation (PAM)-4 short-reach direct detection transmission system. A 100-Gb/s PAM4 optical link is experimentally demonstrated over 15-km standard single-mode fiber (SSMF), using a 16-GHz directly modulated laser (DML) in C-band. Experimental results show that the cascade RNN-based nonlinear equalizer outperforms other feedforward or non-cascade neural network (NN)-based nonlinear equalizers owing to its both cascade and recurrent structure, showing the great potential to effectively tackle the nonlinear signal distortion. With the aid of cascade RNN-based nonlinear equalizer, bit-error rate (BER) lower than 7% hard-decision forward error correction (FEC) threshold can be achieved when the receiver power is larger than 5 dBm. Compared with traditional non-cascade NN-based equalizers, the training time could also be reduced by half with the help of the cascade structure.

An intriguing branching of the maximum position of the absorption cross section in Mie theory explained

Ilia Rasskazov, P. Scott Carney, and Alexander Moroz

DOI: 10.1364/OL.397782 Received 15 May 2020; Accepted 15 Jun 2020; Posted 16 Jun 2020  View: PDF

Abstract: A potential control over the position of maxima of scattering and absorption cross-sections can be exploited to better tailor nanoparticles for specific light-matter interaction applications. Here we explain in detail the mechanism of an appreciable blue shift of the absorption cross-section peak relative to a metal spherical particle localized surface plasmon resonance (LSPR) and remaining scattering and extinction cross sections. Such a branching of cross sections maxima requires a certain threshold value of size parameter x=0.7 and is a prerequisite for obtaining high fluorescence enhancements, because the spectral region of high radiative rate enhancement becomes separated from the spectral region of high non-radiative rate enhancement. A consequence is that the maximum of the absorption cross section cannot be used as the definition of the LSPR position for x> 0.7.

Compact single-end pumped Brillouin random fiber laser with enhanced distributed feedback

Yanping Xu, Ping Lu, and Xiaoyi Bao

DOI: 10.1364/OL.398593 Received 26 May 2020; Accepted 15 Jun 2020; Posted 17 Jun 2020  View: PDF

Abstract: A compact sub-kHz linewidth Brillouin random fiber laser (BRFL) based on a linear cavity scheme with single-end pumping and enhanced distributed Rayleigh feedback from fiber random gratings (FRGs) is proposed and demonstrated. The improved FRGs with low transmission loss make the single-end pumped linear cavity configuration achievable without sacrificing the lasing capability, which contributes to a more compact setup for easy integration and packaging. The enhanced Rayleigh feedback from the FRG enables a high-efficiency random lasing resonance of the Stokes wave via stimulated Brillouin scattering in the lasing cavity. More importantly, the single-end pumped scheme, unlike the previously reported bi-directionally pumped BRFL, significantly alleviates the lasing instabilities and noises induced by the counter-propagating laser beams through the Brillouin-active medium, thus exhibiting lower lasing noises. Single longitudinal mode operation of the proposed random laser is realized with a narrow-linewidth of ~0.97kHz.

Manifestation of Extremely High-Q Pseudo-Modes in Scattering of a Bessel Light Beam by a Sphere

Vasily Klimov

DOI: 10.1364/OL.393570 Received 26 Mar 2020; Accepted 15 Jun 2020; Posted 19 Jun 2020  View: PDF

Abstract: The exact analytical solution of Maxwell equations for a Bessel light beam scattered by a sphere is found. Scattered power, stored energy and a generalized Q factor as a function of frequency, the sphere radius, permittivity, and the Bessel beam angle are found. On the base of this solution, modes and pseudo-modes of a dielectric sphere are extracted by calculation of the generalized Q factor. It is shown that an appropriate choice of Bessel beam parameters can provide excitation of a single given mode and an unlimited value of the radiative Q factor of pseudo-modes

Low-loss fs-laser-written surface waveguide lasers at >2 µm in monoclinic Tm3+:MgWO4

Esrom Kifle, Pavel Loiko, Javier Vazquez de Aldana, Carolina Romero, Victor Llamas, Josep Maria Serres, Magdalena Aguilo, Francesc Diaz, lizhen zhang, zhoubin lin, Haifeng Lin, Ge Zhang, Viktor Zakharov, Andrei Veniaminov, Valentin Petrov, Uwe Griebner, Xavier Mateos, Li Wang, and Weidong Chen

DOI: 10.1364/OL.395811 Received 23 Apr 2020; Accepted 14 Jun 2020; Posted 15 Jun 2020  View: PDF

Abstract: Surface channel waveguides (WGs) based on a half-ring (40–60-μm diameter) depressed-index cladding (type III) geometry are fabricated in monoclinic Tm3+:MgWO4 by femtosecond laser writing at a repetition rate of 1 kHz. The WGs are characterized by confocal laser microscopy and μ-Raman spectroscopy. A Tm3+:MgWO4 WG laser generates 320 mW at ~2.02 μm with a slope efficiency of 64.4%. The WG emits a tranverse single-mode and linear polarization (E || Nm). A remarkable low-loss of <0.1 dB/cm is measured for the WG. Vibronic laser emission at ~2.08 μm is also achieved.

Direct generation of high-power vortex array laser beams

Yung-Fu Chen, H. T. Ke, Y. C. Tseng, M. X. Hsieh, Jung-Chen Tung, Y. H. Hsieh, Hsing-Chih Liang, and K. F. Huang

DOI: 10.1364/OL.399251 Received 02 Jun 2020; Accepted 14 Jun 2020; Posted 15 Jun 2020  View: PDF

Abstract: The frequency degeneracy induced by the astigmatism in a nearly hemispherical cavity is originally exploited to generate the high-power vortex array laser beams. The inhomogeneous Helmholtz equation is employed to derive the wave function for manifesting the characteristics of the lasing modes. The theoretical wave function explicitly reveals the role of Gouy phase in the formation of vortex arrays. Numerical analyses are further performed to confirm that the thermal lensing effect in the laser crystal assists the lasing transverse order to increase with increasing the pump power. It is believed that the high efficiency enables the present laser modes to be useful in the applications of structured vortex beams.

Tunable continuous-wave laser operation of Tm3+:BaY2F8 near 2.3 µm

Abdullah Muti, Isinsu Baylam, Mauro Tonelli, and Alphan Sennaroglu

DOI: 10.1364/OL.394261 Received 01 Apr 2020; Accepted 12 Jun 2020; Posted 15 Jun 2020  View: PDF

Abstract: We report, for the first time to our knowledge, tunable continuous-wave laser action in the Tm3+:BaY2F8 (BYF) crystal near 2.3 μm. In the experiments, a BYF crystal doped with 3 at. % thulium was end pumped with a narrow-linewidth, tunable Ti3+:sapphire laser with up to 920 mW of incident power. Lasing was achieved for two pump polarizations of E//x and E//y. The best power performance was obtained in the case of E//x, double-end pumping, where 100 mW of output power was obtained at 2290 nm with 920 mW of pump power and 1% output coupler. The laser could be continuously tuned from 2 3 to 85 nm. Excitation spectra for E//x and E//y pumping were measured in the 760-810 nm range and the optimum pumping wavelength was determined to be 779 nm for E//x. By using the lifetime and lasing threshold data, the stimulated emission cross section at 2290 nm was further determined to be (1.03±0.21)x10-24 m2.

Buildup dynamics of asynchronous vector solitons in a polarization-multiplexed dual-comb fiber laser

Kangjun Zhao, Chenxin Gao, Xiaosheng Xiao, and Changxi Yang

DOI: 10.1364/OL.398323 Received 22 May 2020; Accepted 12 Jun 2020; Posted 15 Jun 2020  View: PDF

Abstract: Polarization-multiplexed dual-comb fiber lasers enable significant applications in dual-comb spectroscopy and optical sensing. However, the complexity of the underlying formation dynamics of dual-comb solitons has not been unveiled so far. Here, we capture the real-time spectral evolutions of both vector solitons from the initial fluctuations, with the help of time-stretch dispersive Fourier transform technique. Both vector solitons experience the relaxation oscillation, quasimode-locking, beating dynamics and mode locking, accompanying central wavelength shifts in opposite directions which might be induced by the gain saturation during their buildup processes. Moreover, polarization-dependent gain in the gain fiber leads to the different buildup time of both vector solitons. Our findings open new perspectives for dual-comb buildup dynamics and might impact laser design for applications.

Generation of 1-μJ and 40-fs pulses from a large mode area gain-managed nonlinear amplifier

Pavel Sidorenko and Frank Wise

DOI: 10.1364/OL.396683 Received 01 May 2020; Accepted 12 Jun 2020; Posted 16 Jun 2020  View: PDF

Abstract: We demonstrate, numerically and experimentally, a gain-managed nonlinear amplifier with large mode area fiber. The amplifier delivers 1.2-µJ and sub-40 fs pulses with the spectrum spanning from ~1000 to ~1180 nm. We show that longitudinal gain-loss evolution plays an essential role in pulse formation by comparing simulations with different gain models to experimental results.

High-dimensional states of light with full control of transverse path and OAM degrees of freedom

Dudbil Pabon Riaño, Silvia Ledesma, and Lorena Rebon

DOI: 10.1364/OL.397331 Received 20 May 2020; Accepted 12 Jun 2020; Posted 12 Jun 2020  View: PDF

Abstract: We present here a compact scheme for the generation of high-dimensional states of light encoded in the transversepath variable of photons that carry orbital angular momentum. We use a programmable spatial light modulator in phase configuration to create correlations between these two spatial degrees of freedom. With this setup we are able to control, independently, the relative phases and amplitudes of the path superposition in addition to the topological charge of each path. Moreover, we engineer correlations that emulate bipartite quantum states of dimensions $d\times m$. Experimental results from the characterization of different generated states of dimensions up to $9\times 5$ are in excellent agreement with the numerical simulations. Fidelity with the target state is, for all cases, above $95\%$.

InGaN blue light emitting micro-diodes with current path defined by tunnel junction

Krzysztof Gibasiewicz, Agata Bojarska, Grzegorz Muziol, Czesław Skierbiszewski, Szymon Grzanka, Anna Kafar, Piotr Perlin, Stephen Najda, and Suski Tadeusz

DOI: 10.1364/OL.394629 Received 07 Apr 2020; Accepted 10 Jun 2020; Posted 01 Jul 2020  View: PDF

Abstract: We have fabricated tunnel-junction InGaN micro-LEDs using Plasma-Assisted Molecular Beam Epitaxy technology, with top-down processing on GaN substrates. Devices had diameters between 5 µm and 100 µm. All of the devices emitted light at 450nm at driving current density of about 10 A cm-2. We demonstrate that within micro-LEDs’ size range from 100 m down to 5 m, the properties of these devises, both electrical and optical are fully scalable. That means we can reproduce all electro-optical characteristics using a single set of parameters. Most notably, we do not observe any enhancement of nonradiative recombination for the smallest devices. We assign this result to a modification of the fabrication process, i.e, replacement of deep dry etching by a tunnel junction for the current confinement. These devices show excellent thermal stability of their light emission characteristics, enabling operation at current densities up to 1 kA cm-2.

Influence of atmospheric Helium on secondary clocks

Ko-Han Chen, Chien-Ming Wu, Shu-Ron Wu, Hsin-Hung Yu, Tze-Wei Liu, and Wang-Yau Cheng

DOI: 10.1364/OL.394464 Received 06 Apr 2020; Accepted 08 Jun 2020; Posted 09 Jun 2020  View: PDF

Abstract: Glass-cell based secondary frequency standards are the most popular laser-assisted clocks. However, the reported frequency accuracies were always much worse than what was expected, though high laser stabilities have been achieved. The conundrum was left unsolved, even though possible systematic errors had been thoroughly discussed in literature. In this report, a high-precision measurement on the spectral frequency-linewidth relation is first used for revealing a new physics error in secondary clock. We thereby offer a solution to the mysterious inconsistency of secondary clocks that has baffled metrologists for decades.

Photopolymerization with high-order Bessel light beams

Yoshihiko Arita, Jun Hyung Lee, Haruki Kawaguchi, Reimon Matsuo, Katsuhiko Miyamoto, Kishan Dholakia, and Takashige Omatsu

DOI: 10.1364/OL.396012 Received 23 Apr 2020; Accepted 05 Jun 2020; Posted 08 Jun 2020  View: PDF

Abstract: We study photopolymerization with high-order Bessel light beams with phase singularities on-axis. Self-trapping and self-focusing of the propagation-invariant light beams in a photopolymer allow the fabrication of extended helical microfibers with a length scale of a centimeter, which is more than an order of magnitude larger than the depth-of-focus of the Bessel light beams. We show the evolution of microfibers rotating at a rate proportional to the incident optical power, while the periodicity of the helical structures remains constant irrespective of the laser power, suggesting optical momentum transfer playing a predominant role in the growth and rotation of such fiber structures.

2 MW peak power generation in fluorine co-doped Yb fiber prepared by powder-sinter technology

Martin Leich, Andre Kalide, Tina Eschrich, Martin Lorenz, Adrian Lorenz, Katrin Wondraczek, Dörte Schönfeld, Andreas Langner, Gerhard Schoetz, and Matthias Jaeger

DOI: 10.1364/OL.394793 Received 09 Apr 2020; Accepted 04 Jun 2020; Posted 08 Jun 2020  View: PDF

Abstract: We report on the first implementation of a fluorine co-doped large-mode-area REPUSIL fiber for high peak power amplification in an ultrashort-pulse master oscillator power amplifier. The core material of the investigated step-index fiber with high Yb-doping level was fabricated by means of the REPUSIL powder-sinter technology ensuring a very homogeneous dopant and refractive index distribution for large fiber cores and a cost efficient fabrication. For achieving high beam quality by preferentially exciting the fundamental mode and for ensuring a monolithic seed path, one end of the LMA fiber is locally tapered. Since high Yb concentration with appropriate co-doping results in a high refractive index, the core numerical aperture was adjusted by fluorine co-doping of the REPUSIL core glass. A fiber with core diameter of 52 µm and large core-to-clad ratio was realized and we could demonstrate near-diffraction-limited beam quality with M2 = 1.3 up to 2 MW peak power, which is a record peak power from a tapered single core fiber.

Modulational-instability-free pulse compression in anti-resonant hollow-core photonic crystal fiber

Felix Köttig, Francesco Tani, and Philip Russell

DOI: 10.1364/OL.396425 Received 29 Apr 2020; Accepted 03 Jun 2020; Posted 10 Jun 2020  View: PDF

Abstract: Gas-filled hollow-core photonic crystal fiber (PCF) is used for efficient nonlinear temporal compression of femtosecond laser pulses, two main schemes being direct soliton-effect self-compression, and spectral broadening followed by phase compensation. To obtain stable compressed pulses, it is crucial to avoid decoherence through modulational instability (MI) during spectral broadening. Here we show that changes in dispersion due to spectral anti-crossings between the fundamental core mode and core wall resonances in anti-resonant-guiding hollow-core PCF can strongly alter the MI gain spectrum, enabling MI-free pulse compression for optimized fiber designs. In addition, higher-order dispersion can introduce MI even when the pump pulses lie in the normal dispersion region.

Characterization of nanoporous Al2O3 films at terahertz frequencies

Min Zhai, Alexandre Locquet, Mi Jung, Deok Ha Woo, and David Citrin

DOI: 10.1364/OL.390129 Received 26 Mar 2020; Accepted 27 May 2020; Posted 08 Jun 2020  View: PDF

Abstract: A series of nanoporous Al2O3 films grown on Al substrates formed by a two-step anodization process is characterized nondestructively by reflective polarization-resolved pulsed terahertz tomography. Terahertz birefringence of the nanoporous Al2O3 layers and the homogeneity of the samples are investigated. Sparsity-based deconvolution is employed to reconstruct the thicknesses of the films nondestructively from the raw data, showing good agreement with the thicknesses measured directly by destructive cross-sectional field-emission scanning electron microscopy.

High spectro-temporal purity single-photons from silicon micro-racetrack resonators using a dual-pulse configuration

Ben Burridge, Imad Faruque, John Rarity, and Jorge Barreto

DOI: 10.1364/OL.393077 Received 19 Mar 2020; Accepted 07 May 2020; Posted 12 May 2020  View: PDF

Abstract: Single-photons with high spectro-temporal purity are an essential resource for quantum photonic technologies. The highest reported purity up until now from a conventional silicon photonic device is 92% without any spectral filtering. We have experimentally generated and observed single-photons with 98.0 ± 0.3% spectro-tempral purity using a conventional micro racetrack resonator and an engineered dual pump pulse.

**Nonlinear optical properties of semiconductor double quantum wires coupled to a quantum-sized metal nanoparticle: Comment

Sofia Evangelou and Constantinos Angelis

DOI: 10.1364/OL.392610 Received 16 Mar 2020; Accepted 04 May 2020; Posted 07 Jul 2020  View: PDF

Abstract: Some comments about the recently published Optics Letters paper "Nonlinear optical properties of semiconductor double quantum wires coupled to a quantum-sized metal nanoparticle," by Su et al. Opt. Lett. 45, 379 (2020), are provided.

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