February 2020
Spotlight Summary by Paul Steinvurzel
High-power 2 GHz fs pulsed all-fiber amplified laser system at 2.0 µm
High power, high pulse repetition frequency (PRF) fs lasers present a technical challenge: in order to make the repetition frequency high, the cavity round trip time must be short, which limits the length of cavity, and hence the gain medium, to a few cm. The limited available gain begs the question under what conditions can the mode-locking threshold be reached in a short cavity. Researchers at the South China University of Technology in Guangzhou have recently overcome this challenge, demonstrating a fundamentally mode-locked 2 GHz, 126 fs, all-fiber laser amplified to 8 W operating in the 2.0 µm band. In high PRF mode-locked lasers, the frequency comb spectrum becomes sparse, which allows one to isolate individual comb lines. Such lasers have many applications in arbitrary waveform generation, frequency metrology, optical sampling, and spectroscopy (e.g., so-called astrocombs used for exoplanet detection). They are also useful for many nonlinear bioimaging modalities, where using high PRF allows one to increase optical power and, by extension, signal-to-noise ratio or imaging speed, without damaging the sample (in the regime where damage threshold depends on pulse energy rather than average power). The laser oscillator and amplifiers are constructed using commercially available fibers and include lengths of passive fiber to manage dispersion and nonlinearity, which is key both to achieve mode-locking with a short gain fiber and to maintain a short pulse width out of the amplifier chain. The pump-limited three-stage amplifier chain acts to red-shift and frequency broaden the pulses, which are temporally compressed, by a factor of two relative to the 244 fs oscillator pulse width, in a final length of passive fiber. The authors also show that the PRF can be scaled up by using a shorter, more dispersive cavity, demonstrating oscillation at 2.75 GHz PRF, which is the highest to date for a Tm-doped fiber laser.
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Article Information
High-power 2 GHz fs pulsed all-fiber amplified laser system at 2.0 µm
Tian Qiao, Huihui Cheng, Xiaoxiao Wen, Wenlong Wang, Wei Lin, Yi Zhou, Yuankai Guo, Yicai Liu, and Zhongmin Yang
Opt. Lett. 44(24) 6001-6004 (2019) View: Abstract | HTML | PDF