Harmonic mode-locked fiber lasers provide generation of the ultrashort pulse train with high repetition rates up to gigahertz scale. However, setting appropriate parameters for the laser cavity to reach a harmonic mode-locked regime is often a non-trivial task. Depending on the dynamic of adjustment of the cavity elements one may reach unstable, multipulsing or harmonic mode-locked regimes at the same end-point parameters. Here, we demonstrate the state-of-theart fiber mode-locked laser assisted with reinforcement Soft Actor-Critic algorithm that is capable of learning a dynamic strategy of adjusting cavity parameters to maximize the order of harmonic mode-locked regime. Control of the pumping power and nonlinear transmission function of the state-of-the-art single walled carbon nanotube saturable absorber allows reaching a stable harmonic mode-locked regime.
In the present work, the formation of ultrashort pulses in a fiber laser resonator with the effect of slow saturable absorption and spectral filtering was studied. It has been shown that in a resonator with normal chromatic dispersion, the finite relaxation time of a saturable absorber leads to a spectral shift of the generated pulses with respect to the central length of the spectral filter. The numerical results are verified using two experimental laser sources: a ring fiber laser with a semiconductor saturable absrober mirror and a fiber laser with a nonlinear amplifying loop mirror. The results obtained are relevant for designing sources of ultrashort pulses in applications for which the spectral properties of radiation are crucial parameters.
We propose for the first time a new method to electronically control the cavity length of mode-locked fibre lasers, allowing different lasing conditions and performance in a single laser configuration. A special cavity design was developed that combines a multi-kilometre fibre resonator boosting pulse energy to hundreds nJ with an all-PM fibre resonator providing nearly 200-m long environmentally-stable cavity round trip. Switching between the cavities is achieved automatically by adjusting modulation frequency of an optical switcher (used for active mode locking) to match the cavity round-trip time of one of the cavities. We discuss details of our method, its possibilities and limitations, in particular related to stability of pulse parameters.
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