Revealing the Buildup Dynamics of Harmonic Mode‐Locking States in Ultrafast Lasers

Abstract

Harmonic mode‐locking (HML) is an important technique enabling the generation of high‐repetition‐rate ultrashort pulses. Using an emerging time‐stretch dispersive Fourier transform technique, the experimental observation of the entire buildup process of the passive HML state in an ultrafast fiber laser is reported here. It is unveiled that the whole process of HML buildup successively undergoes seven different ultrafast phases: raised relaxation oscillation, spectral beating behavior, birth of a giant pulse, self‐phase‐modulation‐induced instability, pulse splitting, repulsion and separation of multiple pulses, and a stable HML state. It is observed that the multiple HML pulses originate from a single‐pulse splitting phenomenon and a remarkable breathing behavior occurs at an early stage of the HML buildup process. The numerical results confirm that the effects of dispersive wave, gain depletion and recovery, and acoustic wave play key roles in the earlier, middle, and later stages of this HML buildup process, respectively; as well, the acoustic resonance in the single‐mode fiber stabilizes the final HML state of lasers.

Period Doubling of Dissipative-Soliton-Resonance Pulses in Passively Mode-Locked Fiber Lasers

“…This means that the reduction of filtering bandwidth could facilitate the period-doubling of DSR pulses. Recently, an emerging technique called dispersive Fourier transform (DFT) has been proposed to investigate the soliton dynamics in mode-locked fiber lasers [37][38][39]. By virtue of the DFT technique, the single-shot spectra of period-doubled DSR pulses can be measured in our future work, which would be beneficial for further understanding the perioddoubling bifurcation.…”

Intelligent control of mode-locked femtosecond pulses by time-stretch-assisted real-time spectral analysis

“…Through the TS-DFT, more dynamics in MLFLs have come to light in recent studies [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] , deepening the understanding of the internal dynamics of mode-locking pulse formation and assisting the design of MLFLs. The starting dynamics, from the initial noise fluctuations to various regimes, have been intensively reported [23][24][25][26][27][28][29] , but the regime transition dynamics, which are essential processes in mode-locked lasers, are poorly understood. The transition dynamics from Q-switching to mode-locking in a soliton laser have been revealed 34 , and the transition dynamics of the Q-switching, fundamental mode-locking, and harmonic mode-locking regimes 35 have also been observed recently.…”

“…Combined with a real-time oscilloscope, a TS-DFT operates as a real-time OSA, resolving the issue of the low frame rate in a traditional OSA 20 . In numerous works, TS-DFT-based fast spectral analysis has been used with mode-locked lasers to observe soliton dynamics, including soliton explosions 21,22 , build-up processes of various pulsation regimes [23][24][25] , dissipative solitons 26 , soliton molecules [27][28][29] , sophisticated soliton dynamics [30][31][32][33][34] , and the transition dynamics between pulsation regimes 35,36 . In this article, for the first time, we propose using TS-DFT-based fast spectral analysis as the discrimination criterion to achieve rich modelocking regimes.…”

Watt-Level Dual-Wavelength Q-Switched Mode-Locked All-Solid-State Tm:CYA Laser

“…Among them, due to the complex process, narrow working bandwidth, and high cost of semiconductor saturable absorption mirrors, we need to look for other materials. Carbon nanotubes are widely used in fiber lasers [11,12], but their performance is poor in 2 µm solid-state lasers due to their unique structures and properties. Graphene is widely used in 1 µm, but its absorption efficiency at 2 µm is weak, which limits its ability to adjust light, and black phosphorus is easily oxidized in the presence of oxygen and water, leading to its instability in the experiment [13].…”