Description
Corresponding Author: Bo Fu (email: fubo10@buaa.edu.cn)
The conversion between mode-locking (ML) and Q-switching (QS) in a pulsed fiber laser is highly desirable for extending the parameter range of lasers and avoiding unwanted regime transitions [1]. However, such controllable conversion has rarely been achieved in the 2‑μm band, especially in fiber lasers based on real saturable absorbers without active modulators.
Here we propose and demonstrate a feedback‑loop-based method in order to achieve ML‑QS and QS-ML conversions in a 2‑μm fiber laser. The main cavity includes a thulium‑holmium co‑doped as the gain fiber, a saturable absorber, and an output coupler connected with single-mode fibers, while a passive fiber loop with a tunable attenuator constitute the feedback loop integrated into the main cavity. The feedback loop reinjects a fraction of the circulating power when the attenuation is lowered, effectively raising the instantaneous intracavity energy, reaching the gain depletion and thereby triggering the ML-QS conversion. Similar mechanism applies during the QS-ML conversion.
Both numerical simulations and experiments verified the feasibility of the method. In the ML-QS process, the output power slightly oscillated as the attenuation of the feedback loop decreased, and spontaneously transferred to a stable QS state. In the QS-ML process, the increase of the attenuation allowed one dominant noise peak in the QS state to survive gain competition and evolve into a ML pulse, while the peaks in other sections of the cavity vanished. The output power exhibited a rise at the conversion and became stable afterwards. This work provides an all‑fiber solution for controlled conversion between ML and QS at 2 μm, and offers a unified theoretical framework for studying ultrafast fiber laser dynamics.
Reference
[1] G. Wang, B. Fu, J. Sun, et al., “Bridging the Mode-Locked and Q-Switched Regimes in Fiber Lasers,” Laser Photon. Rev. 19, 2402171 (2025).
| I am the presenting author | Yes |
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