Research on output characteristics of SESAM-assisted figure-9 mode-locked fiber laser

Objective Ultrafast mode-locked fiber lasers are important sources of short optical pulses for applications in optical communications, precision measurement, nonlinear optics, and ultrafast photonics. Among various passive mode-locking approaches, nonlinear amplifying loop mirrors (NALMs) have attracted considerable attention because of their intrinsically robust nonlinear optical mechanism, excellent environmental stability, and low sensitivity to external perturbations. However, conventional NALM-based fiber lasers generally exhibit relatively high mode-locking thresholds and limited self-starting capability because the effective nonlinear transmission induced by the NALM is strongly dependent on the accumulated nonlinear phase difference and intracavity phase bias. These characteristics can hinder reliable operation under low-pump-power conditions and reduce their practical applicability in systems requiring repeatable self-starting. Therefore, this work aims to improve the self-starting capability and operational stability of a figure-9 mode-locked fiber laser by introducing a semiconductor saturable absorber mirror (SESAM) as an auxiliary mode-locking element. The primary objective is to realize low-threshold, reliable self-starting, and highly stable mode-locked operation while preserving the inherent stability and nonlinear mode-locking characteristics of the NALM-based figure-9 cavity.

Methods A hybrid-cavity ytterbium-doped mode-locked fiber laser was experimentally constructed by integrating an all-fiber NALM loop with a free-space linear arm, forming a figure-9 cavity configuration. The gain medium was ytterbium-doped fiber pumped by a semiconductor laser diode operating near 980 nm. An asymmetric arrangement of the gain fiber inside the NALM loop was employed to generate different nonlinear phase shifts for the counter-propagating clockwise and counterclockwise waves, thereby producing intensity-dependent transmission through nonlinear interference. A nonreciprocal phase shifter consisting of polarization beam splitters, a Faraday rotator, and wave plates was incorporated into the linear arm to control the relative phase bias between the counter-propagating pulses. A reflective SESAM was further introduced at the end of the single-arm section. This reflective configuration is naturally compatible with the geometry of the figure-9 cavity and enables the SESAM to act as an auxiliary saturable absorber without requiring an additional transmission path. In the hybrid mode-locking mechanism, the NALM provides nonlinear intensity-dependent transmission, whereas the SESAM preferentially suppresses low-intensity continuous-wave components and provides stronger reflectivity for high-peak-power pulses. The combined effect enhances the effective saturable transmission of the cavity, thereby facilitating pulse initiation and improving self-starting reliability. To maintain stable long-term operation and avoid excessive optical loading, the SESAM was deliberately positioned in a relatively low-power region of the cavity. The laser performance was characterized in terms of mode-locking threshold, self-starting behavior, repetition rate, radio-frequency signal-to-noise ratio, pulse characteristics, multi-pulse evolution, output-power stability, and repeated self-starting reliability.

Results and Discussions Stable single-pulse mode-locking was experimentally obtained at a pump power as low as 105 mW, demonstrating a substantial reduction in the mode-locking threshold compared with the conventional NALM configuration. When the pump power was increased to 120 mW, the laser could automatically enter the mode-locked state without external perturbation, confirming reliable self-starting operation. The fundamental repetition rate was measured to be 31.56 MHz, consistent with the expected cavity round-trip frequency of the figure-9 configuration. The radio-frequency spectrum exhibited a clean fundamental peak with a maximum signal-to-noise ratio of 79.5 dB, while no pronounced sidebands or parasitic modulation peaks were observed, indicating stable periodic pulse generation and strong suppression of amplitude fluctuations. As the pump power was further increased, the laser exhibited a well-defined multi-pulse evolution process, with the operating state successively developing from a single-pulse regime to double-pulse and triple-pulse regimes. This behavior can be attributed to the increased nonlinear phase accumulation and gain saturation under higher pump power, which gradually modify the intracavity energy balance and limit the stable single-pulse energy. At a pump power of 200 mW, the maximum average output power reached 14.01 mW. Under this condition, the reported maximum single-pulse energy was 445.13 pJ, corresponding to a peak power of 23.67 W. The measured autocorrelation trace indicated a pulse duration of approximately 8.1 ps, and the fitted temporal profile exhibited a regular shape without severe distortion. Furthermore, long-term operation demonstrated excellent stability. During a continuous 6 h measurement, the root-mean-square fluctuation of the output power was only 0.079%, indicating strong resistance to environmental perturbations and stable intracavity pulse dynamics. To quantitatively evaluate the reliability of self-starting, 100 consecutive startup experiments were performed under the same operating conditions. The laser successfully reached the mode-locked state in all 100 trials, corresponding to a self-starting success rate of 100%. These results demonstrate that the introduction of the SESAM not only assists the initiation of mode-locking at a substantially lower pump power but also provides an additional nonlinear intensity-discrimination mechanism that improves the robustness of the mode-locked state. Meanwhile, the observed transition from single-pulse to multiple-pulse operation provides direct evidence of the influence of pump-induced gain and nonlinear phase accumulation on the intracavity pulse-energy balance.

Conclusions A SESAM-assisted figure-9 ytterbium-doped mode-locked fiber laser was experimentally demonstrated, combining the intrinsic stability of NALM-based nonlinear mode-locking with the reliable pulse-initiation capability of a SESAM. The hybrid configuration achieved a mode-locking threshold of 105 mW and reliable self-starting at 120 mW, with a self-starting success rate of 100% over 100 consecutive trials. A fundamental repetition rate of 31.56 MHz and a maximum radio-frequency signal-to-noise ratio of 79.5 dB were obtained, while the 6-h root-mean-square output-power fluctuation remained as low as 0.079%. The observed controllable transition among single-, double-, and triple-pulse regimes further demonstrates the potential of the proposed configuration for studying and controlling pulse evolution under increased pump power. Overall, the results confirm that introducing a reflective SESAM into the single-arm section of a figure-9 NALM cavity can effectively compensate for the limited self-starting capability of conventional NALM lasers while retaining their inherent environmental robustness. The proposed hybrid mode-locking architecture provides a promising route toward low-threshold, self-starting, stable, and practically deployable ultrafast fiber laser sources.