Enhancement of multipulse generation threshold in SESAM-based PM fiber soliton laser

Ultrafast lasers operating in the short-wavelength infrared (SWIR, 1.4–3 μm) region are essential tools for spectroscopy, telecommunications, microwave photonics, and ranging. SWIR mode-locked fiber lasers - especially those utilizing Er- or Tm-doped fibers - provide significant benefits over bulk laser systems, such as cost-efficiency, compact footprints, reduced noise, and wide gain bandwidths ideal for generating ultrashort pulses. Typically, these lasers are characterized by strong anomalous cavity dispersion, which promotes the formation of optical solitons. A key phenomenon in these soliton-based systems is the transition to multi-pulse regimes at elevated pump powers, resulting in the coexistence of several pulses with similar energies. Focusing specifically on soliton fiber lasers, a key practical challenge is to achieve the broadest possible optical spectrum and the shortest pulse duration, while maintaining stable single-pulse operation. This study provides a joint numerical and experimental analysis of how saturable absorber positioning affects the performance of a polarization-maintaining (PM) soliton fiber laser mode-locked due to a SESAM. Our simulations identify a specific optimal region within the cavity where precise SESAM placement facilitates stable single-pulse operation at increased gain levels, effectively suppressing background noise and delaying multi-pulse transitions. These results are validated by an Er-doped PM fiber laser experiment, showing enhanced pulse energy and spectral width. This approach offers a robust strategy for optimizing soliton lasers, with significant implications for advanced SWIR systems, (especially for Tm- and Ho-doped fiber lasers).