Impact of polarization control on the stability of a Thulium-Doped Figure-8 fiber laser

Figure-8 (F8) cavities of passively mode-locked thulium-doped fiber lasers (TDFLs) operating in the 2-μm wavelength region with polarization-imbalanced nonlinear optical loop mirror (PI-NOLM), which allow the generation of stable trains of pulses with adjustable characteristics using a polarizer, are particularly appealing. However, in the absence of polarization control in such schemes, stable and flexible pulse generation particularly in the noise-like pulse (NLP) regime gives way to chaotic dynamics and transient mode locking, severely limiting their performance and stability, which often precludes their use in real-life applications. These instabilities, driven by birefringence and thermal effects, have not been thoroughly understood or systematically investigated until now. In this work, we combine experimental and theoretical approaches to study the chaotic dynamics arising from the absence of a polarizer in PI-NOLM-based F8 cavities. Our numerical model, which simulates dynamic birefringence effects such as induced by elements such as doped fibers, nonlinear interactions, and thermal effects within the cavity, confirms experimental results. Interestingly, the model indicates that, in the absence of a polarizer, the strong dependence of the PI-NOLM transmission on input polarization leads to some degree of nonlinear intracavity self-polarization during short episodes of pulsed operation. Our theoretical model explains the chaotic behavior observed experimentally in the absence of a polarizer in the cavity and offers a predictive framework for understanding and mitigating these instabilities in PI-NOLM-based F8 configurations with special emphasis on NLP operation. To our knowledge, this is the first systematic investigation of the dynamic instabilities associated with the absence of polarization control in PI-NOLM-based TDFLs. By confirming the interplay between birefringence and polarization dynamics, this work enhances the understanding of mode-locking instabilities and provides actionable insights for designing more robust TDFLs. These findings pave the way for impactful applications in spectroscopy, biomedical imaging, and precision material processing.