Fiber-loop-free, linear-cavity NALM mode-locked fiber laser based on polarization division multiplexing

This paper proposes and demonstrates a fiber-loop-free, linear-cavity nonlinear amplifying loop mirror (NALM) mode-locked fiber laser based on polarization division multiplexing (PDM) technology. By utilizing a PDM module and a Faraday mirror, the fast and slow axes of a single polarization-maintaining (PM) fiber are employed as two independent optical paths. This architecture constructs an equivalent Sagnac interference loop, eliminating the need for a physical fiber ring and integrating the NALM mechanism into a linear-cavity structure. A 750 fs mode-locked pulse train with a repetition rate of 54.6 MHz was achieved. We further show that when the relative delay between pulses in the two axes is sufficiently large, stable, temperature-insensitive NALM mode locking is achieved. In contrast, zero delay activates the nonlinear polarization evolution (NPE) regime, where pulse powers become sensitive to thermal-induced delay variations. This work removes the physical fiber loop and provides a promising route toward compact, high-repetition-rate NALM fiber lasers. Furthermore, this laser provides a platform for the comparison and dynamic study of NALM and NPE.