Revealing the Dynamic Spectral‐Reshaping Effect Induced by the Nonlinear Polarization Evolution in Mode‐Locked Soliton Fiber Lasers

ABSTRACT Passive mode‐locking based on nonlinear polarization evolution (NPE) is among the most widely adopted technique in ultrafast fiber lasers due to the intrinsic fast response, broadband operation and adjustment flexibility, enabling simple implementations in all‐fiber laser cavities. While the NPE‐induced self‐amplitude modulation (SAM) effect is generally regarded as the key mechanism of artificial saturable absorption for initiating passive mode‐locking, the concomitant spectral reshaping, resulted from the NPE effect, has remained elusive, inhibiting in‐depth understanding and precise control of laser dynamics. Here, we report, for the first time to our knowledge, experimental studies on the dynamic spectral reshaping induced by NPE in a soliton fiber laser, which manifests as a time‐delay central‐wavelength shift of pulse spectrum. Such a dynamic spectral reshaping effect has been observed during the build‐up process of soliton mode‐locking as well as in steady states with pump modulations, both featuring intensity‐dependent dynamics of the pulse central wavelength. Moreover, a theoretical model is established to interpret this effect through NPE‐based intensity‐dependent filtering, which exhibits good agreement with experimental observations. Our findings have revealed the critical role of the NPE effect in the determination of the mode‐locked pulse spectrum and may shed new light on stability improvement and precise control of mode‐locked lasers.