Real-time observation of round-trip resolved spectral dynamics in a stabilized fs fiber laser

Fiber-stretcher based phase-lock loop (PLL) is a mature technique in fiber mode-locked lasers for repetition-rate stabilization. However, undesired side effects may be induced if not properly handled, which is easily overlooked owing to the lack of single-shot spectral analyzers. Thanks to the ultrafast spectral analyzing capability of optical time-stretch, an intriguing spectral dynamics is observed in a repetition-rate-stabilized nonlinear polarization rotation (NPR) mode-locked laser. Under the dynamic state, the optical spectra of pulses undergo dramatic evolution in every round trip while the pulse energy is relatively constant. Indicated by estimated cross-spectral densities, such spectral dynamics results in noticeable degradation in optical spectral coherence. The physical origin of the round-trip evolved spectral dynamics is attributed to the local birefringence induced by the fiber stretcher. Therefore, the results are helpful for a proper use of fiber-stretcher based PLL in fiber lasers, particularly when a good spectral coherence is desired. Furthermore, our study has also provided a potentially useful optical source for applications where fast spectral modulation is desired.