Observation of Optical Spectrum Dynamics and Longitudinal-Mode Feature in a Single- Frequency Self-Sweeping Fiber Laser

In this study, we constructed and realized a single-frequency self-sweeping fiber laser that can perform both reverse and normal sweeping. The laser can convert sweeping directions using different lengths of fiber saturable absorber (FSA). The maximum sweeping ranges for normal and reverse sweeping are 5.072 and 3.457 nm, respectively, and are optimized results based on a unidirectional ring cavity. We summarized the optical spectra and pulse dynamics of both normal and reverse sweeping, and we also observed the longitudinal-mode feature of the fiber laser, revealing a phenomenon in which the interval of frequency change is twice the spacing of longitudinal-mode. Furthermore, the reflected spectra of FSA at different lengths and wavelengths were calculated to explain the sweeping direction and longitudinal mode features.