Multi-wavelength-tunable thulium-doped fiber laser generation based on dual-pass Mach-Zehnder and Sagnac loop hybrid filters

A wavelength-tunable thulium-doped fiber laser (TDFL) based on a Mach-Zehnder (MZ) filter and Sagnac loop was proposed in this study. The dual-pass MZ fiber filter, and Sagnac loop were fabricated to operate as reflectors in a linear cavity. The laser operating threshold was set to 138 mW. A single-wavelength laser output with an optical signal-to-noise ratio (OSNR) of up to 38 dB was realized with a tuning range of 1854-1882 nm. The tuning range was obtained with a tuning interval of 6 nm by tuning the polarization controller (PC), which yielded a power shift of less than 0.42 dB. Stability of the single-wavelength center wavelength power fluctuation was less than 0.39 dB within 20 min. When the laser was operated in the dual-wavelength condition, the OSNR was more than 30.5 dB, and the power fluctuation was less than 0.89 dB within 20 min at room temperature. The tunable triple-wavelength laser emission could be realized experimentally by adjusting the PC, which yielded an OSNR of more than 30.8 dB. The proposed TDFL is suitable for fiber-sensing system applications.