Wavelength Switchable Thulium-Doped Fiber Laser Based on Fiber-Core Mismatched Mach–Zehnder Filter

In this study, we propose a wavelength-switchable Thulium-doped fiber laser (TDFL) based on an all-fiber Mach–Zehnder (MZ) filter. The MZ comb interferometer is designed and manufactured by welding two mismatched fiber cores. The proposed ring-cavity TDFL has a working threshold of 140 mW. Single-wavelength switchable laser emission is achieved in the range of 1841.5 to 1892.36 nm, with a minimum wavelength spacing of 3.66 nm and a side-mode suppression ratio (SMSR) of over 32.41 dB. In the experiment, four different dual-wavelength switchable and stable lasers are generated within a 44.56 nm range with 7.12 dB power shifts. Additionally, a triple-wavelength laser at 1864.77, 1888.1, and 1896.76 nm is obtained, with an SMSR of over 30.06 dB. The designed TDFL successfully achieves stable single, dual, and triple-wavelength laser emissions with power fluctuations smaller than 0.51, 1.22, and 1.32 dB, respectively.