Wavelength-switchable single-longitudinal-mode thulium-doped fiber laser at 2.05 µm using a superimposed fiber Bragg grating

• A single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL) is enabled by a three-channel superimposed fiber Bragg grating (SI-FBG). • Three single-wavelength operations can be obtained and switched flexibly. • A maximum optical signal-to-noise ratio (OSNR) is 62 dB. Herein, we present a wavelength-switchable single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL), enabled by a three-channel superimposed fiber Bragg grating (SI-FBG), a compound-subring-cavity (CSC), and an unpumped TDF-based saturable absorber (SA). By adjusting a drop-in polarization controller (DI-PC), three single-wavelength operations can be obtained and switched flexibly. In each operation, the fiber laser exhibited a maximum optical signal-to-noise ratio (OSNR) of 62 dB as well as the wavelength and power fluctuations of ≤0.01 nm and ≤0.641 dB, respectively, in a 50-min time span. Furthermore, the SLM lasing characteristics of the proposed TDFL with the CSC and SA have been investigated in detail.