A switchable multi-wavelength thulium-doped fiber laser (TDFL) using either two-stage cascaded or two-segment Sagnac loop filters are proposed and analyzed. Both filters incorporate a 3-m and 1.7-m long polarization-maintaining fiber, which acts as a comb filter. By adjusting two polarization controllers, when the TDFL used a cascaded Sagnac loop filter, eight stable single-wavelength operations were obtained with easy switching among them. The wavelength range was 16.36 nm during the single-wavelength switchable operation, with the optical signal-to-noise ratio up to 55 dB. This TDFL can achieve at most quintuple-wavelength operation. When the TDFL used a two-segment Sagnac loop filter, we achieved single-wavelength and dual-wavelength switchable. This TDFL can achieve at most stable quadruple-wavelength operation. In dual-wavelength switchable operation, the maximum and minimum wavelength spacings were 21.78 nm and 1.50 nm. Due to its abundant range of wavelengths, the proposed TDFLs has great potential in wavelength-division multiplexing.
