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• A thulium-doped fiber laser with bidirectional single-longitudinal-mode output was proposed. • The proposed TDFL had a simple structure to realize bidirectional output, using only two 2 × 2 optical couplers. • A novel cascaded triple-ring cavity filter was proposed and investigated. The reversibility and the capability of mode selection filtering were investigated by simulation and experiment. • The minimum relative intensity noise was less than −130 dB/Hz at 2 ≥ 2 MHz around the 2-μm band. Single-longitudinal-mode (SLM) fiber lasers play important roles in optical fiber communication. However, there has been little reported research on bidirectional SLM output, especially in the 2-μm band. In the work reported here, a bidirectional SLM output thulium-doped fiber laser (TDFL) using a cascaded triple-ring cavity (CTRC) filter was proposed and investigated. To realize bidirectional output, the proposed TDFL had a simple structure, using only two 2 × 2 optical couplers. To realize SLM output, the transmission spectrum, as well as the reversibility of the CTRC filter were discussed and investigated in detail. The direction-switchable SLM output was realized by polarization-dependent loss. By tuning the polarization controller carefully, each direction could generate SLM output with an optical signal-to-noise ratio (OSNR) > 72 dB. The maximum wavelength and peak power fluctuation over 60 min were, respectively, 0.01 nm and 0.48 dB. The variation of output power with pump power, linewidth, relaxation oscillation, and relative intensity noise were also investigated. To the best of our knowledge, this is the first time that a sub-ring cavity filter has been used to realize bidirectional SLM output. [ABSTRACT FROM AUTHOR]

• A five wavelength-switchable single-longitudinal-mode erbium-doped fiber laser was proposed. • A cascaded PS-FBG was designed and analyzed. To the best of our knowledge, this is the first time that the PS-FBG is used as a multi-channel filter to realize stable multiple SLM single-wavelength outputs. • Five single-wavelength switchable SLM outputs were generated in stable condition over 60 min. • The minimum linewidth at an integration time of 0.001 s was 1.3102 kHz. Wavelength-switchable fiber lasers play an important role in optical fiber communication systems. Although phase-shifted fiber Bragg grating (PS-FBG) has a narrow 3 dB bandwidth, it has rarely been reported as a multi-channel optical filter. Therefore, a five wavelength-switchable single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) was implemented and analyzed, where the cascaded PS-FBG with a 3 dB bandwidth of each channel narrower than 0.024 nm was used as a five-channel filter. The design, fabrication, and characterization of cascaded PS-FBG were implemented and analyzed in detail. By tuning the micro-displacement platform, five-single wavelength switchable SLM outputs with a high optical signal-to-noise ratio (OSNR) were generated and the outputs were monitored over 60 min. The measured maximum wavelength drift and the peak power fluctuation were 0.02 nm and 1.73 dB. The relaxation oscillation peaks were lower than −100 dB/Hz and the minimum linewidth at an integration time of 0.001 s was 1.3102 kHz. To the best of our knowledge, this is the first time that the PS-FBG is used as a multi-channel filter to realize multiple SLM single-wavelength outputs. [ABSTRACT FROM AUTHOR]