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• A highly stable single-longitudinal-mode erbium-doped fiber laser was proposed. • A novel mode-selection filter, composed of two Type-Ⅱ fiber rings and a 1.5 m SA was used to realize SLM output. • The maximum wavelength drift was not observed and the maximum peak power fluctuation was kept at a low level of 0.16 dB. • The minimum linewidth at an integration time of 0.005 s was 2.8809 kHz. Single-longitudinal-mode (SLM) fiber lasers have the advantages of stable output, long coherence length, and low noise. However, several fiber lasers still maintain multiple longitudinal mode outputs. Thus, a highly stable SLM erbium-doped fiber laser (EDFL) was proposed and investigated. The dual-ring compound cavity (DRCC) filter, composed of two Type-Ⅱ fiber rings, and a 1.5 m unpumped erbium-doped fiber (EDF) was used as a mode-selection filter. The mode-selection capability of the DRCC filter and unpumped EDF was investigated and discussed theoretically and experimentally. The proposed EDFL could generate stable SLM output with an optical signal-to-noise ratio of ∼ 64 dB over 60 min. The maximum wavelength drift was not observed on an optical spectrum analyzer and the maximum peak power fluctuation was kept at a low level of 0.16 dB. The output power variations with different pump powers, the relative intensity noise, the relaxation oscillation, and the linewidth at different integration times were investigated. Furthermore, the performance of the proposed EDFL acted as a strain sensor and temperature sensor with sensitivities of 1.0 pm/με and 11.4 pm/℃ were also discussed. [ABSTRACT FROM AUTHOR]

• 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]

• A highly stable single-longitudinal-mode thulium-doped fiber laser. • A linear compound cavity fiber laser utilizing a cascaded dual-coupler double-ring cavity filter. • The peak power fluctuation and wavelength drift over 60 min were less than 0.11 dB and 0.02 nm, respectively. • The minimum linewidth at an integration time of 0.001 s was 8.88 kHz. A linear compound-ring cavity single longitudinal mode (SLM) thulium-doped fiber laser (TDFL) based on a cascaded dual-coupler double-ring cavity filter has been experimentally investigated and is proposed. The cascaded dual-coupler double-ring cavity filter was analyzed theoretically in detail by adjusting the effective free spectral range and transmission bandwidth, which ensures SLM operation and narrow linewidth. The experimental results indicate that the proposed TDFL can generate stable SLM lasing with a high optical signal-to-noise ratio of 69 dB, at 2048.39 nm for more than 60 min. The wavelength drift and peak power fluctuation in that observation time were less than 0.02 nm and 0.11 dB, respectively. The linewidth of the laser was 8.8 kHz at an integration time of 0.001 s. The relative intensity noise was less than −128.88 dB/Hz at frequencies greater than 1 MHz. [ABSTRACT FROM AUTHOR]