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• A highly stable narrow linewidth erbium-doped fiber laser was proposed. • The self-injection locked mechanism was utilized in the ring-cavity fiber laser. • The cascaded Type Ⅱ fiber rings cavity filter was used to keep single-longitudinal-mode output. • The maximum wavelength drift was 0.0041 nm and the maximum peak power fluctuation was kept at a low level of 0.16 dB. • The minimum linewidth at a 0.002 s was 1.82 kHz. Narrow linewidth erbium-doped fiber lasers (EDFLs) hold in significant potential optical fiber sensing, wavelength multiplexing transmission, and optical fiber communication. However, many EDFLs produced outputs with linewidth in several kHz level. Therefore, a high optical signal-to-noise ratio (OSNR), narrow linewidth single-longitudinal-mode (SLM) EDFL was proposed and investigated in this work. The EDFL utilized a combination of compound-ring cavity filter and a self-injection locked mechanism to generate a stable SLM output with an OSNR greater than 72 dB. The maximum wavelength drift and maximum peak power fluctuation over 60 min were maintained at low values of 0.0041 nm and 0.16 dB. Moreover, a narrow linewidth of 1.82 kHz at a measuring time of 0.002 s was realized. Furthermore, the research also investigated output power variations with different pump powers, relaxation oscillation, and the dependence of output central wavelength on the axial strain. [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 high OSNR single-longitudinal-mode thulium-doped fiber laser based on a cascaded dual-ring cavity filter was proposed and demonstrated. • A CDRC filter was used as the mode-selection mechanism. • The proposed TDFL has highly stability and SLM laser output over 60 min, and the linewidth was 11.82 kHz with an integration time of 0.001 s. A high optical-signal-to-noise ratio (OSNR) single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL) is proposed and has been investigated. SLM lasing was realized by utilizing a uniform fiber Bragg grating (UFBG) and a cascaded dual-ring cavity (CDRC) filter. The design, simulation, and characteristic analysis of three compound-cavity filters based on the Vernier effect are presented in detail, and provided the basis for experiments. The single-wavelength SLM lasing with a high OSNR of ∼75.10 dB, operating at 2048.44 nm was pumped by a 793 nm laser diode, and the SLM lasing was continuously monitored for 60 minutes. The peak power fluctuation and wavelength drift were less than 0.685 dB and 0.01 nm, respectively, over this time span. During the test, the relative intensity noise was below −128.49 dB/Hz above 1 MHz and the linewidth was 11.82 kHz with an integration time of 0.001 s. [ABSTRACT FROM AUTHOR]