Thulium-doped fiber laser with bidirectional single-longitudinal-mode output using a cascaded triple-ring cavity filter

•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.