Investigation of a thulium-doped fiber laser with bidirectional output assisted by compound ring cavity filter

A high-performance bidirectional-output wavelength-switchable narrow-linewidth thulium-doped fiber laser (TDFL) is proposed and has been demonstrated. Based on the uniform fiber Bragg grating for wavelength selection, combined with compound ring cavity (CRC) structure for longitudinal mode selection, single longitudinal mode (SLM) laser operation at wavelengths of ∼2048.502 and ∼1942.080 nm are achieved in clockwise (CW) and counterclockwise (CCW) directions, respectively. The multi-objective optimization algorithms, including multi-objective particle swarm optimization (MOPSO) and non-dominated sorting genetic algorithm Ⅱ (NSGA-Ⅱ), are introduced to achieve a trade-off between suppression ratio (SR) and transmittance while determining multiple CRC parameters. Switching among the three states — CW SLM output, CCW SLM output, and simultaneous bidirectional SLM output in both CW and CCW directions — is achieved based on the optical path transmission characteristics of the circulators and the intracavity loss adjustment mechanism. Spectrum stability, optical signal-to-noise ratio (OSNR), linewidth, relative intensity noise (RIN), and relaxation oscillation peak are all investigated for the three states. The CW and CCW SLM output can generate stable laser output with an OSNR larger than 74.11 dB. The fluctuations of the center wavelength and the peak power are less than 0.01 nm and 1.037 dB, respectively, over 60 min. Linewidth does not exceed 1.93 kHz and the RIN is less than −125.03 dB/Hz at frequencies greater than 2 MHz. The proposed TDFL is expected to be integrated with wavelength-division multiplexing and free-space optical communication systems in the future.