Highly stable single longitudinal mode thulium-doped fiber laser utilizing cascaded dual-coupler double-ring cavity filter

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.