Single-Longitudinal-Mode Thulium-Doped Fiber Laser With Sub-kHz Linewidth Based on a Triple-Coupler Double-Ring Cavity

We propose and demonstrate a stable single-longitudinal mode (SLM) thulium-doped fiber laser (TDFL) with Fabry- Pérot (F-P) Bragg grating (FBG) filter triple-coupler based double-ring cavity (TC-DRC) filter. For the first time, this structure of TC-DRC is used to select single from dense longitudinal-modes in ring TDFL. The design fabricate methods are revealed principle SLM selection also analyzed detail. experimental results demonstrated good performance proposed central wavelength TDFL 2049.49 nm its OSNR higher than 35 dB. There no obvious drift during test, power fluctuation less 0.5 operation verified through self-homodyne method, can be stably maintained state after operating for one hour under laboratory condition. In addition, linewidth measured 10 kHz on phase noise demodulation method.