Three-wavelength-switchable narrow linewidth thulium-doped fiber laser enabled by a compound-cavity filter and a sampled fiber Bragg grating

A single-longitudinal-mode (SLM), narrow linewidth thulium-doped fiber laser with a sampled fiber Bragg grating (SFBG), switchable among three wavelengths, with a cascade dual-coupler-ring-based compound cavity (DCR-CC) filter, is proposed and demonstrated. The coupling design, simulation analysis, and characterization of the DCR-CC filter provide the foundation for the experiment. A nonlinear polarization rotation system was injected into the cavity to suppress gain competition and achieve a laser switchable among three wavelengths. The fluctuations of the wavelength and power of the output laser are less than 0.60 nm and 0.91 dBm, respectively. For demonstration, the laser maintained in SLM operation measured by the delayed self-heterodyne method has a linewidth of < 3.7 k H z and relative intensity noise of < − 114 d B / H z .