Switchable Single-Longitudinal-Mode Fiber Laser Based on $\theta$ -Shaped Microfiber Filter

In this letter, we design a wavelength-switchable single-longitudinal-mode (SLM) fiber laser based on a reflective 8-shaped microfiber resonator (8-MR). By cascading the 8-MR with a fiber Fabry-Pérot interferometer, Vernier effect can be generated for SLM filtering. Due to the reflective nature of the 8-MR, a section of unpumped erbium-doped fiber can be directly inserted in front of the 8-MR to further suppress-mode competition. A SLM lasing spectrum with central wavelength of 1564.685nm, and sidemode suppression ratio as high as 56.6 dB is achieved. Stability test shows that the proposed fiber laser has a good long-term stability. Additionally, the lasing wavelength of the proposed laser is switched to 1564.690nm, 1559.392nm, and 1558.99nm when temperature around the 8-MR is set as 30°C, 40°C, and 50°C, respectively, verifying the wavelength switch ability of the 8-MR-based SLM fiber laser.