Switchable Dual-Wavelength Tm-Doped All-Fiber Laser Integrating a Fiber Tip Modal Interferometer

A switchable dual-wavelength thulium-doped all-fiber laser incorporating a fiber tip modal interferometer (FTMI) is reported. The FTMI relies on the splice joint between a standard single-mode optical fiber and a tapered microfiber with 60- $\mu \text{m}$ uniform waist diameter, providing a spectral reflection response that allows the laser to emit beyond 2000 nm, specifically at 2009.20 nm and 2013.65 nm, under a dual-wavelength operation regime. Besides, switching between both laser lines is also observed and controlled by adjustments of the intracavity polarization state, obtaining the ability to operate in a single-wavelength regime at either 2009 nm or 2013 nm. Laser emission lines exhibit linewidths of around 50 pm, side-mode suppression ratios higher than 50 dB and maximum output power of 13.4 mW.