A single-longitudinal-mode narrow-linewidth dual-wavelength fiber laser using a microfiber knot resonator

A stable narrow-linewidth dual-wavelength single-longitudinal-mode (SLM) erbium-doped fiber laser is demonstrated using a microfiber knot resonator (MKR). A MKR made by the microfiber knotting method exhibits the characteristics of flexibility, high quality factor and broad free spectral range and can be used as a comb-like ultra-narrow band pass filter. SLM operation of the fiber laser can be generated as a function of the Vernier effect by cascading a 650 m diameter MKR in the ring cavity. In combination with a common fiber Bragg grating, narrow linewidths (0.016 nm and 0.019 nm) of dual wavelength are generated. Finally, a SLM ring laser with narrow linewidth and low-frequency noise is achieved. Simultaneous generation of first- and second-order optical vortex beams in the dual-wavelength fiber laser using different mode-selective couplers is demonstrated. (letter)