A single-frequency and single-polarization fiber ring laser using a 5-Ghz fiber Bragg grating

A low-threshold, single-frequency fiber laser was achieved without feedback control. This laser was a narrow linewidth fiber ring laser operating in the 1.55-μm regime and pumped by 1.48-μm InGaAsP laser diodes. Mode-hopping instabilities and multiwavelength oscillation due to spatial and polarization hole burning were eliminated by the use of an ultra-narrowband fiber Bragg grating and a polarization-dependent isolator. The laser exhibited stable single-frequency operation at all pump powers. The maximum output power was 1 mW, and the linewidth was less than 20 kHz.