Fiber Ring With Long Delay Used as a Cavity Mirror for Narrowing Fiber Laser

A fiber ring with an extremely low reflectivity fiber Bragg grating inserted as a slow-light component is proposed and demonstrated to be used as a cavity mirror with long delay for narrowing fiber laser linewidth. Due to the recirculating propagation and resonance in the ring composed of a low split ratio coupler, the composite ring provides not only a high enough reflectivity and extremely narrow bandwidth for laser oscillation, but also an enlarged group delay. The effective reflectance and group delay of the composite ring are discussed theoretically. Its effect of laser linewidth narrowing is verified experimentally in a laser with the composite fiber ring and a high reflectivity fiber Bragg grating as cavity mirrors. Its line shape, relative intensity noise (RIN), and frequency noise (FN) are measured, showing the full-width at half-maximum linewidth of 150 Hz, RIN and FN of less than -125 dB/Hz at 500 Hz and 30 Hz/root Hz at 500 Hz, respectively.