Sub-kHz linewidth 1.6-µm single-frequency fiber laser based on a heavily erbium-doped silica fiber

We present a single-frequency erbium-doped fiber laser operated at 1608.8 nm using a homemade, heavily erbium-doped silica fiber as gain medium. The laser configuration is based on a ring cavity, which is combined with a fiber saturable absorber to achieve single-frequency operation. The measured laser linewidth is less than 447 Hz and the optical signal-to-noise ratio exceeds 70 dB. The laser exhibits an excellent stability, without any instance of mode-hopping during 1-hour observing. The fluctuations in both wavelength and power were measured to be 0.002 nm and less than 0.09 dB in a 45-minutes period. The laser produces over 14 mW of output power with a slope efficiency of 5.3%, which, to the best of our knowledge, is currently the highest power directly obtained from a single-frequency cavity based on an erbium-doped silica fiber above 1.6 µm.