Sub-kHz linewidth 1.6-μm single-frequency fiber laser based on 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 optical signal-to-noise ratio exceeds 70 dB. The laser exhibited an excellent stability, without any instance of mode-hopping during 1 hour observing, and 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 produced over 14 mW of output power with a slope efficiency of 5.3%, to the best of our knowledge, this is currently the highest power directly obtained from a single-frequency cavity based on erbium doped silica fiber above 1.6 μm.