Sub-watt threshold, kilohertz-linewidth Raman distributed-feedback fiber laser.

We report a low-threshold, narrow linewidth Raman distributed-feedback (R-DFB) fiber laser at 1109.54 nm based on a 30 cm long center π phase-shifted Bragg grating written directly in a commercially available germano-silica (Ge/Si) fiber. The R-DFB was pumped by a continuous-wave (CW) linearly polarized fiber source at 1064 nm, and the threshold power, full width at half-maximum (FWHM) linewidth and slope efficiency with respect to the incident pump power are measured to be 440 mW, <2.5  kHz (measured with a 29.75 km fiber delay line) and 13.5%, respectively. An rf spectrum analyzer confirms that the R-DFB fiber laser exhibits single-frequency performance.