Narrowing of the generation line of a DFB laser to subhertz width in hybrid configuration

We present the results of characterization of the linewidth of distributed feedback fiber laser based on 30-mm pi-phase-shifted fiber Bragg grating with additional random distributed feedback provided by 25-km SMF-28 fiber spool spliced to the distributed feedback fiber laser. It is shown that for a hybrid resonator, the linewidth becomes more than two orders of magnitude narrower than the linewidth of distributed feedback fiber laser.