Comb-mode-resolvable narrow-bandwidth polarization-maintaining mode-locked fiber laser with large tunability.

Comb-mode-resolvable fiber lasers output a set of evenly spaced and detectable frequencies, enabling precision applications in frequency metrology and high-resolution spectroscopy. Here, we report a narrow-bandwidth down to GHz level, polarization-maintaining mode-locked fiber laser with comb-mode resolvability. Under narrow-bandwidth filtering, the laser owns a minimum spectral width of 12.1 pm (1.49 GHz) at 1560 nm, with a pulse width of 437 ps and a comb spacing of 21 MHz. The comb spectrum is successfully resolved by using low-bandwidth electronics, owing to the high coherence of the combs. The output bandwidth (12.1 pm-3 nm) and center wavelength (1530 nm-1565 nm) can be easily tuned in a large range by setting the intracavity programmable pulse shaper. Therefore, the laser can operate at a broadband range despite its narrow bandwidth. The proposed widely tunable narrow-bandwidth laser has the potential for precise frequency comb metrology and spectroscopy with only a simple detection end.