Ultra-narrow linewidth laser across the C-band using polarization-controlled dual-cavity feedback.

A standard method to reduce the linewidth of semiconductor lasers involves the use of external optical feedback (EOF). However, feedback powers less than 1 % usually trigger coherence collapse (CC), leading to chaotic laser dynamics and linewidth broadening. This paper explores a method to mitigate CC through precise tuning of the feedback polarization depending on the feedback power. We report a semiconductor laser with a sub-100 Hz intrinsic linewidth, achieved via EOF. The laser features a U-shaped cavity with two sampled grating distributed Bragg reflectors (SG-DBRs), enabling broad tunability across a 42 nm wavelength range (1513-1555 nm). By injecting optical feedback into both sides of the laser cavity via an external fiber-based cavity, we reduce the intrinsic linewidth by more than three orders of magnitude, from MHz to sub-kHz across the laser's tuning range. By dynamically tuning the polarization, we demonstrate sub-100 Hz intrinsic linewidths at feedback powers up to 10 %, marking an improvement over prior studies where CC limited performance.