Hertz-linewidth hybrid integrated laser based on high-Q Si3N4 microring reflectors

We report a compact and robust laser linewidth reduction solution for commercially available distributed feedback (DFB) semiconductor lasers by butt-coupling a high-Q Si₃N₄ microring reflector to a DFB semiconductor laser. We fabricate the microring reflector on the 80nm-thick high-aspect-ratio Si3N4 waveguide platform. The 3-dB bandwidth and the free spectral range of the fabricated Si₃N₄ microring reflector are measured to be 50 MHz and 14.13 GHz, respectively, corresponding to a Q factor of over 3.8 million and finesse of 283. In the experiment, the intrinsic linewidth of the hybrid integrated laser is reduced to an ultra-low level of 2.86 Hz. The proposed hertz-linewidth hybrid integrated laser has great potentials in high-speed coherent optical communications systems and high-resolution optical metrology.