Narrow-linewidth frequency combs using hybrid semiconductor-glass waveguide lasers

Chip-based optical frequency comb sources are of high importance in various fields, particularly sensing, metrology, and coherent optical communications. A well-known method for generating frequency combs in chip-based devices is via semiconductor-based mode-locked lasers. A known problem with such lasers, however, is that the comb lines have large optical linewidths, tens to hundreds of MHz in InP lasers and about 400 kHz with heterogeneously integrated InP-Si lasers. We address this problem with a hybrid InP-Si3N4 waveguide laser, where a long and low-loss Si3N4 feedback waveguide is used to notably increase the photon lifetime in the resonator. In a first step, with a smaller number of comb lines, we observe a clearly reduced optical linewidth of the individual comb lines. This finding confirms the potential of our approach for realizing novel chip-based frequency comb sources with extremely narrow linewidths.