Using hybrid integrated InP-Si₃N₄ diode lasers for the generation of sub-GHz repetition rate frequency combs

Optical frequency combs based on broadband-gain bulk lasers, due to the low intrinsic linewidth and sub-GHz repetition rates, have gained tremendous interest for applications such as high-resolution spectroscopy, dual-comb spectroscopy or LIDAR. However, susceptibility to mechanical and acoustic perturbations, the complexity of optical pumping and the larger physical size of these lasers has motivated research toward chip-based integrated extended cavity diode lasers with low-loss Si₃N₄ waveguide feedback circuits for low repetition rates. In diode lasers, mode-locking via saturable absorbers is generally used for generating frequency combs, however, the short upper-state carrier lifetime results in repetition rates of at least a few GHz. Here, we demonstrate absorber-free, passive mode-locking as well as hybrid mode-locking at sub-GHz repetition rates using a long Si3N4 feedback circuit with three highly frequency-selective microring resonators for extending the cavity roundtrip length to more than 0.6 m. This enables frequency-domain mode-locking in the form of a continuous wave, with a line spacing of around 500 MHz. Hybrid mode-locking, in addition to passive mode-locking, is demonstrated by adding a weak AC drive current with a frequency close to 500 MHz. This stabilizes the repetition rate and reduces the Gaussian component of the laser’s RF linewidth attaining a negligible Lorentzian component. Our numerical simulations predict that further lowering of the repetition rate and line spacings might be achievable with further cavity length extension.