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.
We demonstrate the first on-chip laser frequency comb based on hybrid integration with low-loss Si3N4 waveguide circuits. The laser comprises an InP diode amplifier of which a small fraction is reverse biased for passive locking, while a Si3N4 feedback waveguide extends the optical cavity to a roundtrip length of 15 cm. The generated comb densely covers a 25 nm broad spectrum, at a 3 dB level, with more than 1600 comb-lines at 2 GHz spacing. With such properties, hybrid integrated diode lasers show great promise for widespread use in applications such as integrated microwave photonics or metrology.
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