Generation of broadband optical frequency combs from hybrid integrated InP-Si₃N₄ diode lasers

We present the generation of broadband optical frequency combs from hybrid integrated InP semiconductor optical amplifiers and low-loss Si3N4 waveguides. A small fraction of the amplifier is reversed biased to achieve passive mode-locking, while the Si3N4 waveguide allows to extend the cavity to a roundtrip length of up to 15 cm. The generated frequency comb covers a 25 nm broad spectrum, at a 3 dB level (Fig.1a), with more than 1600 comb-lines. Mode-locking is demonstrated at a 2 GHz repetition rate with a RF linewidth down to 110 kHz, at a 3 dB level (Fig.1c). With such properties, hybrid integrated diode lasers show great promise in applications such as integrated microwave photonics or metrology.