Broadband optical frequency comb generation using hybrid integrated InP-Si₃N₄diode lasers

Optical frequency combs based on diode lasers are becoming instrumental for numerous applications based on coherent light, such as sensing, metrology, or integrated microwave photonics. However, standard mode-locked diode lasers show limits in coherence due to intrinsic loss of semiconductor amplifiers and short laser cavity lengths. Extending the cavity with long and low-loss dielectric waveguide feedback circuits, specifically using advanced Si 3 N 4 waveguides [1] , has enabled single-frequency lasers with record-low intrinsic linewidths [2]. This approach was adapted also for demonstrating multi-frequency lasers [3] , but only recent results have proven comb generation [4] using heterogeneous integration [5]. A highly stable repetition rate was achieved and a high comb density (sub-GHz line spacing). The spectral coverage, however, remained low, about 2.5 nm of 3dB bandwidth at 1582 nm, which may be addressed to increased coupling losses and limited pump currents associated with the heterogeneous integration.