Generation of Flat, Tunable Microwave Frequency Combs Using Double-Locked Semiconductor Lasers

This study experimentally demonstrates microwave frequency comb (MFC) generation in a semiconductor laser subjected to double locking via optical injection and intensity modulation. The interaction and subsequent locking between the master-laser modulation frequency and the P1 microwave oscillations induced by optical injection in the slave laser enable the generation of widely tunable MFCs. Operating the optically injected laser near the Hopf bifurcation boundary, through appropriate adjustment of the injection strength and detuning frequency, ensures that applying intensity modulation yields the same P1 oscillation dynamics as under continuous (unmodulated) injection, since the regeneratively amplified frequency component dominates the optical spectrum in this operating regime. In this regime, using strong intensity modulation and selecting the modulation frequency to be an integer fraction of the P1 oscillation frequency enables the realization of microwave frequency combs with continuous line spacings between 2 and 10 GHz. Spectral characterization of the highest-frequency components reveals microwave linewidths below 1 Hz (limited by instrument resolution) and SSB phase noise around –100 dBc/Hz at a 10 kHz offset.