Unveiling frequency combs in semiconductor lasers under intensity-modulated optical injection

Abstract The generation and control of frequency combs in an intensity-modulated, optically injected semiconductor laser are investigated. The underlying dynamics of the unmodulated optically injected laser, particularly its intrinsic relaxation resonance behavior, are responsible for the comb structures generated when intensity-modulated injection is applied. Through the competition between the intrinsic resonance and external modulation frequency, several frequency-locked bands (Arnold tongues) are mapped by sweeping the frequency and amplitude of the intensity-modulated optical injection. Frequency combs are observed around the modulation frequency, as well as its harmonics and subharmonics. Between these locking states, additional frequency-locked bands emerge, following the Farey tree summation relation. The results demonstrate that frequency comb formation is driven by intrinsic oscillation dynamics of the optically injected laser, enabling tunable comb spacing that extends beyond fractional multiples of the relaxation resonance frequency of the free-running slave laser. These findings establish a simple and tunable platform for frequency comb generation, with potential applications in microwave photonics, optical communications, and signal processing.