Densifying free spectral range of a gain switched frequency comb via subharmonic generation

We report the controlled generation of optical frequency combs with rational subdivisions of the gain switching modulation frequency in a monolithically integrated, mutually coupled semiconductor laser system operated under symmetric biasing. Under certain bias conditions, the device can exhibit period doubling and period tripling dynamics that produce comb spacings equal to one half, one third, and one quarter of the modulation frequency. A delay differential rate equation model incorporating phase noise reproduces these bifurcations and identifies clear regions of stable subharmonic operation with regions of optical chaos also present. The observed states are robust to variations in modulation and injection conditions. This work expands the functionality of symmetrically coupled gain switched lasers and enables compact, tunable on-chip frequency division for applications in coherent optical communications.