Subharmonic comb spacing in a gain switched PIC

We report the generation of optical frequency combs through gain switching a three sectioned photonic integrated circuit (PIC) with comb line spacings of simple ratios of the gain switching modulation frequency. The PIC consists of two mutually coupled lasers integrated on an InP epistructure. This PIC, previously shown to generate broadband combs via symmetric coupling, is demonstrated here to exhibit subharmonic spectral regimes consistent with period doubling and other bifurcations, yielding comb spacings equal to one-half, one-third, and one-quarter of the modulation frequency, respectively. A delay-differential rate equation (DDE) model qualitatively reproduces the experimentally observed subharmonic generation. The presence of phase noise is shown to be critical in capturing the full spectral dynamics of the device. Distinct regions of stable subharmonic operation are identified across the driving and injection parameter space. These results extend the functionality of symmetrically coupled gain-switched lasers, offering a compact route to tunable frequency division and dense optical comb generation on chip.