Optical frequency comb bandwidth enhancement of a monolithically integrated gain switched device

We demonstrate bandwidth enhancement of optical frequency combs generated by a monolithically integrated gain switched laser using symmetric optical coupling. Traditionally, gain switched comb sources rely on asymmetrically biased configurations to maintain phase coherence. In this work, we implement a bidirectionally coupled system comprising a two-section discrete mode laser and a Fabry-Pérot gain switched laser integrated on an InP epistructure. By transitioning to a symmetrically coupled regime with symmetric biasing, we achieve significantly broader and flatter optical spectra. Experimental characterisation reveals that the comb bandwidth and power increase significantly across regimes, with stable phase locking preserved. A delay-differential rate equation model, incorporating nonlinear carrier dynamics and modulation, is developed to replicate the observed comb behaviour. Numerical analysis highlights the role of modulation depth, bias current, injection strength, and frequency detuning in determining comb bandwidth. The results indicate that symmetric coupling provides a robust, scalable route for frequency comb generation with spectral performance enhanced by a factor of 4 compared to the asymmetric regime, suitable for high-capacity optical communications and integrated photonic systems.