Carrier dynamics engineering with a saturable absorber in quantum-well semiconductor lasers for frequency comb generation

Optical frequency combs (OFCs) with uniformly equidistant line spacing are highly promising light sources for applications in optical communications and precision metrology. We report a C-band dual-section dumbbell-shaped taper (DST) semiconductor laser based on quantum-well (QW) epitaxy for optical frequency comb generation. The device delivers an RF beatnote signal at 21 GHz, and by incorporating a saturable absorber (SA) section together with a DST waveguide, it introduces diverse carrier dynamics that enable regime transitions among FM mode-locking, AM mode-locking, and Q-switching under different current conditions. We also analyze the differences in the optical and RF spectral profiles across different operation regimes, as well as the influence of structural configurations on the frequency comb performance. By employing a quantum-well epitaxy, the design eliminates the need for complicated epitaxial processes and provides a low-cost, compatible with standard fabrication optical frequency comb source.