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Integrated optical frequency combs based on active cavities such as quantumcascade lasers (QCLs) have emerged as promising light sources in themid-infrared and terahertz spectral regions. Their bandwidths are limited bytwo separate, yet equally important effects: dispersion and diffusion. However,while dispersion has been extensively engineered, diffusion - a phenomenonoriginating from gain variation - has not. We show theoretically andexperimentally that the addition of carefully-engineered diffusive loss canenhance the bandwidth of QCL combs. Adding resonant loss to the cavity of aterahertz QCL can counteract the diffusive effect of the gain medium and allowsbroader bandwidth combs to form, fully exploiting the bandwidth and dynamicrange of the gain medium. These results are well-explained by active cavitymean-field theory. Interestingly, this strategy also permits our structures togenerate soliton-esque pulsed states of light. Our results give a new degree offreedom for the creation of active chip-scale combs, and can be applied to awide array of cavity geometries.

Diffusive loss shaping of quantum cascade laser frequency combs