Scaling limits and bandwidth shaping of frequency-modulated combs in the infrared and terahertz

Frequency-modulated (FM) combs based on active cavities like quantum cascade lasers have recently emerged as promising light sources in the infrared and THz. Unlike passive modelocking, FM combs use phase modulation to generate phase modulation and can be regarded as a phase-domain version of passive modelocking. We show that FM combs are governed by the same fundamental limits, producing combs whose bandwidths are linear in the gain bandwidth. Not only do we show theoretically that the diffusive effect of gain curvature limits comb bandwidth, but we also show experimentally how this limit can be increased.