Fundamental scaling limits and bandwidth shaping of frequency-modulated combs (Invited Paper)

Frequency-Modulated (FM) combs based on active cavities, such as quantum cascade lasers, have recently shown potential as light sources across various spectral regions. Unlike passive mode-locking, which generates amplitude modulation from the field’s amplitude, FM comb formation relies on phase modulation from the field’s phase, essentially acting as a phase-domain version of passive mode-locking. While the fundamental scaling laws of passive mode-locking have been well-established since Haus’s 1975 work showing that the bandwidth of pulses mode-locked by a fast saturable absorber is proportional to the effective gain bandwidth, the limits of FM combs have been less clear. This talk will discuss our recent findings,1 demonstrating that FM combs based on fast gain media adhere to the same fundamental limits, resulting in combs with bandwidths linear in the effective gain bandwidth. Theoretically, I will show that the diffusive effect of gain curvature constrains comb bandwidth and experimentally how this limit can be increased, particularly focusing on terahertz quantum cascade lasers.