Two Frequency-Modulated Comb Regimes in Quantum Cascade Laser Rings

The unique nature of the gain mechanisms in quantum cascade lasers (QCL) facilitates frequency comb generation without external nonlinear cavities, enabling simple on-chip spectroscopy devices in the mid-IR and THz frequencies [1]. QCLs are extremely important in the molecular fingerprint region of the mid-infrared, where they allow a flexible platform for developing frequency-combs with small device footprints with sufficient output power and wide spectral coverage [2]. Due to the fast gain recovery time of QCLs, these types of devices can lase in a frequency comb regime, while their action is often frequency-modulated (FM) and not pulsed [3] due to uniform gain in the cavity. The common comb regime of QCLs is a linearly chirped spectral phase, which maintains approximately a constant intensity over time and thus lower peak power [4]. For homodyne spectroscopy, lower peak power is advantageous, but the most critical features are the linewidth and, more importantly, the bandwidth of the laser output.