Abstract

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.

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