Dispersion engineering of quantum cascade laser frequency combs

Quantum cascade laser (QCL) frequency combs offer the potential for building ultra-compact broadband spectrometers operating in the mid-infrared spectral region, where many light molecules have their fundamental absorption bands. However, key characteristics must be improved for correctly addressing frequency comb spectroscopy applications. In this work, we investigate how the device dispersion influences the comb operation of QCLs. We measure the group delay dispersion of such a device while in operation just below threshold. We then show that by implementing a dispersion compensation scheme based on a Gires–Tournois interferometer integrated into the QCL, the comb operation regime is dramatically improved. In particular, the formation of high-phase-noise regimes is prevented. The continuous-wave output power of these combs can be as high as 150 mW with optical spectra centered at 1330  cm−1 (7.52 μm) with up to 70  cm−1 of optical bandwidth, demonstrating that QCLs are ideal sources for chip-based frequency comb spectroscopy systems.