Quantum Cascade Laser Frequency Combs

TLDR

Broadband quantum cascade lasers can operate as frequency combs under direct electrical pumping at both mid‑infrared and THz frequencies, making them attractive for dual‑comb spectroscopy. The study explores the potential for monolithic integration of nonlinear optical elements and detector functionality to enable chip‑scale dual‑comb systems. Four‑wave mixing, driven by the very short upper‑state lifetime of the gain medium, is the mechanism that generates mode proliferation in these sources. QCL frequency combs now deliver over 100 mW and 100 cm⁻¹ bandwidth in the mid‑IR and 10 mW with 600 GHz coverage in the THz, exhibit FM‑like phase coherence, and have achieved metrologically precise operation with Schawlow‑Townes linewidths and the first dual‑comb spectroscopy measurements.

Abstract

Abstract It was recently demonstrated that broadband quantum cascade lasers can operate as frequency combs. As such, they operate under direct electrical pumping at both mid-infrared and THz frequencies, making them very attractive for dual-comb spectroscopy. Performance levels are continuously improving, with average powers over 100mW and frequency coverage of 100 cm -1 in the mid-infrared region. In the THz range, 10mW of average power and 600 GHz of frequency coverage are reported. As a result of the very short upper state lifetime of the gain medium, the mode proliferation in these sources arises from four-wave mixing rather than saturable absorption. As a result, their optical output is characterized by the tendency of small intensity modulation of the output power, and the relative phases of the modes to be similar to the ones of a frequency modulated laser. Recent results include the proof of comb operation down to a metrological level, the observation of a Schawlow-Townes broadened linewidth, as well as the first dual-comb spectroscopy measurements. The capability of the structure to integrate monothically nonlinear optical elements as well as to operate as a detector shows great promise for future chip integration of dual-comb systems.