Quantum Cascade Laser Frequency Combs

TL;DR

Quantum cascade lasers can operate as broadband frequency combs in the mid-infrared and THz ranges, enabling advanced spectroscopy with ongoing improvements in power and coverage, and promising future chip integration.

Contribution

This paper reviews recent advances in quantum cascade laser frequency combs, highlighting their operational principles, performance improvements, and potential for integrated spectroscopic systems.

Findings

Achieved over 100 mW power and 100 cm$^{-1}$ coverage in mid-infrared.

Demonstrated comb operation with metrological precision and Schawlow-Townes linewidth.

First dual-comb spectroscopy measurements using quantum cascade lasers.

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 100 mW and frequency coverage of 100 cm$^{-1}$ in the mid-infrared. In the THz range, 10 mW 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 monolithically non-linear optical element as well as to operate as a detector show great promise for future chip integration of dual-comb systems.