Quantum cascade laser (QCL) frequency comb-based metrology is a very powerful tool, being able to address the strong fundamental fingerprint absorptions of many molecules in the mid-infrared spectral range. It is nowadays used in various sensitive spectroscopic applications, ranging from trace gas measurements of greenhouse gases and of disease markers in exhaled air for medical diagnostics to the analysis of liquids and in-situ monitoring of chemical reactions.

In this context, the paper of Wang et al. demonstrates and analyzes the effects of fast modulation of QCL frequency combs, a feature inherent to QCLs due to their unique ultrafast gain recovery time, as the authors explain. In particular, they show the first harmonic injection locking of a mid-infrared QCL with output power above 1 W in continuous-wave (CW) operation. In contrast to fundamental injection locking, its harmonic counterpart addresses a higher-order injection frequency, exploiting the picosecond timescale relaxation dynamics of the involved intersubband transitions at the core of a QCL device. This not only allows it to go far beyond the rate of the cavity round trip time, but results, by using such high-power devices, in unlocking the investigation of the traditionally much weaker higher-order harmonic combs.

In conclusion, the study of Wang and coauthors reveals similar values in their ~8.2 μm emitting CW QCLs, i.e. a locking range of ~0.35 MHz vs. ~0.265 MHz, a highly stable locking under temperature fluctuations of ~1°C vs. ~0.5°C and a beat note linewidth of 20 Hz vs. hertz level, when comparing fundamental vs. 2^nd order harmonic injection locking, respectively. This proof of the device capabilities for ultrafast relaxation dynamics while preserving optical frequency comb characteristics, opens the pathway towards next-generation high-speed optical free-space telecommunication and signal processing in the mid-infrared spectral range.

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