In this Optics Letters article, Hideyuki Sera and colleagues combine the optical technologies of fiber frequency combs, solid-state lasers, and nonlinear mixing to realize a widely tunable narrow-linewidth source for sub-Doppler resolution molecular spectroscopy in the mid-infrared spectral region. The realization of high spectral purity lasers in the mid-IR spectral region from 2 to 20 μm, the so-called “molecular fingerprint” region, is a fundamental research topic since molecules exhibit intense and narrow rovibrational transitions in this spectral window. Mid-infrared spectrometers are, indeed, increasingly used in precision tests of physics such us, for example, fundamental symmetries and to measure fundamental constants and their possible variation in time.
This work, which demonstrated a difference frequency generation source tunable in the range from 3.22 to 3.46 μm with an absolute frequency accuracy of 2.7 kHz and an emission linewidth as narrow as 3.5 kHz, may lead to the development of a new class of accurate, broadband, and high resolution mid-IR spectrometers. As a proof-of-principle, the authors use their newly-developed source to demonstrate sub-Doppler methane spectroscopy at 3.28 μm with a frequency resolution of 21 kHz.
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