Abstract
We present a mid-IR high-precision spectrometer capable of performing accurate Doppler-free measurements with absolute calibration of the optical axis and high signal-to-noise ratio. The system is based on a widely tunable mid-IR offset-free frequency comb and a Quantum-Cascade-Laser (QCL). The QCL emission frequency is offset locked to one of the comb teeth to provide absolute-frequency calibration, spectral-narrowing, and accurate fine frequency tuning. Both the comb repetition frequency and QCL-comb offset frequency can be modulated to provide, respectively, slow- and fast-frequency-calibrated scanning capabilities. The characterisation of the spectrometer is demonstrated by recording sub-Doppler saturated absorption features of the CHF3 molecule at around 8.6 μm with a maximum signal-to-noise ratio of ∼7 × 103 in 10 s integration time, frequency-resolution of 160 kHz, and accuracy of less than 10 kHz.
Highlights
The accuracy of the absolute optical frequency axis and signal-to-noise level are key parameters for any precision-spectroscopy measurement
The continuous progress in Quantum-Cascade-Laser (QCL) technology recently led to the availability of room-temperature plug-and-play mid-infrared single-frequency tunable laser sources with an optical power of few tens of mW, 1 MHz emission linewidth, and very high frequency stability[5,6,7] allowing for the detection of strong fundamental absorption lines with a high signal-tonoise ratio (SNR) and resolution
We present a versatile and relatively simple spectrometer based on a QCL laser frequency-locked to a mid-IR optical frequency comb, capable of Doppler-free precisionspectroscopy measurements with high-accuracy and high-sensitivity in both direct-absorption and modulation-spectroscopy approaches
Summary
To partially remove the sources of inhomogeneous spectral line-broadening allowing us to observe Doppler-free absorption features few orders of magnitude narrower than the linear-absorption profiles, with a strong benefit for measurement precision. Balanced detection or other well assessed background-removal techniques can be adopted to account for baseline-related artifacts, such as etalon effects or beam-intensity wavelength dependence, that may affect the overall accuracy. Modulation-spectroscopy techniques[14] on the other hand allow for both technical-noise and background rejection, enabling for very sensitive detection, which is very important especially in the presence of very crowded absorption spectra typical of heavy molecules or molecular mixtures. We present a versatile and relatively simple spectrometer based on a QCL laser frequency-locked to a mid-IR optical frequency comb, capable of Doppler-free precisionspectroscopy measurements with high-accuracy and high-sensitivity in both direct-absorption and modulation-spectroscopy approaches. With respect to our previous investigations,[16,17] in this work, we focused our attention to the analysis of sub-Doppler spectral features using a novel absolute-calibrated scanning approach which is a powerful method to retrieve several spectroscopic parameters
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