Frequency modulation spectroscopy by means of quantum-cascade lasers

In this paper we investigate the performance of quantum cascade (QC) lasers for high frequency modulation spectroscopy, particularly using frequency modulation (FM) and two-tone (2T) techniques. The coupling of the rf signal to the QC laser through the cryostat is studied in detail as well as the noise contributions of both the detector and the laser source to the final spectra. The experimental traces are obtained by spectroscopy on low-pressure N2O and CH4 gases at 8.0 mu m and 7.3 mu m wavelength, respectively, and reproduce the line profiles predicted by theory. As a preliminary result, an enhancement of a factor six is measured with respect to direct absorption line recording.