To our knowledge we present the first experiments with a fully reflective external-cavity quantum- cascade laser system at mid-infrared wavelengths for use as a local oscillator in a heterodyne receiver. The performance of the presented setup was investigated using absorption spectroscopy as well as heterodyne techniques. Tunability over at was demonstrated using a grating spectrometer. A continuous tuning range of was verified by observing the spectra of an internally coupled confocal Fabry–Pérot interferometer and the absorption lines of gas phase . In a second step the output from the system was used as a local oscillator signal for a heterodyne setup. We show that spectral stability and side mode suppression are excellent and that a compact external-cavity quantum-cascade laser system is well suited to be used as a local oscillator in infrared heterodyne spectrometers.
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