Comb-referenced spectroscopy with quantum cascade lasers

In our recent activity we have performed absolute frequency measurements on Doppler-broadened and sub- Doppler CO₂ transitions around 4.3 and 4.4 mum wavelength with DFB QCLs, by referencing their mid-infrared radiation to a near-infrared optical frequency-comb synthesizer. This experiment has pointed out the benefits coming from a frequency stabilization of the QCL and has allowed improvement of the precision of frequency measurements to the kHz level. In particular, we set-up a double-balanced detection technique, both to increase the S/N ratio of the acquired spectra and to erase the residual Doppler-broadened background. The result is a Doppler-free dispersion-like signal with a S/N ratio comparable to that obtained with standard wavelength modulation techniques and lock-in first derivative detection, but with no modulation of the laser and filtering of the detected signal. We have recorded Doppler-free signals narrower than those obtained in our previous experiment based on a standard first-derivative technique. The absence of any modulation and filtering effectively makes the PS signal the ideal candidate as dispersive- like error profile for large bandwidth frequency stabilization of our QCL.