Optical-Feedback Cavity-Enhanced Absorption Spectroscopy with a Quantum Cascade Laser.

International audienceOptical{feedback cavity{enhanced absorption spectroscopy is demonstrated in the mid{IR using a quantum cascade laser (emitting at 4.46 ¹m). The laser linewidth reduction and frequency locking by selective optical feedback from the resonant cavity ¯eld turns out to be particularly important in this spectral range: It allows strong cavity transmission which compensates for low light sensitivity, especially when using room temperature detectors. We obtain a noise equivalent absorption coe±cient of 3£10¡9 cm¡1, for 1 s averaging of spectra composed by 100 independent points. At 4.46 ¹m this yields a detection limit of 35 pptv for N2O at 50mbar, corresponding to 4£107molecules/cm3, or still to 1 femtomoles in the sample volume