Abstract

Optical-feedback cavity-enhanced absorption spectroscopy is demonstrated in the mid-IR by using a quantum cascade laser (emitting at 4.46μm). The laser linewidth reduction and frequency locking by selective optical feedback from the resonant cavity field turns out to be particularly advantageous 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 coefficient of 3×109/cm for 1s averaging of spectra composed by 100 independent points. At 4.46μm, this yields a detection limit of 35 parts in 1012 by volume for N2O at 50mbar, corresponding to 4×107 molecules/cm3, or still to 1fmol in the sample volume.

© 2010 Optical Society of America

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