Sensitivity enhancement in off-axis integrated cavity output spectroscopy.

We report on a detailed model of an improved three mirror off-axis integrated cavity output spectroscopy (OA-ICOS) setup, which re-injects the light reflected by the optical cavity. The model simulates the impact of design parameters on instrument sensitivity and can be used for any off-axis configuration. We demonstrate the application of this model for the real-time detection of ethylene with a pulsed quantum cascade laser (QCL). The three mirror OA-ICOS scheme provides a 10 times increase in signal-to-noise ratio as compared to standard OA-ICOS, resulting in a noise equivalent absorption sensitivity of 1.5 x 10(-8) cm(-1) Hz(-1/2).