Ultra-sensitive cavity ring-down spectroscopy in the mid-infrared spectral region

We describe an ultra-sensitive cavity ring-down spectrometer which operates in the mid-infrared spectral region near 4.5 mu m. With this instrument a noise-equivalent absorption coefficient of 2.6 x 10(-11) cm(-1) Hz(-1/2) was demonstrated with less than 150 nW of optical power incident on the photodetector. Quantum noise was observed in the individual ring-down decay events, leading to quantum-noise-limited short-time performance. We believe that this spectrometer's combination of high sensitivity and robustness make it well suited for measurements of ultra-trace gas species as well as applications in optics and fundamental physics.