title: Ultra-sensitive cavity ring-down spectroscopy in the mid-infrared spectral region.
abstract: We describe an ultra-sensitive cavity ring-down spectrometer which operates in the mid-infrared spectral region near 4.5 μm. With this instrument a noise-equivalent absorption coefficient of 2.6×10-11cm-1Hz-1/2was 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.
authors.forename: D A
authors.initials: DA
authors.lastname: Long
authors.forename: A J
authors.initials: AJ
authors.lastname: Fleisher
authors.forename: Q
authors.initials: Q
authors.lastname: Liu
authors.forename: J T
authors.initials: JT
authors.lastname: Hodges