The sensitivity of molecular fingerprinting is dramatically improved when the absorbing sample is placed in a highfinesse optical cavity, because the effective path length is increased. When the equidistant lines from a laser frequency comb are simultaneously injected into the cavity over a large spectral range, multiple trace gases may be identified within a few milliseconds. However, efficient analysis of the light transmitted through the cavity remains challenging. Here, a novel approach—cavity-enhanced, frequency-comb, Fouriertransform spectroscopy—fully overcomes this difficulty and enables measurement of ultrasensitive, broad-bandwidth, high-resolution spectra within a few tens of microseconds without any need for detector arrays, potentially from the terahertz to ultraviolet regions. Within a period of just 18 ms, we recorded the spectra of the ammonia 1.0 mm overtone bands comprising 1,500 spectral elements and spanning 20 nm, with a resolution of 4.5 GHz and a noise equivalent absorption at 1 s averaging of 1 x 10^-10 cm^-1 Hz^-1/2, thus opening a route to time-resolved spectroscopy of rapidly evolving single events.

Full article:

Nature Photonics, Vol. 4, p. 55 (2010)

doi: 10.1038/nphoton.2009.217