The sensitivity of molecular fingerprinting is dramatically improved when
placing the absorbing sample in a high-finesse optical cavity, thanks to the
large increase of the effective path-length. As demonstrated recently, 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. Analyzing efficiently the light transmitted through
the cavity however still remains challenging. Here, a novel approach,
cavity-enhanced frequency comb Fourier transform spectroscopy, fully overcomes
this difficulty and measures ultrasensitive, broad-bandwidth, high-resolution
spectra within a few tens of $\mu$s. It could be implemented from the Terahertz
to the ultraviolet regions without any need for detector arrays. We recorded,
within 18 $\mu$s, spectra of the 1.0 $\mu$m overtone bands of ammonia spanning
20 nm with 4.5 GHz resolution and a noise-equivalent-absorption at
one-second-averaging per spectral element of 3 10^-12 cm^-1Hz^-1/2, thus
opening a route to time-resolved spectroscopy of rapidly-evolving
single-events
Abstract:The mid-infrared part of the electromagnetic spectrum is the so-called molecular fingerprint region because gases have tell-tale absorption features associated with molecular rovibrations. This region can be for instance exploited to detect small traces of environmental and toxic vapours in atmospheric and industrial applications. Novel Fourier transform spectroscopy without moving parts, based on time-domain interferences between two comb sources, can in particular benefit optical diagnostics and precision spectroscopy.
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