There has been much interest in the spectroscopy of trace gases in recent years. Due to its very great sensitivity, such absorption spectroscopy can be used to identify compounds at very low concentrations. It is used not only in fundamental research but also in fields such as metrology, the physical chemistry of the interstellar medium, in situ detection of trace amounts of air pollutants (whether accidental or illegal), monitoring of industrial processes, etc. However, to develop an effective spectrometer a number of characteristics need to be brought together. It must:
- be able to explore a wide range of wavelengths with a single measurement, which makes it possible to obtain information about many energy levels simultaneously in fundamental spectroscopy, or measure the presence of several molecules simultaneously in applied fields;
- have a good resolution limit, in other words be able to finely distinguish between the different wavelengths that make up the spectrum. This is necessary in fundamental spectroscopy in order to understand the dense spectrum of complex molecules, and in applied fields in order to unambiguously distinguish between the various molecules present in the medium under study;
- have a rapid measurement time so as to be able to observe transient phenomena (chemical reactions, explosions, etc) in real time;
- be highly sensitive in order to observe weak molecular transitions in fundamental spectroscopy, and compounds at low concentrations in the medium under study in applied fields.
Until now, no instrument had combined these four criteria simultaneously.
Now for the first time, the spectrometer developed by the French-German team has found a compromise between these constraints by being based on a high-finesse cavity and two frequency combs (2). This technique makes it possible to record spectra with great sensitivity, and a million times faster than the best current spectrometers (3). In a demonstration, the spectrum of ammonia, a molecule of great importance in environmental and planetary science, was measured in a mere 18 µs: the sensitivity obtained was 20 times better, with a measuring time 100 times shorter, than the demonstration of feasibility that held the previous record. With such high sensitivity, and the possibility of being extended to all the regions of the electromagnetic spectrum, this method would be able to explore dynamically the mid-infrared, the region of 'molecular fingerprints', where no effective spectroscopic technique in real time exists.
There are many other potential applications in a wide range of fields, such as analytical chemistry, plasma physics and laboratory astrophysics, as well as biomedicine, environmental surveys, safety, etc.
© Max Planck Institute for Quantum Optics
Part of a high-finesse passive resonator similar to the one used to enhance the sensitivity of frequency comb-based Fourier transform spectroscopy
The green light coupled to the resonator comes from a frequency-doubled Ytterbium-doped fiber frequency comb.
(1) In collaboration with the University of Tokyo (Japan) and the Ludwig Maximilian University, Munich (Germany).
(2) Femtosecond laser frequency combs were developed at the initiative of Nobel Laureate T.W. Hänsch. They have had a considerable impact in various areas of precision measurement. This type of laser simultaneously delivers several hundred thousand very precisely and equally spaced wavelengths. A laser generally only delivers one single wavelength, with a very fine spectral structure. A frequency comb can be thought of as a laser which delivers the equivalent of the emission of 100 000 conventional lasers in a perfectly controlled and known manner. This technology can, for instance, measure the distance from the Earth to the Moon with a precision equal to 1/100,000th of the thickness of a hair.
(3) Until today, the Fourier spectrometer based on the Michelson interferometer was for decades the most effective instrument in the analytical sciences.
Cavity-enhanced dual-comb spectroscopy,
B. Bernhardt, A. Ozawa, P. Jacquet, M. Jacquey, Y. Kobayashi, T. Udem, R. Holzwarth, G. Guelachvili, T.W. Hänsch, N. Picqué.
Nature Photonics Advance Online Publication, 29 November 2009.