Guillaume MÉJEAN

Daniele ROMANINI

Irène VENTRILLARD

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Principle of detection by absorption spectroscopy
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In order to detect molecules in air by absorption spectroscopy, one may exploit the fact that incident light I_{0} decreases exponentially with the interaction length L as described by the Beer-Lambert law under the form :

I(ν)=I_{0}exp(-α(ν)L)

To detect very weak absorptions lines, one way to go is increasing the interaction length.

In order to increase the interaction length, first thing one can do is using a multipass absorption cell. That allows to go from 1 to 100 meters.

$$F=\frac{\mathrm{\pi}R}{1-R}$$

With high-finesse optical cavities based on high reflectivity mirrors, one can attain interaction lengths above ten km, while the probed sample volume is strongly reduced (100 times).

However, not all optical frequencies are transmitted by the cavity. Resonances, or "modes", are separated by the amount:

Δν = c/2L.