Analytical Absorption Signals in a Fabry-Perot Cavity

2 Analytical Absorption Signals in a Fabry-Perot Cavity 

The effect of admitting an absorbing gaseous sample into a Fabry-Perot, or any resonant cavity, is to lower its Q by an amount proportional to the absorption coefficient a defined as the power attenuation per unit length traversed. Thus the incident power Pq is reduced to P  

The effective path length traversed through the sample before its absorption causes the signal to decay by a factor of 1/e is therefore Qfkjln. For a weak line a 10 m observed under the conditions cited above 0s takes on a value 2 X 10 and 0c changes by only 5 ppm. Thus we are seeking to observe rather small changes in the cavity-0 when a sample is inserted. 

The effect of the sample is not only to increase the cavity loss function but also to broaden its frequency response. Both these cause a modification of the signal observed by an FM spectrometer and will be considered in more detail in Section 6.8. 

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