The photoacoustic principle

The photoacoustic effect is based on the absorption of modulated laser light in a photoacoustic cell, which creates an acoustic wave at the modulation frequency when the laser is tuned to an absorption line of at least one of the gas components. This photoacoustic signal generation happens through a periodic non-radiative relaxation of the molecular excitation resulting in a periodic heating of the irradiated gas volume, followed by a periodic thermally induced expansion and contraction of the gas volume, i.e. sound generation. The amplitude of the acoustic wave, which is sampled with a sensitive microphone, is proportional to the concentration of the absorbing component(s).


Diode laser based photoacoustic gas detection offers a combination of advantages with its simple construction and automatic operation, minimum detectable concentration in the low-ppm or even sub-ppm range (depending on the measured analyte), linear response over more than four orders of magnitude, high selectivity (i.e. insensitivity to the presence of other components), response time below one minute and accuracy in the few percentage range. Besides, our photoacoustic systems have high reliability in critical applications and they have to be calibrated only on a yearly (or even less frequent) time basis, due to the inherent stability of the system's components as well as the operation of the numerous in-built self testing and correction procedures.

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