Photoacoustic detection by means of a differential double resonator cell applied to security and defence

Arturo Vallespi; Verónica Slezak; Alejandro Peuriot; Francisco González; Andrea Pereyra; Guillermo Santiago

doi:10.1117/12.2034482

16 October 2013 Photoacoustic detection by means of a differential double resonator cell applied to security and defence

Arturo Vallespi, Verónica Slezak, Alejandro Peuriot, Francisco González, Andrea Pereyra, Guillermo Santiago

Author Affiliations +

Proceedings Volume 8901, Optics and Photonics for Counterterrorism, Crime Fighting and Defence IX; and Optical Materials and Biomaterials in Security and Defence Systems Technology X; 89010Q (2013) https://doi.org/10.1117/12.2034482
Event: SPIE Security + Defence, 2013, Dresden, Germany

Abstract

The purpose of this article is to present a sensitive optical system for immediate detection of traces of ammonia by means of photoacoustic spectroscopy and study some properties with both a pulsed CO₂ laser (TEA) and a CW CO₂ laser. The laser beam is aimed to an innovative dual resonator differential cell, which lowest resonant frequency is the first longitudinal mode at 1205 Hz, filled with a flowing NH₃ and N₂ mixture. The chosen cell’s material is polypropylene, suitable for reducing the effects of adsorption. As a result of physical adsorption-desorption studies, based on a pulsed CO₂ laser, 5 % PA signal decay from an enclosed sample of 248 ppmV of NH₃ in N₂ is recorded within 1 hour. The setup for CW CO₂ laser excitation takes advantage of a differential microphone connected to both resonators by picking up out of phase signals. For this purpose, the beam is modulated at the cell’s resonance by means of a chopper with a special blade which allows both reflection and transmission of the laser beam; the direct and the reflected beam are alternatively aimed to one resonator and the other. The measurements show that for the double resonator configuration a signal increase is achieved, as expected from the study of the sensitivity of both resonators separately, which have been previously characterized. Measurements with this system indicate a limit of detection of 13ppbV at the 10P(32) laser line, deduced from one standard deviation of the PA signal from pure N₂.

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Arturo Vallespi, Verónica Slezak, Alejandro Peuriot, Francisco González, Andrea Pereyra, and Guillermo Santiago "Photoacoustic detection by means of a differential double resonator cell applied to security and defence", Proc. SPIE 8901, Optics and Photonics for Counterterrorism, Crime Fighting and Defence IX; and Optical Materials and Biomaterials in Security and Defence Systems Technology X, 89010Q (16 October 2013); https://doi.org/10.1117/12.2034482

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KEYWORDS

Resonators

Carbon dioxide lasers

Adsorption

Absorption

Photoacoustic spectroscopy

Signal to noise ratio

Continuous wave operation

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