Author(s)

B. Kost, B. Baumann, M.Wolff & H. Groninga

Abstract

The detection sensitivity of photoacoustic sensors strongly depends on the design of the acoustical cell. It can be considerably improved by taking advantage of cell resonances, i.e., the acoustical eigenmodes of the measuring chamber. In order to optimize a photoacoustic system, it is key to understand precisely the influence of optical excitation, sound wave generation, cell shape and microphone detection on the signal strength in the sample cell. In this paper, first steps towards the optimization of photoacoustic cells are presented. The evaluation of the objective function, the signal strength, is based on a finite element analysis of the pressure field. As a start, independent optimization of some key parameters are investigated. 1 Introduction Photoacoustic spectroscopy finds many applications in the field of concentration measurements of gaseous compounds, e. g. for medical examinations (breath tests), monitoring of emissions and immission control. The photoacoustic effect is based on resonant absorption of light by specified molecules in a gas sample and the transfer of the excitation energy into thermal energy via inelastic collisions of the gas molecules. A modulated irradiation of the sample by a laser beam causes periodic pressure variations that can be detected by a microphone and measured using lock-in technique [1]. The most frequently used type of photoacoustic sensors is based on a cylinder shaped container [2]. In recent years the interest in photoacoustic cells with unconventional shapes is increasing. A so called T-cell, consisting of two intersecting cylinders, has been proposed in [3]. The optical absorption cylinder and the centri-

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