Detection of weakly absorbing gases using a resonant optoacoustic method

Recently developed optoacoustic techniques have greatly expanded our capability of measuring ultralow concentrations of infrared absorbing gases. Devices of this type include both the spectrophone which was introduced by Kreuzer in 1971 and the resonant spectrophone first considered by Dewey, Kamm, and Hackett in 1973. The advantage in the resonant method lies in the capability of accumulating energy in a standing acoustic wave, thereby providing for an effective means of signal amplification. In this paper we present the theory for the resonant acoustic response to an excitation of known characteristics with particular emphasis placed on the dissipative processes which act to limit the extent of resonant enhancement. We also investigate both theoretically and experimentally the effect of a multiple-pass configuration in which the laser beam undergoes numerous reflections, thereby increasing the total energy imparted to the gas. This description leads to a means of objectively comparing a resonant optoacoustic system to other detection devices. This same approach introduces an expression which can be used to optimize the resonant technique with respect to the variable parameters.