Quantitative signal analysis in pulsed resonant photoacoustics

Abstract

The pulsed excitation of acoustic resonances was studied by means of a high-Q photoacoustic resonator with different types of microphone. The signal strength of the first radial mode was calculated by the basic theory as well as by a modeling program, which takes into account the acoustic impedances of the resonator, the acoustic filter system, and the influence of the microphone coupling on the photoacoustic cavity. When the calculated signal strength is used, the high-Q system can be calibrated for trace-gas analysis without a certified gas mixture. The theoretical results were compared with measurements and show good agreement for different microphone configurations. From the measured pressure signal (in pascals per joule), the absorption coefficient of ethylene was calculated; it agreed within 10% with literature values. In addition, a Helmholtz configuration with a highly sensitive 1-in. (2.54-cm) microphone was realized. Although the Q factor was reduced, the sensitivity could be increased by the Helmholtz resonator in the case of pulsed experiments. A maximum sensitivity of the coupled system of 341 mV/Pa was achieved.

Signal-to-Noise Analysis of the Improved Open Photoacoustic Helmholtz Cell

“…To amplify the signal, the acoustic resonance of the chamber may be used. There are a number of open windowless cells reported in the literature, and in most cases, signal amplification is based on the standing wave resonances [1][2][3][10][11][12][13][14]. Such cells are usually of relatively large volumes, so their applications are strongly limited.…”

Pulse Measurements of the Frequency Response of a Photoacoustic Cell

“…The simplest signal (in terms of its generation) that would fulfill such a requirement is a short square pulse. Although pulse operation has been used in photoacoustic measurements for many years [15,16] and there are many papers describing theoretical and practical aspects of pulsed photoacoustic operation [5,6,15,[17][18][19][20], the use of a pulse excitation for the purpose of frequency response measurements requires some additional considerations.…”

“…In such research quite often tens or even hundreds of different cell configurations have to be investigated [1,2], and their frequency responses have to be measured in a wide frequency range. This step cannot be skipped and based on pure modeling, as even in the case of relatively simple cells, their frequency responses can be complex [3][4][5] and quite often different from the theoretical predictions [1,2]. The second case is resonance tracking/locking during target measurements, especially if the cell has a high value of the Q-factor.…”

Optical detection of chemical warfare agents and toxic industrial chemicals: Simulation

“…L-PAS involves absorption of the modulated laser radiation followed by deactivation of the excited molecule via collisions, which convert the absorbed energy into periodic local heating at the modulation frequency, and generate acoustic waves that can be monitored using low-noise microphones. [9][10][11][12][13][14][15][16] The photoacoustic signal S in volts is…”