Pulse Measurements of the Frequency Response of a Photoacoustic Cell

Abstract

The determination of the frequency response of a photoacoustic cell is a typical activity during design and adjustment of a photoacoustic setup. The most common method of such measurements is based on point-by-point checking of the amplitude of the photoacoustic signal at different modulation frequencies. Such an approach can be time-consuming, especially if a lot of measurements at low modulation frequencies are to be performed. The article presents a method based on investigation of a photoacoustic response to short square pulse stimulation instead of continuous modulation of the light source. The method allows for measurements of the frequency response to be substantially shortened. A theoretical description and properties of the pulse measurement method are discussed and illustrated with a comparison of experimental results obtained from both frequency response measurement methods.

Model of the Photoacoustic Helmholtz Resonator with Conical-Ended Duct

“…For the same reason, a 1 W LED diode was used as the light source. The frequency response of the cell was obtained by measuring the output amplitudes point-by-point at different modulation frequencies [29]. From each measurement, two parameters were extracted, i.e., the resonance frequency and quality factor.…”

A Calibration-Free Methodology for Resonantly Enhanced Photoacoustic Spectroscopy Using Quantum Cascade Lasers

“…However, this method takes a significant period of time to acquire data and it cannot account for short-term changes in cell temperature and system pressure. For faster response, the use of a short square pulse [16] or chirped modulation [17] allows the excitation of multiple frequencies simultaneously. By normalization of the Fourier transform of the acoustic response by the laser's multiple drive frequencies the resonance curve can be determined.…”

A Novel Method of Evaluating the Frequency Response of a Photoacoustic Cell

“…The frequency response of the cell can be determined by different methods such as scanning point-by-point, measuring of a pulse response [9,10], or by the chirp stimulation [11]. Each of the mentioned approaches has advantages and disadvantages.…”

“…As a result, such a method is time consuming, especially taking into consideration the need to average measurements. In the pulse method, a measured amplitude and phase response depend on a length of the pulse [10]. Stimulation of the resonant photoacoustic cell with such a signal results in deformation of the obtained characteristic, imposing the requirement of an additional correction.…”