Improved Photoacoustic Generator

Abstract

In conventional photoacoustic setups, the photoacoustic signal results from stimulation of a sample placed in the photoacoustic cell by the light modulated at a selected frequency. The signal can be amplified in a resonance photoacoustic cell. For this purpose, different types of acoustic resonators are used. Acoustic resonators are passive, frequency selective elements. An acoustic resonator used in a photoacoustic cell offers the opportunity to design a system working on a basis similar to that of a self-oscillating generator. The geometrical dimensions of an acoustic resonator, and the temperature, composition, and concentration of substances in the gas filling its interior determine the resonance frequency. In conventional photoacoustic setups, in which the resonance method is used, the variability of parameters requires continuous adjusting of or searching for the actual resonance frequency. Use of a fixed and arbitrary selected modulation frequency of the light beam can cause considerable errors in detection of substances in the sample or in determination of their concentration. Unlike conventional photoacoustic methods, the frequency of a photoacoustic signal in an improved photoacoustic generator is self-tuned to the actual resonant frequency of the photoacoustic cell. The improved photoacoustic generator operates without an external circuit that controls the optical modulator. The improved photoacoustic generator has been tested in different measurements of the concentration of methane in air. The automatic gain control signal can be used for determination of the absorption by the sample.T. Borowski

Measuring Sound Speed in Gas Mixtures Using a Photoacoustic Generator

“…The light source was driven by photoacoustic signals in a positive feedback loop. We used this approach in the concept of the self-excited photoacoustic generator [11,12] earlier. But instead of the acoustic Helmholtz resonator, now a longitudinal resonator was used.…”

The Tracking Resonance Frequency Method for Photoacoustic Measurements Based on the Phase Response

“…This cell is similar to the one used earlier in the measurement of the PA generator [17]. The block diagram of a photoacoustic system is presented in Fig.…”

“…• the pulse technique [14] measuring the amplitudes at several frequencies at the same time by using either the chirp technique [15] or by sweeping frequency, (d) by producing a PA signal based on the PA generator method [16,17]. In this method, the intensity of the light beam is driven by the PA signal in the feedback loop.…”