Simultaneous dual-frequency excitation of a resonant photoacoustic cell

Simultaneous amplitude modulation of the exciting light beam with two different frequencies enables the excitation of two acoustic modes of a resonant photoacoustic cell. The photoacoustic amplitudes at both resonance frequencies and the corresponding photoacoustic amplitude ratio are obtained continuously without changing the modulation frequency. The photoacoustic mode amplitude ratio is affected neither by intensity fluctuations of the light source nor by microphone or electronics drifts and is a measure of the gas concentration. The simultaneous acquisition of signals at both resonance frequencies with the dual-modulation scheme reduces drifts on the timescale needed to obtain the PA amplitude ratio. As an example acetone measurements using this new dual-modulation photoacoustic scheme are presented. A detection limit of 90 ppm m−1 is reached with a simple heated black-body as infrared light source.