Frequency multiplexing in direct, resonator-enhanced photoacoustic gas sensing setups to establish a reference channel

The use of the photoacoustic effect enables the design of selective, miniaturized, low-cost optical gas sensors. The tailoring of selectivity of photoacoustic gas sensor setups depends on the absorption features of the target molecules. If the spectral emission profile of the light source is considerably narrower than the absorption features, then highly selective excitation of sound waves is feasible. However, the so-generated signal depends on the number density of the target molecules and the light intensity. To ensure long term stable sensor operation it is therefore paramount to determine the light intensity independently, which usually requires adding photodiodes to the setup. This contribution presents an acoustic resonator enhanced photoacoustic setup with an integrated reference channel, that is implemented via frequency multiplexing. It leverages the spatial dependence of the coupling efficiency of sound waves into acoustic resonators. The results show that the implementation of a reference channel with no additional components is feasible thus simplifying the setup considerably and enabling long-term stable operation.