Photoacoustic Gas Detection: Optical Excitation Tailored to Acoustic Mode Structure of the Resonant Cell

This work presents a comparative study on how different optical-excitation geometries influence the photoacoustic (PA) response of a resonant gas sensor. Two excitation strategies are experimentally evaluated: a narrow-focused laser diode (LD) beam and a broader distributed light-emitting diode (LED) source at 405 nm using a ring-shaped resonant PA cell designed for NO2 detection. By modifying the optical confinement and the spatial distribution of absorbed energy, the two sources excite the acoustic field in fundamentally different ways in terms of modal excitation, background generation, and gas-sensing performance. Results show that the spatial distribution of absorbed optical energy strongly affects the optical–acoustic coupling, with direct impact on sensitivity and baseline signal. Under comparable optical power levels, LED-based excitation yielded sensitivities on the order of 17–28 mVppm−1, whereas LD excitation achieved sensitivities of 52.6 mVppm−1 and up to 83.0 mVppm−1 depending on the driving conditions. Moreover, the combination of a tightly focused LD excitation and output optical window allowing beam transmission through the cavity,significantly reduced the PA baseline signal level, preserving more than 90% of the available measurement system dynamic range. In contrast, the same geometrical approach provided only marginal baseline improvement for the focused LED source. These results confirm that proper control of the excitation geometry within resonant PA cells substantially improves optical-to-acoustic transduction efficiency and measurement stability, supporting the development of compact photoacoustic sensors suitable for high-sensitivity applications such as trace-gas detection and biomedical sensing.