Sub-ppb level detection of nitrogen dioxide based on an optimized H-type longitudinal acoustic resonator and a lock-in white-light interferometry demodulation algorithm

This paper presents an optimized H-type longitudinal resonant photoacoustic (PA) cell for nitrogen dioxide (NO2) gas detection. The PA field distributions of the H-type resonant PA cell have been evaluated to optimize the construction of the H-type resonant PA cell, using the finite element method and thermoviscous acoustic theory. The optimized H-type resonant PA cell, together with a high-power continuous-wave (CW) multimode laser diode emitting at around ~ 450 nm and a fiber-optic Fabry-Perot (F-P) acoustic sensor, constitutes a PAS system for NO2 detection. The dynamic cavity length of the fiber-optic acoustic sensor is demodulated by the lock-in white-light interferometry (WLI) demodulation algorithm. Experimental results show that the PA signals are proportional to the concentrations of NO2 for the range of 0-2 ppm (parts per million). The sensitivity is measured to be 0.652 nm/ppm and the minimum detectable limit of NO2 is calculated to be 1.26 ppb (parts per billion).