Application of laser photoacoustic spectroscopy for the detection of water vapor near 1.38 μm

Laser photoacoustic (PA) spectroscopy is applied for the determination of the trace content of water vapor using a differential Helmholtz resonant (DHR) cell and a narrow bandwidth diode laser operating near 1380 nm. The PA spectroscopy revealed a rich absorption spectra in this wavelength region and the observed result was compared with the HITRAN database. A multipass optical system was also developed by adopting one aluminum-coated flat mirror with a small uncoated spot for the laser introduction to the detection chamber and one aluminum-coated concave mirror. The multipass optical system enabled the enhancement of the PA signal up to eight times when compared to the single pass case. The calibration curve was plotted by measuring PA signals for various pressures of the water vapor. The sensitivity of the PA detection system is estimated as 7.3×10¹² molecules cm⁻³ with a signal-to-noise ratio (SNR) of 1.