Frequency-dependent gas detection limits of standoff photothermal deflection spectroscopy caused by wind turbulence beam wander

Abstract

The effects of near-ground wind turbulence-induced laser beam wander on photothermal beam deflection performance are presented. The theories of both photothermal deflection and turbulent beam wander are summarized. The semiphenomenological theory and experiment beam wander curve shapes and magnitudes match well over the range of 10 to 5000 Hz. The photothermal deflection modulation curves are set equal to the turbulence beam wander curves, and an effective turbulence equivalent concentration of the vapor of a chemical analyte can be derived as a function of frequency for the range 10 to 5000 Hz. The results show the limitations of near-ground level photothermal deflection measurements when wind is present in the beam path. The frequency points at which 1 ppbv of acetylene gas can be detected at different wind speeds and pump beam power are obtained. The wind is assumed to propagate parallel to the hard surface.