Optical EngineeringDetection limit of chemical gases in ambient air with co-linear photo-thermal deflection spectroscopy
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The frequency response of photo-thermal deflection spectroscopy, caused by infrared resonant absorption in analyte gases in open air using a common collinear pump-probe beam configuration, is studied using experiment and theory. The heat diffusion simulation and the experiment showed a roll-off of 50 to 60 Hz. This is in agreement with work performed by other authors. It was found, that the frequency response curve shapes for the numerical simulations of the deflection angle and the experimental deflection voltage signal disagreed by less than 20 percent over the range of 10 Hz to 1 kHz. A frequency dependent turbulence model is used to determine a noise floor as a function of modulation frequency, between 5 and 5000 Hz, for a probe beam at several different standoff distances from the analyte using parameter scaling. From both experiments, the turbulence equivalent concentration is extracted from the extrapolated curves. The best extrapolations give absorbances of 2·10−5 and 3.2·10−5 for 50°C and 75°C thermal sources, respectively. The most sensitive parameters, which the equivalent concentration depends on, are pump power, pump radius, standoff distance, and absorption length of the gas sample. Lens focusing at the detector allows a smaller detector to be used for probe beams which diverge from the source.