Pulsed photoacoustic calibration of a differential absorption water vapor lidar

We have designed and constructed a photoacoustic (PA) cell to be used for wavelength calibration in a water vapor differential absorption lidar (DIAL) system. This system will be used to remotely determine atmospheric H₂O¹⁶ concentrations. The accuracy of these measurements depends critically on the ability to tune to and detune from lines in the water vapor spectrum. Specifically, the signal-to-noise ratio of the PA signal must be strong enough to locate the center frequency of an absorption line with accuracy and repeatability better than 0.2 times the Lorentzian half width of the absorption (0.1 cm^-1). So, although it represents only a peripheral part of the DIAL experiment, the calibration provided by the PA cell is critical to its overall success. In the environment of a mobile lidar system, problems with mechanical and electronic noise are potentially greater than those in a typical laboratory setting, and the cell is designed to address this situation. The PA cell utilizes a “figure-8” beam tunnel geometry which partially separates the tunnel into two subchambers, each with a microphone mounted in it. The laser beam is allowed to pass through only one of these subchambers. The microphone in the side that receives the laser beam records both PA signal and background noise, while the other microphone records only background noise. A differential lowpass filter and microphone amplifier circuit, of a design dedicated to this application, uses the output of the two microphones to subtract the background noise from the PA signal. The PA cell described within is of simple mechanical design and is constructed at very little cost. © 2003 American Institute of Physics.