Photo-acoustic detection and ranging-a new technique for the remote detection of gases

A new remote technique for range-resolved gas detection is described: photoacoustic detection and ranging (PADAR). The technique involves sending out a laser pulse tuned to an absorption line of the gas. When the pulse hits a localised concentration of the gas an acoustic pulse is generated which can be detected with a parabolic microphone sited close to the laser. The range is inferred from the delay in receiving the acoustic pulse. Range resolution can be better than 10 mm, with a probable maximum range of 100 m. Preliminary demonstrations of the technique are described and a theoretical treatment of the generation and propagation of the acoustic pulse is presented.