Development of a Photo-Acoustic Trace Gas Sensor

The work presented in this paper addresses the problem of creating a low-cost, intrinsically safe, low-level gas sensor for mining and transportation applications. The system is to have no electrical connection to the remote sensor head, and as such relies on optical transmission via fibre. An integral component is the acoustically resonant chamber, which is excited by an appropriate laser excitation. The laser is modulated at the resonant frequency of the chamber to create periodic heating, causing expansion. This is followed by cooling and thus contraction, resulting in a periodic pressure wave. The pressure wave is amplified in the resonant chamber, and then measured using an optical microphone approach. This makes the sensor inherently safe, as there are no electronics at the sensor head nor electrical connections required. A particular challenge associated with this approach is the acoustic signal generation and extraction from very high levels of noise. This paper gives a background on the photo-acoustic method, and describes our approaches to the development of physical hardware and processing algorithms required for an embedded sensor.