Nonlinear Optics Group - University of St Andrews

Gas detection is very important in many contexts from environmental issues to life threatening situations, and so the quicker an unwanted leak can be isolated and serviced the chances of a serious incident involving such a leak are minimised. Current gas detection techniques are based around technologies such as gas chromatography or electronic ‘point and sniff’ detectors. Such techniques have adequate sensitivity and (in the case of electronic gas detectors) portability and so are ideal in a number of small-scale (for instance, domestic) applications. However, as they simply measure point concentration of the gaseous species under investigation, very little feedback is given to the operator as to the source of the leak. This can prove extremely problematic when gas leaks are detected in very large scale industrial installations. A visual picture, relayed to the operator in real time, which indicated both the presence and position of the detected gas would be highly desirable, and it is this feature which is exploited in the Back-scatter absorption gas imaging (BAGI) technique. Gas imaging obtains instant results helping to source the location of the leak as well as an idea of the severity.

This technique, therefore, is a powerful one whereby the strong spectroscopic absorption features in the gaseous species of interest are exploited in order to provide an image of the otherwise invisible (to the naked eye) gas. Such a device has been under development within our research group, in collaboration with the Schools’ Photonics Innovation Centre for the past three years. Methane (CH4) was chosen as the gas of interest because it is both a commonly found source of energy in the commercial and industrial sectors, and is a potent explosive hazard. Also, being the simplest molecule of the hydrocarbon gas family, it exhibits the narrowest absorption feature in the group. It is therefore the most difficult molecule to detect and demonstration of this proves the viability of the BAGI technique.

Diode-Pumped Solid-State Intra-Cavity OPO used in the BAGI detection system (click for larger image).

At the heart of the BAGI detection system lies an illumination source whose output is both narrow enough in ine width to be entirely absorbed by the absorption features in the gasseous species of interest, and powerful enough to yield a strongly back-scattered signal. A novel intra-cavity Optical Parametric Oscillator (OPO) operating in the mid infrared has been developed as such a source in order to take advantage of the very strong absorption features which are exhibited by Methane at 3.31mm. The OPO we have developed has the added advantage of exhibiting broad tunability. Therefore, the absorption features of many different gasses can be accessed from a single device. The system has the added advantage of being compact and highly efficient – over 170mW of mid-IR illumination is produced for only ~3W diode pump power.

Once the scene of interest has been illuminated by this novel spectroscopic laser, some means of image acquisition is required in order to view the scene. Since the very high cost of cryogenically-cooled InSb-based cameras precluded their use in this system, and in any commercially viable based upon this technology, an electro-mechanical polygonal mirror scanning system was realised for this purpose. Such a system operates by rapidly raster-scanning the mid-infrared laser beam over the scene of interest whilst simultaneously measuring the amount of back-scattered light at each pixel point. This light is then sampled via a single-element photodetector and stored digitally for subsequent display.

Publication Records
Optics Express	2004
Hyperspectral imaging of gases with a continuous-wave pump-enhanced optical parametric oscillator Stothard DJM, Dunn MH, Rae CF,


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