Design of a fibre optic sequential multipoint sensor for methane detection using a single tunable diode laser near 1666 nm

The design of a fibre optic sequential multipoint sensor using a single DFB laser source for measurement of average concentration of methane has been reported. A theoretical model for sequential multipoint sensor is developed. The validation of the sensor is conducted using methane gas cells connected in series via fibre optics to form 2-Cell and 3-Cell networks. The second harmonic detection of wavelength modulation spectroscopy (2f-WMS) is employed. Normalisation of the 2f signals with respect to the laser average intensity and the detector gain is used to correct for transmission variation. The measurements are performed at four laser modulation depths to compare the 2f-WMS signals for the target test conditions. The Q(6) transition of the 2v3 band of methane near 1666 nm has been selected because the transition is relatively free of interference from water vapour and absorption by other major gases in that region. The sequential multipoint sensor has some unique advantages over the single cell and multiplexed multipoint sensors. These include the average concentration measurements, the use of only a single laser source, a detector, and a fibre optic to connect series of gas cells, no need for multiplexing techniques to distribute the laser intensity among multi gas cells, and relatively simpler data analysis.