Theoretical analysis of a methane gas detection system, using the complementary source modulation method of correlation spectroscopy

Results from simulations of the response of a methane gas sensor using optical correlation spectroscopy (CoSp) are presented. Predictions of the sensor response, signal/noise performance and detection sensitivity are made for a typical fibre optic-coupled system. Spectral absorption data of the gases are obtained from the publicly available HITRAN database. Emphasis is placed on the effects on the detection sensitivity of varying (a) the optical filter characteristics, i.e. centre wavelength and bandwidth and (b) the concentration (partial pressure) of CH4 gas. Using a 150 nm filter, and assuming a received optical power density at the detector of 10 nW nm(-1), a theoretical noise-limited detection limit below 1 ppm is predicted. The cross-sensitivity to water vapour is derived and compared to that which would occur with a conventional broadband absorption method, i.e. one that would not exhibit the same selectivity as the CoSp method. This work is important for predicting the responsivity, sensitivity and crosstalk performance of practical CoSp gas detection systems.