Analysis of inaccuracy induced by intensity variation of a DFB laser in fibre optic multipoint 2f-WMS measurements of methane near 1666Â nm

The analysis of methane concentration measurement inaccuracy induced by the intensity variation of a DFB laser at 1666 nm in a multipoint fibre optic sensor has been reported. The measurement is based on wavelength modulation spectroscopy (WMS) with second harmonic (2f) detection technique. The 2f signals are normalised to the average laser intensity and the detector gain to eliminate the effect of transmission loss and laser intensity variation on the measurements. The measurements are conducted at two laser intensities, defined as nominated intensity and 73% of the nominated intensity, to quantify the measurements errors due to the intensity change. The experimental results show that a significant change in the laser intensity yields differences in the 2f signals; as a result, errors were induced. A model is developed to quantify the errors in the concentration measurements. The maximum deviations for single cell, 2-Cell, and 3-Cell measurements to the given concentration of 10%, 7.5%, and 5.667% are calculated to be 9.764%, 7.2235%, and 5.368%, respectively. The concentration error increases with the cell number because of the accumulated background transmission loss of the gas cells in 2-Cell and 3-Cell in comparison to a single cell measurement.