Interferometric signals in fiber optic methane sensors with wavelength modulation of the DFB laser source

We examine the performance limitations of a fiber optic methane sensor using microoptic GRIN lens cells in either transmission or reflective mode. We derive the worst case values of sensitivity due to interference effects caused by reflections within the cell as a function of the cell parameters. We also show both theoretically and experimentally how the interference signal may be minimized by suitable choice of the amplitude of the wavelength modulation. Although, theoretically, reflective cells could match the performance of transmission cells, in practice, transmission cells are superior in terms of interferometric noise levels. With reflective cells, two secondary reflections from the cell and secondary cavity effects in the system enhance the interference so that in practice their performance is inferior.