Use of diffuse reflections in tunable diode laser absorption spectroscopy: implications of laser speckle for gas absorption measurements

The use of diffusely scattering materials as a means of eliminating interference fringes has been investigated. Their use introduces laser speckle that can contribute a random, rather than periodic, uncertainty to gas measurements. We have established a method for quantifying the uncertainty due to speckle and investigated ways of reducing it. We characterised the speckle at 823 nm allowing the use of low-cost CCD cameras. We have tested the principle of the model by making gas absorption measurements on the 1650-nm methane absorption line using wavelength-modulation spectroscopy, for which preliminary results are presented.