Abstract:
We have studied the effects of random laser speckle and self-mixing interference
on TDLS based gas measurements made using integrating spheres. Details of the
theory and TDLS apparatus are given in Part 1 of this paper and applied here to
integrating spheres. Experiments have been performed using two commercial
integrating spheres with diameters of 50 mm and 100 mm for the detection of
methane at 1651 nm. We have calculated the expected levels of laser speckle
related uncertainty, considered to be the fundamental limiting noise, and imaged
subjective laser speckle in a sphere using different sized apertures. For
wavelength modulation spectroscopy, noise equivalent absorbances (NEAs) of
around 5x10(-5) were demonstrated in both cases, corresponding to limits of
detection of 1.2 ppm methane and 0.4 ppm methane respectively. Longer-term drift
was found to be at an NEA of 4x10(-4). This lies within our broad range of
expectations. For a direct spectral scan with no wavelength dither, a limit of
detection of 75 ppm or fractional measured power uncertainty of 3x10(-3)
corresponded well with our prediction for the objective speckle uncertainty.
