We have modeled the path-length distribution in an integrating sphere used as a multipass optical cell for absorption measurements. The measured radiant flux as a function of analyte concentration is nonlinear as a result, deviating from that expected for a single path length. We have developed a full numerical model and introduce a new analytical relationship that describes this behavior for high reflectivity spheres. We have tested both models by measuring the optical absorption of methane at 1651 nm in a 50 mm diameter sphere, with good agreement with experimental data in the absorption range of 0-0.01 cm(-1). Our results compare well with previous work on the temporal response of integrating spheres.

We report the effects of self-mixing interference on gas detection using tunable diode laser spectroscopy. For very weak feedback, the laser diode output intensity gains a sinusoidal modulation analogous to that caused by low finesse etalons in the optical path. Our experiments show that self-mixing interference can arise from both specular reflections (eg cell windows) and diffuse reflections (eg Spectralon TM and retroreflective tape), potentially in a wider range of circumstances than etalon-induced interference. The form and magnitude of the modulation is shown to agree with theory. We have quantified the effect of these spurious signals on methane detection using wavelength modulation spectroscopy, and discuss the implications for real gas detectors.

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 823nm allowing the use of low cost CCD cameras. We have tested the principle of the model by making gas absorption measurements on the 1650nm methane absorption line using wavelength modulation spectroscopy, for which preliminary results are presented.

New designs for gas cells are presented that incorporate transmissive or reflective optical diffusers. These components offer simple alignment and can disrupt the formation of optical etalons. We analyse the performance-limiting effects in these cells of random laser speckle (both objective and subjective speckle), interferometric speckle and self-mixing interference, and show how designs can be optimised. A simple, single pass transmissive gas cell has been studied using wavelength modulation spectroscopy to measure methane at 1651nm. We have demonstrated a short-term noise equivalent absorbance (NEA, 1σ) of 2×10 -5 , but longer term drift of up to 3×10 -4 over 22 hours. PACS codes 07