Jane Hodgkinson's latest study on the use of integrating spheres in wavelength modulation spectroscopy has appeared on Applied Physics B's "online first service"
The detection and quantification of gas concentrations by measurement of their optical absorption has considerable great scientific and commercial importance. Extending the optical pathlength in gas sample cells is of interest to improve the signal to noise ratio (SNR) of this technique. Integrating spheres have great promise as multipass gas cells that require minimal optical alignment. However, the exponential distribution of optical pathlengths might be expected to influence the measurement.
In the paper it was found that the gas lineshape becomes distorted at high concentrations, as a consequence of the exponential path-length distribution of the sphere, introducing nonlinearity beyond that expected from the Beer–Lambert law. The effect was for numerically methane absorption at 1.651 μm, with gas concentrations in the range of 0–2.5 %vol in air. The results of this model compared well with experimental measurements. It was shown that if this effect were not accounted for, the resulting error would be approximately 20 % of the reading at a concentration of 2.5 %vol methane.
Using integrating spheres with wavelength modulation spectroscopy: effect of pathlength distribution on 2nd harmonic signals
J Hodgkinson, D Masiyano and R P Tatam
Applied Physics B, (2012).
As always, a full list of publications from the Department of Engineering Photonics is available here.