Abstract

Many substances, frequently of organic origin, are porous and contain free gas distributed throughout the material Wood, fruits, paper, powders, sintered materials, foams and liquids with dissolved gas can be considered. A useful way to analyze gas in situ is by absorption spectroscopy using a sufficiently narrow-banded light source combined with the Beer- Lambertian law. However, this method fails for porous media, since the radiation is heavily scattered in the material containing the gas. Thus, there are no well-defined pathlengths as required by the Beer-Lambertian law. When a laser beam impinges on the gas-containing material, heavy multiple scattering occurs and light emerges diffusely. This situation is much discussed in connection with light propagation in living tissue with applications to optical mammography, dosimetry for photo-dynamic therapy and concentration determinations for tissue and blood constituents. If the frequency of a single-mode laser is slightly changed, the amount of recorded diffusely scattered radiation will stay constant, since absorptive features of liquids and solids are very broad and the cross section for multiple scattering has a very slow wavelength dependence. However, if the scattering medium contains free gas molecules, these will absorb the radiation in a very small wavelength region, giving rise to a tiny, but narrow absorption feature in the intensity of the recorded diffusely scattered light. This observation forms the basis for the present paper. When the volumes of free gas become smaller and smaller, we ultimately come to the point where the molecules do not move freely any longer, and broadening and shifts in the spectral features can be expected. It should thus be possible to observe the transition from bubbles into dissolved gas in liquids, e.g. in water. The new possibility to observe free gas in scattering media does not only allow static gas assessment but also the study of dynamic processes, i e how gas is exchanged with the environment. Clearly, the substance containing gas enclosures must not have substantial absorption to the radiation needed to monitor a particular gas. The interesting aspect of this work is in the regime of strong scattering. Strong scattering means long effective pathlenghts only if the material absorption is weak. A key issue is how much gas is needed to produce a discernable sharp absorption signal in the scattered radiation intensity Techniques allowing the observation of a signal which is a small fraction of the total light are clearly attractive Frequency modulation (FM) spectroscopy, easily applicable to tuneable diode lasers, is such a technique. A preliminary sensitivity analysis shows, that gas signatures should be detectable under many circumstances. We have made preliminary model experiments at 780 nm using diode laser radiation with a sharp spectral imprint due to passage of a cell with rubidium vapour. After passage through several mm of apple tissue or through paper resulting in heavy scattering, the sharp gas feature is clearly visible using two-tone frequency modulation spectroscopy. A set-up for molecular oxygen monitoring at 760 nm using optical fibres for transmission and receiving is being assembled and results will be reported Numerous applications including medical ones can be envisaged with this hitherto unexplored methodology

© 2000 IEEE