GAS IN SCATTERING MEDIA ABSORPTION SPECTROSCOPY — LASER SPECTROSCOPY IN UNCONVENTIONAL ENVIRONMENTS

Abstract

An overview of the new field of Gas in Scattering Media Absorption Spectroscopy (GASMAS) is presented. The GASMAS technique combines narrow-band diode-laser spectroscopy with optical propagation in diffuse media. Whereas solids and liquids have broad absorption features, free gas in pores and cavities in the material is characterized by sharp spectral signatures. These are typically 10,000 times sharper than those of the host material. Many applications in materials science, food packaging, pharmaceutics and medicine have been demonstrated. Molecular oxygen and water vapor have been studied around 760 and 935 nm, respectively. Liquid water, an important constituent in many natural materials, such as tissue, has a low absorption at such wavelengths, allowing propagation. Polystyrene foam, wood, fruits, food-stuffs, pharmaceutical tablets, and human sinus cavities have been studied, demonstrating new possibilities for characterization and diagnostics. Transport of gas in porous media can readily be studied by first immersing the material in, e.g., pure nitrogen gas, and then observing the rate at which normal air, containing oxygen, reinvades the material. The conductance of the human sinus connective passages can be measured in this way by flushing the nasal cavity with nitrogen, while breathing normally through the mouth. A clinical study comprising 40 patients has been concluded. (Less)