In-situ analysis of heterogeneous media with frequency and intensity modulated diode lasers

Electronic intensity and frequency modulation of diode lasers enables new p erspectives for highly sensitive analytical in-situ techniques. For analysi s of "heterogeneous" media, which may consist of coexisting gaseous, liquid , and solid phases and often show multiple light scattering, non-invasive t echniques with outstanding performance characteristics can be developed. An alysis of photon density waves launched by intensity-modulated lasers provi de an elegant way for the determination of absorption and scattering coeffi cients of multiple scattering media. The absorption coefficient can be used for qualitative and quantitative analysis of chemical constituents (which absorb in the range of the laser wavelengths). The scattering coefficient a llows characterization of physical and morphological properties of the samp le. In the current work, applicability of appropriate radiation transport m odels was investigated with simple light scattering solutions and compared with results obtained from the treatment according to the theory of Kubelka and Munk. Measurements of human blood samples are discussed. A new method for determination of water vapor partial pressure in a polyurethane foam wi th a frequency-modulated external cavity diode laser is demonstrated.