Laser Doppler velocimetry in microchannels using integrated optical waveguides

The possibility of laser Doppler velocimetry (LDV) in microchannels, where particles are suspended in a liquid, and where oscillating or evanescent gu ided coherent light wave is present, is examined theoretically. The conditi ons for the observation of the transverse and longitudinal collective phore tic particle movement using the time correlation function (TCF) of the fluc tuations of the light scattered by particles suspended in the channel or it s frequency spectrum (FS) are formulated. It is shown that for the observat ion of the longitudinal phoresis, the heterodyne regime where the light sca ttered by particles is mixed with a reference wave of the observation shoul d be employed. The study carried out shows that heterodyne regime establish ment requires a multimode waveguide, where mode coupling can inherently occ ur. For the observation of the transverse phoretic movement, the homodyne r egime, where only the light scattered by particles reaches the detector, ma y also be employed. The TCF or FS of the light scattered by suspended parti cles may be computed using the light intensity signal in different guided m odes containing fluctuations if the coupling coefficients are influenced by the electric field fluctuations due to presence of the scattering particle s. After the computation of the TCF is carried out, this TCF or its FS may be used to obtain the information on the particle or macromolecule phoretic movement and diffusion in the channel. The information may be used for the electrophoretic analysis of the multicomponent liquid systems in the minia ture integrated optical devices with the continuous introduction of the ana lyzed liquid mixture. (C) 2000 Published by Elsevier Science B.V. All right s reserved.