Single fiber laser-Doppler flowmetry-dependence on wavelength and tip optics

Single fiber, laser-Doppler flowmetry can be used for blood flow measurement in deeply located tissue structures by the insertion of optical fibers into the tissue. The geometry of the monitored volume has been estimated at two different wavelengths and when using two types of fiber tips, one of which has been modified with a lens formed at the fiber end surface. Physical models as well as intramuscular measurements have been used in the experiments. The scattering image was studied in latex solutions of three different scatterer concentrations. The wavelengths 632.8 and 750 nm were used. At higher concentrations of scatterers, the near infrared (NIR) wavelength gave a larger scattering area. At the lower concentration, the difference between the areas was smaller or nonexistent. The NIR wavelength also showed an increased monitoring depth than that of the He-Ne laser in an experimental model study. The properties of the tip optics were evaluated in a flow-through model where the distance between the fiber tips and the flow channel was varied. The flat tip fiber has a sensitivity maximum close to its end surface, whereas the modified fiber ("pear" tip) showed a sensitivity maximum 1.5 mm from the end surface. This property may decrease the influence caused by the insertion trauma in intramuscular measurements. © 1998 Society of Photo-Optical Instrumentation Engineers.