Fiber-coupled self-mixing diode-laser Doppler velocimeter: technical aspects and flow velocity profile disturbances in water and blood flows

The effects of disturbances of the flow pattern in front of the fiber facet of a fiber-coupled self-mixing diode-laser Doppler velocimeter system are investigated. This was done by comparing measurements and calculations of the Doppler frequency spectrum with the expected values. The calculated Doppler spectrum was obtained from the calculation of light scattered (with or without Doppler shift) by the moving particles in front of the fiber facet. The velocity profile of the particles was calculated with a finite-element method. Measurements were done with water (with polystyrene spheres) and whole blood as the samples. Good agreement between measurements and calculations were obtained. The velocimeter was modeled as a five-mirror setup. The reflectivity of the fiber facet closest to the laser turns out to have the most influence on the sensitivity and stability of the laser. Direct reflection of unwanted light back into the laser cavity was avoided by placing a glass plate in front of the fiber. Design considerations are presented.