Laser Doppler velocimeter miniature differential probe for biomedical applications

The dynamic sensors for speed and flow are applied in pathological physiological research of the cardiovascular system and for the study of blood flow in the capillaries. The use of Laser Doppler Anemometry (LDA) method is considered the major prospective for this application. This method is based on the Doppler shift of the frequency of laser radiation scattered by blood particles in movement. However, to have access to inner organs, a small-size delivery system and optical probe are necessary. In this paper, we report a novel miniature optical probe for the differential-type LDA, suitable for use in small blood vessels and in other small channels. For the construction of the probe, two-core single mode optical fibre was used. This fibre had two anizotropic 8 mu cores, located symmetrically with respect to the fibre axis. The separation between them was 25 mu. In the fibre, a directional coupler was integrated near the fibre's remote end. The coupler was fabricated by heating of a small section of the fibre, with simultaneous elongation of this section. For heating, the CO2 laser was utilised. The CO2 laser was also used to fabricate a lens at the fibre tip. At the fibre entrance, the laser light was launched in one of the fibre's cores. The fibre was typically of several meters long. Near the fibre end, this radiation was splitted by the directional coupler, and the second core was excited too. At the fibre tip, the fused lens provided collimation of the emerging beams and secured their intersection in front of the fibre tip. In this intersection volume, the interference field was formed. In the flow, this periodic pattern resulted in Doppler frequency shift of the light scattered by moving particles. This probe was successfully used together with the LDA signal processing equipment for velocity measurement in small tubes for blood-flow simulation experiments. In the probe, the two cores are identical, and the between the directional coupler and the fibre tip is very small. Thereafter, ail extrinsic fields and effects have identical influence on the propagation constant of each core. Hence, the resultant parasitic phase modulation, that is the main problem in the fibre-based differential LDA, is negligible in this probe. This is the principal advantage of this probe. However, it's small size and integrated nature are also of value for biomedical applications.