<title>Laser Doppler velocimeter miniature differential probe for biomedical applications</title>

Abstract

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 fiber was used. This fiber had two anisotropic 8 (mu) cores, located symmetrically with respect to the fiber axis. The separation between them was 25 (mu) . In the fiber, a directional coupler was integrated near the fiber's remote end. The coupler was fabricated by heating of a small section of the fiber, with simultaneous elongation of this section. For heating, the carbon-dioxide laser was utilized. The carbon-dioxide laser was also used to fabricate a lens at the fiber tip. At the fiber entrance, the laser light was launched in one of the fiber's cores. The fiber was typically of several meters long. Near the fiber end, this radiation was splitted by the directional coupler, and the second core was excited too. At the fiber tip, the fused lens provided collimation of the emerging beams and secured their intersection in front of the fiber 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 fiber tip is very small. Thereafter, all 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 fiber-based differential LDA, is negligible in this probe. This is the principal advantage of this probe. However, its small size and integrated nature are also of value for biomedical applications.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.