Development of Miniaturized Fiber-optic Laser Doppler Velocimetry Sensor for Measurement of Local Blood Velocity (Fabrication of Convex or Concave Lens-like Fiber Tip and the Characteristics of Sensor Optical System)

Abstract

A miniaturized fiber-optic laser Doppler velocimetry (LDV) sensor has been newly developed, which is capable of measuring the local velocity in various semi-opaque and opaque fluid flows, particularly local blood velocity in blood vessel. The developed sensor has a convex lens-like fiber tip as a pickup and an improved optical transmission system of remarkably decreased stray light. The present paper describes a method of fabricating a fiber tip and the characteristics of the sensor optical system provided with the fabricated fiber tip. Any fiber-optic LDV sensors up to now developed have not been capable of measuring such opaque fluids because of very low light intensity scattered from the fluid and received by the sensor pickup and of low signal-to-noise ratio. In order to overcome these problems, convex and/or concave lens-like fiber tips have been fabricated by chemical etching, in which quartz fibers have been etched in aqueous solution of hydrogen fluoride. The curvature of the lens-like surface of the fiber tip has depended on the refractive index distribution across the fiber cross section, the concentration of HF-NH4F solution and the etching time. In the present fiber-optic sensor, laser beam emitted from the fiber tip can be focused at any position from about 0.1 to 0.5 mm distant from the fiber tip. Consequently the present sensor is capable of measuring the local velocity in almost all semi-opaque and opaque fluids including blood.