Optical-Fiber Ultrasonic Delay Lines

Abstract

The ultrasonic pulse-propagation characteristics of nonmetallic optical fibers were investigated the 10-Mc/sec region. It was found that the lowest-order longitudinal L(0,1) mode and the lowest torsional or T(0) mode could be propagated, using piezoelectric transducers. The L(0,1) mode was propagated through single fibers as well as flexible bundles of fibers. The T(0) mode was propagated through single fibers. The attenuation was of the order of 0.1 dB/μsec in fused silica fibers of 75 μ diam, and about three times greater in clad glass fibers of the same diameter. In very-short-fiber lines, many echoes were observed and the end-face geometry was not critical. In flexible-fiber bundles with epoxy fused ends, no echoes were seen owing to the high attenuation of the epoxy, which was greater than 10 dB/mm of coated length. Dispersion was negligible in the single-fiber lines and flexible fiber bundles, but severe in rigid, fully fused fiber-optic bundles that behaved essentially as solid rods. There was no significant difference in the ultrasonic behavior of optically coherent and noncoherent flexible-fiber bundles of short length. Flexible fiber-optic bundles with fused ends appear to retain the simple acoustic-mode structure of the single fiber. Various multiple-fiber arrays are considered in parallel and series grouping and with different input-output configurations. A multiple-delay generator using a combination of glass and fused silica fibers of different lengths is described. Further applications are discussed.