Application of optical fibers to wideband differential interferometry and measurements of pulsed waves in liquids

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Virginia Polytechnic Institute and State University


Wideband differential interferometry has been applied to the detection of SAW on specimen surfaces and ultrasonic compressional waves in liquids. Herein is described the performance of a wideband differential system which uses single mode optical fibers to transmit coherent light from input optics to a surface which supports which supports ultrasonic waves. Polarized light from a 2.0 mW helium-neon laser source is divided and coupled to two flexible bundled single mode optical fibers which transmit the light to a small remote detection head. The light at the output end of the fibers is collimated and focused by a varifocal lens system to points on the surface of a specimen to be inspected. Elastic waves on the specimen differentially modulate the relative phases of the two optical beams due to periodic changes in particle displacement at the surface. Upon reflection, the two beams are superimposed, filtered, and detected to produce an optical signal directly proportional to instantaneous displacements.

Also described is the development of two beam and four beam differential systems for the detection of ultrasonic compressional waves in water. Two laser beams are transmitted through a water tank and combined to produce an interference pattern. The detected motion of the pattern yields a differential measure of the acoustic field amplitude at the location of the two probe beams. If a pulsed ultrasonic wave is generated in the tank in a direction perpendicular to and coplanar with the probe beams, each beam is modulated independently and output signals of opposite phase are produced.

The acoustic sensitivity of both the above systems may be adjusted by changing the separation between the two spots on the surface or the two beams in the tank. The system effectively discriminates against low frequency noise vibrations, while the upper acoustic frequency response exceeds 100 MHz. Applications requiring flexibility allowed by a remote detection head can use the fiber system to their advantage while potential applications of the four beam system to three dimensional mapping and ultrasonic field scattering is suggested.