A fiber-optic ultrasound sensor is presented. The sensor consists of a continuous length of single-mode optical fiber with a built-in Fabry-Perot interferometer. The acoustic pressure produces changes in the index of refraction along the interferometer cavity through the strain-optic effect, thus modulating the reflected power of the light propagating in the fiber. The dielectric internal mirrors that form the interferometer are fabricated by joining a fiber coating with a TiO(2) film at one end to an uncoated fiber by electric arc fusion splicing. Experimental results have been obtained for sensors embedded in plastic and graphite composite materials, using ultrasound waves in the range from 100 kHz to 5 MHz. Values for the optical phase shift amplitude as large as 0.5 rad were obtained at an acoustic frequency of 200 kHz for a 1.1-cm-long interferometer embedded in plastic.
Fiber Fabry-Perot interferometers (FFPIS) utilizing internal mirrors have been developed to sense temperature, strain. acoustic waves and other physical perturbations in structural materials, and have been successfully embedded in composites and in metals. The construction, performance and application of the FFPI sensors to smart structures are described
A technique for embedding one or more optical fibers in a cast metal part or structure while maintaining optical transmission through the fiber is presented. This technique provides nondestructive monitor of internal perturbations of the structure. Application of the method to embedded fiber optic sensors in metallic structures and to fiber-embedded metal feedthrough are reported and the performances of temperature and ultrasound fiber sensor embedded in a cast aluminium block are demonstrated.
The sensing of temperature and of ultrasonic pressure with fiber-optic Fabry-Perot interferometers embedded in aluminum is demonstrated. The metal parts are cast in air by using graphite molds. Breakage of the fibers at the air-metal interface during the casting process is avoided through the use of stainless-steel stress-relief tubes. The optical phase in an embedded interferometer is found to be 2.9 times more sensitive to temperature change than for the same interferometer in air, in good agreement with model calculations. An embedded interferometer has also been used to detect ultrasonic waves over the frequency range of 0.1-8 MHz.
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