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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In recent years a strong interest has developed in embedded fiber optic sensors for the monitoring of strain, temperature, and other parameters in structural materials [l].The fiber Fabry-Perot interferometer (FFPI) using internal mirrors [2-41 is a strong candidate for this application because it provides localized (I1pointt1) sensing capability and high sensitivity, and is amenable to time-division multiplexing.Here, new results on the embedding of FFPIs in metals and composites are reported.Previously, metals in which fibgrs have been successfully embedded have had relatively low (< 200 C) melting temperatures [ 5 ] . In some early experiments in our laborators on the casting of metal parts in aluminum (melting temperature = 660 C), the fibers invariably broke at or near the air-metal interface upon cooling to room temperature. Here, a simple solution to this problem is described and the first results on fiber sensors embedded in aluminum are reported.Dielectric mirrors for the FFPIs are formed in single mode Corning fiber by a fusion splicing technique described in detail elsewhere [3].To embed the FFPI sensor, a graphite mold is machined to the desired shape. Stainless steel tubes (1.6 mm OD / 0.5 mm ID) are then positioned to extend about 1 cm into the mold at the bottom and top.The buffer on the fiber containing the interferometer is stripped back so that the total length of bare fiber in the direction of its axis is about 3 cm greater than the dimension of the part. The fiber is then positioned in the mold, passing through both of the tubes, with the interferometer located near the center.A tensile load of about 20 grams is applied to the fiber.The aluminum is heated in air in a crucible with an oxygen/acetylene torch and poured into the mold, as in Fig. 1. After the metal has cooled to near ambient temperature, the mold is removed. Several samples of various geometries containing embedded FFPIs have been produced without breaking the fiber. It is believed that passing the fiber through a tube to enter the metal greatly reduces the stress discontinuity at the air-metal interface, which otherwise causes the fiber to break. Data presented here are for a 1.0 cm long FFPI embedded in a 4 cm x 4 cm x 0.5 cm aluminum block.The transmittance T and reflectance R of the embedded interferometers are evaluated using a single mode 1.3 pm DFB laser with a Faraday isolator in series to suppress feedback, as in Fig. 2. In initial measurements on the embedded sensor near room temperature, the excess loss (= 1 -R -T) was nearly independent of temperature at 10% ( 0 . 4 5 dB), as compared with 8% (0.36 dB) prigr to embedding. Upon temperature cycling from room temperature to 2 5 0 C the excess loss was found to increase slightly, stabilizing at 13% (0.60 dB) after five 368