A reflectively monitored optical-fiber Fabry-Perot interferometer was embedded in a graphite-epoxy composite material. Its performance as a temperature sensor was demonstrated from 20 to 200 degrees C. The change in relative phase shift with temperature, Deltaø/øDeltaT, was measured to be 8.0 x 10(-6)/ degrees C for this embedded sensor. This value is 4% lower than for one employing a similar fiber in an air ambient. A thermal expansion coefficient for the composite material in the direction of the fiber axis is estimated from these data to be 2.1 x 10(-7)/ degrees C.
A reflectively monitored Fabry-Perot temperature sensor is characterized over a 1250 degrees C temperature range. Dielectric mirrors for the interferometer are incorporated into a continuous length of single-mode fiber by a fusionsplicing technique. The change in optical phase per unit temperature is nearly constant above room temperature but decreases at lower temperatures. Phase-shift data for the germania-doped silica fiber used in these experiments are compared with calculations based on previously reported information on the temperature dependence of refractive index and length in bulk fused silica.
Large thermally induced changes in the end-interface reflectance and transmittance of silica fibers coated with vanadium dioxide films have been observed.
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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