Fabrication and characterization of a sapphire based fiber optic microphone for harsh environments.

Abstract

This paper describes the fabrication and characterization of a sapphire based fiber optic microphone designed for the harsh environment of gas turbine engines. The performance requirements are driven by turbine inlet temperatures that have risen in excess of 1500 °C. The harsh environment makes conventional instrumentation unsuitable for time-accurate, continuous, direct measurements. The use of commercially available sapphire substrates and optical fibers allows for performance in extremely high temperature environments due to the high melting point and matched coefficient of thermal expansion. The sensor is based on a fiber optic lever transduction mechanism with a remote photodiode optical readout allowing for isolation of the electronics from the harsh environment. The microphone’s diaphragm consists of a thin sapphire substrate with a sputtered titanium/platinum reflective surface. The back cavity is formed by macro-machining of a 1-mm-thick sapphire substrate. The diaphragm and back cavity are joined using high temperature, alumina based epoxy. The paper focuses on the fabrication and packaging of the sensor with characterization results given using silica based optical fibers at room temperature. The future work is aimed at the implementation of sapphire optical fibers and high temperature characterization. [Work supported by the Air Force.]