A class of developmental photonic temperature-measuring systems is based on the use of miniature, fiber-optic-coupled Fabry-Perot interferometers as temperature transducers. These systems are intended for monitoring and control of advanced aircraft engines, conventional and nuclear powerplants, industrial plants, and other systems in which conditions could be too severe for electronic temperature sensors (thermocouples, thermistors, and bimetallic devices). Unlike electronic temperature sensors, these and other photonic temperature sensors do not pose a sparking hazard and are insensitive to electromagnetic interference at suboptical frequencies.

Figure 1. A Fabry-Perot Fiber-Optic Temperature-Sensor generates a reflected-light spectrum characteristic of the temperature in the sensor head.

In essence, a Fabry-Perot fiber-optic temperature sensor provides a temperature-sensitive reflectance spectrum. Figure 1 illustrates a prototype Fabry-Perot fiber-optic temperature-sensor system that has been built and tested to demonstrate the feasibility of such systems for monitoring aircraft-engine exhaust temperatures from -50 to +600 °C. The sensor head comprises an Inconel" (a nickel-alloy) sheath that contains the Fabry-Perot interferometer, which is located at the tip of a sapphire optical fiber that connects the sensor head to external instrumentation. A small platinum-alloy housing holds a reflector (made of the same alloy) at a short distance, d, from the fiber-optic tip, which is polished. The gap d defines the interferometer cavity.

White light is launched into the optical fiber via a 2:1 fiber-optic coupler at the end opposite the sensor head. The light travels along the fiber to the sensor head. About 4 percent of the incident light is reflected back along the fiber from the polished tip. The remainder of the incident light travels on to the platinum-alloy reflector, and about 90 percent of the light incident on the reflector re-enters the fiber and propagates back along the fiber, along with the light reflected from the fiber tip.

The fiber-optic coupler directs the two backward-propagating light beams to a spectrometer that is integrated with a 1,000-pixel linear charge-coupled-device (CCD) photodetector array on a computer plug-in spectrometer card. Because of interference between the two backward-propagating beams, the CCD output shows characteristic interference fringes; that is, a reflected-intensity-vs.-wavelength spectrum. Because of the difference between the coefficients of thermal expansion of the sapphire optical fiber and platinum-alloy housing and reflector, d varies with temperature, giving rise to a change in the spectrum. Among other things, the number of interference fringes in this spectrum increases with temperature.

Figure 2. The Temperatures in These Plots were calculated from measurements taken during straight and level flight. The two sets of temperatures from measurements taken during severe maneuvers agreed almost as well.

The raw reflectance spectrum is divided by the spectrum of the incident white light to obtain a normalized spectrum. The computer calculates the squared difference between the normalized spectrum and each of 117 stored calibration curves, which are normalized spectra that correspond to known temperatures at 5 °C increments. A parabolic interpolation is used for temperatures between the increments. The measured temperature is taken to be the temperature that yields the minimum squared difference.

In an experiment, the prototype Fabry-Perot fiber-optic temperature-sensor system was used, along with a thermocouple, to monitor the exhaust temperature of an aircraft engine. The Fabry-Perot temperature readings agreed closely with the thermocouple readings (see Figure 2). In more than 50 hours of flight tests, the prototype system proved to be immune to source fluctuations and to deterioration of optical surfaces inside the sensor head. The prototype system functioned under normal flight conditions and during severe maneuvers with accelerations as large as 4 times the normal Earth gravitational acceleration.

This work was done by Takeo Sawatari, Yuping Lin, and Phil Gaubis of Sentec Corp. for Lewis Research Center. Sensor calibration and flight tests were conducted by Margaret L. Tuma of Lewis Research Center and of Kristie A. Elam of Gilcrest Electric Co. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the category, or circle no. 28 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Takeo Sawatari
Sentec Corporation
2000 Oakley Park Road, Suite 205
Walled Lake, MI 48390

NASA Tech Briefs Magazine

This article first appeared in the January, 1999 issue of NASA Tech Briefs Magazine.

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