A recently developed type of fiber-optic temperature sensor utilizes narrow-band near-infrared radiation emitted by rare-earth ions. These sensors are suitable for use in harsh environments at temperatures above the maximum (1,700 °C) that Pt/Rh thermocouples can withstand. The maximum operating temperature for these optical temperature sensors can equal or exceed 2,000 °C, the exact values depending on the choice of fiber-optic and rare-earth-containing radiative materials. The minimum temperature measurable by use of a sensor of this type, related to the minimum detectable radiation, has been found to be ≈700 K (≈427 °C).
Most atoms and molecules at solid-state densities emit electromagnetic radiation in continuous spectra much like those of black bodies. However, even at solid-state densities, the rare earths emit radiation in narrow bands much like those of isolated atoms — a consequence of the electron wave functions peculiar to the rare earths. The development of the present rare-earth optical temperature sensors is a result of prior research on rare-earth-containing selective emitters for thermophotovoltaic energy conversion. In that research, it was found that rare-earth-doped yttrium aluminum garnet (RexY3–xAl5O12, where Re = signifies Yb, Er, Tm, or Ho) is an excellent selective emitter. It is chemically stable at high temperatures (>1,500 °C) and is characterized by emittances of ≈0.7 in the near-infrared wavelength range of interest for measuring temperatures.
A sensor of this type (see figure) is an optical fiber, coated at its input (hot) end with a film that contains a rare earth. The rare-earth-containing tip of the fiber is placed in contact with the object, the temperature of which is to be determined. Infrared radiation emitted at the input end of the optical fiber travels to the output end of the fiber, then through a band-pass filter with a narrow pass band that lies within the emission wavelength band of the rare earth. The filtered radiation impinges on a photodetector, the output of which is processed to obtain an indication of temperature.
Assuming that the photodetector is a photovoltaic device that is operated in a short-circuit configuration, it has been shown theoretically that the absolute temperature, Ts, of the hot sensor tip should be given by where Tc is a known calibration temperature, lf is the middle wavelength of the filter pass band, k is Boltzmann's constant, h is Planck's constant, c0 is the speed of light in a vacuum, ic is the short-circuit detector current measured when the sensor tip is at the calibration temperature, and isc is the short-circuit detector current measured when the sensor tip is at the temperature, Ts, that one seeks to determine.
A prototype sensor was constructed with a sapphire optical fiber tipped by an Er3Al5O12 emitter, a chopper, a filter of lf = 1,012 nm, a silicon photodetector, and a lock-in amplifier for measuring the short-circuit detector current. The prototype sensor was calibrated at a temperature of 1,879 K. Then temperatures calculated from isc readings by use of the equation given above were compared with simultaneous thermocouple measurements. The results of these measurements showed the worst-case fractional temperature error to be only 0.03, thereby confirming the validity of the equation and the underlying temperature-measurement principle.
This work was done by Donald L. Chubb and David S. Wolford of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
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