Abstract: This paper reports the development of a sapphire($alpha -Al$-2$/O$-3$/) single crystal optical fiberthermometer using two wavelength bands. A thin film ofprecious metal or ceramic deposited onto one end of thesapphire fiber forms a mini-radiation cavity. The otherend of the sapphire fiber is coupled to a low-losssilica fiber. Radiation from the small cavity istransmitted along the silica fiber into aphotodetection system which consists of a lens, beamsplitter, two interference filters (820 nm and 940 nmcenter wavelength, 30 nm bandwidth) and two siliconphotocells. The temperature measurement is based on thedetection of radiation from the small cavity. Thesapphire fiber (0.25 - 1.0 mm diameter, 100 - 450 mmlength) was grown by the laser heated pedestal growth(LHPG) methods. Transmission loss in the sapphire fiberwas experimentally measured. Theoretical analysis showsthe apparent emittance of the small cavity with alength to diameter (L/D) ratio greater than eight is aconstant value near to one, so the small cavity can beconsidered as a small black-body cavity. Using thedeveloped sapphire fiber temperature sensor, we havebuilt a sapphire fiber thermometer based on a 8098single-chip microcomputer system. It was calibrated atsome known stable temperature point and uses thefundamental radiation law to extrapolate to othertemperatures. By taking the ratio of the optical powerat two wavelengths, errors due to changes in thesystem, such as emissivity and transmission losses, canbe canceled out. The thermometer has an operatingtemperature range of 800 to 1900 degrees Celsius, andan accuracy of 0.2% at 1000 degrees Celsius. There area number of applications of the thermometer both inscience and industry. !4
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