A fiber thermometer using the cross detection of the fluorescence lifetime and blackbody radiation was presented to measure temperature from -10°C up to 1400°C. Using a long pure YAG crystal fiber as the seed and a 0.1 at. % Cr2O3-doped Y3Al5O12 sintered powder rod as the source rod, a YAG fiber thermal probe with Cr3+ -ions doped end was grown by laser heated pedestal growth method. A blackbody cavity was constructed by sintered a thin ceramic layer around the Cr3+: YAG fiber end. A phase-locked detection scheme was used for the fluorescence lifetime detection. The fluorescence characteristics of the Cr3+-ions doped YAG was analyzed in a temperature range from -10°C up to 500°C. From 350°C to 1400°C the blackbody radiation signal in a narrow waveband were detected. Because the fluorescence lifetime was intensity independent, it should have the long-term stability and would not change if the fiber connectors of the probes were realigned. So the fluorescence lifetime based temperature measurement could be used to recalibrate that based on the blackbody radiation detection. Preliminary experimental results showed that the system could achieve a resolution much better than 1°C over the whole temperature range from -10°C to 1400°C.
The temperature-dependent characteristics of fluorescence of transient-metal doped and/or rare-earth-doped YAG has made these materials the focus of fluorescence thermometer. This article reports growth and fluorescence characteristics of Cr3+: YAG crystal fiber used for thermometer based on fluorescence decay time. Using a long pure YAG crystal fiber as the seed and a 0.1 at. % Cr2O3-doped Y3Al5O12 sintered powder rod as the source rod, a YAG fiber thermal probe with Cr3+-ions doped end was grown by laser heated pedestal growth method. The crystal fiber shows good optical properties and mechanical strength and offers advantages of compact construct, high performance and ability to withstand high temperature. The fluorescence decay characteristics of the crystal fiber, including the temperature dependence of both fluorescence decay time and intensity, were comprehensively investigated. The experimental results indicated the Cr3+:YAG crystal fiber showed a monotonic relationship between the fluorescence lifetime and temperature over a wide temperature range from cryogenic to high temperature(>500°C). The fiber was found to be an excellent candidate material to be used as a fiber thermometer based on fluorescence lifetime. This thermometer may be used as temperature monitor in microwave treatment and Medium Voltage substations.
With a high melting point (over 2000°C), high transmission from ultraviolet to infrared wavelength approaching 4 μm, favorable mechanical strength and chemically inertness, the sapphire fiber is suitable for high temperature optical fiber sensor and near-infrared energy delivery. In this paper, the self-radiation and performance stability of sapphire fiber under high temperature was studied. Experiment results have shown that the self-radiation of sapphire fiber would influence the temperature measurement accuracy. The optical transmission loss increased with the time when the sapphire fiber was exposed to the high temperature environments. The most important factor that resulted in the performance deterioration was the direct contacting of the sapphire fiber with the protective tube and the dirtying of the evaporating substance. A suitable protective tube was applied to assure the performance stability. For further improvement of the optical properties of sapphire fiber, an alumina cladding is being developed to protect the sapphire fiber surface.
A two-step poling technique was presented for the homogenously poling of the large diameter LN wafers. Aluminum was used as the electrode material and confirmed to be suitable for the PPLN poling purpose. PPLN wafers with a homogenously poling area greater than 52 mm in diameter have been fabricated. Experimental parameters for the homogenously poling of the 3-inch LN wafer were also presented.
A sapphire fiber thermometer probe with Cr3+ ion doped tip end was grown from the laser heated pedestal growth method. The fiber probe offers advantages of compact construct, high performance and ability to withstand high temperature. The temperature dependence of fluorescence of the probe was investigated, and a sapphire fiber thermometer based on its fluorescent decay was presented. Among the detection rang from the room temperature to 450 c , the thermometer has an average temperature resolution of 10C.The thermometer may be used in microwave treatment and thermal monitoring of Medium Voltage substations.
Although Y2O3-ZrO2 fiber-optic sensor has been developed for contact measurement of temperature higher than 2000 degree(s)C, its performance is not as good as that of sapphire fiber-optic sensor below 1900 degree(s)C due to the large optical loss of the Y2O3-ZrO2 fiber. In order to improve the Y2O3-ZrO2 fiber-optic sensor for ultra-high-temperature applications, in this work, based on a newly developed rectangular Y2O3-ZrO2 single-crystal waveguide with much lower optical loss, an improved Y2O3-ZrO2 waveguide-fiber-optic sensor has been developed. The sensor has been tested up to near 2300 degree(s)C, we estimate that, the improved sensor has similar performance as the sapphire fiber-optic sensor in accuracy and resolution, except the disadvantage of relatively short waveguide. In addition, in this work, instead of the previous volatile and toxic BeO-coated probe, we use a multi-ions-doped sensor head, which is much stable and safe.
A new designed sapphire fiber-optic sensor, aiming to improve the performance of the traditional sapphire fiber- optic sensors for high temperature measurement, is provided in this paper, in this system, an additional U-shaped sapphire fiber is used together with a modulated LED reference signal. It has advantages of both high sensibility of the single-band type and the high stability of the dual- band system. It is a good method to improve the stability of the traditional radiation based sapphire fiber-optic sensor without sacrificing high sensibility.
This paper reports the development of a sapphire ((alpha) - Al2O3) single crystal optical fiber thermometer using two wavelength bands. A thin film of precious metal or ceramic deposited onto one end of the sapphire fiber forms a mini-radiation cavity. The other end of the sapphire fiber is coupled to a low-loss silica fiber. Radiation from the small cavity is transmitted along the silica fiber into a photodetection system which consists of a lens, beam splitter, two interference filters (820 nm and 940 nm center wavelength, 30 nm bandwidth) and two silicon photocells. The temperature measurement is based on the detection of radiation from the small cavity. The sapphire fiber (0.25 - 1.0 mm diameter, 100 - 450 mm length) was grown by the laser heated pedestal growth (LHPG) methods. Transmission loss in the sapphire fiber was experimentally measured. Theoretical analysis shows the apparent emittance of the small cavity with a length to diameter (L/D) ratio greater than eight is a constant value near to one, so the small cavity can be considered as a small black-body cavity. Using the developed sapphire fiber temperature sensor, we have built a sapphire fiber thermometer based on a 8098 single-chip microcomputer system. It was calibrated at some known stable temperature point and uses the fundamental radiation law to extrapolate to other temperatures. By taking the ratio of the optical power at two wavelengths, errors due to changes in the system, such as emissivity and transmission losses, can be canceled out. The thermometer has an operating temperature range of 800 to 1900 degrees Celsius, and an accuracy of 0.2% at 1000 degrees Celsius. There are a number of applications of the thermometer both in science and industry.
This issue presents a new kind of crystal fiber high temperature sensor. The method of making the mini-blackbody by doping in the fiber end is found to be effective. The experimental performance of the fiber sensor was also reported.
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