AN OPTICAL FIBER TEMPERATURE SWITCHING TECHNIQUE I n recent years, optical fiber sensors are finding increas- ing application for sensing of many physical parameters due to various advantages they offer over the conven- tional ones. These include a dielectric construction that permits usage in high-voltage, electrically noisy, high-tem- perature corrosion or other hostile environments, and geomet- ric versatility that allows arbitrary configurations and inherent compatibility with optical fiber data links. 1–6 These characteristics make optical fiber sensors an attractive choice to monitor the status of temperature in various critical reac- tions, processes, and other industrial and scientific applica- tions. Petrochemical, process control, electrical, oil and gas explora- tion, and mining industries employ various processes and reactors where a particular temperature has to be controlled precisely for obtaining the desired products or composition. Therefore, all these industries are concerned about the possi- ble methods and devices that are useful for in situ monitoring of the exact status of temperature remotely. Thus, there is an important need for a real-time and automatic temperature monitoring technology in industries and power plant environ- ments. 7 Temperature measurement devices such as thermis- tors, thermocouples, and bimetal types are not suitable for use in various industries because they are vulnerable to electro- magnetic interference, are heavy, and may cause sparking. Conventional temperature measuring devices have draw- backs of manual control, large size, fragility, inadequate dynamic range, lack of measurement stability, unacceptably short lifetime, and need for complex calibration procedures when the devices are replaced. Such calibration procedures require a significant amount of time involvement. The optical fiber, the right-angled prism, and the glass capil- lary used in the present technique are made of dielectric material that is nonconductive, noninductive, noncorrosive, and immune to electromagnetic interference/radio frequency interference effects. It is, therefore, desirable to overcome the above-mentioned problems of the conventional techniques and sensors and employ new class of optical sensors for in situ monitoring of point temperature of reactions/processes at remote locations efficiently. 8–10 WORKING PRINCIPLE Figure 1 depicts the basic schematic of the technique investi- gated for in situ monitoring of point temperature of chemical reactions and processes remotely. The technique basically involves a control unit with a light emiting diode (LED) trans- mitter, receiver, right-angled prism, and two optical fibers. A fiber-holding plate covers the hypotenuse surface of the right- angled prism and is provided with two holes for holding the fibers, one of which is connected to a light source and the other being connected to a photodetector. Light from the LED trans- mitter is coupled into a multimode fiber that terminates onto the surface of the right-angled prism. The other fiber picks up the light, which undergoes total internal reflection (TIR) inside the prism, and is connected to a photodetector. Under normal conditions, the light emitted by the light source under- goes TIR at the two reflecting surfaces of the prism and is coupled back to the photo detector. But at the desired temper- ature when the liquid has risen to the calibrated height as per the thermal expansion of the liquid, it spills through the hole on to the prism surface and the light does not undergo TIR but refracts out through the upright reflecting surface. In this case, the light escapes into the liquid, and consequently, the optical power at the receiver decreases as evident from Fig. 2b. DESIGN AND FABRICATION The investigated optical fiber point temperature switching technique basically uses a glass capillary tube (inner diame- ter: 1 mm, outer diameter: 4.35 mm, height: 122 mm) having a top end closed with a cap and a bulb (diameter: 14.50 mm) at a bottom end for holding a liquid having good thermal expan- sion property, which is shown in Fig. 3. A groove is cut in the capillary tube and a small hole of 0.5 mm is made in the groove at a height corresponding to the designed temperature value to facilitate the spill of the liquid on to the prism surface as to disrupt the process of total internal reflection. A right-angled prism is fixed in the groove in such a way that one of its reflecting surfaces covers the hole keeping it at its center. Figure 4 indicates the design parameters of the prism and the process of TIR taking place inside the prism. The versatile feature of the technique is that it can be designed for a given point temperature as per the process to be monitored and the useful temperature range is governed by the type of liquid used and the initial level of the liquid chosen. The switching involves coupling of light from a 50-W quartz halogen lamp into a source fiber, which makes it incident onto the hypotenuse surface of the right-angled prism. The hypot- enuse surface is covered with an aluminum plate for holding the two multimode optical fiber guides (50/125 microns) with a numerical aperture of 0.2. One of the light guides is linked to the light source and the other to the photodetector. The light undergoes TIR at the other two reflecting surfaces and is cou- pled back to the photodetector fiber and finally guided to a Si PIN detector and an optical power meter. The prism is mounted in the groove using suitable epoxy with one of its reflecting surfaces positioned on a small hole drilled in the capillary. Special care was exercised while fixing the prism on the hole so that the epoxy used does not block the hole itself. Isopropyl alcohol has been used as a liquid contained in the bulb because of its good thermal expansion property. For a large range, the initial level of isopropyl alcohol at the room temperature should be taken at a lower value and for a smaller range of temperature, a higher level of isopropyl alcohol TECHNIQUES by N.S. Mehla, S.C. Jain, V. Mishra, G.C. Poddar, P.B. Kassey, and P. Kapur N.S. Mehla, S.C. Jain, V. Mishra, G.C. Poddar, P.B. Kassey, and P. Kapur are affiliated with the Central Scientific Instruments Organisation, Chandigarh, India. doi: 10.1111/j.1747-1567.2007.00158.x Ó 2007, Director C.S.I.O. July/August 2007 EXPERIMENTAL TECHNIQUES 37