Fiber optic sensors on the horizon

By Dan Hebert, Senior Technical EditorFIBER OPTICS are most commonly associated with communications networks, but these materials can also be used to fabricate sensors for process control applications. Compared with electrical and electromechanical sensors, fiber optic sensors are smaller, cheaper, more durable, and can operate at much higher temperatures.And, unlike electrical sensors, fiber-optic sensors are not susceptible to electromagnetic interference and can therefore be reliably used in industrial plants where electrical noise abounds.Fiber optic sensors are not new—they have been around since Corning Glass and Bell Labs first started developing optical fibers for the telecoms industry back in the 1960s. But a consequence of the telecom boom and bust is that prices have tumbled, so fiber optic sensors can be used more widely.According to a recent article in The Economist, the most common type of fiber optic sensor in use today is called a “Bragg grating,” which is the fiber optic equivalent of a strain gauge. A Bragg grating is a region of a fiber where the refractive index has been modified so that it varies in a precise and periodic way, causing the grating to reflect light of a specific wavelength.As the fiber is stretched or compressed the reflected wavelength changes accordingly, and strain can be determined. Changes in temperature also change the fiber's properties in predictable ways. By incorporating several Bragg gratings into a single fiber, each tuned to reflect a different wavelength, it is possible to measure the variations in strain or temperature along the fiber's length.Bragg gratings are used to measure strain in things like turbine blades, and are now cheaper than conventional strain gauges. Indeed, fiber optic sensors are positioned in terms of price and performance to replace electrical and electromechanical sensors in many process control applications.Julian Jones, a professor of engineering optics at Heriot-Watt University in Edinburgh, says that the spread of this technology from the laboratory into everyday use has barely begun. In collaboration with researchers at Aston University in Birmingham, England, and the University of Sheffield in Sheffield, England, he is currently working on several new types of fiber optic sensors.The first is a fiber with multiple cores, an idea that was originally intended to increase fiber optic capacity, but which was soon abandoned. Dr. Jones found that such fibers can be used not just to measure strain, but also to measure the degree of bend and its direction.Another new type of sensor is based on a conventional optical fiber, the end of which has been modified in various ways. In one example, a small hole, just an eighth of a millimeter in diameter, is drilled into the end of a fiber using a high-powered laser. A copper membrane is applied, creating a small air cavity inside the fiber.The optical properties of the fiber then vary depending on the pressure differential across the membrane. The result, says Dr. Jones, may be the fastest reacting pressure sensor ever made.Closer to home, Professor Anbo Wang and his employer, Virginia Tech University in Blacksburg, VA, have created the Center for Photonics Technology to investigate and create sensors that can be used in harsh industrial applications.The center has produced the world’s smallest known pressure sensor. The sensor is a mere 125 microns in diameter and can function at temperatures as high as 700°C. Competing sensors are limited to 500°C.Developed by Yizheng Zhu, a Ph.D. student at the Center, the sensor is fabricated directly on the tip of a fiber by micromachining and thermal fusion, giving it the same thickness as the optical fiber.The sensor has minimal cross-sensitivity to temperature, resulting in a simplified sensor system with a wide temperature range. Its small size and low mass give the sensor an extremely high resonant frequency, resulting in a flat response over a very wide range of frequencies.Sensitivity can be adjusted for different applications, with a pressure range as low as a few psi or as large as 10,000 psi. The sensor is another step toward the Center’s goal of developing pressure sensors that can operate above 1000°C.The Center has also developed new ways to get a single fiber to function as a very large number of independent sensors. In one experiment, Wang and his colleagues demonstrated a technique that can read 1,000 different Bragg gratings along a single fiber.Optical fibers have already slashed the cost of communications, and may be poised to do the same for certain sensing applications.