10 April 2001
Fiber-optic sensors ready to take off?
by Bob Felton
Analysts predict new applications for old technology will boost use.
There's no overstating the importance of fiber optics in the Internet age: When Corning, Inc., a large producer of fiber-optic cable, announced late 24 January that its fourth-quarter 2000 profits had exceeded analysts' estimates but that first-quarter 2001 profits might be down, its stock dropped to $61 a share before opening in New York, down 13%.
Some analysts predict that fiber optics are about to assume a comparably important role in sensing. According to a report prepared by the International Technology Research Institute at Loyola University, the international market for fiber-optic sensors could be as high as $5 billion by 2010.
Impervious to environment
Fiber-optic sensors are of two types: extrinsic sensors, which use the glass thread for conveying information collected by another device, and intrinsic sensors, which make measurements by detecting perturbations in the light carried in the fiber. Both types offer advantages that aren't available from conventional wired devices.
First, radio and electromagnetic interference do not affect them, nor do they radiate energy that can affect other devices.
Second, they are efficient, transmitting huge amounts of information at stupendous speeds with virtually no transmission losses. In addition, they are lightweight, are safe in explosive environments, don't corrode, and are unaffected by temperature and pressure.
The military has been the primary developer of fiber-optic sensors, exploiting their lightweight ruggedness for applications as disparate as detecting chemicals in air, water, and soil; evaluating temperatures; measuring strain and displacements; and measuring the intensity of electrical and magnetic fields. So far, though, the sensors have gained a significant commercial foothold, primarily in chemical and structural monitoring applications.
Fiber-optic strain gauges have found wide application for structural monitoring because in addition to their other benefits, they are exceptionally light. Structural engineers embed them in concrete in order to monitor bridge deflections under traffic loads, and aeronautical engineers use them to monitor the deformation of aircraft components. In both uses, the monitors provide useful data about performance under service loads, which is valuable for future designs and may even provide life-saving warnings of imminent problems. When used as chemical sensors, fiber optics are extremely sensitive to the presence of reflection-diminishing films that are byproducts of chemical reactions.
There are numerous other applications. Wound coils of fiber-optic cable serve as gyroscopes, detecting changes in orientation as phase changes in counterpropagating wave fronts. Auto companies are using fiber-optic pressure sensors in automobile engines. A Frost & Sullivan study estimated that the market for fiber-optic pressure sensors alone will increase to $22 million by 2006.
A Navy research laboratory has developed a fiber-optic magnetometer that monitors harbor traffic, and Sandia National Laboratory is exploring the use of fiber-optic sensors in security applications. When buried in the ground, the fibers are invisible to intruders and detect automobiles or foot traffic. The control box costs about $2,400, and the sensing cable costs less than $1 per foot. IT