Optical Fiber Sensors Could Enhance Building Security and other Applications

This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.


Researchers at Heriot-Watt University have developed fiberoptic sensors that have potential for withstanding high pressure and temperature, facilitating development of buildings capable of surviving a strong explosion. The three types of fiberoptic sensors developed, which require some engineering development before they are ready for the marketplace, use specially-engineered optical fibers and could be suitable for such applications as monitoring blast-waves from high explosives, structural safety in tunnels, bridges and buildings, bending in critical aircraft components, or deterioration in weapons stockpiles.

Electronic sensors--in which the measurement of such parameters as pressure, temperature, or stress affects the sensor's electrical behavior--can be unsafe in explosive environments or in certain medical applications and, moreover, are vulnerable to interference in environments with strong magnetic fields (such as power plants or magnetic resonance imaging). Fiberoptic sensors, which work with light, can address such limitations of electronic sensors.

Fiberoptic interferometry involves using a pair of optical fibers--one of which takes the measurement, and the other serves as a reference. Light beams travel along the fibers, are reflected at the end, and travel back to the starting point where they merge together. This process produces an interference pattern (in similar fashion to the fringes formed when one folds a net curtain in two). The exact pattern will depend on the difference in distance that the two beams travel. If the path length of the measurement fiber varies, the interference pattern changes and the variation in length can be calculated.

Such fiberoptic sensors have been used, for example, to measure strain in airplane wings and to detect movements in large civil engineering projects, such as bridges and dams. However, the fibers custom-designed for specific sensing applications by researchers at Heriot-Watt University could enable a range of fiberoptic with enhanced performance and greater suitability for challenging applications, such as explosive-proof structures.

One type of specialty fiber has multiple cores, which are highly suitable for measuring how a parameter changes over short distances, by comparing the difference between the adjacent cores. A suitable application, for example, entails measuring how a structure bends, where one side of the fiber stretches more than the other one.

Special gratings can be inscribed with a laser beam along the length of a fiber to produce mirrors tuned to a single color of light. White light can then be sent up the fiber, from which each of the component colors will be reflected at a different grating. "Taken together, this tells you exactly where the fiber is bending, by how much, and in which direction," stated Julian Jones, professor of engineering optics and head of the School of Engineering and Physical Sciences at Heriot-Watt University. "That's enough to measure the strains on all parts of a wing or a mast just by using the light coming from a single glass fiber."

Previously, such a measurement would have purportedly required hundreds or even thousands of electrical sensors. The new sensors have been developed in collaboration with NASA Langley to monitor flexible aerodynamic wings and for safety monitoring of tunnels by measuring changes in their shape.

The collaboration with NASA Langley arose from their interest in the possibilities of using gratings in multicore fibers, based on Heriot-Watt's previous work with multicore fiber obtained from France Telecom, noted James Barton of the School of Engineering and Physical sciences at Heriot-Watt University. NASA funded the production of multicore fibers, which were better suited to Heriot Watt's requirements. Heriot-Watt University has used such multicore fibers in experiments on bend and transverse load measurement. The gratings are written by Heriot-Watt's collaborators at Aston University, Birmingham, UK.

In collaboration with the Civil Engineering Department at the University of Birmingham, the researchers have conducted tunnel monitoring experiments aimed at developing improved techniques for monitoring the long-term stability of, for example, subway rail tunnels (particularly if they are subjected to nearby building construction).

The second class of special fibers are made of plastic. Glass fibers have their limits, Jones noted, and optical strain gauges could be used in considerably more situations if only the fibers were more resilient. Plastic fibers are well-suited for monitoring plastic and composite structures for indications of excessive stress. However, according to the researchers, such fibers are only now being made sufficiently slender for interferometry.

The third class of fibers, for use in situations where communications fibers are most suitable, have tiny sensing structures constructed at the tip of a communication-type fiber. For example, the researchers use a laser to drill a hole that is only thousandths of a millimeter wide in the end of the fiber and cap it with a light-weight membrane.

"These microsensors may be the fastest-reacting pressure sensors in the world," declared Jones. "And they're so robust that we'll be using them to measure blast waves. In the current climate of increased terror threat, there's a huge demand for technology which could help to design bomb-proof buildings."

There is considerable interest in the micromachined miniature pressure sensors for instrumenting test models in explosive trials to investigate how airborne blast waves interact with structures. Such sensors use a micromachined silicon with silica or silicon nitride diaphragm. Heriot-Watt is collaborating with University of Sheffield, UK in such activities. Barton noted that their small size, fast response and immunity to electromagnetic interference give such fiber sensors an advantage over commercially available piezoceramic pressure gauges. Improved data from systematic trials will improve the numerical models used in design codes for blast effects.

The fiber sensors containing a membrane at the tip are hand-assembled and their performance will depend on the thickness and diameter of the diaphragm. In blast experiments, such sensors have achieved a pressure measurement capability of 6 bar, with a rise time of about three microseconds. Such sensors are capable of surviving temperatures of about 100 degrees C for short periods, limited by the epoxy used to seal the fiber into the sensor body.

Jones has noted that fiberoptic sensors are finding key opportunities to address real-word applications in such areas as, for example, power generation, air and sea guidance systems, food safety, and medicine. In medical applications, the fiberoptic sensor's small size and all-dielectric construction are advantageous, noted Barton.

According to Frost & Sullivan's World Optoelectronics Markets research service (published November 2003), global revenues for the optoelectronics market are projected to exceed $13.23 billion in 2009. The segmentation of the total optoelectronics revenues in 2009 by product type is projected to be (figures may not add due to rounding): high-brightness LEDs--59.7%; optocouplers--11.9%; photodetectors--10.3%; and laser diodes--18.0%.

According to Frost & Sullivan's North American Pressure Sensors Markets research service (published December 2003), overall revenues for the North American pressure sensors market are projected to reach $2.105 billion in 2009. The distribution of the overall North American pressure sensor revenues in 2009 by product type is expected to be (figures may not add due to rounding): bourdon tube gauges--12.3%; electronic pressure sensors--14.9%; micro-electromechanical systems (MEMS) pressure sensors--60.3%; vacuum pressure sensors--8.2%; and other types of pressure sensors (including linear variable differential transformer), piezoelectric, and fiberoptic pressure sensors--4.2%.

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