Good sense

Fiber-optic sensors are being used in a wide variety of applications.^1-5 Sagnac interferometers, which can determine movement by measuring the shift in interference fringes from two counterpropagating coherent beams in a ring, support fiber gyros on aircraft, missiles, rockets, and robots. Michelson interferometers, in which interference fringes indicate the length relationship between two legs of the interferometer, support strain and acoustics measurements for civil structures and underwater applications. Another type of fiber sensor, fiber-grating sensors, is emerging as a potentially low-cost solution to a wide variety of point-sensor measurements such as axial strain and temperature, transverse strain, shear strain, moisture, pressure, acoustics, vibration, and chemical content. Applications areas for these sensors include aerospace and structural monitoring.

UV light shining into the germanium-doped core of an optical fiber induces a local change in the index of refraction. One can create a grating in the core of the fiber by imaging an interference pattern (created either holographically or by a phase mask) through the side of the fiber. The resulting grating acts as a wavelength filter. The period of the induced index of refraction modulation in the optical fiber core determines the wavelengths that will be reflected.

When the fiber grating is subject to strain, two effects cause the period of the fiber grating to change. One effect is that the overall length of the grating changes, and the other is that the index of refraction changes. The net result is that the overall period of the grating changes, resulting in a shift in the reflected wavelength, which provides a mechanism to measure strain. The wavelength shift in most silicon-based fibers is extremely repeatable and accurate, as the composition of the fibers are very nearly pure quartz, which provides excellent mechanical properties.

Fiber gratings are widely used to support wavelength division multiplexing (WDM) and dispersion compensation in the telecommunications industry. These volume applications have significantly reduced the price of fiber gratings and have triggered the production of fiber gratings with greatly improved reliability and performance. Because the long-term trend in optical networking is toward bringing fiber to the home with networks incorporating these devices, other fiber-grating users will continue to benefit from telecommunications development, which will improve the performance and lower the cost of these devices.

Each fiber grating may be made to reflect at a specific wavelength, so one can design a simple system that performs WDM along a single fiber line. This technique can readily be used to multiplex 10 or more fiber-grating sensors along a single fiber. By combining time-division multiplexing with WDM, one can support hundreds of fiber Bragg grating sensors in a single fiber.

Figure 1. A multi-axis fiber-grating strain sensor is formed by writing a fiber grating into polarization-maintaining fiber. The resultant spectral peaks reflecting from what are essentially two fiber gratings correspond to the different indices of refraction along each axis. With a uniform transverse load, the peaks maintain their spectral shape.

As a first example of the unique capabilities of fiber grating sensors, consider a multi-axis fiber grating strain sensor that offers the ability to measure 2-D and 3-D strain fields along a single fiber line as well as measuring transverse strain gradients and shear strain (see figure 1). Strain is the change in length divided by the original length of the object under test. It can be presented as a percentage or as a unitless number. A fiber grating written into polarization-preserving fiber effectively acts as two separate gratings, one for each polarization because light in each polarization experiences a different refractive index. This duality allows users to quantitatively measure both transverse strain and axial strain. One can implement a series of similar sensors simply by writing fiber gratings tuned to different wavelengths at different points along the optical fiber.

When the fiber grating is illuminated by a broadband source such as an LED, the reflection spectrum shows two peaks as measured by an optical spectrometer or other spectral measurement device. The spacing between these peaks is a measure of transverse strain along the birefringent axes of the optical fiber. In the case of figure 1, the multi-axis fiber grating strain sensor is subject to uniform transverse strain along one of its axes. This results in two clear, distinct spectral peaks whose spectral separation is proportional to the transverse strain.

While fiber-grating strain sensors provide an absolute measurement of strain in the optical fiber, they are often embedded in materials that provide residual strains during manufacture. This residual strain can be measured, and for many applications the parameter of interest is the relative strain change after the structure has been fabricated, which involves recording the reflected wavelengths after the structure is fabricated and noting wavelength changes thereafter. By fastening this type of fiber-grating sensor onto a structure with an appropriate bonding material, the user can record an initial strain state. Changes in the axial strain and transverse strain, then, may indicate changes in material properties, dimensions, pressure, or temperature of the structure. These indications of third-party damage and/or changes that affect structural integrity reference the baseline value. Blue Road Research (Gresham, OR) has typically achieved strain measurement accuracy of less than 1% for surface-mounted fiber-grating strain sensors, compared to their electrical equivalents.

Figure 2. If the fiber grating is partially unloaded along a transverse strain axis (perhaps due to adhesive-bond failure) then transverse strain gradients occur. This results in clear splitting of the appropriate spectral peak.

If the transverse strain along one of the axes is partially unloaded (as would be the case during an adhesive-bond failure or internal failure of a composite material), then transverse strain gradients occur that result in clear splitting of the appropriate spectral peak (see figure 2). The amplitude of the transverse strain is encoded in the spectral shift. The length over which a specific load occurs can be determined by the amplitude intensity of the peak. The position of the load can be localized either by using short gauge-length fiber-grating sensors (typical dimensions are on the order of 5 mm) or by using distributed techniques such as chirped fiber gratings (in which the period of the fiber grating varies along its length, allowing position information to be wavelength-encoded).

Figure 3. Multi-axis fiber grating strain sensor mounted in the edge of an adhesive bond can measure transverse strain, transverse strain gradients, and axial strain simultaneously within an adhesive bond. The purple line shows zero load.

A multi-axis fiber-grating strain sensor can also be used for testing joints by mounting the sensor in the edge of the adhesive bond, with its transverse axes aligned at 45° to the plane of the part in order to offer optimal response to shear. For zero load, the spectral response is simple. As the load increases, the spectral response becomes more complex, with peaks separating to show increased transverse forces or splitting to show bond failure (see figure 3). At higher loads, all the peaks move, which indicates axial strain.

We tested this application by placing the ends of a test coupon containing a fiber-grating sensor in an Instron universal test machine and applying a load. As the test machine pulled the joint apart, shear force (transverse strain) increased, causing the principal peaks to move apart. When the joint began to fail, one of the principal peaks split, indicating that a portion of the bond was unloaded by approximately 600 microstrain. The intensity of the two split peaks was approximately equal, indicating that half of the fiber-grating length had been unloaded. When the joint was loaded to higher levels, all the peaks moved, indicating axial strain.

Thus, we demonstrated simul- taneously measuring transverse strain, transverse strain gradients, and axial strain interior to an adhesive bond. This technique offers unique capabilities for accurate diagnostics of previously inaccessible structures.

By putting fiber gratings in tubes and placing them under axial tension, one can form long strain sensors. Blue Road Research has installed instrumentation on two bridges in Oregon using this technology. On the Horsetail Falls bridge, instrumented in 1998, we used long gage-length fiber-grating strain sensors in tubes 0.7- and 1-m long to monitor the deflection of concrete beams as well as motion of composite overwraps that were added in an attempt to strengthen the bridge.⁶ The small size of the fiber-grating assemblies allowed us to add instruments for long-term monitoring without altering the appearance of this historic bridge. The results of static testing from 1998 to 2001 confirmed the success of a composite overwrap procedure for strengthening the bridge.

Figure 4. Signatures obtained on a variety of vehicles passing over the Horsetail Falls bridge allowed us to indicate vehicle weight (deflection of the bridge beam in microstrain) and speed (full width half maximum). The sensor is sensitive enough to register the presence of a jogger running, jumping, and walking off the bridge.

We later used this bridge as an evaluation bed for dynamic testing that allowed the weight and speed of traffic to be monitored. We determined traffic speed using the full width half maximum of the signature, and the weight by the deflection of the bridge beam in microstrain (see figure 4). The sensors were sensitive enough to detect a jogger running, jumping, and walking off the bridge in 1999. As a result of these tests, Blue Road Research has instrumented the Interstate 84 freeway with embedded fiber-grating sensors and has shown the sensors' ability to monitor a wide range of traffic.

In addition, fiber-grating strain sensors incorporated in both the bridges and the I-84 segment have demonstrated their ability to perform over long periods of time. The 26 fiber-grating sensors installed in 1998 have shown no degradation since their installation. We obtained similar results on other bridges with more recent installations. The I-84 freeway sensors, once installed, also have shown unchanged performance over a period of approximately one year. This lifetime is comparable to the best achieved by current technology. We expect the lifetime of the installed fiber-grating sensors to be on the order of decades. Besides mechanical testing and civil structures, examples of fiber-grating sensor applications also exist in oil and gas, aerospace, and power utilities. oe

References

1. E. Udd, ed., Fiber Optic Sensors: An Introduction for Engineers and Scientists, Wiley 1991.

2. B. Culshaw and J. Dakin, eds., Optical Fibre Sensors: vols. I, II, III and IV, Artech House Inc., 1996.

3. E. Udd, ed., Fiber Optic Smart Structures, Wiley, New York, 1995.

4. J. M. Lopez-Higuera, ed., Handbook of Optical Fibre Sensing Technology, Wiley, New York, 2002.

5. E. Udd and Richard Claus, eds., Proceedings of Optical Fiber Sensors 15, Portland Oregon, IEEE Press, May 6–10, 2002.

6. E. Udd, J. Seim, et al., Proc. SPIE 4185, p. 872, 2000.

Eric Udd

Eric Udd is president of Blue Road Research, Gresham, OR.