Optical fiber sensors—particularly fiber Bragg gratings—have become a practical, reliable and effective sensing technology for strain sensing and damage detection in a variety of structural monitoring applications and are poised for broader commercial use and market growth in the near future.
The fiber optics field has undergone tremendous growth and advancement over the past 40 years. Initially conceived as a medium to carry light and images for medical endoscopic applications, optical fibers were later proposed in the mid 1960’s as an adequate information-carrying medium for telecommunication applications. The outstanding success of this concept is embodied in the millions of miles of telecommunications fiber that have spanned the earth, the seas, and utterly transformed the means by which we communicate. This has all been documented with awe over the past several decades. Some of the reasons why optical fibers are such an attractive technology is due to their low loss, high bandwidth, EMI immunity, small size, lightweight, safety, relatively low cost, low maintenance, and much more.
As optical fibers cemented their position in the telecommunications industry and its technology and commercial markets matured, parallel efforts were carried out by a number of different groups around the world to exploit some of the key fiber features and utilize them in sensing applications. Initially, fiber sensors were lab curiosities and simple proof-of-concept demonstrations. However, optical fibers are increasingly making an impact as well as creating serious commercial inroads in other fields besides communications such as in industrial sensing, bio-medical laser delivery systems, military gyro sensors, as well as automotive lighting & control—to name just a few—and spanned applications as diverse as oil well downhole pressure sensors to intra-aortic catheters. This transition has taken the better part of 20 years and reached the point where fiber sensors enjoy increased acceptance as well as a widespread use for structural sensing and monitoring applications in civil engineering, aerospace, marine, oil & gas, composites, smart structures, bio-medical devices, electric power industry and many others [1,2]. Optical fiber sensor operation and instrumentation have become well understood and developed, and a variety of commercial discrete sensors based on Fabry-Perot (FP) cavities and fiber Bragg gratings (FBGs), as well as distributed sensors based on Raman and Brillouin scattering methods, are readily available along with pertinent interrogation instruments. Among all of these, FBG based sensors—more than any other particular sensor type—have become widely known, researched and popular within and out the photonics community and seen a rise in their utilization and commercial growth.
Since their fortuitous discovery by Ken Hill back in 1978  and subsequent development by researchers at the Canadian Research Center, United technologies, 3M and several others , intra-core fiber gratings have been used extensively in the telecommunication industry for dense wavelength division multiplexing, dispersion compensation, laser stabilization, and erbium amplifier gain flattening, mostly at the 1550 nm, C-band wavelength range. But given their intrinsic capability to measure a multitude of parameters such as strain, temperature, pressure, chemical and biological agents—and many others—coupled with their flexibility of design to be used as single point or multi-point sensing arrays and their relative low cost, FBG devices were recognized since its early beginnings as ideal sensing elements and have been used extensively as on-line monitoring devices in Structural Health Monitoring (SHM) applications. However, some technical hurdles and market barriers need to be overcome in order for this technology—and fiber sensors in general—to gain more commercial momentum and achieve faster market growth.
As shown in Figure 1, FBG-based sensors have been developed for a wide variety of physical sensing and Structural Health Monitoring (SHM) applications including monitoring of civil structures (highways, bridges, buildings, dams, etc.), smart manufacturing and non-destructive testing (composites, laminates, etc.), remote sensing (oil wells, power cables, pipelines, space stations, etc.), smart structures (airplane wings, ship hulls, buildings, sports equipment, etc.), as well as traditional strain, pressure and temperature sensing.
The main advantage of fiber gratings for mechanical sensing is that these devices perform a direct transformation of the sensed parameter to optical wavelength, independent of light levels, connector or fiber losses, or other FBGs at different wavelengths. For instance, when compared to one of the most common and popular basic electronic sensors—the foil strain gage—the relevant advantages of FBG-based sensors become evident:
- totally passive -> no resistive heating or local power needed
- small size -> can be embedded or laminated
- narrowband with wide wavelength operating range -> can be multiplexed
- non-conductive -> immune to electromagnetic interference
- environmentally more stable -> glass compared to copper
- low fiber loss at 1550 nm -> remote sensing
Figure 1. SHM applications of FBG-based sensors.
Future applications of FBG sensors will rely heavily on cost reduction and development of specialized and application-specific packaging. It is expected that more conventional and popular applications such as discrete strain and temperature sensing will continue to evolve and grow and acquire greater market shares. Similarly, applications calling for multi-grating arrays will become more popular as prices come down, allowing to compete more directly with truly distributed fiber sensing approaches based on Raman and Brillouin scattering techniques.
Although FBG-based sensors and, for that matter, fiber sensors in general have attracted commercial interest and developed some lucrative niche markets, there are a number of significant technical hurdles and market barriers to overcome. In general, there is still a pervasive lack of awareness and understanding about the operation and benefits of using fiber optic sensors and fiber gratings. Many customers and end-users still distrust the “subconscious” fragility of optical fibers. However, by far, the most significant barriers that have prevented a more widespread use and commercial diffusion of FBG sensors are inadequate reliability of some existing products and excessive cost.
Reliability is a key feature that needs to be taken very seriously and incorporated in every aspect of the fiber sensing design and production facets. It is the reliability that can make or brake the commercial acceptance and rapid adoption of a given design or product and the one limiting factor that can slow down the utilization of a given product or technology. Many industries are naturally conservative and adverse to failures—such as the electric power, mining and biomedical industries—such attitudes demand that devices demonstrate proven reliability and a solid record of performance established via prototype testing and field trials.
- Udd, E., “Overview of Fiber Optic Applications to Smart Structures”, Review of Progress in Quantitative Nondestructive Evaluation, Plenum Press, 1988.
- Culshaw, B. and Dakin, J., Eds, Optical Fiber Sensors: systems and applications, Vol.II, 1989, Artech House.
- K.O. Hill et al., “Photosensitivity in Optical Fiber Waveguides: Application to reflective Filter Waveguide”, Appl. Phys. Lett., Vol. 32, pp. 647-649, 1978.
- Meltz, G., Morey, W.W., Glenn, W.H., “Formation of Bragg gratings in optical fiber by transverse holographic method”, Opt. Lett. 14:823, 1989.
Author: Alexis Mendez
MCH Engineering, LLC