Optical fiber sensors - fiber Bragg gratings, in particular - have become a practical, reliable and useful sensing technology for strain sensing and damage detection in a range of structural monitoring applications. This technology is set to achieve greater commercial use and market growth in the coming years.
Overview of Fiber Optics
The field of fiber optics has experienced impressive growth and development over the past four decades. Originally developed as a medium for transmitting light and images in medical endoscopic devices, optical fibers were proposed as the next big thing in the telecommunications industry as early as the mid-1960’s, with some considering them to be an optimal medium for transmitting information.
The millions of miles of telecommunications fiber that has since spanned the globe confirms the incredible success of this notion. Some reasons why optical fibers are such an appealing technology include their low loss, elevated bandwidth, EMI immunity, small size, light weight, safety, comparatively low cost, low maintenance - among others.
Along with the strengthening position of optical fibers in the telecommunications industry, and the maturation of its technology in commercial markets, similar attempts have been made by several different groups, globally, to exploit some of the main features of fiber and use them in sensing applications.
Fiber Sensors vs. Optical Fibers
Originally, fiber sensors were curiosities found in the laboratory and used for simple ‘proof-of-concept’ demonstrations.
However, optical fibers are making a substantial 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, automotive lighting and control, as well as covered applications as diverse as oil well downhole pressure sensors to intra-aortic catheters.
This transition has taken almost two decades and has reached the point where fiber sensors have been accepted and used in a broad range of applications; for example, in structural sensing and monitoring 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 has become well understood and developed, and a range of commercial discrete sensors - founded on Fabry-Perot (FP) cavities and fiber Bragg gratings (FBGs), as well as dispersed sensors based on Raman and Brillouin scattering methods - are now easily available alongside relevant interrogation instruments.
Applications of FBG-Based Sensors
Among these, FBG-based sensors have become broadly known, researched and popular within and beyond the photonics community, and have experienced a growth in their utilization and commercial growth.
From the time of their unexpected discovery by Ken Hill back in 1978 , and in later developments by researchers at the Canadian Research Center, United Technologies, 3M and several others , intra-core fiber gratings have been widely employed in the telecommunication industry for dense wavelength division multiplexing, dispersion compensation, laser stabilization, and erbium amplifier gain flattening, largely at the 1550 nm, C-band wavelength range.
However, given their inherent ability to measure a multitude of parameters such as strain, temperature, pressure, chemical, biological agents and others, along with their flexibility of development to be used as single point or multi-point sensing arrays and their comparative cheapness, FBG devices were recognized early on as possessing ideal sensing elements. They have since seen wide use as on-line monitoring devices in Structural Health Monitoring (SHM) applications.
However, certain technical hurdles and market barriers need to be overcome before fiber sensors gain more commercial momentum and achieve faster market growth.
As displayed in Figure 1, sensors based on FBG have been developed for a broad range of physical sensing and Structural Health Monitoring (SHM) uses, such as 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 conventional strain, pressure and temperature sensing.
Advantage of FBG-Based Sensors
The main advantage of using fiber gratings for mechanical sensing is that they can perform a direct conversion of the sensed parameter to an optical wavelength, regardless of light levels, connector/fiber losses, or other FBGs at varying wavelengths. For example, when compared to one of the most commonly-used electronic sensor, the foil strain gage, the comparative advantages of FBG-based sensors become clear:
- Totally passive, which means no resistive heating or local power is required
- Small size, which means it can be embedded or laminated
- Narrowband with broad wavelength operating range, meaning it can be multiplexed
- Non-conductive, which means it is resistant to electromagnetic interference
- More environmentally stable, as it uses glass, as opposed to copper
- Low fiber loss at 1550 nm, enabling remote sensing
Figure 1. SHM applications of FBG-based sensors.
Future use of FBG sensors will heavily depend on reducing costs and developing specialized and application-specific packaging. It is anticipated that more traditional and popular uses, such as discrete strain and temperature sensing, will continue to grow and acquire further market shares.
Applications that demand multi-grating arrays will become similarly popular as prices lower, which will enable them to contend directly with truly distributed fiber sensing approaches based on Raman and Brillouin scattering methods.
While FBG-based sensors (and fiber sensors in general) have attracted commercial interest and spawned some profitable niche markets, there are several technical obstacles and market barriers that must be overcome. In general, there remains a persistent lack of awareness and understanding regarding the advantages of using fiber optic sensors and fiber gratings.
A large portion of customers and end-users remain suspicious of the “subconscious” instability of optical fibers. However, the largest hurdle - by far – is the insufficient reliability of some current products and their relatively high costs. To date, this has prevented the widespread use and commercial spread of FBG sensors.
Reliability is a major characteristic that needs to be taken seriously and included in every aspect of the fiber sensing design and production process. Increasing reliability could lead to commercial approval and fast implementation of a given design (or product) – ignoring this aspect could hinder the production process and limit commercial appeal.
Several industries are conservative by nature and adverse to risk, for example the electric power, mining and biomedical industries. These widely-held attitudes require that devices demonstrate proven reliability and a consistent record of performance, and this is achieved 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.
This information has been sourced, reviewed and adapted from materials provided by Fiberguide Industries.
For more information on this source, please visit Fiberguide Industries.