Fiber Bragg Grating Sensors: Recent Advancements, Industrial Applications and Market Exploitation, 2011, 313-320 313

Andrea Cusano, Antonello Cutolo and Jacques Albert (Eds) All rights reserved - © 2011 Bentham Science Publishers Ltd.

CHAPTER 16

Fiber Bragg Grating Sensors: Market Overview and New Perspectives

Jeff Wayne Miller1 and Alexis Méndez2,*

1Micron Optics Inc., 1852 Century Place, Atlanta, GA 30345, USA and 2MCH Engineering LLC, 1728 Clinton Avenue, Alameda, CA 94501, USA

Abstract: Over the last few years, optical fiber sensors have seen increased acceptance and widespread use. Among the multitude of sensor types, Fiber Bragg Grating (FBG) based sensors—more than any other particular sensor type—have become widely known and popular. FBGs have an intrinsic capability to measure a variety of parameters along a single fiber, such as: strain, temperature, pressure, chemical and biological agents, and many others. These multi-point sensing arrays of many relative low cost FBGs, provide great flexibility of design and make them ideal devices to be adopted for a multitude of different sensing applications and implemented in different fields and industries. 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 and more pronounced market growth. Other relevant factors are the need for industry standards on FBGs and FBG-based sensors, adequate and reliable packaging designs, as well as training and education of prospective customers and end-users.

INTRODUCTION

The fiber optics field has undergone a 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 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 and the seas, utterly transforming the means by which we communicate. This has all been documented with awe over the past several decades. Among the reasons why optical fibers are such an attractive communication medium are their low loss, high bandwidth, EMI immunity, small size, lightweight, safety, relatively low cost, low maintenance, etc.

As optical fibers cemented their position in the telecommunications industry—and its associated 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, nowadays, optical fibers are making an impact and 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, and seen a rise in their utilization and commercial growth.

FIBER BRAGG GRATINGS AS SENSORS

Since their fortuitous discovery by Ken Hill back in 1978 [3] and subsequent development by researchers at the Communications Research Centre, United technologies, 3M and several others [4], intra-core fiber gratings have been

*Address correspondence to this author Dr. Alexis Méndez: MCH Engineering LLC, 1728 Clinton Avenue, Alameda, CA 94501, USA; Tel: +1 (510) 521-1069; Fax: +1 (510) 521-5079; Email: alexis.mendez@mchengineering.com

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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 within the C-band wavelength range. However, one of the primary FBG benefits is the ability to be multiplexed, allowing multiple sensors and multiple parameters to be measured along a single fiber. These, multi-point, FBG sensing arrays provide great measurement flexibility and make them ideal devices to be adopted for a multitude of different sensing applications and implemented in different fields and industries.

FBG-based sensors have been developed for a wide variety of sensing applications (see Fig. 1) 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. To date, there are a diverse number of commercial FBG-based sensors designed and packaged to measure a variety of different mechanical, electrical and chemical parameters, as shown in (Fig. 2).

Figure 1: FBG sensors are used in a variety of industrial applications.

Figure 2: Diverse types of commercial FBG-based sensors.

The main advantage of fiber gratings for mechanical sensing is that these devices perform a direct transformation of the sensed parameter into optical wavelength—independent of light levels, connector or fiber losses, or other FBGs operating at distinct 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

Accelerometer Displacement meter

Strain meter Pressure meter

Incline meter Thermometer

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• Non-conductive immune to electromagnetic interference

• Environmentally more stable glass compared to copper

• Low fiber loss at 1550 nm remote sensing

Table 1: Pros and cons of FBG sensors.

Table 1 summarizes several of the many benefits to intra-core FBGs used as sensing elements. However, there are also some limitations and cons associated with gratings. Most fundamentally, it is the fact that they are simultaneously sensitive to strain, temperature, pressure and radiation effects. Hence, adequate temperature compensation is always essential in the design and commercial offering of reliable and repeatable physical sensors.

Although the theory and use of FBGs dates back to the late 1980s, the actual commercial transition did not happen until the mid-1990s, when it was strongly driven by the vast communications needs at the time coupled with the rampant demand for components brought on by the so-called telecommunications bubble, which saw a tremendous explosion on the number of companies and research groups engaged with the design, fabrication, packaging and use of gratings. The significant milestones and timeline evolution of the FBG industry over the past 30 years is illustrated in (Fig. 3).

Figure 3: Historical evolution of fiber Bragg gratings.

Soon after the telecommunications bubble collapse (circa 2002), there was a significant shift by many players in the industry from communications to sensing applications. At the time, this was a prudent and strategic move on the part of FBG manufacturers to keep exploiting the technical and manufacturing infrastructure they had available and ride the telecomm crisis until a possible comeback. At the present time, with the dust from the telecomm “bubble” already settled, it is clear that the original expectations and market potential originally envisioned for FBG components in the communications sector has not materialized and that the industry is operating at more realistic levels that match the trends at pre-bubble years.

Nevertheless, the sensing sector benefited tremendously from this shift and resulted in an increase in activity and demand of FBG-based sensors. However, the impact of the frenzy of mergers and acquisition at the peak of the

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bubble coupled with the industry contraction after its collapse, have resulted in a number of valuable companies and groups disappearing from the industry altogether as is the case of 3M and Bragg Photonics—to name a few—and diminished the pool of experienced and knowledgeable personnel.

It is expected that a resurgence of FBG related activity and companies will take place over the next 3 to 5 years as the demand for FBG sensors increases and more attention is drawn to this sector.

FBG SENSORS MARKET

As FBGs made the transition, from optical communications devices to sensing elements in the 1990s, the bulk of the sensing applications centered on discrete, single-point sensing of specific parameters—such as strain and temperature—using sensors based on embedded or packaged gratings. These early gratings were typically written using phase masks or side exposure interferometric techniques. These fabrication methods initially relied heavily method mostly on manual skills and labor, severely limiting many of the features and performance of the gratings in terms of production capacity, repeatability, mechanical strength, as well as number and quantity of FBGs written on a continuous fiber. Furthermore, during the boom years of the telecommunication industry in the late 1990s, it was possible to absorb the cost of the low yield from such manufacturing.

However, the sensor industry is much more cost sensitive, demanding multiple sensing points and greater mechanical strength. Such requirements also call for the capability to fabricate an array of multiple FBGs at different locations along a same length of optical fiber. These needs are being addressed by more sophisticated, on-line, reel-to-reel fabrication processes and systems that allow the writing of complex FBG arrays along a continuous single fiber spool. Additional flexibility is being offered by FBG written on a draw tower. Both approaches will result in gratings with better tolerance, higher yields and lower production costs—factors that should increase demand and promote wider industrial adoption.

In general, fiber optic sensors markets are in a relatively early development stage and are difficult to assess and forecast with accuracy. Furthermore, the market segments are fragmented due to the variety of sensing applications and industries where they are applied. Several niche markets have been established—such as fiber gyros and distributed temperature sensing systems (DTS). (Fig. 4a) provides forecast estimates of the total fiber sensors market (discrete and distributed) worldwide, while (Fig. 4b) provides a breakdown of the distributed FOS market alone. Estimates vary, but the overall FOS market was estimated at around $1Billion dollars in 2008. Over the past 5 years, the compounded annual growth (CAGR) rate has been in the 20 to 35%.

However, there has been a steady growth and demand for FBG-based sensing systems in the oil & gas industry for downhole pressure and temperature sensing, as well as in reservoir monitoring. To date, there are around 20 to 30 small companies around the world offering diverse FBG products and technology and addressing the needs of specific applications and customers.

The present FBG sensor market is primarily composed of 3 key segments: 1) sensing devices, 2) instrumentation, and 3) system integration & installation services. The sensing devices segment is composed of bare FBGs for sensing applications, packaged FBG sensors and FBG arrays. The instrumentation market segment is composed of FBG interrogating instruments and related ancillary components such as multiplexers, switches, data acquisition systems, software and graphical user interfaces. Finally, the third segment is mostly covering services—rather than products—and entails all project management and engineering aspects related to implementing sensing solutions and system installations such as design, planning, system integration, customer training, service and on-site installation.

Estimates of worldwide volume demand for bare and packaged FBG sensors are greater than 10,000 pieces per year. The worldwide volume demand for FBG arrays is estimated at several 100s to 1,000+ arrays per year. The combined present global market size of this segment is estimated to be in the range of $15M to $35M USD a year, with an annual growth rate of 15% to 25%. The instrumentation market has been growing steadily over the past three years, in part due to a variety of new fiber sensing projects and installations throughout Asia. Furthermore, as more people get interested in the use and application of FBGs, there will always be a need for interrogating equipment for R&D work, measurements, testing and qualification, as well as actual sensor interrogation. The global volume for FBG

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interrogating instruments is estimated at several hundred units a year, with an annual growth rate of 20% to 30%. The total market size is estimated to be in excess of $50M USD.

Figure 4: (a) Worldwide total fiber sensors market (discrete & distributed). (b) Breakdown of distributed fiber sensors market. (Sources: iNEMI and Lightwave Venture, LLC)

For its part, the service segment enjoys the benefit or providing additional value added to the actual cost of the FBG sensors and instruments. Furthermore, in any sensing project the expenses associated with design, planning, system integration and installation can easily be 50% or more of a total project cost. Hence, on an equal basis, service projects and complete sensing solution products will result in a greater value amount and overall revenue. However, the number of available and active service providers in the FBG sensors industry is still very small. Therefore, for the time being, the market is estimated at $15 to $25M USD.

FUTURE APPLICATIONS & OPPORTUNITIES

Future applications of FBG sensors will rely heavily on cost reduction—of both interrogating electronics and sensors—and development of specialized and application-specific, packaged devices. 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, and that hybrid systems combining FBG and distributed sensing instruments will start surfacing in the market.

Newer applications in chemical, biological and medical sensing will lead to a new generation of devices and products that will perform specific agent or parameter sensing functions relying on specially developed coatings and

(a)

(b)

318 Fiber Bragg Grating Sensors Miller and Méndez

reagents. Initially such devices will tend to be of moderate cost, due to slow market diffusion and expenses associated with qualification and industry certifications.

Applications in the energy sector for wind turbine blade monitoring will undoubtedly grow, given the recent demand for wind turbines and the global push for green, renewable energy sources. Similarly, there is a growing interest and increasing awareness for the use and deployment of infrastructure monitoring technology to assess the condition or bridges, pavements, pipelines and other vital civil infrastructure works. Historically, FBG-based monitoring systems have been very effective and useful in this type of applications, and there continues to be a steady growing number of new structural health monitoring (SHM) projects around the world that make use of commercial FBG sensors and instruments to implement such systems. This application will also be a key market segment and driver of business to the FBG sensing industry for years to come.

A review of US patents, both applications and filings, related to FBG sensors up to 2003, reveal that, by far, the bulk of the interest and applications remains centered on civil structure, aerospace and oil & gas applications, with the rest of the areas, as is the case with the fiber sensors in general, being spread over a diverse number of fragmented areas (see Fig. 5).

High-temperature resistant FBGs—such as chemical composition gratings (CCG); gratings written on N-doped, pure Si fibers; or those written with femtosecond 800 nm laser pulses—will open up opportunities in harsh environment sectors such as power plants, turbines, combustion, and aerospace. Similarly, the prospects of using polymer optical fibers (POF) in sensing applications is expected also to open up the door to the development of POF FBGs to be used as inexpensive, simple and low-cost disposable sensors.

Regardless of the application of FBG type, two common factors remain: reliability and packaging. Further strides and engineering is still needed to provide effective and suitable packaging to meet the needs of each sensing application.

THE NEED FOR STANDARDS

As observed in the telecommunications industry, standards can become a barrier but more often than not, they help simplify and normalize devices, formats and protocols. At the present time, there is no Bragg grating or FBG sensor standards in place. This has lead to a broad variability in available grating designs and specifications offered by commercial vendors, as well as a variation in the performance of FBG-based sensors when used in conjunction with instruments from different vendors. Furthermore, there is a tendency to make specific FBGs or FBG arrays from scratch which results in tedious and time consuming up-front phases of any sensing related project, not to mention making the components unnecessarily more expensive. In general, custom products are always more expensive and difficult to manufacture than standardized ones. Hence, sensor interrogation systems need to be standardized as well.

Several groups in North America, Europe and Asia are active in standards for fiber optic sensors including OIDA (Optoelectronic Industry Development Association, in Washington, D.C.) [5], ISIS Canada [6], the European Union COST 270/299 Committee [7], and RILEM [8]. For its part, OIDA has made initial attempts at formulating standards for FBGs for sensing applications for WDM and TDM interrogating schemes. However, there is not, as of yet, a formal standard development process or group.

MARKET BARRIERS

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 to end-users.

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26%

25%18%

9%

6%4%

4%4%

Civil Infrastructure Oil & Gas Aerospace

Nuclear Medical ProcessElectric Automotive Chemical

Figure 5: Breakdown of application field of US patents related to FBG sensors and sensing up to 2003.

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 break 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.

Furthermore, the lack of standards and sophisticated manufacturing processes and systems results in small production volumes as opposed to the batch fabrication of standardized products, with high yields and low cost.

Another significant barrier is the fact that most of the sensor developers and manufacturers only provide one piece of the complete sensing solution puzzle. Customers and end users require, in most cases, complete turn-key solutions that encompass all the necessary sensing components, data telemetry & acquisition systems, as well as all the necessary software and data processing algorithms and, most importantly, the actual sensing system design and installation.

The following list summarizes some of the most relevant set of market hurdles and barriers encountered:

• Unfamiliarity with the technology

• Conservative/no-risk attitude of some industries & customers

• Need for a proven field record

• Cost

• Availability of trained personnel

• Turn-key type systems (complete sensing solution)

• Lack of standards

• Quality, performance, packaging & reliability deficiencies across vendors

Another common entry barrier observed is the lack of knowledge about the technology and its use in sensing applications. There is, in general, a strong tendency among most technology companies to assume that customers are coming from the same industry and background and to think that they know already the “ins and outs” of FBGs. However, when it comes to sensing applications, there is a wide variety of backgrounds and technical disciplines from which customers will be coming from. Hence, it is important to articulate properly the operation and value of FBG technology and systems in terms that a given customer within a specific industry can understand and relate to. Therefore, it is important to also better understand the needs and common practices commonly encountered in industries such as oil & gas, civil engineering, composites, aerospace and others.

4%

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CONCLUSIONS

FBGs have an intrinsic capability to measure a multitude of parameters along a single fiber, such as: strain, temperature, pressure, chemical and biological agents, and many others. These multi-point sensing arrays of many relative low cost FBGs, provide great flexibility of design and make them ideal devices to be adopted for a multitude of different sensing applications and implemented in different fields and industries.

The present total FBG sensors market (devices and instrumentation) is estimated at over $100M USD, with an annual growth rate of 15% to 25%. Initially, instrumentation markets will growth fast, but eventually it will be the sensor and services segments that will display greater rates of growth and increased market size as the technology is further accepted and applications take root in the real world. 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 such as the need for industry standards on FBGs and FBG-based sensors, adequate packaging designs, as well as training and education of prospective customers and end-users.

REFERENCES

[1] Udd E. Overview of Fiber Optic Applications to Smart Structures. Review of Progress in Quantitative Nondestructive Evaluation. New York: Plenum Press; 541, 1988.

[2] Culshaw B, Dakin J Eds. Optical Fiber Sensors: Systems and Applications. USA: Artech House; Vol. II, 1989. [3] Hill KO, Frujii Y, Johnson DC, Kawasaky BS. Photosensitivity in optical fiber waveguides: Application to reflection filter

fabrication. Appl. Phys. Lett. 1978; 32(10): 647-9. [4] Meltz G, Morey WW, Glenn WH. Formation of Bragg gratings in optical fibers by a transverse holographic method. Opt.

Lett. 1989; 14(15): 823-5. [5] www.oida.org [6] www.isiscanada.com [7] www.cost299.org [8] www.rilem.net