Fiber Bragg Grating Sensors: Recent Advancements, Industrial Applications and Market Exploitation

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The development of Fiber Bragg Gratings (FBGs) is closely associated with the field optical fiber sensors. This chapter presents a personal overview of the history of FBGs, with particular emphasis on the interrelation and impact of FBGs with sensing. The major milestones for FBG-based sensing are identified, from the very discovery of fiber gratings to current developments that are described in full detail in the following chapters of this book.

Successful commercialization of a technology such as Fiber Bragg Gratings requires the ability to manufacture devices repeatably, quickly and at low cost. Although the first report of photorefractive gratings was in 1978 it was not until 1993, when phase mask fabrication was demonstrated, that this became feasible. More recently, draw tower fabrication on a production level and grating writing through the polymer jacket have been realized; both important developments since they preserve the intrinsic strength of the fiber. Potentially the most significant recent development has been femtosecond laser inscription of gratings. Although not yet a commercial technology, it provides the means of writing multiple gratings in the optical core providing directional sensing capability in a single fiber. Femtosecond processing can also be used to machine the fiber to produce micronscale slots and holes enhancing the interaction between the light in the core and the surrounding medium.

Common methods for modeling, analysis, and synthesis of Fiber Bragg Gratings are reviewed in detail, including coupled-mode theory, transfer matrix methods, and layer-peeling algorithms.

The incredible growth of Fiber Bragg Gratings (FBGs) from both the research and the commercial points of view, has forced the development of several methods able to tailor their spectral characteristics for specific applications. Basically, two different approaches can be adopted to specialize the grating spectra: one is based on complex grating profiles induced directly at the fabrication stage. Another, more interesting, approach relies on post processing methodologies that introduce localized defects along the grating, breaking its periodic structure. The presence of the defects leads to the formation of allowed bands or defect states within the grating bandgap. The introduction of finer scale spectral features results in new interesting perspectives in both telecommunications and sensing fields. In addition, the possibility to accurately control the defect states spectral features - depth, bandwidth, and spectral position - could allow the complete engineering of the FBG bandgap, opening the way to the realization of several new advanced and attractive photonic devices. This chapter is focused to review the main advancements in FBGs post processing to easily control the spectral features of the final device for specific applications.

Fiber Bragg Gratings are structures with remarkable characteristics that have induced new qualitative developments in the broad field of optical fiber technology, most notably in optical communications and in optical sensing. When these devices are applied for sensing, the underlying concept is the modulation of the grating Bragg wavelength by the measured and, therefore, a central issue is the sensitive and accurate conversion of the resonant wavelength into a proportional electrical signal with the adequate format for further processing. This topic is broadly known as Fiber Bragg Grating interrogation and is the subject of the present chapter. It is organized in two parts: in the first one, the techniques developed by the scientific community looking for this functionality are reviewed, with emphasis on the identification of general conceptual classes where they fit; in the second part, illustrative and state-of-the-art commercial Fiber Bragg Grating interrogation systems are described.

One of the main goals of fiber optic sensor technology is to multiplex together a high number of sensors in the same fiber in order to share expensive terminal equipment and reduce the size and weight of the optical cable. Both passive and active multiplexing techniques are used for telemetry in sensor networks. The different multiplexing techniques for Fiber Bragg Grating sensors will be described and compared here, including networks using optical amplification and lasing multiplexing systems.

Due to the lateral inscription process, Fiber Bragg Gratings (FBGs) written into standard single-mode fiber can exhibit birefringence. The birefringence value is however too small to be perceived in the FBG spectral response but it can lead to significant polarization-dependent properties such as polarization dependent loss (PDL) and differential group delay (DGD). A transverse force applied on the FBG can enhance the birefringence, which increases both the PDL and DGD values. Hence, although these properties are not desired in telecommunication applications, they can be advantageously used for transverse force sensing measurements, allowing the use of FBGs written into standard single-mode optical fiber, which fail to work when they are interrogated through amplitude spectral measurements. This chapter first analyzes the normalized Stokes parameters, PDL and DGD evolutions with wavelength when the FBG parameters and the birefringence value are modified. It then focuses on the realization of a transverse force sensor based on the monitoring of the PDL and DGD evolutions.

Fiber Bragg Gratings (FBG) have been regarded as one of the most promising local monitoring sensors and are widely deployed in civil infrastructures. In this chapter, those FBG-based sensors aiming at civil structures have been briefly presented, including direct packaged strain and temperature sensors, indirect sensors constructed using FBG as a sensing element, and FBG based smart civil structures. Specific issues concerning methods of effectively and correctly applying the FBG sensors to civil infrastructures have been discussed. Those issues involve sensor installation technique, strain transfer based error modification, and temperature compensation. Finally, more than ten case studies of critical infrastructures outfitted with FBG sensors have been reported. Both research and practical applications show that FBG sensors are now competitive with conventional electrical sensors in long-term structural health monitoring.

Fiber optic sensors, Fiber Bragg Grating (FBG) sensors in particular, are among most promising sensors for structural health monitoring of aerospace structures in order to assess the safety and durability during a long period of time in use. These sensors are expected to provide a low-cost maintenance methodology for carbon fiber reinforced composite structures which are now extensively being used for the primary structures. This chapter presents an overview of current use of FBG sensors in aeronautics and astronautics.

Fiber Bragg Grating sensing systems have found application in numerous market sectors, wherein the unique capabilities of the technology are either displacing existing, limited sensing technologies, or providing solutions where sensing was hitherto impractical or impossible. Here, we review the energy market for the technology, a market where some of the most challenging measurement conditions are presented, yet some of the largest value propositions can be seen.

Fiber Bragg Grating (FBG) sensor technology has been attracting substantial industrial interests for the last decade. FBG sensors have seen increasing acceptance and widespread use for structural sensing and health monitoring applications in composites, civil engineering, aerospace, marine, oil & gas, and smart structures. One transportation system that has been benefited tremendously from this technology is railways, where it is of the utmost importance to understand the structural and operating conditions of rails as well as that of freight and passenger service cars to ensure safe and reliable operation. Fiber-optic sensors, mostly in the form of FBGs, offer various important characteristics, such as EMI/RFI immunity, multiplexing capability, and very long-range interrogation (up to 230 km between FBGs and measurement unit), over the conventional electrical sensors for the distinctive operational conditions in railways. FBG sensors are unique from other types of fiber-optic sensors as the measured information is wavelength-encoded, which provides self-referencing and renders their signals less susceptible to intensity fluctuations. In addition, FBGs are reflective sensors that can be interrogated from either end, providing redundancy to FBG sensing networks. These two unique features are particularly important for the railway industry where safe and reliable operations are the major concerns. Furthermore, FBGs are very versatile and transducers based on FBGs can be designed to measure a wide range of parameters such as acceleration and inclination. Consequently, a single interrogator can deal with a large number of FBG sensors to measure a multitude of parameters at different locations that spans over a large area.

FBG is the most promising, cost-effective and distributed sensor technology that provides an ideal platform to monitor the condition and structural health of tracks, carriages and underframe equipment in railway systems. In the last few years, a number of field trial railway projects using FBG sensors for axle counting, track and train vibration measurements, monitoring of bogie conditions, structural health monitoring of train bodies, and interaction between overhead contact lines and current collectors (pantograph) were successfully conducted by a few research institutions. These studies demonstrated the superiority of FBG sensors over conventional sensors in many crucial aspects. However, major barriers, such as lack of proprietary and custom specifications, packaging and reliability standards, insufficient field experience, have yet to be resolved before major railway operators are to embrace the FBG sensor technology.

Fiber Bragg Grating sensors are evaluated for applications in environments where the presence of highly energetic radiation is a concern, such as space and nuclear industry. To assess the feasibility of using this sensor technology in these particular application environments it is essential to evaluate the behavior of Fiber Bragg Gratings when exposed to different types of ionizing radiation. We, therefore, review the effects of ionizing radiation on several types of Fiber Bragg Gratings.

While Fiber Bragg Grating (FBG) sensors continue to act as valuable sensing platforms specially when physical parameters have to be measured in single or multipoint configuration, great efforts have been focused in the last decade to convey the great advantages of FBG technology towards the development of new devices and components employable in modern chemical and biological applications. Since the first attempt carried out in 1996 by Meltz et al., many sensing schemes have been proposed based on the evanescent wave effect arising from the surrounding refractive index sensitization of the final device by tailoring either the grating structure or the host fiber. In this chapter, we review the main advances in the area of FBG evanescent wave sensors, with particular emphasis on principles of operation, technological developments, overall performances and discussing in details perspectives and challenges that lie ahead.

The unusual properties of Fiber Bragg Gratings in microstructured fibers deliver significantly improved performance in some respect to traditional fibers. They are also superior in one or few characteristics and eventually unveil the novel properties of fiber Bragg grating sensors, leading to the superiority of implementations and original applications, which are briefly discussed in this chapter. Moreover, a very promising functionality of doped core microstructured fibers in perspective of material and geometrical guiding mechanism competition is clarified and encouraged. Furthermore, such fibers are greatly advantageous for Fiber Bragg Grating inscription and instead of scarifying important properties they pave the way to novel applications fulfilling the industrial requirements. Additionally, very interesting and hopeful research and development of pure silica microstructured Fiber Bragg Gratings are presented as well.

This chapter deals with gratings recorded in polymeric optical fibers (POFs); predominantly those based on poly (methyl methacrylate) (PMMA). We summarize the different mechanical and optical properties of POFs which are relevant to the application of POF Bragg gratings and discuss the existing literature on the subject of the UV photosensitivity of PMMA. The current state of the art in POF grating inscription is presented and we survey some of the emerging applications for these devices.

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 FBGbased sensors, adequate and reliable packaging designs, as well as training and education of prospective customers and end-users.