Fiber Bragg Gratings: Theory, Fabrication, and Applications

The development of optical fibers has revolutionized not only telecommunications but also the way monitoring and sensing is conducted, particularly in remote or harsh environments. In this context, the discovery of photosensitivity in optical fibers led to the establishment of fiber Bragg gratings (FBGs), optical filters that have been widely employed in telecom and as measurement elements.

Here we offer a short explanation of FBGs provided as excerpts from the SPIE Tutorial Text, Fiber Bragg Gratings: Theory, Fabrication, and Applications.

Bragg gratings are one of the most useful, reliable, versatile, practical, and attractive passive devices in the fields of optical fiber communications and fiber optic sensors. Their simplicity of operation coupled with attractive and unique features, such as all-fiber construction, self-wavelength-value referencing, absolute encoding, capability for multi-point cascading, and batch fabrication, make FBGs perform key roles in any measurement or monitoring plant.

In this sense, a multitude of FBG devices can be found in different optical communication applications, such as dispersion compensators, gain lockers, spectral filters, wavelength references, and several more. Similarly, there is a broad variety of different sensors based on FBG elements that allow for the measurement of temperature, pressure, strain, acceleration, moisture, bio-chemical agents, and many other parameters in diverse civil and defense applications that range from aerospace structural health monitoring, to oil- and gas-well sensing, to miniature intra-aortic pressure and temperature probes. In this article, basic rules of thumb and and practical aspects concerning the use of FBGs are presented.

Operational Characteristics
FBG technology is one of the most popular choices for optical fiber sensors, particularly for strain or temperature measurements due to their simple manufacture, the relatively strong reflected signal, and the fact that the wavelength, its measuring parameter, is something absolute in the universe. Therefore, when using a FBG as a sensor, its measuring parameter, the Bragg wavelength, does not change with time, with temperature, or even with any kind of fiber losses. In addition, due to their inherent advantages, they can be used to replace conventional measurement techniques and devices (Chapter 1, pages 6-7).

FBGs started being used in the sensing world for measuring and monitoring several parameters, such as strain, temperature, pressure, displacement, voltage, electric current, or chemical substances in a number of applications and environments. Particularly, these transducers represent enhancements and benefits to power-system operators and users because they enable the development of practical and lightweight devices. In addition, the general safety of the measurement system can be improved due to the insulating characteristics of optical fibers, while the integration with the emerging smart-grid technology can be easily achieved. Thus, FBGs are considered key devices and important alternatives when compared with conventional technologies such as strain gauges or solid-state sensing. Many other uses of FBGs exist, such as biosensors for bacteria or chemical sensors for measuring H2S; prospective applications are almost infinite, and the only restriction is one's inspiration (Chapter 1, page 7; Chapter 2, page 16).

In its essence, an FBG acts as a dichroic mirror, reflecting part of the incoming spectrum (Chapter 3, page 22), as it can be seen in fig.1 (fig. 3.8). By injecting a spectrally broadband source of light into the fiber, a narrowband spectral component at the Bragg wavelength will be reflected by the grating. This spectral component will be missed in the transmitted signal, but the remainder of this light may be used to illuminate other FBGs in the same fiber, each one tuned to a different Bragg wavelength. The result: all of the Bragg peak reflections of each FBG return to the beginning of the fiber, each one in its specific wavelength range (Chapter 3, page 27).

Figure 1 Diagram of an FBG in an incident-light spectrum (fig. 3.8)

The following equation, known as the classical Bragg grating equation (1), teaches that these types of optical sensors are influenced by temperature and strain variantions:

                                                            

where λB is the Bragg wavelength; while the parameters for a silica fiber with a germanium-doped core are ρe = -0.22, α = 0.55 x 10-6/°C and η = 8.6 x 10-6°C.

Thus, fiber Bragg gratings present inherent advantages in fields such as instrumentation, sensing, and automation systems, playing an important role not only for industry professionals but also for academics.

Conclusions
An FBG is essentially a sensor of temperature or strain, but, by designing the proper interface, many other measurands can be made to impose a perturbation on the grating and produce a shift in the Bragg wavelength, which can then be used as a parameter transducer. Therefore, a FBG used as a sensor can measure strain, force, temperature, pressure, vibration or displacement. These mentioned fields, however, are not intended to exhaust possible applications of FBGs but rather offer a sample of what can be done and developed with these types of filters/sensors (Chapter 2, page 16).

Essencial information concerning FBGs to professionals and researchers with an approach based on rules of thumb, practical aspects, and detailed information about how to fabricate, operate and use these novel sensors can be found in reference [1] (preface).

Marcelo M. Werneck received a BSc in electronic engineering from the Pontifícia Universidade Católica of Rio de Janeiro in 1975 and a MSc from the Biomedical Engineering Program, Federal University of Rio de Janeiro (UFRJ) in 1977. His PhD was obtained from the University of Sussex, Brighton, U.K., in 1985. He has been with UFRJ since 1978, where he is currently a Lecturer and Researcher.

Regina Célia da Silva Barros Allil received her BSc in electronic engineering from Faculdade Nuno Lisboa, Rio de Janeiro, in 1988, and her MSc from the Biomedical Engineering Program, UFRJ, in 2004. Her DSc was obtained from the Electronic Engineering Program, Instrumentation and Photonics Laboratory, UFRJ, in 2010. Since 1985, she has been a researcher with the Institute of Chemical, Biological, Radiological, and Nuclear Defense at the Brazilian Army Technological Center, Rio de Janeiro.

Fábio Vieira Batista de Nazaré received his BSc in electronic engineering from the Universidade Federal de Pernambuco, Recife, Brazil, in 2007, his MSc in electrical engineering from the Institute for Post-Graduate Studies and Research in Engineering, UFRJ, in 2010, where he also received his DSc at the Instrumentation and Photonics Laboratory, Electrical Engineering Program, in 2014. He currently works as an Industrial Property Researcher at the Instituto Nacional da Propriedade Industrial (INPI), Rio de Janeiro.