Fiber Bragg Grating

"FBG is a simple device used for the separation of wavelength from a group of wavelengths."  Rayc


A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength specific dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.

The first in-fiber Bragg grating was demonstrated by Ken Hill in 1978. Initially, the gratings were fabricated using a visible laser propagating along the fiber core. In 1989, Gerald Meltz and colleagues demonstrated the much more flexible transverse holographic inscription technique where the laser illumination came from the side of the fiber. This technique uses the interference pattern of ultraviolet laser light to create the periodic structure of the fiber Bragg grating.

Fiber Bragg gratings are created by "inscribing" or "writing" systematic (periodic or aperiodic) variation of refractive index into the core of a special type of optical fiber using an intense ultraviolet(UV) source such as a UV laser. Two main processes are used: interference and masking. The method that is preferable depends on the type of grating to be manufactured. Normally agermanium-doped silica fiber is used in the manufacture of fiber Bragg gratings. The germanium-doped fiber is photosensitive, which means that the refractive index of the core changes with exposure to UV light. The amount of the change depends on the intensity and duration of the exposure as well as the photosensitivity of the fibre. To write a high reflectivity fiber Bragg grating directly in the fiber the level of doping with germanium needs to be high. However, standard fibers can be used if the photosensitivity is enhanced by pre-soaking the fiber in hydrogen. More recently, fiber Bragg gratings have also been written in polymer fibers, this is described in the PHOSFOS entry

Fiber bragg grating can easily designed by using a simple fiber in presence of the interference of two ultravoilet light beam. Which give constructive and destructive interferences. At constructive interference refractive index decrease and at destructive interference refractive index remains same so we get two different refractive indices. Which periodically makes FBG.

FBG  are of two types:

1. Simple FBG 

2. Chirped FBG

 Simple FBG  in this refractive index of the dense medium is fixed. In this reflected light is the function of length of a period of refractive index.

 Chirped FBG in this refractive index of the dense medium is getting denser.In this refracted light is combination of different wavelengths are refracted at different time intervals.

Fiber Bragg Gratings are made by laterally exposing the core of a single-mode fiber to a periodic pattern of intense ultraviolet light. The exposure produces a permanent increase in the refractive index of the fiber's core, creating a fixed index modulation according to the exposure pattern. This fixed index modulation is called a grating.

At each periodic refraction change a small amount of light is reflected. All the reflected light signals combine coherently to one large reflection at a particular wavelength when the grating period is approximately half the input light's wavelength. This is referred to as the Bragg condition, and the wavelength at which this reflection occurs is called the Bragg wavelength. Light signals at wavelengths other than the Bragg wavelength, which are not phase matched, are essentially transparent.

Therefore, light propagates through the grating with negligible attenuation or signal variation. Only those wavelengths that satisfy the Bragg condition are affected and strongly back-reflected. The ability to accurately preset and maintain the grating wavelength is a fundamental feature and advantage of fiber Bragg gratings.

The central wavelength of the reflected component satisfies the Bragg relation: λrefl=2nΛ, with n the index of refraction and Λ the period of the index of refraction variation of the FBG. Due to the temperature and strain dependence of the parameters n and Λ, the wavelength of the reflected component will also change as function of temperature and/or strain, see Figure 2. This dependency is well known what allows determining the temperature or strain from the reflected FBG wavelength.

There have been many advances in the methods used to perform strain measurements. The three most prevalent technologies today are electrical foil gages, electrical vibrating wire, and fiber Bragg grating (FBG) optical sensors. 

For most standard strain sensing applications, electrical sensing has been and will continue to be the best and most effective solution. However, optical sensors can offer an important alternative in traditionally challenging applications. Harsh environments, distributed systems, or long-term deployments are a few examples where the characteristics of an optical sensing system can make it a more effective solution compared to conventional electrical sensors. 

Applications often require measurement solutions that encompass the benefits and attributes of multiple sensing technologies. It is therefore important to consider a hybrid approach. This paper provides a brief overview of each technology and weighs their benefits and drawbacks.

Electrical Sensing: Metal Foil Gages

Foil strain gages use the relationship between electrical resistance and conductor length to measure changes in strain. As the foil is stretched, its length is increased, which translates into a minute increase in resistance. To accurately measure these small changes in resistance, additional signal conditioning is necessary, often in the form of a Wheatstone bridge resistance network. A constant voltage is applied across the resistance network, and the varying proportional drop in voltage across the foil can be translated to strain.