A Bragg Fiber Grid (FBG) is a type of distributed Bragg reflector built into a short segment of optical fiber that reflects particular wavelengths of light and transmits all the others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a dielectric mirror specific to the wavelength. Therefore, a fiber Bragg grating can be used as an in-line optical filter to block certain wavelengths or as a specific wavelength reflector.
The first Bragg grating in fiber was demonstrated by Ken Hill in 1978. Initially, gratings were made using a visible laser that propagated along the fiber core. In 1989, Gerald Meltz and his colleagues demonstrated the much more flexible transverse holographic inscription technique where laser illumination came from the fiber side. This technique uses the interference pattern of the ultraviolet laser light to create the periodic structure of the fiber Bragg grating.
Fiber Bragg gratings are created by "inscribing" or "writing" a systematic (periodic or aperiodic) variation of the refractive index in the core of a special type of optical fiber using an intense ultraviolet (UV) source such as a laser UV. Two main processes are used: interference and masking. The preferred method depends on the type of grid to be manufactured. Usually a silicon fiber doped with germanium is used in the manufacture of Bragg fiber gratings. The germanium doped fiber is photosensitive, which means that the refractive index of the nucleus changes with exposure to UV light. The amount of change depends on the intensity and duration of exposure, as well as the photosensitivity of the fiber. To write a Bragg high reflectivity fiber grid directly into the fiber, the level of doping with germanium must be high. However, standard fibers can be used if photosensitivity is improved by prior immersion of the fiber into hydrogen. More recently, Bragg fiber gratings have also been written on polymer fibers, this is described in the entry PHOSFOS.
This was the first widely used method for manufacturing Bragg fiber gratings and used two-beam interference. Here the UV laser is divided into two beams that interfere with each other creating a periodic intensity distribution along the interference pattern. The refractive index of the photosensitive fiber changes according to the intensity of light to which it is exposed. This method allows fast and simple changes to the Bragg wavelength, which is directly related to the interference period and a function of the incident angle of the laser light.