A Fiber Bragg Grating (FBG) is a type of Bragg reflector (distributed type), which is constructed as a short segment of an optical fiber to reflect light of a certain wavelength and transmit the rest. Bragg gratings are inscribed into a single mode optical fiber.
A Bragg grating is constructed by the periodic variation of the refractive index of the optical fiber core.
The single mode fiber used to construct FBG’s is made up of a very small inner core (diameter of 4-9µm) and an outer cladding, made up of pure glass. The core of the fiber exhibits a higher refractive index, due to high Germanium doping. Due to the difference in the refractive index of core and cladding, the incident light propagates within the small core.
The fiber is coated with acrylate or ORMOCER. In order to incorporate the grating inside the fiber core, the fiber must be dismantled from the coating and recoated. The recoating must be done thoroughly, to ensure the stability of the fiber.
In some cases the gratings are apodized, meaning the refractive index is graded such that it approaches zero at the end of the grating.
Manufacture of FBGs
The manufacture of the grating may be done by any of the following methods:
- Inteference – Incorporates the two-beam interference method for inscribing the grating
- Sequential writing – Gratings are formed by the sequential exposure of fiber to UV radiation
- Photomasking – This method uses a photomask that bears the intended features of the grating
- Point-by-point – A single UV laser beam is used for inscribing the grating
Some of the applications of FBs are listed below:
- Fiber Bragg Grating sensors
- Fiber lasers
- Optical communication
- Matching of active and passive fibers