FBG (Fiber Bragg Grating) Filters

FBG filters, one kind of applications of fiber Bragg grating, have become very important for commercial applications because of its compactness and low cost.

As the picture shows, it is made by using ultraviolet light with phase mask to write gratings in the photosensitive fiber. When the gratings are formed by the external UV light source, it is a small perturbation of fiber refractive index to occur. This fiber grating will reflect all the light beams have a certain wavelength range if the Bragg condition is met.

At here, the Bragg condition is: 2Λ = λ/n (assign the theta is half of pi) and Λ is the spatial period of grating, λ/n is the wavelength in the propagation medium. For example, we need to reflect light at 1550nm in the opposite direction then we can let Λ be nearly 530nm and consider n to be about 1.46 in a typical single mode fiber.