Interestingly, there are two Braggs, father (W.H) and son (W.L.), for whom the Bragg
Grating is named. Actually, the grating is named after the
Bragg equation, developed in 1912, that describes the
reflection of light in a
three-dimensional grating:
nλ=2dsinΘ
Where d is the distance between parallel planes of crystal and Θ is the glancing angle.
What this basically means is that a Bragg Grating is a lattice of crystals arranged in a way that they filter light by internal reflection. In other words, you can arrange a grid of crystals (or other highly-structured material) so that light going into it will bounce around in such a way that only the light with the wavelength you want will come out of it.
An excellent example of this occurs in nature, visible in the colors of a butterfly's wing. They don't have any pigment in them, as their color comes from the way light is selectively absorbed and reflected by microscopic structures on their surfaces. Another example is the colors you see on the surface of a soap bubble.
A Bragg Grating is extremely useful, as it allows one to filter light without using pigment-based filters. Applications range from filters for fiber-optic communications, where they can be created in the fiber itself, to the recent experimentation in iridescent display technology, where each pixel in the display is a tunable Bragg Grating, reflecting the color desired to create the image.