Before reading this you may find the following tutorials useful:
Optical Networks, Optical Fiber, Wavelength Division Multiplexing (WDM)

Have you ever gone to a trendy nightclub and been denied entry? Perhaps it was the duck boots, maybe last week’s bath was beginning to lose effect — or you may just look like a narcotics agent. For whatever reason, you entered the line outside the club, and when you reached the front you were told "You ain’t comin' in" by the bouncer. You could say that you were treated like an optical signal reflected back from a fiber Bragg grating (FBG).

A fiber Bragg grating is a short length of regular optical fiber that has been slightly modified. The fiber core has been exposed to ultraviolet radiation in a regular pattern, which has caused the refractive index of the fiber core to be altered in a regular pattern, too. The result is that light traveling through these refractive index changes is reflected back slightly, but the maximum reflection usually only occurs at one particular wavelength. The reflected wavelength — known as the “Bragg wavelength” — depends on the amount of refractive index change that has been applied and also on how distantly spaced these changes are.

So let's imagine that I am an optical signal at 1550 nanometers, and you are an optical signal at 1560nm. We are two friends, together in a WDM (wavelength-division multiplexing) system, traveling down the same fiber. We arrive at a club, and the bouncer, for some reason, hates you. Hey, sorry, but that's just the way it is. (I think it is those shoes). Anyway, the bouncer denies you entry, and you are then sent back down the fiber whence you came. If I liked you better I would join you. I would join you. But the bouncer likes me. So I pretend I don’t know you and carry on down the fiber into the club. The fiber Bragg grating bouncer completely reflected you at 1560nm, and didn't reflect me at 1550nm.

In practice there is a slight reflection at some other wavelengths too, but the main reflection is at the Bragg wavelength, and the grating can be designed to give almost 100 percent reflection at that color.

These devices may have an important part to play in optical networks of the future. The eventual hope is to incorporate them into things like demultiplexers, where you need to separate individual wavelengths in a system. FBGs are already used in some advanced amplifier designs where they play the role of a filter in order to “flatten” the amplification so that signals at different wavelengths are amplified by equal amounts. For this application, fairly complex arrangements of different fiber gratings can be joined together to reflect just a small percentage of certain wavelengths, in order to reduce the signals to match the intensity of others.

Key Points

Further Reading

Optical Crossconnects, Arrayed Waveguide Gratings (AWGs)