A short length of optical fiber that filters out a particular wavelength. Periodically spaced zones in the fiber core are altered to have different refractive indexes slightly higher than the core. This structure selectively reflects a very narrow range of wavelengths while transmitting others. Fiber Bragg gratings are used to stabilize the output of a laser and to filter out wavelengths in a WDM system.
The name comes from Bragg's Law, which is used to create the spacing of the changes. Sir William Lawrence Bragg, noted British physicist (1890-1971) discovered this in his study of x-rays and crystal structures. See diffraction grating
A Fiber Bragg Grating
Fiber Bragg gratings such as this one from JDS Uniphase can be customized for a variety of applications covering a range of wavelength bands from 850 to 1650 nm. (Image courtesy of JDS Uniphase.)
The Gratings Reflect
In this example, the red wavelength matches the grating period and is reflected. The blue wavelength is transmitted because it does not match the grating spacings. (Illustration assistance courtesy of Jeff Hecht.)
Since the desired wavelength is reflected back by the grating, another device known as an "optical circulator" detects and switches it. (Illustration courtesy of Jeff Hecht.)