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 and WDM.