ooves. The reflected spectral components interfere at the grating output and, at certain angles, only one wavelength interferes constructively; all others cancel by destructive interference. This allows the desired wavelength to be selected by placing a spatial filter (essentially a hole) filter at the proper angle. A grating can be also impressed onto a guiding structure, like a fiber or a monolithic waveguide. These gratings can be either transmissive or reflective. 4.4.1.2 Fiber Bragg Gratings A fiber Bragg grating is a periodic or pseudo-periodic perturbation of the effective refrac- tive index in the core of an optical fiber [16]. Typically, the perturbation is approximately periodic over a certain length from a few millimeters to centimeters for long gratings, and the period is on the order of hundreds of nanometers, or much longer for long-period fiber gratings. The refractive index perturbation leads to the reflection of light in a narrow range of wavelengths, for which a Bragg condition is satisfied (4.91) where A is the grating period 2 is the vacuum wavelength Neff is the effective refractive index of light in the fiber Essentially, the condition means that the wave number of the grating matches the differ- ence of the (opposite) wave vectors of the incident and reflected waves. In that case, the complex amplitudes corresponding to reflected field contributions from different parts of the grating are all in phase SO that they can add up constructively. Even a weak modulation index (with an amplitude of, e.g., 10-4) is sufficient for achieving nearly total reflection, if the grating is sufficiently long (e.g., a few millimeters). Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices Light at other wavelengths, not satisfying the Bragg condition, is nearly not affected by the Bragg grating, except for some side lobes that frequently occur in the reflection spec- trum (but can be suppressed by apodization of the grating [47]). The reflection bandwidth of a fiber grating, which