Gratings should be at least as large as the incident light cone or beam, or else light from the edges will be lost. A set of reftectors like this is called a grating reftector and can be produced in an optical fiber by imposing a variation in the refractive index of the core periodically along the fiber axis. This can be achieved by making use of fiber photosensitivity. Typically, the perturbation is approximately periodic over a certain length of e. This versatility has stimulated a number of signifi rent back reflection). Reflectivities ap-proaching 100% are possible, with the.