 It is an optical fiber structure with the properties periodically varying along the fiber, such that the conditions for the interaction of several copropagating modes are satisfied.The period of such a structure is of the order of a fraction of a millimeter.In contrast to the fiber Bragg gratings, LPFGs couple copropagating modes with close propagation constants; therefore, the period of such a grating can considerably exceed the wavelength of radiation propagating in the fiber.