Photorefractive intermodal exchangers in optical fiber

Photorefractive intermodal exchangers (PRIME) in optical fiber are optically written refractive index gratings that couple two copropagating modes of the fiber. The author examines the characteristic feature of these devices. He shows how mode coupling automatically occurs at the wavelength λ_irr used to write the refractive index grating, and calculates the bandwidth of mode coupling as a function of the fiber parameters. A discussion is presented to determine under what conditions mode coupling can also be obtained at wavelengths different from λ_irr, and between modes other than those used to write the grating. The refractive index change required for 100% power transfer over a given length is also calculated for different situations