Mode-coupling characteristics of UV-writtenBragg gratings in depressed-cladding fibre

Abstract
Additional grating resonances are observed near the fundamental-mode Bragg wavelength in depressed-cladding fibre. They occur above and below the LP11 cutoff. Images of the reflected and transmitted fields and calculations of the phase-matching conditions show that they are due to coupling between LP01 and bound, or slightly leaky, LP11 modes.