Bragg grating induced cladding mode coupling due to asymmetrical index modulation in depressed cladding fibers

摘要

UV-written Bragg gratings find wide spread use as wavelength selective components. In reflection high extinction ratios are routinely obtained. However, coupling to cladding modes gives excess loss on the short wavelength side of the main reflection. Different fiber-designs have been proposed to reduce this problem. None of these designs seems to give complete solutions. In particular, the otherwise promising depressed cladding design gives a pronounced coupling to one LP/sub 01/ mode, this has been referred to as a Ghost grating. To find the modes of the fiber we have established a numerical mode-solver based on the staircase-approximation method. The Bragg grating causes coupling between the fundamental LP/sub 01/ mode and higher order LP/sub 1p/ modes that satisfy phase-matching. The coupling strength is determined by the overlap integral of the LP/sub 01/, the LP/sub 1p/ mode, and the UV-induced index perturbation. For LP/sub 0n/ the index perturbation is set to one in the core and zero elsewhere. For LP/sub 1n/ it is simplified to the worst case, i.e., opposite sign of the field.