Investigation of influence of grating shapes on diffracted beam profiles

Abstract

The influence of grating shapes on the diffracted and the transmitted beam profiles for right-angle guided-wave diffraction was investigated both numerically and experimentally. Numerically, by using the first order 2-D coupled mode equations and the finite-difference method, the diffracted and the transmitted beam distributions from corrugated surface-relief gratings with different shapes were investigated both for weak and strong index modulations. Experimentally, different shaped photoresist gratings were fabricated on BK7 ion-exchanged single-mode glass waveguides, which were then covered with a layer of Nile-blue-dye-doped PMMA. The fluorescent beam profile revealed by dye:PMMA was recorded by an optical multichannel analyzer. Both numerical and experimental results show that square or rectangular gratings, which result in a distorted profile for the diffracted beam, are not suitable for right-angle diffraction and that a truncated shape can improve both diffracted and transmitted beam profiles significantly. About 50% diffraction effciency was achieved by using a shaped grating 300 μm wide.