Grating Coupler Design for Reduced Back-Reflections

A grating coupler having asymmetric grating trenches for low back reflections is experimentally demonstrated. Conventional and asymmetric-trench grating couplers have been fabricated on a silicon nitride waveguide platform. Both grating couplers have fully etched trenches, which normally result in higher back reflections than shallow-etched trenches. For evaluating the back reflection characteristics, test structures based on a 3-dB multimode interference power splitter have been measured and the backreflection has been extracted from each grating coupler using an equivalent optical circuit. The designed grating coupler has no critical penalty (<;0.2 dB) in coupling efficiency and ~5 dB lower back reflections than a conventional grating coupler design. Using ray transfer matrix modeling, further improvements to the back reflection characteristics of the asymmetric grating coupler are expected.