Ultra short two-section vertical directional coupler switches with high extinction ratios

We show that both cross and bar states with high extinction ratios larger t han 30 dB can be achieved at the same end of ultra short fused vertical dir ectional coupler switches (FVCSs) with two sections by changing the refract ive indices of cores and inner cladding layer less than 1%. The improvement of extinction ratios for various combinations of the refractive indices in a two-section FVCS is studied using the improved coupled theory and beam p ropagation method. Design guidelines to achieve high extinction ratios with large tolerances are presented.