Polarization Characteristics of a silicon-on-insulator rib waveguide Directional Coupler

The polarization characteristics of a silicon-on-insulator (SOI) rib waveguide directional coupler are studied. The fundamental TE and TM-polarized modes are calculated by a semivectorial beam propagation method (BPM). The phase difference (Phi) between the coupling coefficients of TE and TM modes depends on the rib parameters of the SOI waveguide and also the gap (G) between the waveguides. The variation of the coupling lengths Ls (splitting lengths) with the reduction of rib width W, slab height H, the normalized slab height r, and G is shown. The simulated result shows the remarkable decrease of the value of Ls when H is decreased and also with the lower values of r and G. The negative and positive values of the phase difference Phi between the coupling coefficients of TE and TM modes depend appreciably on G with the fixed values of R = W/H. It shows that under certain condition of rib parameters, Phi becomes zero or very small to obtain a nonbirefringent SOI rib waveguide coupler. The slope of the curves decrease when G increases up to a certain critical value of Go. The results also show that for each value of R there is a value of G = Go, when we get the nonbirefringent SOI rib waveguide coupler with larger fabrication tolerance. The effect of extinction ratios with the variation of these parameters, and their improvements with the decrease of the normalized slab height are discussed. The fabrication tolerance of the splitters is also discussed.