We report on new types of sub-wavelength grating gradient index structures for efficient mode coupling in high index contrast slab waveguides. Using sub-wavelength gratings (SWGs), an adiabatic transition can be formed at the interface between slab waveguides of different core thicknesses. The SWG transition region minimizes both mode mismatch loss and coupling to higher order modes. By creating the adiabatic gradient effective index region in the direction of propagation, we demonstrate that vertical mode size transformation is readily achieved in structures that can be fabricated using a single etch step. Using 3D finite-difference time-domain simulations we study the loss, polarization dependence and the higher order mode excitation for two types (triangular and triangular-transverse) of SWG transition regions between silicon-on-insulator slab waveguides of different core thicknesses. We also demonstrate two solutions to mitigate polarization dependent loss, namely using a partial transverse SWG and a SWG in the region between the triangular teeth. Our mode transformer designs are optimized for applications in polarization compensator in echelle grating and arrayed waveguide grating (AWG) multiplexers and a gradient index interface between the ridge waveguide and the slab combiner region in a curved waveguide grating demultiplexer.

Date Published: 18 February 2009
PDF: 7 pages
Proc. SPIE 7220, Silicon Photonics IV, 722005 (18 February 2009); doi: 10.1117/12.809736

Published in SPIE Proceedings Vol. 7220:
Silicon Photonics IV
Joel A. Kubby; Graham T. Reed, Editor(s)