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17 February 2009 Sub-wavelength grating gradient index mode transformers in high index contrast slab waveguides
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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.
© (2009) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
P. J. Bock, P. Cheben, J. H. Schmid, D.-X. Xu, S. Janz, and T. J. Hall "Sub-wavelength grating gradient index mode transformers in high index contrast slab waveguides", Proc. SPIE 7220, Silicon Photonics IV, 722005 (17 February 2009);

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