Simulations revealed that the bending of the waveguide causes a redistribution of the energy within the core. Small radii push the main energy to the outer region of the core increasing the coupling efficiency. On the other hand, at excessive lowered bend radii additional losses occur (due to a coupling into the cladding), which is why an optimum has to be found. Based on the simulation results it is possible to derive requirements and design rules for the coupling element.
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Lukas Lorenz, Krzysztof Nieweglowski, Klaus-Jürgen Wolter, Karlheinz Bock, "Simulation of bended planar waveguides for optical bus-couplers," Proc. SPIE 9889, Optical Modelling and Design IV, 988902 (27 April 2016);