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A compact waveguide crossing structure with low transmission losses and negligible crosstalk is demonstrated for silicon-on-insulator circuits. The crossing structure is based on a mode expander optimized by means of a genetic algorithm leading to transmission losses lower than 0.2 dB and crosstalk and reflection losses below 40 dB in a broad bandwidth of 20 nm. Furthermore, the resulting crossing structure has a footprint of only 6×6 μm^2 and does not require any additional fabrication steps.