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 $0.2\mathrm{dB}$ and crosstalk and reflection losses below $40\mathrm{dB}$ in a broad bandwidth of $20\mathrm{nm}$ . Furthermore, the resulting crossing structure has a footprint of only $6\times 6\mu {\mathrm{m}}^2$ and does not require any additional fabrication steps.