Highly-efficient apodized grating coupler in visible spectrum for backward coupling

A novel optimizing algorithm for designing apodized grating coupler for coupling $-1^{\mathrm{st}}$ order to waveguide using ZEP/SiO2 platform is described. The grating is optimized to achieve maximum coupling efficiency at $\lambda=532$ nm(green) and $\lambda=632$ nm(red) simultaneously.