Fabrication of aberration-free focusing grating couplers

An almost aberration-free focusing grating coupler with a grating area of 1 x 1 mm(2) and a 2-mm focal length was made by using an electron beam writing system with accurately calibrated electron beam deflection and an accurate parameter setting of the effective refractive index of the waveguide. The smallest focused spot size achieved was 1.2 microm (FWHM), which is 1.06 times the value of the diffraction-limited spot size.