Fabrication of a concentric-circular focusing grating coupler by a conic-wave-front interference method and light-convergence experiments using the coupler.

An interference method utilizing conic-wave-front light for the fabrication of a concentric-circular and chirped grating is proposed. The design method and fabrication of an interference lens that generates conic-wave-front light are also shown. A focusing element is constructed from a concentric-circular grating coupler with a 0.4-mm diameter and a concentric-circular focusing grating coupler with an annular aperture of 2-mm focal length and 2.0-4.0-mm diameter. Light-convergence experiments using the focusing element were able to obtain a focusing spot of 0.5 µm × 0.7 µm at half-intensity widths for a wavelength of 820 nm in combination with liquid-crystal polarization elements.