Optical head employing a concentric-circular focusing grating coupler.

An optical head employing a concentric-circular grating coupler (CGC) and a concentric-circular focusing grating coupler (CFGC) is proposed, and its operating principle and characteristics are reported. Satisfaction with a prerequisite for the head, i.e., the removal of aberrations caused by deviations in wavelength and the effective index, is theoretically achieved by application of the concept of optimization of an annular aperture. With CGC and CFGC fabricated by an electron-beam-writing method, we experimentally confirmed its fundamental characteristics of light input, waveguiding, output, and convergence, with an elliptical focusing spot converging at half-intensity widths of 1.8 and 4.0µm.