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. Satisfa ction with a prerequisite for the head, i.e., the removal of aberratio ns caused by deviations in wavelength and the effective index, is theo retically achieved by application of the concept of optimization of an annular aperture. With CGC and CFGC fabricated by an electron-beam-wr iting method, we experimentally confirmed its fundamental characterist ics of light input, waveguiding, output, and convergence, with an elli ptical focusing spot converging at half-intensity widths of 1.8 and 4. 0 mum.