Focusing grating coupler for blue laser light

A focusing grating coupler (FGC) for use with a blue laser of 400-nm wavelength as a light source was fabricated for the first time. The FGC was designed to have a numerical aperture of 0.48 and a minimum period of 0.2 μm when the focal length and the grating area were 900 μm and 1×1 mm
²
, respectively. Grating pattern of minimum line of 0.1 μm was fabricated on a waveguide based on the boron phosphor-silicate glass material by electron-beam lithography process using the vector scan method. The spot size at the full width (1/e
²
) was measured as 0.92 and 0.85μm in x and y directions and was nearly the same as the diffraction limit.