Design of a high-efficiency volume grating coupler for line focusing

A volume grating for outcoupling and line focusing of waveguided infrared light is designed and optimized. A local grating vector approach is used in combination with the rigorous coupled-wave analysis. By design, this volume grating coupler is holographically constructed on top of a waveguide by the interference of two coherent 364-nm ultraviolet waves formed with two aberration-optimized cylindrical lenses. This focusing coupler exhibits preferential-order coupling (92.9%) into the cover as well as very low focal intensity side lobes. This is accomplished through a chirped, slanted-fringe volume grating with a designed spatial variation in the attenuation coefficient (describing the outcoupling of the guided mode) along the length of the grating. This is achieved by a specific variation in the grating slant angle along the grating length. By design, the 1000-mum-length coupler focuses an 850-nm infrared guided wave to a line with an intensity FWHM of 3.32 mum and a 90% power width of 5.53 mum at a focal distance of 4 mm directly above the grating. Its performance is compared with that of a corresponding electron-beam-written surface-relief coupler design.