Optimization of finite-length input volume holographic grating couplers illuminated by finite-width incident beams.

A finite volume holographic grating coupler (VHGC) normally illuminated with various incident-beam profiles (such as a Gaussian beam, a flat cosine-squared beam, and an exponential-decay beam) with finite beam widths for input coupling is rigorously analyzed by use of the finite-difference frequency-domain method. The effects of the incident-beam width, the incident-beam position, the incident-beam profile, and the incident-beam angle of incidence on the input coupling efficiency are investigated. The optimum conditions for input coupling are determined. Both a VHGC embedded in the waveguide film region and a VHGC placed in the waveguide cover region are investigated. For a given finite VHGC, the input coupling efficiencies are strongly dependent on incident-beam widths, incident-beam positions, and incident-beam angles of incidence, but are only weakly dependent on incident-beam profiles.