Position Dependence of Coupling Efficiency of Grating Coupler in Waveguide Cavity

A grating coupler (GC) in a thin film waveguide is an integrated-optic component for coupling a guided wave to/from a free-space wave via the waveguide surface. A GC in a waveguide cavity resonator can provide high-efficiency vertical coupling with small aperture. The coupling efficiency was discussed with a dependence on a position of the GC in the cavity resonator by using an analytical model based on the coupled mode theory. The maximum efficiency can be obtained by centering the GC in the cavity. The efficiency can be eliminated by shifting the position of the GC from the center by a quarter of the grating period of the GC. GCs with and without the quarter-period shift were designed and fabricated in a Si-N waveguide on a SiO
₂
substrate for operating at 1.55 μm wavelength. The cavity resonator consisted of a pair of distributed Bragg reflectors and was integrated with the GC by the same process simultaneously. The output coupling and its elimination were demonstrated experimentally.