A new integrated-optic coupler was proposed for coupling a guided wave to a free-space wave propagating vertically from the waveguide surface. The coupler consists of a grating coupler in a cavity formed by two distributed Bragg reflectors, and has a small aperture. The cavity was designed to eliminate both transmission and reflection of the incident guided wave, resulting in 100 % radiation by a several-micron aperture. Design consideration was theoretically discussed based on the coupled mode analysis. Predicted performance was simulated and confirmed by the finite difference time domain method.