This paper proposes and demonstrates a plane-wave generator which consists of a compact taper array and expands the aperture of the grating waveguide (GWG) of optical leaky wave antenna, resulting in a high antenna gain. The device size is reduced to half of a conventional slow taper without degrading the aperture efficiency. We confirm the well correspondence of the antenna gain between the full numerical simulation and the estimation using measured parameters for the device fabricated by silicon photonics.