In this paper, we propose a 2x2 optical coupler on a Si photonic platform designed using a covariance matrix adaptation evolution strategy (CMA-ES). The optimized 2x2 coupler has a small footprint (1.5 um x 10 um), low excess loss (less than 0.03 dB), and much better fabrication tolerance and thermal stability than conventional ones. These features make the proposed coupler promising for a building block of large-scale photonic integrated circuits.