This paper describes the optical design of a new type surface encoder, which consists of a 2D angle grid and a 2D slope sensor for 2D position detection. The sensitivity of the new type surface encoder is two times higher than that of a conventional surface encoder through employing a double pass. A wave optical model of the surface encoder is established for the optical design. Simulations based on the wave optical model are carried out to analyze the behavior of the diffraction pattern on the photo-detector of the slope sensor. Basic experiments are also carried out to confirm the
feasibility of the double pass surface encoder.