The coupled field expansion method (CFEM) has been proposed, and validated recently by applying to frequency selective surfaces (FSS) and printed artificial magnetic conductor (AMC) surfaces. In this paper, we demonstrate another novel application of the CFEM - designing a 2D FSS to behave as a circular polarizer as well as a reflector over a wide operating frequency band. The polarizer transforms a linearly polarized incident wave into a circularly polarized reflected wave. For one example design at 11.8 GHz, its 3 dB axial ratio (AR) bandwidth is 43% and 1 dB AR bandwidth is 33%. It can be compactly printed on a lightweight substrate like FR4. To gain insight into the features and properties of the polarizer, the results of a comprehensive parametric study are discussed.