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A simple on-chip integrated structure is proposed to generate optical orbital angular momentum (OAM) beams, where only silicon waveguides and couplers are involved. The operating principle is based on mode converting and proper controlling the phase shift of second-order propagating mode so that OAM modes with charge numbers of 1 or -1 can be selectively generated with mode purity > 90%. The proposed device is very compact with footprint of <; 120 μm × 50 μm and can be easily fabricated with CMOS technology. Thus, such OAM mode generator is a promising candidate for applications ranging from optical tweezers and optical spanners to optical communications.