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We demonstrate a method based on a unique sinusoidally shaped phased grating for efficient and nearly alignment free detection of both signs and modulus of the orbital angular momentum (OAM) of light. OAM detection efficiency is almost the same over the whole grating area. The capability and robustness of this method are demonstrated by detection of optical vortices with OAM topological charge up to 150 using a reflective phase-only liquid crystal on a silicon spatial light modulator.

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