It is proved that there exists π/2 phase difference between the transmitted beam and the diffracted (or reflected) beam of one incident wave when they leave a thick grating (or another beam splitter). It is the phase difference that makes energy coupling between input-beam possible when there is the phase shift between phase grating and the inter ference pattern of the incident wave in the photorefractive crystal, and the diffraction efficiency of phase grating and modulation amplitude of photorefractive index are calculated based on the theory.