The influence of a periodical spatial modulation of asurfaceorder parameter on the distribution of the order parameter in a nematic liquid crystal bulk is studied. The problem is treated in the framework of the Landau - de Gennes theory with an anchoring energy in the formFS∼ (S-Q)2, where S is the NLC order parameter and Qis the surface order parameter. It is shown that the order parameter grating, which damps exponentially with the distance to the cell plates, appears in the cell bulk. The damping coefficient is inversely proportional to the coherence length ξ of the nematic-isotropic phase transition. Diffraction efficiency of this grating is proportional to (ξΔQ)2, where ΔQis an amplitude of the order parameter modulation, and is sufficient for experimental observations.