The photorefractive interaction of collinear TE and TM leaky modes in a structure “lithium niobate planar waveguide - prism coupling element” is experimentally investigated and discussed. At light propagation along X axis within Y cut waveguide formed in Cu - doped crystal plate the photorefractive gratings were produced due to the photogalvanic current component orthogonal to the grating wave vector and the waveguide plane.