Introduction of diffractive optical elements (DOEs) opened the possibilty to control amplitude-phase distribution in the cross-section of laser beam. In fact, by use of DOE one can form the beam with pregiven behaviour during propagation through waveguide medium. The diffractive microrelief can be realized either on the separated substrate or directly on the waveguide surface. This work is devoted to the theoretical and experimental investigation of diffractive microrelief intended for waveguiding beam control. Experimental results of step-like optical waveguide mode excitation and selection by DOE are presented. Different approaches for synthesis of high-efficient DOEs matched with waveguide modes are discussed. The strategy of the search of waveguide mode with amplitude distribution closed to the illuminating beam amplitude distribution is presented. The phase of found mode can be chosen as DOE phase function.