We study director structures of electrically-controlled cholesteric diffraction gratings (CDG's). The cholesteric liquid crystal (LC) is confined between two transparent non-patterned electrodes with either planar or homeotropic anchoring. In cells with planar alignment, the applied electric field causes reorientation of the initial (zero held) planar state and creates one-dimensionally modulated structures in the plane of the cell. The modulations occur via two distinct scenarios: (I) nucleation and expansion of 'stripes' and (II) undulation of quasinematic layers in the plane normal to the cell's plates. In the cells with homeotropic surface alignment, modulated structures exist without applied voltage. Magnetic field is used to provide uniform orientation of these structures. Confocal microscope studies and computer simulations are carried out to reveal the fine structure of the CDG's.