| 摘要 |
PROBLEM TO BE SOLVED: To make it possible to improve the efficiency of light in a backward path without degrading the light utilization efficiency in a forward path by arranging a first substrate forming rugged parts of optically anisotropic diffraction gratings on the side (optical recording medium side) opposite to a light source. SOLUTION: The optically anisotropic diffraction gratings 2 are formed of optically anisotropic materials, such as liquid crystals, held between the first substrate and a second substrate 9. At this time, the substrate 9 provided with the rugged parts is arranged on the light source 1 side. A phase difference plate 3 is thereafter laminated on the outside surface of the substrate 8. In such a case, the ordinary ray refractive indices of the grating part and the liquid crystal part are nearly equal with respect to the P waves (the light of a polarization direction parallel with the plane of Fig.) from the light source 1 and, therefore, the light transmits these parts in the forward path. In the backward path, the polarization direction is changed by the plate 3 and S waves (perpendicular to the plane of Fig.) are made incident so that the rugged parts function as diffraction gratings and the diffraction of the light takes place. In such a case, a significant difference is not admitted even if the gratings are formed at any of the substrates in the forward path, but the higher diffraction efficiency than the diffraction obtainable when the gratings are disposed on the lights source side is obtd, by providing the substrate 8 on the optical recording medium 6 side with the gratings in the backward path where the light diffracts. |
