| 摘要 |
PURPOSE:To stably detect a signal by using a Fresnel zone plate with a long focus to a diffracting element, and performing differential detection in a far field. CONSTITUTION:An outside-axis two-focus diffracting element 6 is interposed between lenses 2 and 3, and in an out going path, its zero-order transmitted light is converged on a disk 4. A beam reflected on the disk 4 passes through the lens 3 again on its returning path, it is made into the approximately parallel light, thereafter, made incident on the diffracting element 6, generates two wave surfaces 61 and 62 with different focuses outside the axes in addition to the zero-order transmitted light, made convergent by the collimator lens 2, includes the convergent point of the zero-order transmitted light, and respectively focuses on the two surfaces of the position before and after a vertical surface 111 to the optical axis of the lens 2. Further, it is a detector to receive the light, and consists of detectors 51 and 52. In addition, the two wave surfaces generated by the diffracting element 6 is used for a light control, and the far field pattern of the spot converged on the optical disk 4 is differential-detected by an optical detector. Thus, the signal can be stably detected, and the device can be miniaturized and is reduced in weight. |
