| 摘要 |
PURPOSE:To reduce mixture of a pertinent signal to a reproduced signal by setting a proportion of the transmitted + or - 1st-order light of the + or - 1st-order reflected light, which is mixed with the transmitted 0th-order light of the 0th- order reflected light made incident on a photodetector, to a prescribed value or smaller. CONSTITUTION:With respect to a phase type diffraction grating 30a, a rectangular grating 32 having a refractive index ng is formed on a surface 30 of the substrate of a luminous flux separating element 40 having a refractive index ns, and the refractive index ng, a thickness (d), and a wavelength lambda of a light source are set to desired values to give a prescribed phase difference to the diffracted light. A phase type diffraction grating 32a is used where a desired effect is attained only by forming the rectangular grating 32 on a plane plate optical element. Thus, the thickness (d) of the rectangular grating 32 is determined when materials of the luminous flux separating element 40 and the angle of inclination of this element itself are set, and the luminous flux separating element 40 is formed in accordance with these values to considerably reduce the intensity of the transmitted + or - 1st-order diffracted light of the reflected light from a disk. |
