| 摘要 |
PURPOSE:To form a good convergence of a beam onto a recording medium by providing two regions which have the shape having a slope to set a blaze angle and vary in grating pitch from each other and combining these regions in such a manner that the zero order diffraction efficiencies and 1st order diffraction efficiencies of the respective regions are nearly equaled to each other. CONSTITUTION:The optical diffraction grating element 2 has a 1st region 2a and 2nd region 2b. The grating pitch d1 in the 1st region 2a is set at (2.2+ or -0.2)lambda with respect to the wavelength lambda of incident light and the average grating pitch d2 in the 2nd region 2b is set at (2.8+ or -0.2)lambda. The depth of respective valley parts and blaze angle theta are set the same between these two regions and the sectional shape of the valley parts is so formed that the angle in the bottom of the valley parts is perpendicular. Further, the ranges where the depth of the valley parts generate 0.7 to 0.9 pi optical phase difference among the ranges of 30 to 45 deg. Blaze angles are set at such a combination with which the zero order diffraction efficiencies and 1st order diffraction efficiencies of the 1st region 2a and the 2nd region 2b are nearly equaled to each other. The above- mentioned ranges are so set that the incident light is diffracted in different directions by the 1st region and the 2nd region. |
