| 摘要 |
A diffractive optical element includes a plurality of diffraction gratings. A diffraction grating within the plurality of diffraction gratings includes a plurality of grating parts having a curved grating surface and grating tips which connect to define a curved envelope face. The grating surface has a radius of curvature larger than a radius of curvature of the envelope face. In addition, two diffraction gratings within the plurality of diffraction gratings are composed of different materials having different dispersions. Therefore, even when the radius of curvature of the envelope face is small, the degradation of the diffraction efficiency is suppressed.
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