| 摘要 |
PROBLEM TO BE SOLVED: To obtain a diffraction optical element (BOE) in which a diffraction efficiency is prevented from being lowered by dully forming ridgelines of fine difference in level parts of a cyclic step structure to reduce undesired lights. SOLUTION: Ridgelines 11 of difference in level parts having a height h=λ/(n-1) for every one cycle are sharply formed, however, ridgelines 12 of fine difference in level parts having a height d-h/2M being in one cycle are dully formed to be dully formed with respect to conventional ridgelines 15. Providedλis a wavelength to be used, n is the refractive index of glass and M is the number of sheets of masks to be used in a lithographic process. Then, the phase function of the wave front passed through the BOE approaches the phase function 25 of an ideal wave front as compared with the phase function 24 of the wave front passed through the BOE by dulling ridgelines of the fine difference in level parts of the step structure. Besides, a part in which the phase of the wave front 19 of one part of a wave front 17 is delayed by 2πand a part in which the phase of the wave front 20 of one part of a wave front 18 is advanced by 2πare combined at discontinuity parts 21. Thus, it is required that phase functions for every one cycle are to be given sharply so that phases of different wave fronts are easy to connect.
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