| 摘要 |
A grating portion has a plurality of convex portions provided on at least one principal plane of a transparent substrate having a first layer containing a first transparent material and a second layer containing a second transparent material, and extended in parallel to each other in a first direction as well as periodically arranged in a second direction orthogonal to the first direction. A filling portion fills a third layer containing a third transparent material at least between the plurality of convex portions. Assuming that the change rate of the refractive index with the change of temperature of the first transparent material is αA, the change rate of the refractive index with the change of temperature of the second transparent material is αB, and the change rate of the refractive index with the change of temperature of the third transparent material is αC, the relationship αA<αC<0 and |αC|>|αB| is satisfied.
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