DIFFRACTION ELEMENT, OBJECTIVE LENS UNIT, OPTICAL PICKUP, AND OPTICAL DISK DEVICE

摘要

PROBLEM TO BE SOLVED: To hardly change a diffraction efficiency EF regardless of usage temperature.

SOLUTION: In the diffraction element, which is composed of a base layer 20C having a diffraction pattern PTc for a CD and a cover layer 20D covering the base layer 20C, a cover material RA and a base material RB are selected so that a change amount of the refractive index nA of the cover material RA and a change amount of the refractive index nB of the base material RB when temperature is changed from -40[°C] to 75[°C] are nearly equal to each other.

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