Patent ID: 8483023

Claim:
An objective lens comprising: a diffraction structure configured to diffract a laser beam emitted from a light source so as to generate diffraction lights of a plurality of orders, focus a zero-order diffraction light generated by the diffraction structure onto an information recording surface of a first information recording medium, focus a plus-first-order diffraction light onto an information recording surface of a second information recording medium, and focus a minus-first-order diffraction light onto a position other than a surface of the first information recording medium, wherein an optical path of the laser beam before being reflected on the information recording medium is an advancing path, an optical path of the laser beam after being reflected on the information recording medium is a returning path, a position on an optical axis of a condensed point in which a diffraction light with the zero-order diffraction light in the advancing path and the zero-order diffraction light in the returning path, reflected on the information recording surface of the first information recording medium, is focused at the condensed point by a condensing lens for focusing the laser beam reflected on the information recording surface of the first information recording medium and is different from a position on the optical axis of the condensed point in which a diffraction light with the zero-order diffraction light in one path either the advancing path or the returning path and the minus-first-order diffraction light in the other path, reflected on the surface of the first information recording medium, is focused at the condensed point by the condensing lens, and wherein a protective material design thickness of the information recording surface of the first information recording medium is t 1 , a protective material design thickness of the information recording surface of the second information recording medium is t 2 , a refractivity of the protective material of the first and second information recording medium is n, a first protective material thickness of the information recording surface of the first information recording medium is t 1 _L 1 , a second protective material thickness of the information recording surface of the first information recording medium is t 1 _L 2 , a working distance for write or read of the first information recording medium is WD 1 , a working distance for write or read of the second information recording medium is WD 2 , and a focal position interval difference between a plus-first-order diffraction light and the minus-first-order diffraction light is k, then WD 1 −WD 2≠( t 2 −t 1−2 ×t 1 — L 1)/ n−k and WD 1 −WD 2≠( t 2 −t 1−2 ×t 1 — L 2)/ n−k are satisfied.