Objective lens for optical pickup device, and optical pickup device

An objective lens used in an optical pickup device, the optical pickup device comprising a first light source which emits a first light flux at a first wavelength λ1 (nm), a second light source which emits a second light flux at a second wavelength λ2 (nm) wherein λ2>λ1, and a third light source which emits a third light flux at a third wavelength λ3 (nm) wherein λ3>λ2 for recording and/or reproducing information of a first optical disc comprising a protective substrate with a thickness of t1 by using the first light flux, recording and/or reproducing information of a second optical disc comprising a protective substrate with a thickness of t2 wherein t1<t2 by using the second light flux, and recording and/or reproducing information of a third optical disc comprising a protective substrate with a thickness of t3 wherein t2<t3 by using the third light flux, and the objective lens comprising an optical surface:

• wherein the objective lens is a single lens,
• the optical surface comprises at least a central region, an intermediate region around the central region, and a peripheral region around the intermediate region,
• the objective lens converges the first light flux, which passes through the central region, on an information recording surface of the first optical disc such that the information is recorded and/or reproduced, converges the second light flux, which passes through the central region, on an information recording surface of the second optical disc such that the information is recorded and/or reproduced, and converges the third light flux, which passes through the central region, on an information recording surface of the third optical disc such that the information is recorded and/or reproduced,
• the objective lens converges the first light flux, which passes through the intermediate region, on the information recording surface of the first optical disc such that the information is recorded and/or reproduced, converges the second light flux, which passes through the intermediate region, on the information recording surface of the second optical disc such that the information is recorded and/or reproduced, and does not converge the third light flux, which passes through the intermediate region, on the information recording surface of the third optical disc such that the information is recorded and/or reproduced,
• the objective lens converges the first light flux, which passes through the peripheral region, on the information recording surface of the first optical disc such that the information is recorded and/or reproduced, does not converge the second light flux, which passes through the peripheral region, on the information recording surface of the second optical disc such that the information is recorded and/or reproduced, and does not converge the third light flux, which passes through the peripheral region on the information recording surface of the third optical disc such that the information is recorded and/or reproduced,
• the central region comprises a first optical path difference providing structure obtained by superimposing a first base structure, which is a blazed structure, and a second base structure, which is the blazed structure, on each other,
• the intermediate region comprises a second optical path difference providing structure obtained by superimposing a third base structure, which is the blazed structure, and a fourth base structure, which is the blazed structure, on each other,
• the first base structure makes an amount of A-th order diffracted light of the first light flux, which passes through the first base structure, larger than an amount of diffracted light of any other order, makes an amount of B-th order diffracted light of the second light flux, which passes through the first base structure, larger than the amount of the diffracted light of any other order, and makes an amount of C-th order diffracted light of the third light flux, which passes through the first base structure, larger than the amount of the diffracted light of any other order,
• the second base structure makes an amount of D-th order diffracted light of the first light flux, which passes through the second base structure, larger than the amount of the diffracted light of any other order, makes an amount of E-th order diffracted light of the second light flux, which passes through the second base structure, larger than the amount of the diffracted light of any other order, and makes an amount of F-th order diffracted light of the third light flux, which passes through the second base structure, larger than the amount of the diffracted light of any other order,
• the third base structure makes the amount of A-th order diffracted light of the first light flux, which passes through the third base structure, larger than the amount of the diffracted light of any other order, makes the amount of B-th order diffracted light of the second light flux, which passes through the third base structure, larger than the amount of the diffracted light of any other order, and makes the amount of C-th order diffracted light of the third light flux, which passes through the third base structure, larger than the amount of the diffracted light of any other order,
• the fourth base structure makes the amount of D-th order diffracted light of the first light flux, which passes through the fourth base structure, larger than the amount of the diffracted light of any other order, makes the amount of E-th order diffracted light of the second light flux, which passes through the fourth base structure, larger than the amount of the diffracted light of any other order, and makes the amount of F-th order diffracted light of the third light flux, which passes through the fourth base structure, larger than the amount of the diffracted light of any other order,
• A, B, C, D, E, and F satisfy
• |A|=1,
• |B|=1,
• |C|=1,
• |D|=2,
• |E|=1, and
• |F|=1, respectively, and
• a pitch of each of the first base structure, the second base structure, the third base structure, and the fourth base structure is with a positive sign when a step of the base structure faces opposite to an optical axis and with a negative sign when the step of the base structure faces to the optical axis,
• there is a boundary between the central region and the intermediate region,
• a pitch P2 of the second base structure in a position closest to the boundary and a pitch P4 of the fourth base structure in a position closest to the boundary satisfy a following equation (1) in consideration of the sign of the pitch:
• P4−P2<0 (1)
• where,
• pitch P(h)=λB/(Σ(2i×C2i ×h2i−1)) is satisfied when an optical path difference function, which defines the base structure, is represented by Φ(h)=Σ(C2ih2i ×λ×m/λB), where λ represents a wavelength used, m represents a diffraction order, λB represents a manufacturing wavelength, and h represents a distance in a direction perpendicular to the optical axis from the optical axis.