Light converging optical system, optical pickup apparatus, aberration correctng element and objective lens

A light-converging optical system for recording and/or reproducing for an optical information recording medium to converge a light flux emitted from a light source onto an information recording plane of the optical information recording medium, comprising:

• an objective lens arranged at a position to be opposite to the optical information recording medium and including at least one plastic lens, and
• an aberration correcting element provided on an optical path between the light source and the objective lens and structured by two groups of a positive lens and a negative lens in which one of the positive lens and the negative lens is a plastic lens and the other one is a glass lens;
• wherein a total paraxial power of the aberration correcting element has a temperature dependency satisfying a formula of:
• PT1<PT0<PT2, or PT1>PT0>PT2,
• where PT0 is the total paraxial power (mm−1) of the aberration correcting element under the temperature of 25° C.,
• PT1 is the total paraxial power (mm−1) of the aberration correcting element under the temperature of −5° C., and
• PT2 is the total paraxial power (mm−1) of the aberration correcting element under the temperature of 55° C.

A light-converging optical system for converging a light flux emitted from a light source on an information recording plane of a optical information recording medium for recording and/or reproducing for the optical information recording medium, comprising:

• an objective lens arranged at a position to be opposite to the optical information recording medium and including at least one plastic lens, and
• an aberration correcting element provided on an optical path between the light source and the objective lens and structured by two groups of a positive lens and a negative lens both being a plastic lens;
• wherein at least one of the positive lens and the negative lens has at least one diffractive surface on which a diffractive structure is formed and the following formulas are satisfied:
• PD>0Δ3SAOBJ/ΔTOBJ<0Pλ1<Pλ0<Pλ2,
• where PD is a total sum (mm−1) of diffractive powers defined by a formula of (PD=Σ(−2·n·b2)) when an optical path difference provided to a wavefront having passed through the diffractive structure is represented by a formula of (Φb=n·(b2·h2+b4·h4+b6·h6+ . . . ))(here, n is a diffraction order of a diffracted ray having a maximum light amount among diffracted rays generated by the diffractive structure, h represents a height from an optical axis (mm), and b2, b4 and b6 . . . are respectively 2nd, 4th and 6th . . . order optical path difference function coefficients (or called diffractive surface coefficients),
• ΔTOBJ represents an fluctuation amount of the ambient temperature of the objective lens,
• Δ3SAOBJ represents a change amount of the third order spherical aberration component in the case of developing a residual aberration of the objective lens into a Zernike's polynomial expression when a refractive index of at least one plastic lens among plastic lenses included in the objective lens fluctuates due to a fluctuation ΔTOBJ of the ambient temperature of the objective lens, wherein the sign of the third order spherical aberration component is plus (+) when the third order spherical aberration component is over-corrected and is minus (−) when the third order spherical aberration component is under-corrected,
• Pλ0 is the total paraxial power (mm−1) of the aberration correcting element with a wavelength of a light flux emitted from the light source,
• Pλ1 is the total paraxial power (mm−1) of the aberration correcting element with a wavelength shorter by 10 nm than the wavelength of the light flux emitted from the light source, and
• Pλ2 is the total paraxial power (mm−1) of the aberration correcting element with a wavelength longer by 10 nm than the wavelength of the light flux emitted from the light source.

A light-converging optical system for converging a light flux emitted from a light source on an information recording plane of a optical information recording medium for recording and/or reproducing for the optical information recording medium, comprising:

• an objective lens arranged at a position to be opposite to the optical information recording medium and including at least one plastic lens, and
• an aberration correcting element provided on an optical path between the light source and the objective lens and structured by two groups of a positive lens and a negative lens at least one of which is a plastic lens, wherein the lens being the plastic lens among the positive lens and the negative lens has at least one diffractive surface on which a diffractive structure is formed;
• wherein the aberration correcting element corrects a change of the spherical aberration of the objective lens caused by a fluctuation of the refractive index of at least one plastic lens among plastic lenses included in the objective lens due to a fluctuation of the ambient temperature of the light-converging optical system with a change of the degree of divergence of a light flux emitted from the aberration correcting element caused by the fluctuation of the refractive index of the plastic lens of the aberration correcting element and the following formula is satisfied:
• Pλ1<Pλ0<Pλ2,
• where Pλ0 is the total paraxial power (mm−1) of the aberration correcting element with a wavelength of a light flux emitted from the light source,
• Pλ1 is the total paraxial power (mm−1) of the aberration correcting element for with a wavelength shorter by 10 nm than the wavelength of the light flux emitted from the light source, and
• Pλ2 is the total paraxial power (mm−1) of the aberration correcting element with a wavelength longer by 10 nm than the wavelength of the light flux emitted from the light source.

An aberration correcting element provided on an optical path between a light source and an objective lens which has a structure of two elements in two groups including a first lens having a positive refractive power and a second lens having a positive refractive power arranged in this order from a light source, wherein at least one lens including the first lens is a plastic lens, comprising:

• a positive lens and a negative lens both being a plastic lens and at least one of the positive lens and the negative lens has at least one diffractive surface on which a diffractive structure is formed,
• wherein the following formulas are satisfied:
• Δθ/ΔTAC<0Pλ1<Pλ0<Pλ2,
• where Δθ is a change amount of an inclination angle of an upper marginal ray of a light flux emitted from the aberration correcting element when the ambient temperature of the aberration correcting element is fluctuated by ΔTAC, where the sign is plus (+) when Δθ changes in a clockwise direction on a basis of an optical axis and is minus (−) when Δθ changes in an anti-clockwise direction,
• Pλ0 is the total paraxial power (mm−1) of the aberration correcting element with a wavelength of a light flux emitted from the light source,
• Pλ1 is the total paraxial power (mm−1) of the aberration correcting element for with a wavelength shorter by 10 nm than the wavelength of the light flux emitted from the light source, and
• Pλ2 is the total paraxial power (mm−1) of the aberration correcting element with a wavelength longer by 10 nm than the wavelength of the light flux emitted from the light source.

An aberration correcting element provided on an optical path between a light source and an objective lens including at least one plastic lens, comprising:

• a positive lens and a negative lens;
• wherein the total paraxial power of the aberration correcting element has a temperature dependency satisfying a formula of:
• PT1<PT0<PT2, or PT1>PT0>PT2,
• where PT0 is the total paraxial power (mm−1) of the aberration correcting element under the temperature of 25° C.,
• PT1 is the total paraxial power (mm−1) of the aberration correcting element under the temperature of −5° C., and
• PT2 is the total paraxial power (mm−1) of the aberration correcting element under the temperature of 55° C.