Objective lens for optical disc

1. An objective lens for recording data to and/or reproducing data from first and second optical discs, the first optical disc having a recording density higher than that of the second optical disc, a first light beam being used for the first optical disc, a second light beam being used for the second optical disc, the second light beam having a wavelength longer than that of the first light beam,

the objective lens comprising at least one surface including a plurality of annular zones divided by steps, the steps having functions of phase shifting a wavefront of light passing through the plurality of annular zones so that a beam spot formed at a converging point of an average wavefront of the wavefront phase shifted by the steps is used for recordation and/or reproduction for the first and second optical discs,

the plurality of annular zones of the at least one surface including at least one annular zone structure,

the at least one annular zone structure including:

first and second annular zone groups each of which has at least three steps for phase shifting the wavefront in a first direction; and

a first return step which is a step formed at a boundary between the first and second annular zone groups, the first return step phase shifting the wavefront in a second direction opposite to the first direction,

wherein an absolute value |Δψ1| of a phase shifting amount of the first light beam given by the first return step satisfies a condition:

6π<|Δψ1|<14π  (1),

wherein the absolute value  Δψ1| of the phase shifting amount of the first light beam given by the first return step is smaller than larger one of an absolute value of a sum of phase shift amounts given by the first annular zone group and an absolute value of a sum of phase shift amounts given by the second annular zone group.

2. The objective lens according to claim 1 ,

wherein the at least one surface includes an inner area contributing to convergence of both of the first and second light beams and an outer area contributing to only convergence of the first light beam, and

wherein the first return step is formed within the inner area.

3. The objective lens according to claim 2 ,

wherein the first optical disc is compliant with a DVD standard,

wherein the second optical disc is compliant with a CD standard, and

wherein the absolute value |Δψ1| of the phase shifting amount of the first light beam given by the first return step satisfies a condition:

9.4π<|Δψ1|<10.6π  (2).

4. The objective lens according to claim 2 , wherein the at least one surface includes a second return step which is a step formed at a boundary between the inner area and the outer area, the second return step phase shifting the wavefront in the second direction opposite to the first direction.

5. The objective lens according to claim 2 , wherein a wavefront aberration of the first light beam passing through the inner area is lower than or equal to ±0.1λ at a vicinity of a recording surface of the first optical disc.

6. The objective lens according to claim 1 , wherein the objective lens is a single-element lens.

7. An objective lens for recording data to and/or reproducing data from first and second optical discs, the first optical disc having a recording density higher than that of the second optical disc, a first light beam being used for the first optical disc, a second light beam being used for the second optical disc, the second light beam having a wavelength longer than that of the first light beam,

the objective lens comprising at least one surface including a plurality of annular zones divided by steps, the steps having functions of giving optical path length differences to light passing through the plurality of annular zones so that a beam spot formed by light passing through the at least one surface is used for recordation and/or reproduction for the first and second optical discs,

the plurality of annular zones of the at least one surface including at least one annular zone structure,

the at least one annular zone structure including:

first and second annular zone groups each of which has at least three steps for giving optical path length differences in a first direction; and

a first return step which is a step formed at a boundary between the first and second annular zone groups, the first return step giving an optical path length difference in a second direction opposite to the first direction,

wherein an absolute value |ΔOPD| of an optical path length difference given to the first light beam by the first return step satisfies a condition:

3

<



Δ

⁢

⁢

OPD

λ

⁢

1



<

7

(

3

)

where λ1 represents a wavelength of the first light beam,

wherein the absolute value |ΔOPD| of the optical path length difference given to the first light beam by the first return step is smaller than larger one of an absolute value of a sum of the optical path length differences given by the first annular zone group and an absolute value of a sum of the optical path length differences given by the second annular zone group.

8. The objective lens according to claim 7 ,

wherein the at least one surface includes an inner area contributing to convergence of both of the first and second light beams and an outer area contributing to only convergence of the first light beam, and

wherein the first return step is formed within the inner area.

9. The objective lens according to claim 8 ,

wherein the first optical disc is compliant with a DVD standard,

wherein the second optical disc is compliant with a CD standard, and

wherein the absolute value |ΔOPD| of the optical path length difference given to the first light beam by the first return step satisfies a condition:

4.7

<



Δ

⁢

⁢

OPD

λ

⁢

1



<

5.3

.

(

4

)

10. The objective lens according to claim 8 , wherein the at least one surface includes a second return step which is a step formed at a boundary between the inner area and the outer area, the second return step giving an optical path length difference in the second direction opposite to the first direction.

11. The objective lens according to claim 8 , wherein a wavefront aberration of the first light beam passing through the inner area is lower than or equal to ±0.1λ at a vicinity of a recording surface of the first optical disc.

12. The objective lens according to claim 7 , wherein the objective lens is a single-element lens.

13. An objective lens for recording data to and/or reproducing data from first and second optical discs, the first optical disc having a recording density higher than that of the second optical disc, a first light beam being used for the first optical disc, a second light beam being used for the second optical disc, the second light beam having a wavelength longer than that of the first light beam,

the objective lens comprising at least one surface including a diffracting structure,

the diffracting structure including a plurality of annular zones divided by steps so as to provide a function of suitably converging the first and second light beams onto recording surfaces of the first and second optical discs, respectively,

the diffracting structure including at least first and second annular zone groups, the first annular zone group and the second annular zone group being located in that order from an optical axis of the objective lens,

wherein the diffracting structure is configured to satisfy conditions (5) trough (7) or to satisfy conditions (5), (8) and (9):

4<|φ i+1 ( h i )−φ i ( h i )|<8  (5)

|φ i ( h i )−φ i ( h i−1 )|>|φ i+1 ( h i )−φ i ( h i )|  (6)

{φ i ( h i )−φ i ( h i−1 )}/{φ i+1 ( h i )−φ i ( h i )}<0  (7)

|φ i+1 ( h i+1 )−φ i+1 ( h i )|>|φ i+1 ( h i )−φ i ( h i )|  (8)

{φ i+1 ( h i+1 )−φ i+1 ( h i )}/{φ i+1 ( h i )−φ i ( h i )}<0  (9)

where Φi(h) represents an additional optical path length given by the diffracting structure in an i-th annular zone group, and hi represents a height of a boundary position between the i-th annular zone group and a (i+1)-th annular zone group,

wherein Φi(h) is expressed by a following equation:

Φ i ( h )=( P 0i +P 2i h 2 +P 4i h 4 +P 6i h 6 + . . . )× m

where P 0i , P 2i , P 4i and P 6i are coefficients of 0-th, second, fourth and sixth orders regarding the additional optical path length of the i-th annular zone group, h represents a height from the optical axis, m represents a diffraction order used for recording and/or reproducing operation, and h i−1 is 0 if i is 0.

14. The objective lens according to claim 13 ,

wherein the at least one surface includes an inner area contributing to convergence of both of the first and second light beams and an outer area contributing to only convergence of the first light beam, and

wherein the boundary position is formed within the inner area.

15. The objective lens according to claim 14 ,

wherein the first optical disc is compliant with a DVD standard,

wherein the second optical disc is compliant with a CD standard, and

wherein the objective lens satisfies a condition:

5.7<φ i+1 ( h i )−φ i ( h i )|<6.3  (10).

16. The objective lens according to claim 15 , wherein the at least one surface has a first step formed at a boundary between the inner area and the outer area, the first step giving an optical path length difference in a direction equal to a direction of the optical path length difference given by the boundary position between the i-th annular zone group and a (i+1)-th annular zone group.

17. The objective lens according to claim 14 , wherein a wavefront aberration of the first light beam passing through the inner area is lower than or equal to ±0.1λ at a vicinity of a recording surface of the first optical disc.

18. The objective lens according to claim 13 , wherein the objective lens is a single-element lens.