Optical system of optical pick-up

1. An optical system of an optical pick-up, comprising:

a light source that emits a light beam; and

an objective lens that converges the light beam emitted by said light source onto a data recording layer of an optical disc,

wherein said optical system satisfies a condition:

0.75< d ·( M− 1)/( M·L )<1.0  (1)

where d (mm) represents a thickness of said objective lens, M represents magnification of said optical system, and L (mm) represents an O/I distance which is defined as a distance between an object and an image,

wherein a beam spot suitable for the optical disc is formed by said objective lens on the data recording layer of the optical disc without using a coupling lens.

2. The optical system according to claim 1 ,

wherein when a numerical aperture NA of said objective lens on an optical disc side is larger than or equal to 0.55, said optical system satisfies a condition:

−0.0016< M·NA 2 /L<− 0.0007  (2).

3. The optical system according to claim 1 ,

wherein the O/I distance L and the magnification M respectively satisfy conditions:

30<L<50  (3), and

−1/10< M<− 1/12.5  (4).

4. The optical system according to claim 3 ,

wherein the O/I distance L satisfies a condition:

30<L<45  (5).

5. An optical system of an optical pick-up used for recording data to and/or reproducing data from a plurality of types of optical discs having different thicknesses of cover layers, comprising:

a light source having a plurality of light emitting portions that respectively emit light beams having different wavelengths; and

an objective lens that converges each of the light beams emitted by the light emitting portions onto a data recording layer of corresponding one of the plurality of types of optical discs,

wherein said optical system satisfies a condition:

0.75< d ·( M i −1)/( M i ·L i )<1.0  (6)

where d (mm) represents a thickness of said objective lens, M i represents magnification of said optical system, L i (mm) represents an O/I distance which is defined as a distance between an object and an image, and i (i is a natural number ranging from 1 to j; j≧2) represents a type of each of the plurality of types of the optical discs, given that i (i=1,2, . . . , j) respectively correspond to a 1 st , 2 nd , . . . , j-th ones of the plurality of types of the optical discs, the thicknesses of cover layers of the plurality of types of the optical discs satisfying a relationship 1 st disc<2 nd disc< . . . <j-th disc,

wherein a beam spot suitable for each of the plurality of types of the optical discs is formed by said objective lens on the data recording layer of the corresponding one of the plurality of types of the optical discs without using a coupling lens.

6. The optical system according to claim 5 ,

wherein when a numerical aperture NA of said objective lens on an optical disc side is larger than or equal to 0.55, said optical system satisfies a condition:

−0.0016< M k ·NA k 2 /L k <−0.0007  (7)

where M k , NA k and L k respectively represent the magnification, the numerical aperture and the O/I distance when a k-th (1≦k≦j) optical disc having the numerical aperture larger then or equal to 0.55 is used.

7. The optical system according to claim 5 ,

wherein the O/I distance L i and the magnification M i respectively satisfy conditions:

30<L i <50  (8), and

−1/10< M i <−1/12.5  (9).

8. The optical system according to claim 7 ,

wherein the O/I distance L i satisfies a condition:

30<L i <45  (10).

9. The optical system according to claim 8 ,

wherein the plurality of light emitting portions are aligned in a line which is perpendicular to a direction in which said objective lens is moved by a focusing operation and is perpendicular to a direction in which said objective lens is moved by a tracking operation.

10. The optical system according to claim 5 ,

wherein both of lens surfaces of said objective lens are aspherical surfaces,

wherein said objective lens has a diffracting structure on one of the lens surfaces, the diffracting structure having a function of forming a beam spot suitable for each of the plurality of types of optical discs.