Optical system for optical disc
There is provided an optical system for an optical disc which requires a numerical aperture larger than or equal to 0.60. The optical system is provided with a light source, an objective lens, and a driving system that controls a position and an attitude of the objective lens. The driving system operates to move the objective lens in a direction perpendicular to a central axis of the objective lens for a tracking operation, and to change the attitude of the objective lens during the tracking operation so that the central axis on a light source side tilts toward the light source.
1. A finite optical system for recording data to and/or reproducing data from an optical disc having a numerical aperture on an optical disc side larger than or equal to 0.60, comprising:
a light source that emits a diverging light beam;
an objective lens that converges the diverging light beam onto a data recording surface of the optical disc; and
a driving system that controls a position and an attitude of the objective lens,
wherein the driving system operates to move the objective lens in a direction perpendicular to a central axis of the objective lens for a tracking operation, and to change the attitude of the objective lens during the tracking operation so that the central axis on a light source side tilts toward the light source, and
wherein the driving system controls the position and the attitude of the objective lens to satisfy a condition:
0.25 ≦d· tan θ/ TR≦ 0.75 (1)
where d (mm) represents a distance on an air basis between the light source and the data recording surface of the optical disc, TR (mm) represents a shifting amount of an object image caused by tracking shift movement of the objective lens by the tracking operation, and θ (°)represents a tilting amount of the objective lens with respect to a reference axis of the optical system.
2. The optical system according to claim 1 ,
wherein the objective lens is configured to satisfy a condition:
−0.75≦ CM D /CM L ≦−0.15 (2)
where CM L represents sensitivity of a coma caused when only the objective lens tilts with respect to the light beam from the light source, and CM D represents sensitivity of a coma caused when only the optical disc tilts with respect to the light beam passed through the objective lens.
3. The optical system according to claim 1 ,
wherein the optical system satisfies a condition:
−0.30≦( CM D /CM L )· d· tan θ/ TR≦ −0.15 (3)
where CM L represents sensitivity of a coma caused when only the objective lens tilts with respect to the light beam from the light source, and CM D represents sensitivity of a coma caused when only the optical disc tilts with respect to the light beam passed through the objective lens.
4. A finite optical system for recording data to and/or reproducing data from a plurality of types of optical discs, comprising:
a plurality of light sources that emit diverging light beams respectively corresponding to the plurality of types of optical discs;
an objective lens that converges each of the diverging light beams onto a data recording surface of corresponding one of the plurality of types of optical discs; and
a driving system that controls a position and an attitude of the objective lens,
wherein the driving system operates to move the objective lens in a direction perpendicular to a central axis of the objective lens for a tracking operation, and to change the attitude of the objective lens during the tracking operation so that the central axis on a light source side tilts toward a corresponding one of the plurality of light sources at least when a first optical disc having a thinnest cover layer of the plurality of types of optical discs is used, and
wherein when the first optical disc is used, the driving system controls the position and the attitude of the objective lens to satisfy a condition:
0.25≦ d 1 ·tan θ 1 /TR ≦0.75 (6)
where d 1 (mm) represents a distance on an air basis between a first light source of the plurality of the light sources used for the first optical disc and a data recording surface of the first optical disc, TR (mm) represents a shifting amount of an object image caused by tracking shift movement of the objective lens by the tracking operation, and θ 1 (°) represents a tilting amount of the objective lens with respect to a reference axis of the optical system when the first optical disc is used.
5. The optical system according to claim 4 ,
wherein the driving system changes the attitude of the objective lens to satisfy a condition:
−0.1≦θ 2 /θ 1 ≦1 (4)
where θ 2 (°) represents a tilting amount of the objective lens during tracking shift movement of the objective lens by the tracking operation when a second optical disc of the plurality of types of optical discs having a cover layer thicker than that of the first optical disc is used.
6. The optical system according to claim 5 ,
wherein the driving system changes the attitude of the objective lens to satisfy a condition:
θ 2 /θ 1 =0 (5).
7. The optical system according to claim 4 , wherein the first optical disc requires a numerical aperture on an optical disc side larger than or equal to 0.60.
8. The optical system according to claim 5 , wherein the second optical disc has a thickest cover layer of all of the plurality of types of optical discs.
9. The optical system according to claim 4 ,
wherein the objective lens is configured to satisfy a condition:
−0.75 ≦CM D1 /CM L1 ≦−0.15 (7)
where CM L1 represents sensitivity of a coma caused when only the objective lens tilts with respect to a first light beam emitted by one of the plurality of light sources for the first optical disc, and CM D1 represents sensitivity of a coma caused when only the first optical disc tilts with respect to the first light beam passed through the objective lens.
10. The optical system according to claim 4 ,
wherein the optical system satisfies a condition:
−0.30≦( CM D1 /CM L1 )·d 1 ·tan θ 1 /TR ≦−0.15 (8)
where CM L1 represents sensitivity of a coma caused when only the objective lens tilts with respect to a first light beam emitted by the first light source, and CM D1 represents sensitivity of a coma caused when only the first optical disc tilts with respect to the first light beam passed through the objective lens.
11. The optical system according to claim 4 ,
wherein the objective lens is configured to satisfy a condition:
−1.50 ≦CM D2 /CM L2 ≦−0.50 (9)
where CM L2 represents sensitivity of a coma caused when only the objective lens tilts with respect to a second light beam emitted by one of the plurality of light sources used for a second optical disc of the plurality of types of optical discs having a cover layer thicker than that of the first optical disc, and CM D2 represents sensitivity of a coma caused when only the second optical disc tilts with respect to the second light beam passed through the objective lens.
12. The optical system according to claim 4 ,
wherein the plurality of light sources are located at positions shifted with respect to each other in a plane perpendicular to an emitting direction of each of the diverging light beams, and
wherein the plurality of light sources are aligned in a direction perpendicular to a direction in which an object image formed by the objective lens shifts in accordance with tracking shift movement of the objective lens by the tracking operation.
13. The optical system according to claim 4 , further comprising a detecting system that detects a type of an optical disc being used,
wherein the driving system sets a changing amount of the attitude of the objective lens responsive to tracking shift movement of the objective lens by the tracking operation and a detection result of the detecting system.
14. The optical system according to claim 4 ,
wherein the plurality of light sources are located at positions shifted with respect to each other in a plane perpendicular to an emitting direction of each of the diverging light beams, and
wherein the driving system moves the objective lens in the tracking operation with respect to a reference position which is defined for each of the plurality of light sources, the reference position being defined as a position at which the central axis of the objective lens passes through one of the plurality of light sources corresponding to a type of an optical disc being used.
15. The optical system according to claim 14 ,
wherein the plurality of light sources are aligned in a direction in which an object image formed by the objective lens shifts in accordance with tracking shift movement of the objective lens by the tracking operation.
16. The optical system according to claim 14 , further comprising a detecting system that detects the type of the optical disc being used,
wherein the driving system sets a changing amount of the attitude of the objective lens responsive to tracking shift movement of the objective lens by the tracking operation and a detection result of the detecting system, and determines the reference position for the optical disc being used based on the detection result of the detecting system.
17. A finite optical system for recording data to and/or reproducing data from two types of optical discs including a first optical disc and a second optical disc whose cover layer is approximately twice as thick as that of the first optical disc, comprising:
a plurality of light sources that emit diverging light beams respectively corresponding to the first and second optical discs;
an objective lens that converges each of the diverging light beams onto a data recording surface of corresponding one of the first and second optical discs; and
a driving system that controls a position and an attitude of the objective lens, wherein the objective lens has sensitivity of a coma caused by a cover layer of the second optical disc,
wherein the driving system operates to move the objective lens in a direction perpendicular to a central axis of the objective lens for a tracking operation,
wherein when the first optical disc is used, the driving system operates to change the attitude of the objective lens during the tracking operation so that the central axis on a light source side tilts toward one of the light sources used for the first optical disc by an amount which is approximately half of a tilting amount by which astigmatism caused by tracking shift movement of the objective lens by the tracking operation is substantially completely corrected,
wherein when the second optical disc is used, the driving system operates not to change the attitude of the objective lens during the tracking operation, and
wherein when the first optical disc is used, the driving system controls the position and the attitude of the objective lens to satisfy a condition:
0.25≦ d· tan θ/ TR≦ 0.75 (1)
where d (mm) represents a distance on an air basis between the one of the light sources used for the first optical disc and the data recording surface of the first optical disc, TR (mm) represents a shifting amount of an object image caused by tracking shift movement of the objective lens by the tracking operation, and θ (°) represents a tilting amount of the objective lens with respect to a reference axis of the optical system when the first optical disc is used.
18. The optical system according to claim 17 ,
wherein the objective lens is configured to satisfy a condition:
−0.75 ≦CM D /CM L ≦−0.15 (2)
where CM L represents sensitivity of a coma caused when only the objective lens tilts with respect to a first light beam from the one of the light sources for the first optical disc, and CM D represents sensitivity of a coma caused when only the first optical disc tilts with respect to the first light beam passed through the objective lens.