JP2011065720A - Optical system and optical pickup device using the same - Google Patents

Abstract

An optical system capable of suitably focusing a laser beam on each of a plurality of information recording surfaces provided on an information recording medium is provided.
An optical system for focusing a laser beam on each of a plurality of information recording surfaces of an information recording medium having a plurality of information recording surfaces parallel to each other in order from the light source side toward the information recording surface. , A magnification conversion element and an objective lens, and by changing the imaging magnification by moving the magnification conversion element in the optical axis direction, the amount Δm of change in the imaging magnification is expressed by the following equation: Δm> 0.015 is satisfied.
[Selection] Figure 2

Description

  The present invention relates to an optical system and an optical pickup device using the same. Specifically, the present invention relates to an optical system for focusing laser light on each information recording surface of an information recording medium having a plurality of information recording surfaces, and an optical pickup device using the optical system.

  In recent years, research and development of an information recording medium having a plurality of information recording surfaces (for example, an optical disc such as a BD (Blu-ray Disc (registered trademark)) has been advanced. However, when a plurality of information recording surfaces are provided, since the distance from the light source side surface of the information recording medium to each information recording surface is different from each other, usually, one objective lens is used for each of the plurality of information recording surfaces. When trying to focus the laser beam, spherical aberration occurs due to the difference in distance from the light source side surface of the information recording medium to each information recording surface. It is difficult to focus the light. In view of such a problem, for example, Patent Document 1 discloses a light source for an information recording medium by changing a divergence angle of a beam incident on an objective lens by driving a beam expander lens or a collimator lens in the optical axis direction. A technique for correcting spherical aberration due to a difference in distance from the side surface to each information recording surface is disclosed.

JP 2006-31901 A

  However, when the objective lens has a two-piece configuration as in the technique disclosed in the above-mentioned document, it is necessary to perform assembly adjustment, which increases the cost. Further, when the number of information recording surfaces is further increased in order to increase the storage capacity of one information recording medium, a large amount of spherical aberration is generated, which cannot be corrected and cannot be stably recorded and reproduced.

  The present invention has been made in view of such a point, and an object of the present invention is to suitably focus laser light on each of a plurality of information recording surfaces provided on an information recording medium. An optical system and an optical pickup apparatus are provided.

In order to achieve the above object, an optical system according to the present invention is an optical system that focuses laser light on each of the plurality of information recording surfaces of an information recording medium having a plurality of information recording surfaces parallel to each other. The imaging magnification is changed by a light source, an objective lens, and a magnification conversion element arranged between the light source and the objective lens, and the imaging magnification change rate Δm is as follows: It consists of an optical system characterized by satisfying the equation.
Δm> 0.015

An optical system according to the present invention is an optical system that focuses laser light on each of the plurality of information recording surfaces of an information recording medium having a plurality of information recording surfaces parallel to each other. An optical system in which an imaging magnification is changed by a lens, and a magnification conversion element disposed between the light source and the objective lens, wherein the imaging magnification change rate Δm satisfies the following expression: And
Δm> 0.02

In addition to the configuration of the optical system described above, the optical system according to the present invention is an optical system for focusing laser light on each of the plurality of information recording surfaces of an information recording medium having a plurality of information recording surfaces parallel to each other. An optical system in which an imaging magnification is changed by a light source, an objective lens, and a magnification conversion element disposed between the light source and the objective lens, and is the largest of the imaging magnifications When the imaging magnification is m1 and the minimum imaging magnification is m2, the following expression is satisfied.
(M1 + m2) <0.002

  ADVANTAGE OF THE INVENTION According to this invention, the optical system which can focus a laser beam suitably with respect to all the information recording surfaces provided in the information recording medium, and an optical pick-up apparatus using the same are realizable.

The block diagram showing the structure of the principal part of the optical system which concerns on embodiment of this invention Sectional drawing which expanded the structure of the objective optical system which concerns on embodiment of this invention Optical path diagram of objective lens according to Numerical Example 1 of the present invention Aberration diagram showing spherical aberration and unsatisfactory sine condition of objective lens according to Numerical Example 1 of the present invention

(Embodiment)
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a configuration diagram showing a configuration of a main part of an optical system according to an embodiment of the present invention. FIG. 2 is an enlarged cross-sectional view of the configuration of the objective optical system according to the embodiment of the present invention.

  The optical system 1 according to the present embodiment is for focusing laser light on the information recording surface 21 of the information recording medium 20. In detail, the information recording medium 20 includes a plurality of information recording surfaces 21 arranged in parallel to each other and spaced apart from each other, and the optical pickup device 1 includes each of the plurality of information recording surfaces 21. Is for focusing the laser beam on. Specifically, the information recording medium 20 includes a first information recording surface 21a, a second information recording surface 21b, a third information recording surface 21c, and a fourth information disposed in this order from the light source 10 side. The information recording surface 21 has four information recording surfaces 21 and 21d. A first protective layer 22a is provided on the light source 10 side of the first information recording surface 21a. A second protective layer 22b is provided between the first information recording surface 21a and the second information recording surface 21b. A third protective layer 22c is provided between the second information recording surface 21b and the third information recording surface 21c. A fourth protective layer 22d is provided between the third information recording surface 21c and the fourth information recording surface 21d. The first protective layer 22a, the second protective layer 22b, the third protective layer 22c, and the fourth protective layer 22d may have substantially the same layer thickness or different layer thicknesses. It may be. Here, the case where the four information recording surfaces 21 are provided will be described as an example, but the present invention is not limited to this. The optical system according to the present invention focuses laser light on each information recording surface of an information recording medium having two or three information recording surfaces or five or more information recording surfaces, for example. May be.

  Here, the type of the information recording medium 20 is not particularly limited. The information recording medium 20 may be, for example, a CD (Compact Disk Rerecordable), a CD-RW (Compact Disk Read Only Memory), a CD-ROM (Compact Disk Read Only Memory), a DVD (Digit Discrete Memory), or a DVD (Digit Digital Memory). DVD-R (Digital Versatile Disc Recordable), DVD-RW (Digital Versatile Disc Rewriteable), DVD-ROM (Digital Versatile Disc Read Only Memory), DVD-ROM (Digital Versatile Disc Read Only Memory), DVD-ROM (Digital Versatile Disc Rewriteable Memory) EVD (Enhanced Versatile Disc), EVD-R (Enhanced Versatile Disc Recordable), EVD-RW (Enhanced Versatile Disc ReWritable), EVD-ROM (Enhanced Versatile Disc Read Only Memory), EVD-RAM (Enhanced Versatile Disk Random Access Memory) BD (Blu-ray Disc Read Only Memory), BD-R (Blu-ray Disc Recordable), BD-RW (Blu-ray Disc Rewritable), BD-ROM (Blu-ray Disc Read Only Memory), BD-RAM lu-ray Disc Random Access Memory) or the like of the optical disk (or more, may be all registered trademark).

  The optical system 1 includes a light source 10, a beam shaping lens 11, a beam splitter 12, a collimator lens 13, an objective optical system 14, and a detection system 16. The light source 10 emits laser light (diverged light) having a wavelength corresponding to the type of the information recording medium 20. For example, when the information recording medium 20 is BD (registered trademark), a laser beam having a wavelength of 378 to 438 nm (may be less than 420 nm) can be emitted. On the other hand, when the information recording medium 20 is a DVD (registered trademark), a laser beam having a wavelength of 630 to 690 nm can be emitted. When the information recording medium 20 is a CD (registered trademark), a laser beam having a wavelength of 750 to 810 nm can be emitted. Further, for example, when the optical system 1 is for focusing laser light on three types of optical discs of CD, DVD, and BD, the light source 10 depends on the type of optical disc disposed with respect to the optical system 1. It is possible to selectively emit light having different wavelengths.

  A beam shaping lens 11 is disposed in front of the light source 10. The laser beam emitted from the light source 10 by this beam shaping lens 11 is shaped into a desired shape. The laser beam shaped by the beam shaping lens 11 is reflected toward the information recording medium 20 by the reflecting surface 12 a of the beam splitter 12. Here, the beam shaping lens may be omitted. Between the beam splitter 12 and the information recording medium 20, a collimator lens (which may be constituted by a single lens or may be constituted by a plurality of lenses) 13 and each of the information recording medium 20. An objective optical system 14 for focusing the laser beam on the information recording surface 21 is arranged. In the present embodiment, the objective optical system 14 is composed of only the objective lens 15, but the objective optical system 14 may be replaced with the objective lens 15 and another one such as a phase correction element or a beam expander lens as necessary. You may comprise with a some optical element.

  The NA of the objective lens 15 is not particularly limited, but is particularly 0.8 or more when the optical system 1 is for focusing laser light on BD (registered trademark) or the like. Is preferred.

  The laser beam focused on the information recording surface 21 of the information recording medium 20 by the objective optical system 14 is reflected by the information recording surface 21. The reflected light from the information recording surface 21 is again transmitted through the objective optical system 14, the collimator lens 13, and the beam splitter 12 and is incident on the detection system 16. The detection system 16 includes a detector 18 and a detection lens 17 for focusing on the detector 18, and is configured so that the reflected light focused by the detection lens 17 is detected by the detector 18. Has been.

  In the present embodiment, the collimator lens 13 has a function as an aberration correction element, and is located at a reference position between the beam splitter 12 and the objective lens 15 (objective optical system 14), and the optical axis from the reference position. It is configured to be displaceable on AX. Then, the objective lens 15 (objective optical system 14) is displaced from the reference position on the optical axis AX so that the laser beam is focused on the information recording surface 21 to be focused on the laser beam among the plurality of information recording surfaces 21. The laser beam is suitably focused on the information recording surface 21. In other words, the position on the optical axis AX of the collimator lens 13 as an aberration correction element is adjusted according to the position on the optical axis AX of the information recording surface 21 to focus the laser beam on each information recording surface 21. The laser beam is suitably focused. Specifically, for example, when the collimator lens 13 is positioned at the reference position, the objective lens 15 is optically designed so that the laser light is suitably focused on the first information recording surface 21a by the objective optical system 14. In the case where the laser light is focused on the information recording surface 21 other than the first information recording surface 21a, for example, the second information recording surface 21b, the objective optical system 14 causes the second information recording surface to be focused. The position of the collimator lens 13 on the optical axis AX is adjusted so that the laser beam is suitably focused on 21b, and the shape (divergence angle, etc.) of the laser beam is adjusted. Therefore, in the optical system 1 according to the present embodiment, the laser beam can be suitably focused on each information recording surface 21 of the information recording medium 20 having the plurality of information recording surfaces 21.

  At this time, the light beam incident on the objective lens is configured to be divergent light and convergent light. Usually, when the spot is focused on multiple information recording surfaces with the same objective lens, the thickness of the protective layer from the surface of the information recording medium to the information recording surface is different, so it differs from the protective layer thickness at the design center Causes spherical aberration. In particular, when the numerical aperture of an objective lens such as a BD is large, the amount of generation of this spherical aberration becomes very large, making it difficult to perform stable recording and reproduction. Therefore, in order to form a good spot without aberration, the incident angle of the light beam incident on the objective lens is changed, and spherical aberration is generated in the objective lens. The generated spherical aberration and the spherical aberration generated by the change in the thickness of the protective layer are canceled out, and a good spot without aberration is formed.

  In addition, in the BD optical system, an infinite conjugate configuration with little spot deterioration with respect to the objective lens shift for tracking is usually used. It is necessary to change this imaging magnification. In addition, since it is necessary to increase the number of information recording media in order to further increase the capacity in the future, it is necessary to increase the fluctuation range of the magnification.

  In order to increase the recording capacity per information recording medium, it is essential to increase the number of recording layers. However, if the thickness of the protective layer between the recording layer and the adjacent recording layer is reduced, crosstalk between layers occurs and it is difficult to detect a good signal. For this reason, the protective layer between the recording layer and the adjacent recording layer needs to have a certain thickness.

  Therefore, when the information recording surface is increased, the width of the change in the thickness of the protective layer from the surface to the recording surface is increased, that is, the amount of change in the generated spherical aberration is increased. When the generated spherical aberration is corrected by changing the magnification, increasing the width of the magnification change enables correction of a large spherical aberration, which contributes to obtaining good spot performance.

  Further, the spherical aberration to be corrected here is not limited to the spherical aberration caused by the change in the protective layer thickness. It is also possible to correct spherical aberration caused by any optical element included in the optical system, wavelength change of the light source, spherical aberration caused by temperature change in the atmosphere, and the like. In such a case, as described above, the problem of aberration that occurs when the objective lens is shifted occurs. Therefore, it is preferable that the absolute values of the maximum magnification and the minimum magnification are as small as possible even for the same magnification fluctuation range. That is, infinite conjugate when the protective layer thickness from the light source side surface of the information recording medium to the nearest information recording surface and the protective layer thickness from the farthest information recording surface is exactly the middle thickness (half the sum of the layer thicknesses). It is preferable that the optical system is configured as follows.

  With this configuration, aberration deterioration (spherical aberration, various aberrations caused when off-axis laser light is incident, the objective lens 15 is tilted with respect to the optical axis AX) on all of the plurality of information recording surfaces 21. In the optical recording medium 20 and deterioration due to various aberrations that occur when the information recording medium 20 is tilted with respect to the optical axis AX can be effectively reduced. Further, with this configuration, the amount of displacement on the optical axis AX of the collimator lens 13 as an aberration correction element can be made relatively small, and the amount of displacement from the reference position toward the light source 10 and the information recording medium The difference from the displacement amount toward the 20 side can be reduced. Therefore, stable reproduction of the information recording medium 20 can be performed. These effects can also be obtained when there are two information recording surfaces 21. However, when there are three or more information recording surfaces 21, there are four or more information recording surfaces 21. Especially great. Further, it is particularly effective in an optical pickup device used for DVD (registered trademark), BD (registered trademark), etc., in which the wavelength of the laser beam used is relatively short.

  Note that, here, an example of an optical system configured to focus laser light on only one type of information recording medium has been described, but laser light is focused on each of a plurality of types of information recording media. The structure which can be focused may be sufficient. In this case, the objective optical system 14 may be configured by the objective lens 15 and a phase correction element that corrects the phase of light incident on the objective lens 15, a beam expander lens, or the like. In addition, each information recording medium may have a dedicated objective lens.

  Although the case where the collimator lens 13 is used as an aberration correction element has been described as an example here, the aberration correction element is a single beam expander lens disposed between the collimator lens and the objective lens, or a beam expander. You may comprise with a lens and a collimator lens. Further, a liquid crystal lens, a liquid lens, or the like may be used as the aberration correction element.

  In the present embodiment, the optical element such as a lens may be constituted by only a refractive surface having substantially only a refractive action. For example, other optical elements such as a diffraction surface and a phase step surface may be used. It may have a functional aspect. Furthermore, the material of the optical element such as a lens is not particularly limited, and may be made of, for example, glass or resin. The optical system 1 may further include an element between the light source 10 and the information recording medium 20 that does not substantially affect the transmitted wavefront aberration.

但し、
Ｘ：光軸からの高さがｈである非球面状の点の非球面頂点の接平面からの距離、
ｈ：光軸からの高さ、
Ｃ_j：レンズ第ｊ面の非球面頂点の曲率（Ｃ_j＝１／Ｒ_j）、
Ｋ_j：レンズ第ｊ面の円錐定数、
Ａ_j,n：レンズ第ｊ面のｎ次の非球面定数、
であり、ｊ＝１、２、３、４、
である。 Hereinafter, the optical system 1 embodying the present invention will be described more specifically with reference to construction data, aberration diagrams, and the like. In each numerical example, it is shown that the surface to which the aspheric coefficient is given is an aspherical refractive optical surface or a surface having a refractive action that is transparent to the aspherical surface (for example, a diffractive surface). It is defined by the following conditional expression representing the surface shape.

However,
X: distance from the tangent plane of the aspherical vertex of the aspherical point whose height from the optical axis is h,
h: height from the optical axis,
C _j : curvature of the aspherical vertex of the lens j-th surface (C _j = 1 / R _j ),
K _j : Conic constant of the lens j-th surface,
A _{j, n} : nth-order aspherical constant of the lens j-th surface,
J = 1, 2, 3, 4,
It is.

(Numerical example 1)
The construction data in Numerical Example 1 is shown in Table 1.

  In Numerical Example 1, the distance from the surface 20a of the information recording medium 20 to the first information recording surface 21a: 40 μm, the distance from the surface 20a of the information recording medium 20 to the second information recording surface 21b: 67 μm, information recording Distance from the surface 20a of the medium 20 to the third information recording surface 21c: 86 μm, distance from the surface 20a of the information recording medium 20 to the fourth information recording surface 21d: 100 μm, design center thickness: 70 μm, of the objective lens 15 Focal length: 1.2 mm, effective diameter of objective lens 15: 2.04 mm. In the construction data, optical elements other than the objective lens data, such as a collimating lens, are omitted. Moreover, it is shown as the distance of the optical virtual object point which changes when the collimating lens is moved in the optical axis direction.

  FIG. 3 is an optical path diagram of the objective lens according to Numerical Example 1. FIG. 4 is an aberration diagram showing the spherical aberration of the objective lens according to Numerical Example 1 and the unsatisfactory amount of the sine condition. In FIG. 4, the left side is a spherical aberration diagram, and the right side is a sine condition diagram. Table 2 shows the optical virtual object point distance, the imaging magnification of the optical system, and the total aberration at that time when the light is condensed on each information recording surface.

  As the amount of change in the imaging magnification, Δm is 0.026 because the maximum magnification is 0.011 and the minimum magnification is −0.013. The center magnification is (0.011−0.013) /2=−0.001. From this, it can be seen that the center of variation in magnification is in an almost infinite system, and the optical system has an optimum configuration.

  Since the optical system and the optical pickup device according to the present invention can suitably focus laser light on each of a plurality of information recording surfaces, a CD (Compact Disc), a DVD (Digital Versatile Disc), Recording and playback of various information recording media such as BD (Blu-ray Disc), EVD (Enhanced Versatile Disc), and HD-DVD (High Definition Digital Versatile Disc), for example, information devices such as computers, video Useful for equipment, audio equipment, etc.

DESCRIPTION OF SYMBOLS 1 Optical pick-up optical system 10 Light source 11 Beam shaping lens 12 Beam splitter 12a Beam splitter reflective surface 13 Collimator lens 14 Objective optical system 15 Objective lens 16 Detection system 17 Detection lens 18 Detector 20 Information recording medium 21 Information recording surface 22 Protective layer

Claims (4)

An optical system for focusing laser light on each of the plurality of information recording surfaces of an information recording medium having a plurality of information recording surfaces parallel to each other, in order from the light source side to the information recording surface,
A magnification conversion element and an objective lens;
An optical system in which the imaging magnification is changed by moving the magnification conversion element in the optical axis direction, and the amount Δm of change in the imaging magnification satisfies the following expression:
Δm> 0.015 The optical system according to claim 1 satisfying the following formula:
Δm> 0.02 An optical system for focusing laser light on each of the plurality of information recording surfaces of an information recording medium having a plurality of information recording surfaces parallel to each other, in order from the light source side to the information recording surface,
A magnification conversion element and an objective lens;
When the magnification conversion element is moved in the optical axis direction to change the imaging magnification, and when the maximum imaging magnification is m1 and the minimum imaging magnification is m2, the following expression is obtained. Meet the optical system:
| M1 + m2 | <0.002 A light source;
Condensing means for condensing the light emitted from the light source on the surface of the information recording medium;
Separating means for separating a light beam modulated by the information recording medium;
Comprising light receiving means for receiving light modulated by the information recording medium,
The optical system including the light collecting unit from the light source is an optical pickup device according to any one of claims 1 to 3.

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