JP2000090470A - Optical system for recording and reproducing optical disks - Google Patents

Abstract

(57) [Problem] To provide an optical system for a low-density optical disk with a divergent finite system while using an objective lens optimally designed in consideration of manufacturing tolerance for an optical system for a high-density optical disk. Coma at the time of lens shift is reduced. A first optical path for condensing a light beam from a light source on a high-density optical disk, and a second optical path for condensing a light beam from a light source on a low-density optical disk are provided. Has an objective lens 15 that converges a light beam from the light source 11 onto the optical disc 16, and the second optical path has
The objective lens 15 is a finite conjugate optical system that shares the objective lens 15 with the first optical path as a lens that converges the light beam from the light source 41 to the optical disk 26. The position is optimally designed, and the position of the light source 41 in the optical axis direction is displaced by a predetermined amount from the position 21 where the wavefront aberration on the information recording surface of the optical disk 26 is minimized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
Alternatively, the present invention relates to an optical system for recording and / or reproducing optical disks having different corresponding wavelengths.

[0002]

2. Description of the Related Art In recent years, standards for optical disks have been increased due to the increase in density and the spread of recordable media, and recording and / or reproducing optics have been developed in order to cope with differences in disk substrate thickness and wavelength dependence of reflectance. It is necessary to change the numerical aperture (hereinafter, NA) of the objective lens of the system (hereinafter, abbreviated as an optical system) and the used wavelength. For example, the thickness of a CD substrate is 1.2 mm, while that of a DVD is 0.6 mm. The reflectivity of the disk recording material used for the CD-R is 65% or more at a wavelength of 780 to 830 nm, but drops to 20% or less at a wavelength of 635 to 650 nm. Therefore, in the optical system corresponding to DVD, the light source wavelength 635-650 nm and the objective lens NA
0.6 is used. In an optical system corresponding to CD-R, a light source wavelength of 780 to 830 nm and an objective lens NA are used.
Typically 0.45 is used.

[0003] It is desirable that the same optical disk having different optical systems can be recorded and reproduced by the same optical disk device, and it is required to reduce the size and cost of the optical disk device. A method of sharing an optical system has been proposed.

As a typical method, Japanese Patent Laid-Open Publication No. 8-55
There is an optical head disclosed in JP-A-363-363. In this method, a condensing lens for condensing a light beam emitted from a light source and an objective lens for converging the light beam on an information recording surface of the optical disk are shared, and an optical system corresponding to a high-density optical disk such as a DVD is used. An infinite conjugate system (hereinafter abbreviated as an infinite system) is used, and an objective lens which is an infinite system and optimally designed for the substrate thickness of a high-density optical disk is used. CD or C
When recording and reproducing optical discs having different substrate thicknesses and relatively low densities, such as DR, the optical system is defined as a finite conjugate system (hereinafter, abbreviated as a finite system) to reduce spherical aberration caused by the difference in substrate thickness. By canceling out, good signal recording / reproducing characteristics are obtained.

[0005]

However, in the system disclosed in Japanese Patent Application Laid-Open No. 8-55363, although the optical system is optimized for recording and reproducing on a high-density optical disk, recording on a low-density optical disk is performed. For reproduction, there is a problem that coma aberration increases when the objective lens is moved (lens shift) in a direction perpendicular to the optical axis for tracking control.

[0006] This increase in coma aberration occurs when the aperture stop moves integrally with the objective lens, but becomes more remarkable when the aperture stop is installed at a fixed portion of the optical system. When the aperture stop moves with the objective lens, the luminous flux incident on the objective lens always passes through a circle with the same diameter as the aperture stop centered on the lens optical axis even if there is a lens shift, but the aperture stop is fixed In the case of being installed on the side, the center of the narrowed light beam is shifted from the lens optical axis at the time of lens shift, and transmits through the outer peripheral portion of the objective lens where aberrations are likely to occur. For this reason, the amount of generation of coma aberration is further increased as compared with the case where the aperture stop moves integrally with the objective lens. Recently, it has been required to reduce the thickness of the optical head in order to reduce the thickness of the apparatus. However, as the thickness of the optical head is reduced, it becomes difficult to dispose the aperture limiting means near the objective lens. It needs to be installed on a fixed part. Therefore, in order to satisfy the demand for thinning, it is inevitable to generate coma at the time of lens shift, and this greatly deteriorates the recording or reproducing performance of the optical system.

In order to solve this problem by designing an objective lens, not only the conditions for optimizing the performance of a high-density optical disc but also the lens shift in a finite optical system corresponding to different substrate thicknesses are set as objective lens design conditions. There has been proposed a design method for performing an optimal design in consideration of a condition for suppressing the occurrence of aberration in the case of (for example, JP-A-9-179).
No. 021). However, in the design of an objective lens, in general, the center of the first surface and the second surface of the lens are displaced in order to satisfy the on-axis performance and the off-axis performance for one type of optical disk and to secure tolerance during manufacturing. In this case, it is necessary to satisfy the condition that large aberration does not occur even in the case of decentering. If the three conditions of on-axis performance, off-axis performance, and decentering are to be satisfied, the Usually, the degree of freedom is greatly restricted. Conversely, if it is attempted to achieve both on-axis performance and off-axis performance of an infinite system and a finite system, it becomes difficult to satisfy the decentering condition. Therefore, although such an objective lens can be designed at the expense of tolerance during manufacturing, it is difficult to realize.

As another method for suppressing the occurrence of aberration at the time of lens shift in a finite system, two optical systems corresponding to optical disks having different substrate thicknesses are both finite systems, and each objective lens has a lateral magnification. There has been proposed a method of reducing the coma aberration at the time of lens shift by reducing the divergence angle or convergence angle of the incident light of the objective lens by configuring to have the opposite sign (Japanese Patent Laid-Open No. 9-316123).
No.). For example, the optical system for a high-density optical disk is a finite system in which the objective lens incident light is convergent light (hereinafter, referred to as a convergent finite system), and the optical system for a low-density optical disk having a larger substrate thickness emits the objective lens incident light. This is a system that is a finite system that is light (hereinafter, referred to as a divergent finite system).

However, in this system, there is a problem that the adjustment process of the optical head is complicated and lengthened by using a convergent finite optical system for recording / reproducing a high-density optical disk. Generally, when the divergence angle or convergence angle of the incident light of the objective lens deviates from the design value, the aberration on the information recording surface of the optical disk increases. Therefore, an optical system using a condenser lens and an objective lens, particularly a high-density optical disk sensitive to aberration. In the optical system for use, it is necessary to adjust the divergence angle or convergence angle of the light beam by changing the distance between the light source and the condenser lens.

If the focal length of the condenser lens is sufficiently long, this adjustment can be omitted by determining the positions of the light source and the condenser lens with mechanical accuracy. In addition to this, if the lens diameter is the same, the light transmission efficiency decreases in proportion to the focal length of the condenser lens.

Therefore, in order to reduce the size of the optical head and to ensure the accuracy required for recording and reproduction on a high-density optical disc, the focal length of the condenser lens must be set to an appropriate value, and then the light source and the condenser lens must be set. It is necessary to adjust the distance. If the optical system is an infinite system, the light flux after passing through the condenser lens is almost parallel, and it is easy to adjust the state using only the light source and the condenser lens using an autocollimator or the like.

However, when the optical system is a finite system, it is difficult to accurately adjust only the light source and the condensing lens. Must be adjusted so that In order to reproduce the information on the optical disk, the detection optical system for detecting the light reflected on the optical disk must be adjusted, and for that, the distance between the light source and the condenser lens must be fixed.

Therefore, in the case of a finite optical system, the adjustment of the distance between the light source and the condenser lens and the adjustment of the detection optical system must be repeated alternately to gradually increase the accuracy. However, there is a problem that it is complicated and the time is long.

In view of the above-mentioned problems of the optical system for recording / reproducing an optical disk, the present invention provides an objective system which is optimally designed for an optical system for a high-density optical disk in consideration of manufacturing tolerance. An object of the present invention is to provide an optical disc recording / reproducing optical system that reduces coma aberration during lens shift in a low-density optical disc optical system that is a divergent finite system while using a lens.

[0015]

In order to solve the above-mentioned problems, a first invention (corresponding to the first invention) is provided.
A first optical path for condensing a light beam from a first light source on a first optical disc, and a light beam from a second light source on a second optical disc having a substrate thickness different from that of the first optical disc. A second optical path that emits light, wherein the first optical path has an objective lens that converges a light beam from a first light source on the first optical disc, and the second optical path includes the second optical path. A finite conjugate optical system that shares the objective lens with the first optical path as a lens that converges a light beam from a light source on the second optical disk, wherein the objective lens is connected to the first optical path and the first optical path. And the second light source is positioned such that its position in the optical axis direction is at a position where the wavefront aberration on the information recording surface of the second optical disk is minimized. Displaced by a predetermined amount A reproducing optical system of the optical disk to be.

According to a second aspect of the present invention (corresponding to the second aspect of the present invention), when the predetermined amount is λ where the wavelength of the second light source is λ, the information recording surface of the second optical disk is The recording / reproducing optical system for an optical disc according to the first aspect of the present invention, wherein the rms value WA of the wavefront aberration satisfies the condition of Formula 1.

[0017]

0.02 ≦ WA / λ ≦ 0.06 According to a third aspect of the present invention (corresponding to the third aspect of the present invention), the substrate thickness of the first optical disk is t1, the second optical disk is The recording / reproducing optical system for an optical disk according to the first or second aspect of the present invention, wherein t1 <t2 when the substrate thickness is t2.

According to a fourth aspect of the present invention (corresponding to the fourth aspect of the present invention), an optical disk side numerical aperture corresponding to the first optical path is set to NA1, and an optical disk corresponding to the second optical path is set to NA1. When the side numerical aperture is NA2, the wavelength of the first light source is λ1, and the wavelength of the second light source is λ2, NA
4. The optical system for recording / reproducing an optical disk according to claim 3, wherein 1> NA2 and λ1 <λ2.

A fifth aspect of the present invention (corresponding to the fifth aspect of the present invention) has an aperture function only for the light beam from the second light source, and for the light beam from the first light source. A fourth aspect of the present invention is an optical system for recording / reproducing an optical disk according to the fourth aspect of the present invention, comprising an aperture limiting means having no influence.

According to a sixth aspect of the present invention (corresponding to the sixth aspect of the present invention), the aperture limiting means is supported so as to be controlled to move integrally with the objective lens, and the second light source is provided with A fifth feature is that the position in the optical axis direction is located farther from the objective lens than the position where the wavefront aberration on the information recording surface of the second optical disc is minimized. 2 is an optical system for recording and reproducing the optical disk of the present invention.

According to a seventh aspect of the present invention (corresponding to the seventh aspect of the present invention), the aperture limiting means is supported independently of the movement of the objective lens, and the second light source is provided with the light source. The fifth book is arranged such that an axial position is closer to the objective lens than a position where the wavefront aberration on the information recording surface of the second optical disc is minimized. 4 is an optical system for recording and reproducing the optical disc of the invention.

The operation of the recording / reproducing optical system for an optical disk having the above configuration is as follows.

When recording / reproducing a second optical disk having a different substrate thickness using an objective lens optimally designed in consideration of the manufacturing tolerance of the first optical disk, the optical system is made to be a finite system. 1st, 2nd
The spherical aberration generated due to the difference in the substrate thickness of the optical disk is corrected and set to a minimum. In this case, if the objective lens is shifted in a direction perpendicular to the optical axis by tracking control, coma aberration is likely to occur.

Since the coma generated at this time has a relation to cancel the spherical aberration when the objective lens is on the axis of the optical system, it is necessary to correct the coma until the spherical aberration on the axis can be almost completely removed. In other words, coma aberration generated at the time of lens shift can be reduced by remaining (under) or excessively correcting (over) to the extent that the quality of information recording / reproduction does not deteriorate.

Whether the correction of the spherical aberration is under or over, the effect of reducing the coma upon lens shift depends on the distribution of the coma, and if the aperture stop moves integrally with the objective lens, the under-correction occurs. When the aperture stop is provided on the fixed side of the optical system, overcorrection is performed. This is because, in the former case, the light beam incident on the objective lens always passes through a circle having the same diameter as the aperture stop centered on the lens optical axis even if there is a lens shift, whereas in the latter case, the light beam is narrowed down. This is because the center of the light flux is shifted from the lens optical axis during the lens shift and passes through the outer edge of the objective lens where aberrations are likely to occur, so that the distribution of coma aberration is different.

As described above, when the aperture stop moves integrally with the objective lens, the correction of the spherical aberration is under, and when the aperture stop is installed on the fixed side of the optical system, the correction is over. In addition, coma generated at the time of lens shift can be reduced.

The above equation (1) is a condition for defining the amount of remaining or overcorrecting the spherical aberration component on the axis for this purpose. When the amount of on-axis wavefront aberration is smaller than the lower limit of Expression 1, the effect of reducing coma upon lens shift cannot be expected because the amount of spherical aberration component is insufficient. If the amount of wavefront aberration is larger than the upper limit of Equation 1, the amount of spherical aberration component is excessive, so that the quality of information recording / reproducing on the axis is impaired and the quality of recording / reproducing is stable when the lens is shifted. May exceed the Marechal's reference value of 0.07.

[0028]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) First, the first embodiment of the present invention will be described.
An embodiment will be described with reference to the drawings.
For simplicity, a description of a detection optical system that detects light reflected from an optical disk is omitted.

FIG. 1 is a configuration diagram of an optical system according to the first embodiment of the present invention. In FIG. 1, (a) shows a high-density optical disk 16 having a substrate thickness of 0.6 mm (the first optical disk of the present invention).
(B) is a relatively low-density optical disc 26 having a substrate thickness of 1.2 mm.
(Corresponding to the second optical disk of the present invention) is shown for recording and reproducing, respectively.

In FIG. 1A, a light beam emitted from a light source 11 having a wavelength of 650 nm (corresponding to the first light source of the present invention) is converted into a substantially parallel light beam by a condenser lens 12 and passes through a beam splitter 13. Then, after being stopped down to an appropriate light beam diameter by the aperture stop 14, the light enters the objective lens 15 (corresponding to the objective lens of the present invention) and forms an image as a light spot 17 on the information recording surface of the optical disk 16. At this time, the NA of the optical disk 16 is 0.6, and the objective lens 15 is designed in consideration of the tolerance at the time of manufacturing so that the best performance can be obtained with respect to this optical system. FIG.
The optical path shown in (a) corresponds to the first optical path of the present invention.

In FIG. 1B, the wavelength 780 n
The light beam emitted from the light source 21 (corresponding to the second light source of the present invention) passes through the beam splitter 13 and is then narrowed down to an appropriate light beam diameter by the aperture limiting means 24 and then enters the objective lens 15. Then, an image is formed as an optical spot 27 on the information recording surface of the optical disk 26. At this time, the NA of the optical disk 26 is 0.45. The aperture restricting means 24 (corresponding to the aperture restricting means of the present invention) is supported so as to move integrally with the objective lens 15, performs an aperture function only on the light beam from the light source 21, and does not affect the light beam from the light source 11. (For example, in a light beam incident on the objective lens 15, when the NA is in the range of 0.45 or less, the light is totally transmitted at both the wavelengths of 650 nm and 780 nm, and the NA is 0. Wavelength 6 in the range larger than 45
An optical thin film having spectral characteristics such that it is totally transmitted for light of 50 nm and totally reflected for light of wavelength 780 nm. ). Note that the optical path shown in FIG.
This corresponds to the optical path of FIG.

Reference numeral 31 denotes a light source position at which the wavefront aberration on the optical disk 26 is minimized, and the light source is conventionally set at this position. In the present embodiment,
The light source 21 is located farther from the objective lens 15 than the position 31, and is set so that correction of spherical aberration due to a difference in substrate thickness between the optical disc 16 and the optical disc 26 is on the under side. The position of the light source 21 is set so as to satisfy Equation 1, and in the present embodiment, as described later, the position is such that WA / λ = 0.044.

FIG. 2 shows the relationship between the aberration on the information recording surface of the optical disk 26 and the shift amount of the objective lens 15 when the light source is located at the position 31 in the optical system of FIG. That is, it is the lens shift characteristic of the aberration in the conventional optical system. When the objective lens 15 is on the axis of the optical system, the aberration is almost zero, and the spherical aberration component due to the difference in substrate thickness is well corrected.
When 5 is shifted in the direction perpendicular to the optical axis by tracking control, a coma component is generated, and it can be seen that the coma component is dominant with respect to the wavefront aberration.

FIGS. 3 to 6 show the relationship between the aberration of the light spot 27 and the shift amount of the objective lens 15 in the optical system shown in FIG. 1B, and FIGS. 3, 4, 5, and 6 show the relationship. They show a wavefront aberration, an astigmatism component, a coma aberration component, and a spherical aberration component, respectively. For comparison, the aberration when the light source is at the position of 31 is also shown. The value (WA / λ) obtained by dividing the rms value WA of the on-axis wavefront aberration by the wavelength λ is 0.044, which satisfies the condition of Expression 1. The aberration at the time of lens shift has a spherical aberration component generally remaining compared to when the light source is at the position of 31, whereas the coma aberration component is about 3%.
It can be seen that the astigmatism component hardly changes.

FIG. 7 shows a result of calculating a jitter value of a reproduced signal when a CD is reproduced by the optical system of FIG. 1B. For comparison, the calculated jitter value when the light source is at the position of 31 is also shown. Lens shift is 0.
When the distance is 3 mm or less, the jitter is increased as compared with the case where the light source is at the position of 31, but the increase amount is 0.5 point, and there is almost no influence on the reproduction quality of the signal. On the other hand, when the lens shift is 0.3 mm or more, the magnitude relation of the jitter is reversed. When the lens shift is 0.5 mm, the difference is 1.
5 points. Reducing the coma aberration by leaving the spherical aberration in this way is effective in improving the quality of the reproduced signal.

In this embodiment, the position of the second light source of the present invention in the optical axis direction is determined by the rms value Wm of the wavefront aberration.
A and the wavelength λ of the second light source are represented by WA / λ = 0.
044 is described as being in a position that satisfies
It has been confirmed that if the range of Expression 1 is satisfied, coma aberration can be reduced without adversely affecting the reproduction quality of a signal and other optical performances, which are practically problematic.

(Second Embodiment) Next, a second embodiment of the present invention will be described.
An embodiment will be described with reference to the drawings.
Note that, for simplicity, the description of the detection optical system is omitted as in the first embodiment. The present embodiment is different from the above-described first embodiment only in the point relating to the opening restricting means of the present invention, and the rest is the same as the first embodiment. Therefore, in the present embodiment,
Components that are not particularly described are the same as those in the first embodiment, and those components that are assigned the same reference numerals as those in the first embodiment are the same as those in the first embodiment unless otherwise described. Assume that they have similar functions.

FIG. 8 is a configuration diagram of an optical system according to the second embodiment of the present invention. In FIG. 8, (a) shows the case of recording / reproducing a high-density optical disk 16 having a substrate thickness of 0.6 mm, and (b) shows an optical disk 26 having a substrate thickness of 1.2 mm.
Respectively when recording and reproducing.

FIG. 8 (a) has the same configuration as FIG. 1 (a). In FIG. 8B, the light beam emitted from the light source 41 is converted into the aperture limiting means 28 (corresponding to the aperture limiting means of the present invention).
After the aperture is adjusted to an appropriate luminous flux diameter with the objective lens 15
Incident on. The aperture limiting means 28 is provided for the objective lens 15
Of the light source 41
It is configured by appropriate means so as to stop the light beam from the light source 11 and not to affect the light beam from the light source 11 (for example, NA out of the light beam incident on the objective lens 15).
In the range of 0.45 or less, the wavelengths 650 nm and 780
An optical thin film having spectral characteristics such that it is totally transparent to both wavelengths of nm and that when the NA is greater than 0.45, it is totally transmitted for light of wavelength 650 nm and totally reflected for light of wavelength 780 nm. . ).

The light source 41 is closer to the objective lens 15 than the light source position 31 at which the wavefront aberration on the optical disk 26 is minimized, and correction of spherical aberration due to the difference in substrate thickness between the optical disk 16 and the optical disk 26 is performed. It is set to be on the over side. The position of the light source 41 is given by
In the present embodiment, the position is such that WA / λ = 0.039, as described later.

FIG. 9 shows the relationship between the aberration on the information recording surface of the optical disk 26 and the shift amount of the objective lens 15 when the light source is located at the position 31 in the optical system of FIG. That is, it is the lens shift characteristic of the aberration in the conventional optical system. When the objective lens 15 is on the axis of the optical system, the aberration is almost zero, and the spherical aberration due to the difference in substrate thickness is well corrected, but the objective lens 15 is shifted in a direction perpendicular to the optical axis by tracking control. Then, a coma aberration component and an astigmatism component are generated, and it is understood that the coma aberration component and the astigmatism component are dominant with respect to the wavefront aberration.

FIGS. 10 to 13 show the relationship between the aberration of the light spot 27 and the shift amount of the objective lens 15 in the optical system of FIG. 8B, and FIG. 10, FIG. They respectively show the total wavefront aberration, astigmatism component, coma aberration component and spherical aberration component. For comparison, the light source is 31
The aberration at the position of is also described. The value obtained by dividing the rms value WA of the on-axis wavefront aberration by the wavelength λ (WA
/ Λ) is 0.039, which satisfies the condition of Expression 1. The aberration at the time of lens shift reduces astigmatism component, coma aberration component and spherical aberration component compared to when the light source is at the position of 31, and the coma aberration component which has a great effect on the deterioration of signal quality is about It turns out that it becomes half.

FIG. 14 shows a CD formed by the optical system shown in FIG.
11 shows the result of calculating the jitter value of the reproduced signal when. For comparison, the calculated jitter value when the light source is at the position of 31 is also shown. When the lens shift is 0.1 mm or less, the jitter slightly increases as compared with the case where the light source is at the position of 31, but the increase is 0.3 points and can be ignored. On the other hand, when the lens shift is 0.1 mm or more, the magnitude relationship of the jitter is reversed, and it can be seen that the effect of reducing the coma aberration appears remarkably.

In the present embodiment, the position of the second light source of the present invention in the optical axis direction is determined by the rms value Wm of the wavefront aberration.
A and the wavelength λ of the second light source are represented by WA / λ = 0.
039 has been described as being in a position that satisfies
It has been confirmed that if the range of Expression 1 is satisfied, coma aberration can be reduced without adversely affecting the reproduction quality of a signal and other optical performances, which are practically problematic.

In the above-described first and second embodiments, an example is shown in which the condenser lens is used only for the infinite optical system, and the divergent finite optical system is composed only of the light source and the objective lens. The optical lens may be shared with a finite system. An example of the configuration in this case is shown in FIGS. FIGS. 15 and 16 show examples in which the condenser lens is shared based on the optical systems of FIGS. 1 and 8, respectively.
Consists of the same components. In FIG. 15B, the position of the light source 51 is set in consideration of the refractive power of the condenser lens 12 so that the same wavefront aberration as in the optical system of FIG. The position 61 is the position of the light source where the wavefront aberration of the light spot 27 is minimized, and corresponds to the position 31 in FIG. The lens shift characteristics of the aberration and the jitter of this optical system are the same as those shown in FIGS. In FIG. 16B, the position of the light source 71 is set in consideration of the refractive power of the condenser lens 12 so that the same wavefront aberration as in the optical system of FIG. The lens shift characteristics of aberration and jitter of this optical system are the same as those in FIGS. 10 to 13 and 14. FIGS. 15 and 16 are optically equivalent to FIGS. 1 and 8, respectively, but the projection area can be made smaller, which is suitable for downsizing the optical head.

In the first and second embodiments described above, the first optical system corresponding to an optical disk having a substrate thickness of 0.6 mm is an infinite system. Can be set independently of the configuration of the first optical system. Therefore, if there is an advantage in making the first optical system a finite system, the present invention can be applied to such a first optical system as well. A similar effect can be obtained by applying the invention.

The optical recording / reproducing optical system for an optical disk according to the present invention has been described as including the aperture limiting means according to the present invention in the first and second embodiments described above. Even if it is not provided, the effect that coma aberration at the time of lens shift can be reduced as compared with a conventional optical system for recording / reproducing an optical disk which does not have an aperture limiting means can be obtained. In short, a first optical path for condensing a light beam from the first light source on the first optical disc, and a first light path for condensing the light beam from the second light source on the first optical disc having a different substrate thickness and / or corresponding wavelength from the first optical disc. A second optical path for converging light onto a second optical disk, wherein the first optical path has an objective lens for converging a light beam from a first light source on the first optical disk, and the second optical path is A finite conjugate optical system that shares the objective lens with the first optical path as a lens that converges a light beam from the second light source onto the second optical disc, wherein the objective lens is Optimally designed for the combination of the optical path and the first optical disc, wherein the second light source is positioned so that its position in the optical axis direction has a minimum wavefront aberration on the information recording surface of the second optical disc. From the position where If arranged Configurations, as compared to the recording reproducing optical system of the conventional optical disk, the effect is obtained of reducing the coma when the lens shift.

[0049]

As is apparent from the above description, the present invention provides an optical system for a high-density optical disk while using an objective lens which is optimally designed in consideration of manufacturing tolerance. It is possible to provide an optical system for recording / reproducing an optical disk, which reduces coma upon lens shift in an optical system for a low-density optical disk.

That is, according to the present invention, even when an optical system corresponding to a high-density optical disk is made into an infinite system that can be easily adjusted, and an objective lens of an optimal design that takes into account the tolerance during manufacturing is used, the thickness of the corresponding substrate or the corresponding Good tracking performance can be obtained in a finite optical system that records and reproduces optical disks having different wavelengths. In addition, the present invention provides not only a case where the aperture limiting means is supported so as to move integrally with the objective lens as in the related art, but also a more remarkable effect when it is installed on the fixed side of the optical system. This makes it possible to provide a very advantageous optical system.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical system according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a relationship between aberration and a lens shift amount in a conventional optical system.

FIG. 3 is an explanatory diagram illustrating a relationship between a wavefront aberration and a lens shift amount according to the first embodiment of the present invention.

FIG. 4 is an explanatory diagram illustrating a relationship between an astigmatism component and a lens shift amount according to the first embodiment of the present invention.

FIG. 5 is an explanatory diagram illustrating a relationship between a coma aberration component and a lens shift amount according to the first embodiment of the present invention.

FIG. 6 is an explanatory diagram illustrating a relationship between a spherical aberration component and a lens shift amount according to the first embodiment of the present invention.

FIG. 7 is an explanatory diagram illustrating a relationship between a jitter and a lens shift amount according to the first embodiment of the present invention.

FIG. 8 is a configuration diagram of an optical system according to a second embodiment of the present invention.

FIG. 9 is an explanatory diagram showing a relationship between aberration and a lens shift amount in a conventional optical system.

FIG. 10 is an explanatory diagram illustrating a relationship between a wavefront aberration and a lens shift amount according to the second embodiment of the present invention.

FIG. 11 is an explanatory diagram illustrating a relationship between an astigmatism component and a lens shift amount according to the second embodiment of the present invention.

FIG. 12 is an explanatory diagram illustrating a relationship between a coma aberration component and a lens shift amount according to the second embodiment of the present invention.

FIG. 13 is an explanatory diagram illustrating a relationship between a spherical aberration component and a lens shift amount according to the second embodiment of the present invention.

FIG. 14 is an explanatory diagram illustrating a relationship between a jitter and a lens shift amount according to the second embodiment of the present invention.

FIG. 15 is a configuration diagram of a modified example equivalent to the optical system according to the first embodiment of the present invention.

FIG. 16 is a configuration diagram of a modification equivalent to the optical system according to the second embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 1st light source 12 condensing lens 13 beam splitter 14 aperture stop 15 objective lens 16 1st optical disk 17 1st light spot 21 2nd light source 24 aperture limiting means 26 2nd optical disk 27 2nd light spot 28 Aperture limiting means 31 Second light source position in conventional optical system 41 Second light source 51 Second light source 61 Second light source position in conventional optical system 71 Second light source

Claims (7)

[Claims] 1. A first optical path for condensing a light beam from a first light source on a first optical disk, and a second light beam from a second light source having a substrate thickness different from that of the first optical disk. A second optical path for converging light on an optical disk, wherein the first optical path has an objective lens for converging a light beam from a first light source on the first optical disk, and the second optical path is A finite conjugate optical system that shares the objective lens with the first optical path as a lens that converges a light beam from a second light source onto the second optical disc;
The second light source is optimally designed for the combination of the first optical path and the first optical disc, and the position of the second light source in the optical axis direction is on the information recording surface of the second optical disc. An optical system for recording / reproducing an optical disk, wherein the optical system is arranged so as to be displaced by a predetermined amount from a position where the wavefront aberration is minimized. 2. The predetermined amount is such that when the wavelength of the second light source is λ, the rms value WA of the wavefront aberration on the information recording surface of the second optical disk satisfies the condition of Formula 1. The optical system for recording / reproducing information on an optical disk according to claim 1, wherein: ## EQU1 ## 0.02 ≦ WA / λ ≦ 0.06 3. The substrate thickness of the first optical disk is t
3. The optical system for recording / reproducing an optical disk according to claim 1, wherein t1 <t2 when a substrate thickness of the second optical disk is t2. 4. An optical disk side numerical aperture when corresponding to the first optical path is NA1, an optical disk side numerical aperture when corresponding to the second optical path is NA2, a wavelength of the first light source is λ1, When the wavelength of the second light source is λ2,
4. The optical system according to claim 3, wherein NA1> NA2 and λ1 <λ2. 5. An aperture limiting means having an aperture function only for a light beam from the second light source and having no effect on a light beam from the first light source. 5. The optical system for recording / reproducing an optical disk according to item 4. 6. The aperture limiting means is supported so as to be controlled to move integrally with the objective lens, and the position of the second light source in the optical axis direction on the information recording surface of the second optical disc is controlled. 6. The optical disc recording / reproducing optical system according to claim 5, wherein the optical disc is arranged so as to be located farther from the objective lens than a position where the wavefront aberration is minimized. 7. The aperture limiting means is supported independently of the movement of the objective lens, and the position of the second light source in the optical axis direction is a wavefront on the information recording surface of the second optical disc. 6. The optical system according to claim 5, wherein the optical system is arranged so as to be closer to the objective lens than a position where aberration is minimized.