JP4722472B2 - Optical system for optical disc - Google Patents

Description

  An optical system for an optical disc used in an optical information recording / reproducing apparatus for recording and / or reproducing information on a plurality of types of optical discs having different protective layer thicknesses and recording densities, and does not particularly use a coupling lens such as a collimating lens. The present invention relates to an optical system for an optical disc having a configuration.

  There are multiple standards for optical disks with different recording densities and protective layer thicknesses. For example, a DVD (digital versatile disk) has a higher recording density and a protective layer is thinner than a CD (compact disk). Therefore, for optical discs with different standards, recording is performed by changing the numerical aperture (NA) of light used for recording and reproducing information while correcting spherical aberration that changes depending on the thickness of the protective layer of each disc. It is necessary to use an optical system for an optical disc configured to obtain an effective beam diameter corresponding to the difference in density.

  Conventional optical systems for optical discs are composed of a light source, a coupling lens, an objective lens, and the like. Here, the coupling lens is a collimating lens that collimates the light emitted from the light source, or a lens having a function of converting the divergence (magnification) of the light beam. Coupling lenses have been used for the purpose of suppressing the occurrence of aberrations on the recording surface or increasing the light utilization efficiency. However, in recent years, it has been studied to reduce the number of components in order to reduce the cost and further reduce the size of the optical system for optical disks. Therefore, an optical system for an optical disc of a so-called finite conjugate system (finite system) has been proposed in which the coupling lens is omitted as in Patent Documents 1 to 4 below.

JP-A-8-62496 JP-A-8-334686 JP-A-64-25113 JP-A-2-223906

  Each of the optical systems for optical discs disclosed in the above Patent Documents 1 to 4 is premised on the use of an optical disc having a low recording density such as CD or CD-R and a thick protective layer. That is, each optical system is configured to record and reproduce information by forming a relatively large spot with a low NA.

  In the optical systems for finite optical disks disclosed in the above Patent Documents 1 to 4, aberration occurs when the relative positional relationship between the light source and the objective lens is changed. Specifically, when the objective lens is moved along the central axis direction for focusing, spherical aberration occurs. Further, when the objective lens is moved in a direction orthogonal to the optical axis direction for tracking (tracking shift), coma and astigmatism due to incidence of off-axis light on the objective lens is generated. Here, the optical axis is a state where the light source, the objective lens, and the optical disk are aligned in a straight line without being tilted or laterally displaced (even if they are folded by a mirror or the like, they are substantially in a straight line). It is a reference axis in a state that can be regarded as being in a line. In general, in order to record and reproduce information with respect to an optical disk with high accuracy, it is desired to suppress the occurrence of various aberrations as much as possible. In particular, non-rotationally symmetric aberrations such as coma and astigmatism are a major factor in degrading the quality of information recording and reproduction signals, so the optical disc optical system can sufficiently correct the non-rotationally symmetric aberrations. It is desirable to be configured as follows.

  In the text, the movement of the objective lens along the optical axis direction for focusing is called focusing shift. Further, moving the objective lens in the direction orthogonal to the optical axis direction (circumferential direction of the optical disk) for tracking is called tracking shift.

  Here, when the optical disc is tilted with respect to the optical axis, coma aberration generated in the protective layer of the optical disc has a property of changing in accordance with the thickness of the protective layer. Therefore, in each of the above-mentioned patent documents, an aplanatic objective lens having both aspherical surfaces is adopted in the optical disc optical system in order to cancel the coma generated in the protective layer. This suppresses the occurrence of coma aberration during optical information recording or reproduction.

  However, it is extremely difficult to sufficiently correct astigmatism, which is another non-rotationally symmetric aberration, even if a double-sided aspheric objective lens is used. In other words, each of Patent Documents 1 to 4 is capable of recording and reproducing information with a relatively low NA, and an allowable amount for aberration to the extent that information can be recorded and reproduced even if astigmatism remains. This is a configuration specialized for an apparatus for an optical disc (such as a CD) having a large size. In other words, each of the above Patent Documents 1 to 4 is not applicable to an optical disc apparatus that has a high recording density such as a DVD and needs to form a small-diameter spot with a high NA when recording or reproducing information. There was a problem I said.

  In recent years, many optical information recording / reproducing apparatuses are equipped with optical systems for optical discs that are compatible with DVDs and CDs. Here, taking into account that the amount of coma changes depending on the thickness of the disk protective layer, such optical disc optical systems that are compatible with a plurality of types of optical discs are disclosed in the above Patent Documents 1 to 4. Like an optical system, it is impossible to correct coma generated when each disk is used only by the configuration of the objective lens. In addition, astigmatism remains in a compatible optical system for optical discs as in the above-mentioned Patent Documents 1 to 4. Therefore, if the configurations of the above-mentioned Patent Documents 1 to 4 are applied to a compatible optical system for optical discs, there is a problem in that high-precision information cannot be recorded or reproduced on an optical disc having a high recording density.

  Therefore, in view of the above circumstances, the present invention is directed to an optical system for an optical disc that can satisfactorily suppress various aberrations that occur during information recording / reproduction even when a coupling lens is omitted, and particularly to a plurality of types of optical discs having different protective layer thicknesses. An object of the present invention is to provide an optical system for an optical disc that realizes recording and / or reproduction of information.

In order to solve the above problems, an optical system for an optical disc according to the present invention is an optical system for an optical disc that can use an optical disc having a numerical aperture of 0.60 or more on the optical disc side required for recording and / or reproducing information. A light source for irradiating divergent light, an objective lens for condensing the divergent light from the light source on the optical disc, and a drive means for tracking and shifting at least the objective lens and changing the attitude of the objective lens. The driving means is characterized in that the posture of the objective lens is changed so that the central axis of the objective lens is directed toward the light source at the same time as the tracking shift is performed.

As described above, by changing (tilting) the posture of the objective lens simultaneously with the tracking shift, astigmatism generated in the objective lens by tracking can be reduced by the tilt. That is, according to the present invention, even in an optical disc that requires a relatively high NA for recording and / or reproducing information, the influence of aberration can be reduced and stable information recording and / or reproducing can be executed. it can. Further, according to the present invention, astigmatism is corrected by changing the posture of the objective lens without giving any aberration to the objective lens itself. Therefore, there is no element that degrades the performance in the state before the tracking shift that is the reference state.

In order to effectively suppress the generation of astigmatism and to make an optical disc optical system suitable for an optical disc apparatus that requires a high NA during use, the air equivalent distance from the light source to the recording surface of the optical disc is d [ mm], the amount of movement of the image by tracking shift is TR [mm], and the amount of change in the attitude of the objective lens during tracking shift (ie, the amount of lens tilt) is θ [°], the following conditional expression (1) is satisfied. It is desirable to do.
0.25 ≦ d · tanθ / TR ≦ 0.75 (1)

  Conditional expression (1) is a condition for defining the lens tilt amount. Here, in order to completely correct astigmatism, the lens tilt amount may be defined so that d · tan θ / TR = 1. However, if the lens tilt amount is defined so that d · tan θ / TR = 1, a large amount of coma generated in the protective layer of the disk remains. Therefore, it is desirable to satisfy the conditional expression (1) in order to satisfactorily correct astigmatism while suppressing the occurrence of coma.

  If the lower limit of conditional expression (1) is not satisfied, that is, the lens tilt amount is small, the astigmatism correction effect is small, which is not preferable. If the amount of lens tilt is small, when a large amount of higher-order coma aberration occurs in the objective lens, the coma aberration remains large without being sufficiently corrected. Conversely, if the upper limit of conditional expression (1) is exceeded, that is, the lens tilt amount is large, the coma aberration generated in the objective lens remains, or the amount of coma aberration generated due to an error in the lens tilt amount increases. Too much. In order to reduce the error, it is conceivable to improve the driving accuracy of driving means such as an actuator, but this is not preferable because it leads to further cost increase.

In the optical system for optical discs, in order to effectively suppress the occurrence of coma aberration, it is desirable to use an objective lens having a characteristic that satisfies the following conditional expression (2).
−0.75 ≦ CM _D / CM _L ≦ −0.15 (2)
However, _CML is the sensitivity of coma aberration that occurs when only the objective lens is tilted with respect to the light beam emitted from the light source.
CM _D is the sensitivity of the coma aberration generated when only the inclined disc for the light flux transmitted through the objective lens is irradiated from the light source, each representing.

In general, when an objective lens is mounted on an optical system for an optical disc, the relative position of the objective lens relative to the light source or the optical disc is such that the central axis substantially coincides with the principal ray of the light beam from the light source and is orthogonal to the recording surface of the optical disc. Adjustments are made. The objective lens is designed so as not to cause coma even when off-axis light is incident on the objective lens in the position-adjusted state. More specifically, in the objective lens, the coma aberration generated when the objective lens is tilted with respect to the incident light beam has the same amount and a different sign (coma aberration generated when the optical disk is tilted with respect to the incident light beam). It is designed so that CM _D / CM _L ≈-1). As a result, the two coma aberrations cancel each other, and as a result, the occurrence of coma aberration can be suppressed.

  However, when the objective lens is tilted to correct astigmatism, the inclination of the objective lens with respect to the principal ray of the light beam emitted from the light source is different from the inclination of the optical disc with respect to the principal ray. Therefore, if the two coma aberrations have the same amount and different signs, the astigmatism is corrected by tilting the lens, but the coma aberration increases on the contrary. Therefore, it is desirable to satisfy the conditional expression (2) in order to satisfactorily correct both coma and astigmatism.

  In conditional expression (2), if the sensitivity of the coma aberration is too small, i.e., if the objective lens is tilted, the coma aberration is undesirably increased even if the astigmatism can be corrected. In conditional expression (2), if the sensitivity of coma aberration exceeds the upper limit, that is, the objective lens is tilted, the amount of coma aberration generated due to the lens tilt amount error becomes too large, which is not preferable. .

In the present text, the coma aberration sensitivities CM _L and CM _D are obtained by expanding the wavefront aberration generated when the objective lens and the optical disc are tilted by 1 ° with respect to the incident light flux by a Zernike polynomial. Among the term coefficients, the term coefficient corresponding to the third-order coma aberration is used. Incidentally, when the amount of wavefront aberration is expressed as an rms value, the following relationship is established between the third-order coma aberration coefficient CM3 and the rms value.
rms value = 1 / √8 × | CM3 |

Further, by satisfying the following conditional expression (3), it is possible to simultaneously and effectively suppress the generation of astigmatism and coma.
−0.30 ≦ (CM _D / CM _L ) ・ d ・ tanθ / TR ≦ −0.15 (3)

  Outside the range defined by the conditional expression (3), it is not preferable because the astigmatism correction effect and the coma aberration correction effect cannot be properly balanced.

The present invention can also be applied to an optical system for an optical disc capable of recording and / or reproducing information on a plurality of types of optical discs having different disc protective layer thicknesses. That is, the optical system for an optical disc according to the present invention includes a plurality of light sources that emit divergent light having wavelengths corresponding to information recording and / or reproduction on a plurality of types of optical discs having different thicknesses of the disc protective layer, and An objective lens that is used in common during recording and / or reproduction and collects divergent light from a plurality of light sources on each optical disc; and at least a drive unit that performs tracking shift of the objective lens and changes the posture of the objective lens The drive means is configured so that the central axis of the objective lens faces the light source simultaneously with the tracking shift at the time of recording / reproducing information with respect to the first optical disc having the thinnest protective layer thickness among the plurality of types of optical discs. The configuration is such that the posture is changed.

In the optical system for an optical disc having the above-described configuration, the change amount of the posture of the objective lens at the time of tracking shift when recording / reproducing information with respect to the first optical disc is θ ₁ [°], which is larger than that of the first optical disc. Assuming that the change amount of the posture of the objective lens at the time of tracking shift when recording / reproducing information with respect to the second optical disk having a relatively thick protective layer is θ ₂ [°], the following conditional expression ( 5 ) is obtained. It is desirable to satisfy. More preferably, each change amount satisfies the conditional expression ( 6 ).
−0.1 ≦ θ ₂ / θ ₁ ≦ 1 ( 5 )
θ ₂ / θ ₁ = 0 ( 6 )

Outside the range defined by conditional expression ( 5 ), coma aberration remains, which is not preferable because information cannot be recorded or reproduced with high accuracy. When configured to satisfy the conditional expression ( 6 ), there is an advantage that control of the driving means becomes easy at the time of recording and reproducing information on the second optical disk.

  Specifically, an optical disc having a numerical aperture of 0.60 or more required for recording and / or reproducing information can be used as the first optical disc. Also, the optical disk with the thickest protective layer thickness among the plurality of types of optical disks can be used as the second optical disk.

In the optical system for optical discs that can record and / or reproduce information on a plurality of types of optical discs with different disc protective layer thicknesses as described above, the conditional expressions (1) to (3) are respectively the following conditional expressions (4 ), (7), (8).

0.25 ≦ d ₁・ tanθ ₁ /TR≦0.75 ( 4 )
−0.75 ≦ CM _D1 / CM _L1 ≦ −0.15 (7)
_{-0.30 ≦ (CM D1 / CM L1} ) · d 1 · tanθ 1 /TR≦-0.15 ... (8)
However, the number subscripted at the end of each coefficient means the value when the first optical disc is used.

Of the plurality of light sources, when only the objective lens is tilted with respect to the light beam emitted from the second light source that emits the divergent light having a wavelength corresponding to the recording and / or reproduction of information on the second optical disk. When the sensitivity of the generated coma aberration is CM _L2 , and the sensitivity of the coma aberration generated when only the second optical disc is tilted with respect to the light beam irradiated from the second light source and transmitted through the objective lens is CM _D2 , It is desirable that the conditional expression (9) is satisfied.
−1.50 ≦ CM _D2 / CM _L2 ≦ −0.50 (9)

  In conditional expression (9), if the value is below the lower limit, coma aberration generated by the tracking shift becomes too large at the time of recording and / or reproducing information on the second disk. If the upper limit is exceeded, the sensitivity of the coma aberration with respect to the tilting of the objective lens becomes too large, and the amount of coma aberration generated due to an error in the lens tilt amount becomes too large.

Of the numerical values in the above conditional expressions, when d · tan θ / TR is set to 0.5 and CM _D / CM _L is set to −0.5, the astigmatism correction effect and the coma aberration correction effect Both work best. Based on this, the present invention is defined from another viewpoint as follows.

That is, the optical system for an optical disc according to the present invention records and / or reproduces information with respect to a first optical disc and a second optical disc having a protective layer thickness approximately twice the protective layer thickness of the first optical disc. A plurality of light sources for irradiating diverging light having a wavelength corresponding to recording and / or reproduction of information on the first optical disc and the second optical disc, and a protective layer thickness of the second optical disc The coma aberration sensitivity is almost the same as the sensitivity of coma aberration generated by, and is used in common when recording and / or reproducing information to and from the first optical disc and the second optical disc. The objective lens to be moved, and at least the objective lens is tracking-shifted, and the posture of the objective lens is changed so that the central axis of the objective lens is directed toward the light source. And driving means for correcting the astigmatism generated by the tracking shift almost completely at the same time as the tracking shift at the time of recording and / or reproducing information on the first optical disc. The posture of the objective lens is changed by approximately half of the change amount corresponding to the posture of the objective lens, and the posture of the objective lens is not changed at the time of tracking shift when recording and / or reproducing information on the second optical disc. And

  In the optical system for optical discs corresponding to the above-described plural types of optical discs, the plurality of light sources are arranged at positions shifted in a plane orthogonal to the light emission direction, and images of the objective lenses of the plurality of light sources are tracked. You may arrange | position so that it may rank in a direction substantially orthogonal to the moving direction of the image by a shift.

  The optical system for optical discs corresponding to the above-mentioned plural types of optical discs may further comprise disc discriminating means for discriminating the types of optical discs used. In this case, the driving unit can set the amount of change in the posture of the objective lens according to the tracking shift amount of the light source to be used and the objective lens among the plurality of light sources based on the determination result by the disc determination unit.

  In the optical system for optical discs corresponding to the above-described plural types of optical discs, the plurality of light sources are arranged at positions shifted in a plane orthogonal to the light emission direction, and the driving means is an optical disc used among the plurality of light sources. The position where the objective lens is moved so that the light source corresponding to the type is positioned on the central axis of the objective lens may be used as the tracking shift reference position of the optical disk to be used. When the optical system for an optical disc has such a configuration, the plurality of light sources may be arranged in the image moving direction by tracking shift. Furthermore, the optical system for optical discs may include disc discriminating means for discriminating the type of optical disc used. In this case, the driving unit sets the amount of change in the posture of the objective lens according to the tracking shift amount of the light source to be used and the objective lens among the plurality of light sources based on the discrimination result by the disc discrimination unit, and performs tracking. The reference position for shift can be set.

  As described above, according to the present invention, the astigmatism generated when off-axis light is incident on the objective lens by the tracking shift is reduced by the astigmatism generated by changing the posture of the objective lens, It is possible to provide a finite optical disk optical system capable of recording and / or reproducing information even if the optical disk requires a high NA for recording and / or reproducing information.

  The optical system for optical discs according to the present invention can correct not only astigmatism but also coma. Specifically, the sensitivity of the coma aberration of the objective lens when off-axis light is incident has a predetermined relationship with the sensitivity of the coma aberration of the optical disc when the off-axis light is incident through the objective lens. The objective lens is configured. As a result, coma generated by the tracking shift can be corrected well while balancing with the astigmatism correction effect.

  Hereinafter, several embodiments of an optical system for an optical disc according to the present invention will be described together with specific examples with reference to the drawings. FIG. 1 is a diagram showing an optical system 100 for an optical disc according to the first embodiment and an optical disc 20A that is a target for recording / reproducing information. The optical system for optical disc 100 includes a light source 1, a beam splitter 2, a lens driving mechanism 3, a sensor 4, and an objective lens 10. The optical disk 20A means an optical disk (eg, DVD) having a thin protective layer and a high recording density. In other words, the optical system for optical disc 100 is an optical system that is mounted on an optical information recording / reproducing apparatus that records and reproduces information with respect to the optical disc 20A having a high recording density. The optical disc 20A is placed on a turntable (not shown) and driven to rotate.

  As shown in FIG. 1, the optical system for optical disc 100 is a finite system that does not have a coupling lens between the light source 1 and the objective lens 10. That is, the laser light (divergent light) emitted from the light source 1 is converged only by the objective lens 10. The same applies to the optical system 200 for optical discs of the second embodiment described below.

  The light source 1 oscillates a laser beam having a relatively short wavelength. The laser light emitted from the light source 1 forms a small-diameter spot on the recording surface of the optical disc 20 </ b> A via the beam splitter 2 and the objective lens 10. Light (signal light) from the optical disk 20 </ b> A is received by the sensor 4. The signal light incident on the sensor 4 is processed by a signal processing circuit (not shown), and information writing control and information reading on the optical disc 20A are performed.

  The lens driving mechanism 3 includes a lens holder 31 that holds the objective lens 10 and an actuator 32 that finely adjusts the position and posture of the objective lens 10 held by the lens holder 31. FIG. 2 is a diagram for explaining the operation of the lens driving mechanism 3. FIG. 2 shows the optical paths of the optical system 100 for optical disc shown in FIG. 1 in a straight line for convenience of explanation. The objective lens 10 is subjected to a focusing shift (shifted in the X direction in FIG. 2) or a tracking shift (shifted in the Y direction in FIG. 2) by the actuator 32. Thereby, the optical system for optical disc 100 realizes stable information recording and reproduction even when a focus shift or a track shift occurs due to warpage, inclination, eccentricity, or the like of the optical disc.

The lens driving mechanism 3 tilts the objective lens 10 so that the central axis CL is directed toward the light source 1 simultaneously with tracking shift. More specifically, the direction of the light source 1 is a direction where there is a virtual light emitting point viewed from the objective lens 10. As a result, astigmatism caused by the tracking shift is effectively reduced by astigmatism caused by the tilt of the objective lens 10. At this time, the lens tilt amount is set to an amount smaller than the virtual light emitting point located on the extension line of the central axis of the objective lens so that the coma aberration can be sufficiently corrected. In this embodiment, the air conversion distance from the light source 1 to the recording surface of the optical disc is d [mm], the amount of image movement by tracking shift is TR [mm], and the lens tilt amount (the optical axis Ax and the central axis CL of the objective lens). When the angle between the two lens units is θ [°], the lens driving mechanism 3 drives the objective lens 10 so as to satisfy the following conditional expression (1).
0.25 ≦ d · tanθ / TR ≦ 0.75 (1)

  If the objective lens 10 is driven so as to satisfy the conditional expression (1), the astigmatism generated by the off-axis light entering the objective lens 10 after the tracking shift is effectively suppressed. For example, when the lens tilt amount is set so that the value of conditional expression (1) is 0.3, the amount of astigmatism is about ½, and the value of conditional expression (1) is 0.5. Since the lens tilt amount can be reduced to ¼, the information can be sufficiently corrected to the extent that recording and reproduction of information with respect to the optical disk having a high numerical aperture is realized with high accuracy.

The objective lens 10 has both aspherical surfaces on the light source 1 side and the optical disc 20A side. The objective lens 10 has characteristics that satisfy the following conditional expression (2).
−0.75 ≦ CM _D / CM _L ≦ −0.15 (2)
However, _CML is the sensitivity of coma aberration that occurs when only the objective lens is tilted with respect to the light beam emitted from the light source.
CM _D is the sensitivity of the coma aberration generated when only the inclined disc for the light flux transmitted through the objective lens is irradiated from the light source, each representing.

  By providing the objective lens 10 with a characteristic that satisfies the conditional expression (2), coma aberration generated when off-axis light is incident on the objective lens 10 after the tracking shift is effectively suppressed. In particular, when the objective lens 10 is given a characteristic in which the value of the conditional expression (2) is in the vicinity of −0.5, the balance between the astigmatism correction effect and the coma aberration correction effect is improved.

When conditional expressions (1) and (2) are summarized, the optical system for optical disc 100 is configured to satisfy the following conditional expression (3), thereby suppressing astigmatism and coma simultaneously and satisfactorily. ing.
−0.30 ≦ (CM _D / CM _L ) ・ d ・ tanθ / TR ≦ −0.15 (3)

  The above is the description of the optical system 100 for optical discs of the first embodiment. Next, the optical system 200 for optical disc according to the second embodiment of the present invention will be described. FIG. 3 is a diagram showing the optical system 200 for the optical disc according to the second embodiment, and the first optical disc 20A and the second optical disc 20B to be recorded and reproduced. That is, the optical system for optical disc 200 of the second embodiment is an optical system applicable to a plurality of types of optical discs having different protective layer thicknesses. Of the optical system 200 for optical discs of the second embodiment, for the same configuration as the optical system 100 for optical discs described above, refer to the description in the first embodiment, and the description here is simplified or omitted. .

  Of the applied optical disks, the first optical disk 20A is the same as the optical disk 20A described in the first embodiment. The second optical disk 20B is an optical disk having a thicker protective layer than the first optical disk 20A, and is exemplified by CD, CD-R, and the like.

  The light source 1 of the second embodiment includes a plurality of (here, two) light emitting units capable of oscillating a laser beam having a wavelength corresponding to the use of each of the optical disks 20A and 20B. Each light emitting point is disposed at a position slightly shifted in a plane orthogonal to the optical axis of the objective lens 10 when the optical path of the optical system for optical disc 200 is developed on a straight line. In FIG. 3, the optical path of the light beam used when the first optical disc 20A is used is indicated by a solid line, and the optical path of the light beam used when the second optical disc 20B is used is indicated by a broken line.

  A diffraction grating 5 is disposed between the light source 1 and the beam splitter 2. The diffraction grating 5 is provided to generate a sub beam for obtaining a track deviation signal by the three beam method.

  As in the first embodiment, the objective lens 10 of the second embodiment has two aspheric surfaces. At least one surface is provided with an annular diffractive structure having a plurality of fine steps centered on the central axis.

  The objective lens 10 is compatible with each of the optical disks 20A and 20B having different protective layer thicknesses by the diffractive action of the diffractive structure. Furthermore, the objective lens 10 also has an aperture limiting function that has an aperture diameter corresponding to the numerical aperture on the disc side required when using each of the optical discs 20A and 20B. As a result, the laser beam from the light source 1 converges on the recording surface of the optical disc 20A or 20B with a numerical aperture suitable for recording and reproducing information.

  The lens driving mechanism 3 of the second embodiment performs focusing shift, tracking shift, and tilt of the objective lens 10 as in the first embodiment. As a result, stable information recording and reproduction is possible not only when the optical disc is warped or tilted but also when there is a focus shift or track shift due to a difference in working distance due to a difference in the protective layer thickness of the optical disc used. Realized. In FIG. 3, the objective lens 10 at the position when the first optical disc 20A is used is indicated by a solid line, and the objective lens 10 at the position when the second optical disc 20B is used is indicated by a dotted line.

  Astigmatism and coma caused by tracking shift at the time of recording and reproducing information on the first optical disc 20A are corrected well by the lens tilt as in the first embodiment. That is, at the same time that the lens shift is performed for tracking, the central axis of the objective lens is a virtual light emitting point viewed from the objective lens 10 (more specifically, a laser beam having a wavelength corresponding to the first optical disc). The lens tilt is performed in the direction of the oscillating emission point.

By using the lens tilt as described above, the optical system 200 for the optical disc according to the second embodiment also satisfies the conditional expressions (1) to (3) in order to suppress simultaneously and satisfactorily the coma and astigmatism that occur when using the optical disc 20A. ) Are configured to satisfy conditional expressions (4), (7), and (8) corresponding to. Note that the number “1” subscripted at the end of each coefficient used in each conditional expression means a coefficient when the first optical disc 20A is used.

0.25 ≦ d ₁・ tanθ ₁ /TR≦0.75 ( 4 )
−0.75 ≦ CM _D1 / CM _L1 ≦ −0.15 (7)
_{-0.30 ≦ (CM D1 / CM L1} ) · d 1 · tanθ 1 /TR≦-0.15 ... (8)

  Further, the numerical aperture on the disk side required for recording or reproduction of the second optical disk is relatively small. For this reason, even when the amount of lens tilt is small, astigmatism does not remain so large, and there is no practical problem. Therefore, at the time of recording and reproducing information on the second optical disc 20B, even if a lens shift is performed for tracking, no lens tilt is performed, or the amount of lens tilt at the time of recording and reproducing information on the first optical disc Keep it below the same amount.

In this way, by using the lens tilt as necessary, the objective lens 10 of the second embodiment further satisfies the following conditional expression (in order to suppress coma and astigmatism generated when the optical disc 20B is used simultaneously and satisfactorily: 9). Note that the number “2” subscripted at the end of each coefficient used in each conditional expression means a coefficient when the second optical disc 20B is used.
−1.50 ≦ CM _D2 / CM _L2 ≦ −0.50 (9)

  If a configuration in which the lens tilt is not performed when the second optical disc 20B is used, the coma aberration is best corrected by giving the objective lens 10 a characteristic in which the value of the conditional expression (9) is -1. The

The following conditional expressions ( 5 ) and ( 6 ) define the lens tilt amount.
−0.1 ≦ θ ₂ / θ ₁ ≦ 1 ( 5 )
θ ₂ / θ ₁ = 0 ( 6 )
However, θ ₁ is the lens tilt amount of the objective lens 10 at the time of tracking shift when recording or reproducing information with respect to the first optical disc 20A.
θ ₂ represents the lens tilt amount of the objective lens 10 at the time of tracking shift when recording or reproducing information on the second optical disc 20B.

  As described above, the plurality of light emitting units of the light source 1 are arranged at slightly shifted positions in a plane orthogonal to the optical axis of the objective lens 10. One example of the arrangement of the plurality of light emitting units is such that the images of the plurality of light emitting units by the objective lens 10 are arranged in a direction substantially orthogonal to the moving direction of the image by the tracking shift. In this case, each light emitting unit is arranged so as to be substantially orthogonal to the tracking shift direction, so that an image height (object height) component generated when the position of the light emitting unit is shifted and an image generated due to the tracking shift. The high components are orthogonal to each other. Therefore, it can be prevented that these two components are added together to increase the influence on the performance of satisfactorily suppressing the aberration.

  As another example of the arrangement of the plurality of light emitting units, the plurality of light emitting units are arranged in the moving direction of the image by tracking shift.

  The lens driving mechanism 3 is used at a position where the objective lens 10 is moved so that the light emitting part corresponding to the type of the optical disk to be used is positioned on the central axis of the objective lens 10. You may operate | move so that it may become the reference position (the position of the objective lens 10 in the state which is not tracking-shifted) of the tracking shift of an optical disk. By shifting the reference position of the tracking shift according to the position of the light emitting unit to be used, the state where the light emitting unit and the objective lens 10 are aligned on the central axis of the objective lens 10 is realized at the reference position. Thereby, the generation of aberration can be minimized.

  When the lens driving mechanism 3 is configured to adjust the tracking shift reference position as described above, the plurality of light emitting units are preferably arranged in the image moving direction by the tracking shift. In this case, since the light sources are arranged along the tracking shift direction, the moving direction of the objective lens 10 when the tracking shift reference position is shifted in accordance with the light emitting unit to be used matches the tracking shift direction. It will be. Therefore, in order to shift the reference position of the tracking shift in accordance with the light emitting unit to be used, it is not necessary for the lens activation mechanism 3 to have a separate drive shaft, and the lens activation mechanism 3 can be prevented from becoming complicated. .

  A discriminating function for automatically discriminating the type of optical disc to be used may be added to the optical disc optical system 200 of the second embodiment. In this case, the lens driving mechanism 3 can perform setting of light emission in the light source 1 having a plurality of light emitting units and control of the posture of the objective lens 10 using the result of automatic discrimination. When the lens driving mechanism 3 is configured to adjust the tracking shift reference position, the tracking shift reference position may be further adjusted using the determination result of the determination function. The discriminating function of the optical disc type can be realized by using a signal output from the sensor 4 that receives signal light from the optical disc being used. The discrimination function may be realized by a signal processing circuit that processes a signal from the sensor 4. As one specific example of the discrimination function, the protective layer thickness is obtained from the distance on the sensor 4 between the reflected light from the optical disk surface returning to the sensor 4 and the reflected light from the disk recording surface, There is also a technique for discriminating the type of optical disc.

  The above is the description of the optical system 200 for optical discs of the second embodiment. Hereinafter, one specific example based on the first embodiment and five specific examples based on the second embodiment will be described in order.

  The optical system 100 for optical discs of the first embodiment is specifically shown in Example 1. A schematic configuration of the optical system 100 for an optical disc according to the first embodiment is shown in FIG. Tables 1 and 2 show specific numerical configurations of the optical disc optical system 100 of the first embodiment and the optical disc 20A used.

  In Table 1, the design wavelength λ is the most suitable wavelength for recording and reproducing information on the optical disc 20A. In Table 2, r is a radius of curvature (unit: mm) of each surface of the optical member, d is a lens thickness or lens interval (unit: mm) at the time of recording / reproducing information with respect to the optical disc 20A, and n is a d line (588 nm). Is the Abbe number at the d-line.

各非球面を規定する円錐係数と非球面係数は、表３に示される。なお、表３における表記Ｅは、１０を基数、Ｅの右の数字を指数とする累乗を表している。以下に示す各表においても同様である。 The first surface (surface number 3) and the second surface (surface number 4) of the objective lens 10 are aspheric. The shape is such that the distance (sag amount) from the tangential plane on the aspherical optical axis of the coordinate point on the aspherical surface where the height from the optical axis is h is X (h), on the aspherical optical axis. Is the curvature (1 / r) of C, the conic coefficient is K, the fourth, sixth, eighth, tenth, and twelfth aspheric coefficients are A ₄ , A ₆ , A ₈ , A ₁₀ , A ₁₂ , It is expressed by the following formula.

Table 3 shows conic coefficients and aspheric coefficients that define each aspheric surface. In addition, the notation E in Table 3 represents a power in which 10 is a radix and the number on the right of E is an exponent. The same applies to each table shown below.

In the optical disc optical system 100 of the first embodiment configured as described above, the objective lens 10 has a characteristic of CM _D1 / CM _L1 = −0.528. The lens driving mechanism 3 (actuator 32) is designed so as to drive and control the objective lens 10 at d ₁ · tan θ ₁ /TR=0.45 during tracking shift and tilt. Therefore, the optical system 100 for the optical disc of Example 1 is (CM _D1 / CM _L1 ) · d ₁ · tan θ ₁ /TR=−0.238. As described above, the optical disc optical system 100 of Example 1 satisfies all the conditional expressions (1) to (3).

  FIG. 4 is a graph showing the amount of non-rotationally symmetric aberration caused by the tracking shift. In FIG. 4, the horizontal axis represents the tracking amount, and the vertical axis represents the aberration amount. The same applies to the following drawings. As shown in FIG. 4, the optical disc optical system 100 of Example 1 that satisfies all of the conditional expressions (1) to (3) has coma aberration even when the tracking amount increases during recording or reproduction of information with respect to the optical disc 20A. It can be seen that generation of non-rotationally symmetric aberrations such as astigmatism and astigmatism is effectively prevented. That is, when the optical disc 20A is used in the optical system 100 for optical disc of the first embodiment, information recording and reproduction with high accuracy is realized.

  The optical system 200 for optical discs of the second embodiment is specifically shown in Examples 2-6. A schematic configuration of the optical system 200 for optical discs of Examples 2 to 6 is shown in FIG.

  Table 4 shows performance specifications of the optical system 200 for the optical disc of Example 2. The specific numerical configuration of the optical disc optical system 200 of Example 2 when using the first optical disc 20A is shown in Table 5, and the optical system 200 of Example 2 when using the second optical disc 20B is shown in Table 5. Specific numerical configurations are shown in Table 6, respectively.

  In the optical disc optical system 200 of Example 2, each surface (surface numbers 5 and 6) of the objective lens 10 is aspheric. Table 7 shows the conical coefficient and the aspheric coefficient that define each aspheric surface.

Further, a diffractive structure is formed on the first surface (surface number 5) of the objective lens 10. The diffractive structure is represented by the following optical path difference function φ (h).

The optical path difference function φ (h) represents the function of the diffractive lens in the form of an additional optical path length at a height h from the optical axis. P ₂ , P ₄ , P ₆ ,... Are secondary, fourth, sixth,. The optical path difference function coefficients P ₂ ,... That define the diffractive structure are shown in Table 8. m represents the order of the diffracted light to be used. In this embodiment, m = 1.

  As shown in Tables 5 to 8, the first surface (surface number 5) of the objective lens 10 has a curvature depending on an inner region (h ≦ 1.34) and an outer region (h> 1.34) of the lens. The radius r, the shape of the aspherical surface, and the diffraction structure are different.

In the optical disc optical system 200 of Example 2 configured as described above, the objective lens 10 has the characteristics of CM _D1 / CM _L1 = −0.493 and CM _D2 / CM _L2 = −0.961. The lens driving mechanism 3 (actuator 32) is designed to drive and control the objective lens 10 with d ₁ · tan θ ₁ /TR=0.50 when tracking shift and tilt are performed when the first optical disc 20A is used. . Therefore, the optical system 200 for the optical disc of the second embodiment when the first optical disc 20A is used is (CM _D1 / CM _L1 ) · d ₁ · tan θ ₁ /TR=−0.247. As described above, the optical disc optical system 200 of Example 2 satisfies all the conditional expressions (4), (7), (8), and (9).

FIG. 5 is a graph showing the amount of non-rotationally symmetric aberration caused by tracking shift when using the first optical disc 20A. As shown in FIG. 5, the optical disc optical system 200 of Example 2 that satisfies all of the conditional expressions (4), (7), and (8) has a large tracking amount when recording / reproducing information with respect to the first optical disc 20A. Even so, it can be seen that the generation of non-rotationally symmetric aberrations such as coma and astigmatism is effectively prevented. That is, when the optical disk 20A is used in the optical system 100 for the optical disk according to the second embodiment, information can be recorded and reproduced with high accuracy.

Further, when the second optical disc 20B is used, the optical disc optical system 200 according to the second embodiment does not perform lens tilt. That is, both conditional expressions ( 5 ) and ( 6 ) are satisfied. FIG. 6 is a graph showing the amount of non-rotationally symmetric aberration caused by tracking shift when using the second optical disc 20B. As shown in FIG. 6, the optical system 200 for the optical disc of Example 2 that satisfies the above conditional expressions has practical problems in both astigmatism and coma even when the second optical disc 20B is used, even if the tracking amount increases. It is sufficiently suppressed to a certain extent.

  The optical disc optical system 200 according to the third embodiment has the same configuration as the optical disc optical system 200 according to the second embodiment. In the optical disc optical system 200 of the third embodiment, lens drive control when using the first optical disc 20A is exactly the same as that of the second embodiment. Therefore, the optical disk optical system 200 according to the third embodiment can obtain the same effects as those of the second embodiment when the first optical disk 20A is used. However, the optical system for optical disc 200 of the third embodiment is different from the second embodiment in that lens tilt is performed when the second optical disc 20B is used.

The optical system 200 for the optical disk according to the third embodiment performs lens tilt so as to satisfy θ2 / θ1 = 0.4 simultaneously with tracking shift when the second optical disk 20B is used. That is, the optical system 200 for an optical disc according to the third embodiment satisfies only the conditional expression ( 5 ) among the conditional expressions that define the lens tilt amount.

  FIG. 7 is a graph showing the amount of non-rotationally symmetric aberration caused by tracking shift when the second optical disc 20B is used in the optical disc optical system 200 of the third embodiment. As shown in FIG. 7, when the second optical disk 20B is used, the optical system 200 for the optical disk of Example 3 is sufficiently suppressed to the extent that astigmatism and coma are not practically problematic even if the tracking amount increases. ing.

  Table 9 shows the performance specifications of the optical system 200 for the optical disc of Example 4. The specific numerical configuration of the optical disc optical system 200 of Example 4 when using the first optical disc 20A is shown in Table 10, and the optical disc optical system 200 of Example 4 when using the second optical disc 20B is shown in Table 10. Specific numerical configurations are shown in Table 11, respectively.

Also in the optical disk optical system 200 of Example 4, each surface (surface numbers 5 and 6) of the objective lens 10 is aspheric. Table 12 shows the conical coefficient and the aspheric coefficient that define each aspheric surface. In addition, a diffractive structure is formed on the first surface (surface number 5) of the objective lens 10. Table 13 shows optical path difference function coefficients P ₂ ,... That define the diffraction structure.

  As shown in Tables 10 to 13, the first surface (surface number 5) of the objective lens 10 has a curvature depending on an inner region (h ≦ 1.36) and an outer region (h> 1.36) of the lens. The radius r, the shape of the aspherical surface, and the diffraction structure are different.

In the optical disc optical system 200 of Example 4 configured as described above, the objective lens 10 has the characteristics of CM _D1 / CM _L1 = −0.653 and CM _D2 / CM _L2 = −1.191. The lens driving mechanism 3 (actuator 32) is designed to drive and control the objective lens 10 at d ₁ · tan θ ₁ /TR=0.30 when tracking shift and tilt are performed when the first optical disc 20A is used. . Therefore, the optical system 200 for the optical disc of Example 4 when the first optical disc 20A is used is (CM _D1 / CM _L1 ) · d ₁ · tan θ ₁ /TR=−0.196. From the above, the optical disc optical system 200 of Example 4 satisfies all the conditional expressions (4), (7), (8), and (9).

FIG. 8 is a graph showing the amount of non-rotationally symmetric aberration caused by tracking shift when using the first optical disc 20A. As shown in FIG. 8, the optical disc optical system 200 of Example 4 that satisfies all of the conditional expressions (4), (7), and (8) has a large tracking amount when recording / reproducing information with respect to the first optical disc 20A. In this case, it can be seen that coma is suppressed satisfactorily and only astigmatism that does not affect practically occurs. That is, when the optical disk 20A is used in the optical system 100 for the optical disk according to the fourth embodiment, information recording / reproduction with high accuracy is realized.

When the second optical disc 20B is used, the optical disc optical system 200 according to the fourth embodiment does not perform lens tilt. That is, both conditional expressions ( 5 ) and ( 6 ) are satisfied. FIG. 9 is a graph showing the amount of non-rotationally symmetric aberration caused by tracking shift when using the second optical disc 20B. As shown in FIG. 9, the optical system 200 for optical disc of the second embodiment that satisfies the above conditional expressions has practical problems both in astigmatism and coma even when the tracking amount increases when the second optical disc 20B is used. It is sufficiently suppressed to a certain extent.

  The optical disc optical system 200 according to the fifth embodiment has the same configuration as the optical disc optical system 200 according to the fourth embodiment. In the optical system 200 for the optical disc of the fifth embodiment, lens drive control when using the second optical disc 20B is exactly the same as that of the fourth embodiment. Therefore, the optical disk optical system 200 according to the fifth embodiment can obtain the same effects as those of the fourth embodiment when the second optical disk 20B is used. However, the optical system 200 for optical disc of the fifth embodiment differs from the second embodiment in the tracking amount and the lens tilt amount when using the first optical disc 20A.

The lens driving mechanism 3 (actuator 32) of the optical disc optical system 200 according to the fifth embodiment is designed so as to drive and control the objective lens 10 with d ₁ · tan θ ₁ /TR=0.45 when the first optical disc 20A is used. Is done. Therefore, the optical system 200 for the optical disc of Example 2 when the first optical disc 20A is used is (CM _D1 / CM _L1 ) · d ₁ · tan θ ₁ /TR=−0.294. As described above, the optical disc optical system 200 according to the fifth embodiment satisfies all the conditional expressions (4), (7), and (8) as in the fourth embodiment even when the tracking amount and the lens tilt amount are changed.

  FIG. 10 is a graph showing the amount of non-rotationally symmetric aberration caused by tracking shift when the first optical disc 20A is used in the optical disc optical system 200 of the fifth embodiment. As shown in FIG. 10, the optical system 200 for the optical disc of Example 5 has some astigmatism when the tracking amount increases when the first optical disc 20A is used, but the astigmatism is extremely small. It is suppressed. Therefore, it can be said that the optical system 200 for the optical disc of Example 5 is a suitable configuration when the signal characteristics are easily affected by astigmatism.

  Table 14 shows performance specifications of the optical system 200 for an optical disc according to Example 6. The specific numerical configuration of the optical disc optical system 200 of Example 6 when using the first optical disc 20A is shown in Table 15, and the optical system 200 of Example 6 when using the second optical disc 20B is shown in Table 15. Specific numerical configurations are shown in Table 16, respectively.

Also in the optical disc optical system 200 of Example 6, each surface (surface numbers 5 and 6) of the objective lens 10 is aspherical, and a diffractive structure is formed on the first surface (surface number 5). Table 17 shows conical coefficients and aspheric coefficients defining each aspheric surface, and Table 18 shows optical path difference function coefficients P ₂ ,.

  As shown in Tables 15 to 18, the first surface (surface number 5) of the objective lens 10 has a curvature depending on the inner region (h ≦ 1.48) and the outer region (h> 1.48) of the lens. The radius r, the shape of the aspherical surface, and the diffraction structure are different.

In the optical disc optical system 200 of Example 6 configured as described above, the objective lens 10 has the characteristics of CM _D1 / CM _L1 = −0.442 and CM _D2 / CM _L2 = −0.918. The lens driving mechanism 3 (actuator 32) is designed to drive and control the objective lens 10 with d ₁ · tan θ ₁ /TR=0.55 when tracking shift and tilt are performed when the first optical disc 20A is used. . Therefore, the optical system 200 for the optical disc of Example 6 when using the first optical disc 20A is (CM _D1 / CM _L1 ) · d ₁ · tan θ ₁ /TR=−0.243. As described above, the optical disc optical system 200 of Example 2 satisfies all the conditional expressions (4), (7), (8), and (9).

FIG. 11 is a graph showing the amount of non-rotationally symmetric aberration caused by tracking shift when using the first optical disc 20A. As shown in FIG. 11, the optical system for optical disc 200 of Example 6 that satisfies all the conditional expressions (4), (7), and (8) has a tracking amount at the time of recording and reproducing information with respect to the first optical disc 20A. It can be seen that when it is increased, the occurrence of non-rotationally symmetric aberrations such as coma and astigmatism is effectively prevented.

When the second optical disc 20B is used, the optical disc optical system 200 according to the sixth embodiment performs lens tilt so as to satisfy θ ₂ / θ ₁ = 0.2 simultaneously with the tracking shift. That is, the optical disc optical system 200 of Example 6 satisfies only the conditional expression ( 5 ) among the conditional expressions that define the lens tilt amount.

  FIG. 12 is a graph showing the amount of non-rotationally symmetric aberration caused by the tracking shift when the second optical disc 20B is used in the optical disc optical system 200 of the sixth embodiment. As shown in FIG. 12, the optical system 200 for the optical disc of Example 6 can sufficiently suppress both astigmatism and coma even when the tracking amount increases when the second optical disc 20B is used. ing.

  The above is the embodiment of the present invention. The optical systems 100 and 200 for optical disks according to the embodiments are not limited to the above configuration. For example, the optical system 200 for an optical disc according to the second embodiment may be configured as shown in FIG. In the optical system 200 for optical disc shown in FIG. 13, light sources 1A and 1B corresponding to the optical discs 20A and 20B are separately provided. The light beams from the light sources 1A and 1B are combined in the optical path by the prism 2 'and travel toward the objective lens. The signal light from each of the optical disks 20A and 20B is incident on the sensors 4A and 4B provided separately via the diffraction gratings 6A and 6B.

  In any configuration, the optical system for optical disc 200 having compatibility with a plurality of types of optical discs is used for recording and reproduction on an optical disc having the thinnest protective layer thickness among the plurality of types of optical discs that can be used. Simultaneously with the lens shift, the lens tilt is performed so that the central axis of the objective lens 10 is directed toward the light emitting point. As a result, regardless of which optical disk is used, information is recorded / reproduced without being affected by aberrations.

It is a figure which shows the optical system for optical disks of 1st embodiment, and the optical disk used as the object of information recording and / or reproduction | regeneration. It is a figure for demonstrating an effect | action of the lens drive mechanism of this invention. It is a figure which shows the optical system for optical discs of 2nd embodiment, the 1st optical disc used as the object of recording and / or reproduction | regeneration of information, and a 2nd optical disc. 6 is a graph showing the amount of non-rotationally symmetric aberration caused by a tracking shift when using the first optical disc of Example 1. 6 is a graph showing the amount of non-rotationally symmetric aberration caused by a tracking shift when using the first optical disc of Example 2. 6 is a graph showing the amount of non-rotationally symmetric aberration caused by a tracking shift when using the second optical disk of Example 2. 10 is a graph showing the amount of non-rotationally symmetric aberration caused by a tracking shift when using the second optical disk of Example 3. 10 is a graph showing the amount of non-rotationally symmetric aberration caused by tracking shift when using the first optical disk of Example 4. 10 is a graph showing the amount of non-rotationally symmetric aberration caused by a tracking shift when using the second optical disk of Example 4. 10 is a graph showing the amount of non-rotationally symmetric aberration caused by a tracking shift when using the first optical disc of Example 5. 10 is a graph showing the amount of non-rotationally symmetric aberration caused by a tracking shift when using the first optical disc of Example 6. 14 is a graph showing the amount of non-rotationally symmetric aberration caused by a tracking shift when using the second optical disk of Example 6. It is a modification of the optical system for optical discs of 2nd embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 3 Lens drive mechanism 10 Objective lens 20A, 20B Optical disk 100, 200 Optical system for optical disks

Claims (17)

An optical system for an optical disc capable of using an optical disc having a numerical aperture of 0.60 or more required for recording and / or reproducing information,
A light source that emits divergent light;
An objective lens for condensing the diverging light on the optical disc;
At least driving means for tracking and shifting the objective lens and changing the posture of the objective lens so that the central axis of the objective lens is directed toward the light source ;
Have
The air conversion distance from the light source to the recording surface of the optical disk is d [mm], the amount of image movement by tracking shift is TR [mm], and the amount of change in the attitude of the objective lens during the tracking shift is θ [°]. Then, the optical system for optical discs satisfying the following conditional expression (1):
0.25 ≦ d · tanθ / TR ≦ 0.75 (1) In the optical system for optical discs according to claim 1,
CML sensitivity when only the objective lens is tilted with respect to the light beam irradiated from the light source, CML, when only the optical disk is tilted with respect to the light beam irradiated from the light source and transmitted through the objective lens If the sensitivity of the coma aberration generated in the above is CMD, the following conditional expression (2) is satisfied:
−0.75 ≦ CMD / CML ≦ −0.15 (2) In the optical system for optical discs according to claim 1 or 2 ,
CML sensitivity when only the objective lens is tilted with respect to the light beam irradiated from the light source, CML, when only the optical disk is tilted with respect to the light beam irradiated from the light source and transmitted through the objective lens An optical system for optical discs satisfying the following conditional expression (3):
−0.30 ≦ (CMD / CML) ・ d ・ tanθ / TR ≦ −0.15 (3) An optical system for an optical disc capable of recording and / or reproducing information on a plurality of types of optical discs having different disc protective layer thicknesses,
A plurality of light sources for irradiating divergent light having a wavelength corresponding to information recording and / or reproduction for each optical disc;
An objective lens that is used in common when recording and / or reproducing information on each optical disc, and that condenses the divergent light from the plurality of light sources on each optical disc;
At least driving means for tracking and shifting the objective lens and changing the posture of the objective lens;
Have
At the time of recording and / or reproducing information on the first optical disc having the thinnest disc protective layer thickness among the plurality of types of optical discs, the driving means moves the central axis of the objective lens toward the light source simultaneously with tracking shift. Change your posture as you go,
Of the plurality of light sources, d1 [mm] is an air-converted distance from the first light source that irradiates divergent light having a wavelength corresponding to information recording / reproduction to the first optical disc, and tracking. If the amount of movement of the image due to the shift is TR [mm], and the amount of change in the attitude of the objective lens during the tracking shift when recording / reproducing information with respect to the first optical disk is θ1 [°], An optical system for an optical disc characterized by satisfying conditional expression (4).
0.25 ≦ d1 / tanθ1 / TR ≦ 0.75 (4) In the optical system for an optical disk according to claim 4 ,
The amount of change in the posture of the objective lens during the tracking shift when recording and / or reproducing information with respect to the second optical disk having a relatively thick protective layer than the first optical disk is θ2 [°]. Then, the optical system for optical discs satisfying the following conditional expression ( 5 ):
-0.1 ≦ θ2 / θ1 ≦ 1 ( 5 ) In the optical system for an optical disk according to claim 5 ,
An optical system for an optical disc, further satisfying the following conditional expression ( 6 ):
θ2 / θ1 = 0 ( 6 ) In the optical system for optical discs according to any one of claims 4 to 6 ,
An optical system for an optical disk, wherein the first optical disk has an optical disk side numerical aperture of 0.60 or more required for recording and / or reproducing information. In the optical system for an optical disk according to any one of claims 5, 6, and 7 that cites claim 5 ,
The optical system for an optical disc, wherein the second optical disc is an optical disc having a thickest protective layer thickness among the plurality of types of optical discs. In the optical system for optical discs according to any one of claims 4 to 8 ,
Of the plurality of light sources, when only the objective lens is tilted with respect to a light beam emitted from a first light source that emits divergent light having a wavelength corresponding to recording and / or reproduction of information on the first optical disc The sensitivity of the coma aberration generated in the above is CML1, and the sensitivity of the coma aberration generated when only the first optical disc is tilted with respect to the light beam irradiated from the first light source and transmitted through the objective lens is CMD1. An optical system for an optical disc satisfying the following conditional expression (7):
−0.75 ≦ CMD1 / CML1 ≦ −0.15 (7) In the optical system for optical discs according to any one of claims 4 to 9 ,
Of the plurality of light sources, when only the objective lens is tilted with respect to a light beam emitted from a first light source that emits divergent light having a wavelength corresponding to recording and / or reproduction of information on the first optical disc The sensitivity of the coma aberration generated in the first lens is CML1, and the sensitivity of the coma aberration generated when only the first optical disc is tilted with respect to the light beam irradiated from the first light source and transmitted through the objective lens is CMD1. An optical system for an optical disc satisfying the following conditional expression (8):
−0.30 ≦ (CMD1 / CML1) ・ d1 ・ tanθ1 / TR ≦ −0.15 (8) In the optical system for an optical disc according to any one of claims 7 to 10 , which refers to claim 5, claim 6, or claim 5 .
Of the plurality of light sources, when only the objective lens is tilted with respect to a light beam emitted from a second light source that emits divergent light having a wavelength corresponding to recording and / or reproduction of information on the second optical disc The sensitivity of the coma generated in the above is CML2, and the sensitivity of the coma generated when only the second optical disc is tilted with respect to the light beam irradiated from the second light source and transmitted through the objective lens is CMD2. An optical system for optical discs that satisfies the following conditional expression (9):
−1.50 ≦ CMD2 / CML2 ≦ −0.50 (9) An optical system for an optical disc capable of recording and / or reproducing information with respect to a first optical disc and a second optical disc having a protective layer thickness approximately double the protective layer thickness of the first optical disc,
A plurality of light sources for irradiating divergent light having a wavelength corresponding to recording and / or reproduction of information on the first optical disc and the second optical disc;
The sensitivity of the coma aberration generated by the protective layer thickness of the second optical disc is approximately the same as the sensitivity of the coma aberration, and is commonly used when recording and / or reproducing information on the first optical disc and the second optical disc. An objective lens used to collect the divergent light on each optical disc;
At least driving means for tracking and shifting the objective lens and changing the posture of the objective lens so that the central axis of the objective lens is directed toward the light source;
Have
The drive means corresponds to the posture of the objective lens so as to substantially completely correct astigmatism generated by the tracking shift at the same time as the tracking shift at the time of recording and / or reproducing information on the first optical disc. An optical disk characterized in that the posture of the objective lens is changed by approximately half of the amount of change, and the posture of the objective lens is not changed during tracking shift during recording and / or reproduction of information with respect to the second optical disc. Optical system. In the optical system for optical discs according to any one of claims 4 to 12 ,
The plurality of light sources are arranged at positions shifted in a plane orthogonal to the light emission direction, and the images of the plurality of light sources by the objective lens are in a direction substantially orthogonal to the moving direction of the image by tracking shift. An optical system for an optical disk, wherein the optical system is arranged in a line. In the optical system for optical discs according to any one of claims 4 to 13 ,
A disc discriminating means for discriminating the type of optical disc used;
The drive means sets a change amount of the posture of the objective lens according to a tracking shift amount of the light source to be used among the plurality of light sources and the objective lens, based on a discrimination result by the disc discrimination means;
An optical system for optical discs. In the optical system for optical discs according to any one of claims 4 to 12 ,
The plurality of light sources are arranged at positions shifted in a plane orthogonal to the light emission direction of the light beam,
The drive means tracks the position of the optical disk used so that the position of the objective lens is moved so that the light source corresponding to the type of optical disk to be used is located on the central axis of the objective lens. To be the reference position for the shift,
An optical system for optical discs. The optical system for an optical disk according to claim 15 ,
The optical system for an optical disc, wherein the plurality of light sources are arranged in an image moving direction by tracking shift. The optical system for an optical disk according to claim 15 or 16 ,
A disc discriminating means for discriminating the type of optical disc used;
The drive unit sets a change amount of the posture of the objective lens according to a tracking shift amount of the light source to be used and the objective lens among the plurality of light sources based on a determination result by the disc determination unit, Setting a reference position for the tracking shift;
An optical system for optical discs.

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