JP2008181657A - Optical disk and its recording and playing-back device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical disk suitable for high density recording by designating effective ranges of the thickness of a light transmission layer and the refractive index of the light transmission layer in a next generation optical disk etc. and to provide its recording and playing-back device. <P>SOLUTION: In the optical disk 51 including a substrate 52, an information recording layer 53, an intermediate layer 54, an information recording layer 55 and a cover layer 56, the thickness t of the cover layer 56 is set to be f(n)-t1 or more by using a function f(n) which is related to the refractive index n of the cover layer 56, and constants t1 and t2 determined based on the maximum permissible value of aberration in the light transmission layer including the cover layer 56, the information recording layers 53 and 55 and the intermediate layer 54 which can be corrected by a spherical aberration correcting mechanism 24 that is included in an optical disk device, and the total thickness of the cover layer 56, the intermediate layer 54 and the information recording layer 55 excluding the information recording layer 53 that is the nearest to the substrate 52 is set to be f(n)+t2 or less. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an improvement in an optical disc that enables high-density recording of information and a recording / reproducing apparatus thereof.

  As is well known, in recent years, a DVD (Digital Versatile Disk) having an information recording capacity of 4.7 GB (Giga Byte) per layer on one side has been put to practical use as an optical disk capable of high-density recording of information.

  For this DVD, a plurality of types such as a read-only DVD-ROM (Read Only Memory) and a rewritable DVD-RAM (Random Access Memory) are prepared.

  In a DVD, an information recording layer is formed on a transparent substrate having a thickness of 0.6 mm, and laser light is transmitted through the transparent substrate and focused on the information recording layer, thereby writing and reading information. It has become.

  In this case, the numerical aperture NA (Numerical Aperture) of the objective lens that focuses the laser light is 0.6 as a reference value. The refractive index n of the transparent substrate is specified in the range of 1.45 to 1.65 for laser light having a wavelength of 650 nm.

  For this reason, a substrate material that meets the above conditions is selected for the transparent substrate. As the substrate material, polycarbonate is generally used. In this case, the refractive index n of the transparent substrate is 1.58.

  As described above, the reference value of the thickness of the transparent substrate constituting the DVD is 0.6 mm. However, in practice, it is inevitable that manufacturing variations occur in the thickness of the transparent substrate.

  However, if the optical system side for recording / reproducing with respect to the DVD is designed with a transparent substrate thickness of 0.6 mm, which is a reference value, an aberration occurs when the thickness of the transparent substrate varies from 0.6 mm. appear.

  The aberration of this optical system increases the diameter of the beam spot focused on the information recording layer of the DVD and adversely affects the recording / reproducing of information, so it is necessary to keep it below a certain value.

  Here, the aberration of the optical system caused by the variation in the thickness of the transparent substrate is a deviation from the design value of the thickness of the transparent substrate and a deviation from the design value of the refractive index of the transparent substrate. It depends on both.

  For this reason, in the case of DVD, in order to suppress the aberration of the optical system due to the variation in the thickness of the transparent substrate to a certain value or less, the thickness range allowed for the transparent substrate is set to the refractive index n. It is defined as a two-dimensional range.

  This two-dimensional range is disclosed in Patent Document 1, for example. That is, in this two-dimensional range, the error with respect to the reference value of the thickness of the transparent substrate is ± 0.02 mm with respect to the range where the refractive index n is 1.45 to 1.65.

  This two-dimensional range is not a simple rectangle when the refractive index n is taken on the horizontal axis and the thickness of the transparent substrate is taken on the vertical axis, and is transparent when the refractive index n is smaller than the lens load specification. It is defined in a form shifted in the direction of increasing the thickness of the substrate.

  Currently, technical developments for higher density recording of DVDs are in progress. In this case, the diameter of the beam spot focused on the information recording layer of the optical disc is proportional to the wavelength of the laser beam and inversely proportional to the numerical aperture NA representing the aperture angle of the objective lens.

  Therefore, in order to reduce the diameter of the beam spot focused on the information recording layer of the DVD with the aim of improving the information recording density of the DVD, it is necessary to increase the numerical aperture NA of the objective lens.

  On the other hand, when the optical disk is tilted with respect to the incident direction of the laser beam due to the influence of warp or the like, the optical path of the laser beam transmitted through the transparent substrate becomes asymmetrical, so that coma aberration is added to the laser beam focused on the information recording layer. Occurs.

  Since the amount of coma is approximately proportional to the third power of the numerical aperture NA of the objective lens, if the numerical aperture NA is increased to achieve high density recording, a very large coma aberration can be caused by a slight characteristic change of the optical disk. Will be generated.

  The coma aberration is proportional to the thickness of the light transmission layer of the optical disk. Therefore, in the next generation optical disk for high density recording, the thickness of the light transmission layer should be reduced in order to secure a margin for the inclination of the optical disk. Is being considered.

  As examples of the numerical aperture NA of the objective lens and the thickness of the light transmission layer in such a next-generation optical disc, the numerical aperture NA of the objective lens = 0.85, and the reference value of the thickness of the light transmission layer = 0. .1 mm.

In addition, regarding the thickness error of an optical disc that has been recorded at a higher density than the current DVD, there is one proposed in Patent Document 2, for example. That is, in Patent Document 2, when the wavelength of the laser beam is λ, the thickness unevenness Δt of the transparent substrate is
| Δt | ≦ 5.26λ / NA ^ 4
It is proposed that This is a range derived based on the thickness unevenness standard in a CD (Compact Disk) system.

  However, the above formula is not appropriate from the following two viewpoints.

(1) Originally, the thickness range of the light transmission layer should be defined by an absolute value in consideration of the refractive index n of the light transmission layer as in the current DVD, whereas in the above equation, Only the thickness range of the light transmission layer is defined.

(2) In next-generation optical discs, the introduction of a mechanism for correcting spherical aberration that occurs with higher NA is considered, and it is no longer possible to define the thickness range of the light transmission layer based on the CD system. Not appropriate.

  First, (1) is a necessary condition when the entire optical disc drive system is considered. That is, the optical disk drive uniquely defines the thickness and refractive index of the light transmission layer of the optical disk to be recorded and reproduced as the specifications of the objective lens constituting the optical system.

  Therefore, as a matter of course, it is desirable that the optical disc loaded in the optical disc drive has the same light transmission layer thickness and refractive index as the specification values of the optical disc drive.

  However, the thickness and refractive index of the light transmission layer of the optical disk loaded in the optical disk drive have a certain width in the manufacture of the optical disk and in the selection of the material of the light transmission layer used in the optical disk by the manufacturer. Inevitable. The question is how much the width is allowed.

  This allowable width is defined as the range of the thickness and refractive index of the light transmission layer that gives a certain wavefront aberration by deviation from the reference value of the load specification value of the objective lens of the optical disk drive described above. It is reasonable.

  On the other hand, the above formula merely defines the thickness range of the light transmitting layer, and naturally, considering the optical system specifications of the optical disk drive, naturally, the concept of refractive index that must be studied simultaneously. This is an inadequate specification for a system that takes into account the optical disk and its drive.

  In the DVD, a certain range between the thickness of the transparent substrate (light transmission layer) and the refractive index is already defined as a standard based on the above-described idea. However, it is needless to say that this standard cannot be used at all with next-generation optical discs that have a high NA and the specifications of the thickness of the light transmission layer differ greatly.

  The above (2) has been a precondition specific to the next generation optical disc. As described above, in the next-generation optical disk, the increase in coma aberration accompanying the increase in NA can be compensated by making the light transmission layer thin.

  On the other hand, as the NA increases, the increase in spherical aberration due to the thickness error of the light transmission layer becomes remarkable. This is because spherical aberration is proportional to the fourth power of the NA value as an approximation.

  Since this spherical aberration cannot be compensated for by reducing the thickness of the light transmission layer, the next generation optical disc is considering introducing a new mechanism for correcting spherical aberration into the optical system.

  This spherical aberration correction mechanism was not introduced at all in the conventional DVD or CD system, and there is a difference from the conventional system. For this reason, it is apparent that an unsuitable result will be obtained if the conventional DVD or CD system is considered as a standard for defining the error range of the thickness of the light transmission layer.

  That is, in the next-generation optical disc, it is necessary to select a wavefront aberration different from the conventional one on the basis of a spherical aberration correction mechanism and to define the range of the thickness of the light transmission layer.

  For example, even if the standard value of wavefront aberration is 0.04 λrms or less in the past, in the next generation, the wavefront aberration reference may be relaxed to 0.10 λrms in consideration of the effect of the spherical aberration correction system. it can.

  As described above, when a spherical aberration correction system is introduced in a drive system for a next-generation optical disc, using the conventional system as a reference will give erroneous results. The above formula is based on the CD system, which is not appropriate for the next generation optical disc.

For example, when λ = 405 nm and NA = 0.85 as the specifications of the next-generation optical disc, the above formula specifies a thickness unevenness range of | Δt | ≦ 4.08 μm. This is an unreasonably narrow range in view of the spherical aberration correction system described above, and the yield in the production of the optical disk is unnecessarily reduced.
JP-A-8-273199 JP 2000-11454 A

  Therefore, the present invention has been made in consideration of the above circumstances. For example, it specifies the effective thickness of the light transmission layer and the range of the refractive index of the light transmission layer in the next generation optical disk, and is extremely suitable for high density recording. It is an object of the present invention to provide a good optical disk and a recording / reproducing apparatus therefor.

In the optical disk according to the present invention, two information recording layers are laminated on a substrate with a light-transmitting intermediate layer interposed therebetween, and the outer information recording layer is covered with a light-transmitting cover layer. Intended for things. When the numerical aperture of the lens that irradiates the laser light incident on the cover layer is 0.85, the function f (n) relating to the refractive index n of the cover layer.

And based on the maximum allowable value of aberration in the light transmission layer composed of the cover layer, the information recording layer, and the intermediate layer, which can be corrected by the spherical aberration correction mechanism included in the optical disk device that performs recording and reproduction with respect to the optical disk. Using the determined constants t1 and t2, the thickness t of the cover layer is set to f (n) −t1 or more, and the information recording layer except the cover layer, the intermediate layer, and the information recording layer closest to the substrate The total thickness is set to f (n) + t2 or less.

In the optical disk recording / reproducing apparatus according to the present invention, two information recording layers are laminated on a substrate with a light-transmitting intermediate layer interposed therebetween, and the outer information recording layer is made light-transmitting. A function f (n) relating to the refractive index n of the cover layer when the numerical aperture of the lens that covers the cover layer and irradiates the laser beam incident on the cover layer is 0.85.

Determined based on the maximum allowable value of aberration in the light transmission layer composed of the cover layer, the information recording layer, and the intermediate layer, which can be corrected by the spherical aberration correction mechanism of the apparatus for recording and reproducing the optical disk. The thickness t of the cover layer is set to f (n) −t1 or more using the constants t1 and t2, and the sum of the cover layer, the intermediate layer, and the information recording layer excluding the information recording layer closest to the substrate Is recorded on and reproduced from a predetermined information recording layer by irradiating an optical disc having a thickness of f (n) + t2 or less with a laser beam from the cover layer side. Spherical aberration correction means for correcting the spherical aberration in is provided.

  According to the above-described invention, the amount of allowable aberration can be increased by correcting the spherical aberration due to the thickness error of the light transmission layer including the cover layer, the information recording layer, and the intermediate layer. When the thickness of the light transmissive layer and the refractive index range of the light transmissive layer of the multilayer optical disk suitable for recording are specified, a manufacturing margin can be increased, so that the yield of the optical disk can be improved. It becomes possible.

  Hereinafter, a first embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional view showing the structure of an optical disk 11 described in the first embodiment.

  In the optical disc 11, an information recording layer 13 including a phase change recording film is formed on a substrate 12 made of polycarbonate or the like. When the optical disk 11 is a read-only disk, an information recording layer 13 made of a metal reflective film is formed instead of the phase change recording film.

  A cover layer (light transmission layer) 14 having a thickness t is formed on the information recording layer 13. The cover layer 14 is a sheet having a thickness t made of, for example, a plastic material, and is adhered to the information recording layer 13 formed on the substrate 12 via an adhesive, an ultraviolet curable resin, or the like.

  Next, FIG. 2 shows an optical disc apparatus that performs recording / reproduction on the optical disc 11. This optical disk apparatus uses a short wavelength semiconductor laser light source 20 as its light source. The wavelength of the emitted light 100 from the semiconductor laser light source 20 is, for example, in the violet wavelength band in the range of 395 nm to 415 nm.

  The emitted light 100 from the semiconductor laser light source 20 is converted into parallel light by the collimator lens 21, sequentially passes through the polarization beam splitter 22 and the λ / 4 plate 23, then passes through the relay lens system 24, and enters the objective lens 25. Incident. As a result, the light passes through the cover layer 14 of the optical disc 11 and is focused on the information recording layer 13.

  Then, the reflected light 101 from the information recording layer 13 of the optical disk 11 is transmitted again through the cover layer 14 of the optical disk 11, transmitted through the objective lens 25, the relay lens system 24, and the λ / 4 plate 23, and is substantially eliminated by the polarization beam splitter 22. After being reflected at a right angle, the light passes through the light detection system 26 and enters the light detector 27.

  The light detector 27 has a light receiving portion divided into a plurality of regions, and outputs a current corresponding to the light intensity from each light receiving region. The current output from each light receiving region of the photodetector 27 is converted into a voltage by an I / V amplifier (not shown) and then supplied to the arithmetic circuit 30.

  The arithmetic circuit 30 performs arithmetic processing on voltage signals corresponding to the respective light receiving regions of the photodetector 27 to thereby perform an HF (High Frequency) signal, a focus error signal, a tracking error signal, and a control signal for the relay lens system 24. Etc. are generated.

  Among these, the focus error signal and the tracking error signal are supplied to the drive unit 29 via the servo driver 31 to be used for controlling the focus direction and the tracking direction of the objective lens 25.

  The relay lens system 24 includes a bottom lens 24a and a top lens 24b. The top lens 24 b is controlled in the optical axis direction when a control signal for the relay lens system 24 from the arithmetic circuit 31 is supplied to the drive unit 28 via the servo driver 31.

  The relay lens system 24 is designed so that laser light is incident on the objective lens 25 as substantially parallel light when the thickness of the cover layer 14 is a specified value (for example, 100 μm).

  However, when the thickness of the cover layer 14 deviates from the specified value, spherical aberration due to the thickness error of the cover layer 14 occurs. At this time, since the shape of the beam spot focused on the information recording layer 13 of the optical disc 11 is distorted, stable and accurate recording / reproduction becomes difficult.

  On the other hand, spherical aberration is generated by making the incident light to the objective lens 25 into convergent light or divergent light. Further, by moving the top lens 24b of the relay lens system 24 in the optical axis direction, the incident light to the objective lens 25 can be made into convergent light or divergent light.

  For this reason, the top lens 24b of the relay lens system 24 is moved in the optical axis direction according to the thickness error amount of the cover layer 14, and the incident light to the objective lens 25 is converted into convergent light or divergent light, whereby the cover layer. The spherical aberration caused by the thickness error of 14 can be corrected.

  Specifically, when the thickness of the cover layer 14 is larger than a specified value, the top lens of the relay lens system 24 is set so that incident light to the objective lens 25 becomes divergent light according to the thickness error amount of the cover layer 14. 24b is moved in the optical axis direction.

  Further, when the thickness of the cover layer 14 is thinner than the specified value, the top lens 24b of the relay lens system 24 is light-transmitted so that the incident light to the objective lens 25 becomes convergent light according to the thickness error amount of the cover layer 14. Move in the axial direction.

  As described above, it is assumed that an optical disc apparatus that performs recording / reproduction with respect to the next-generation optical disc includes means for correcting spherical aberration due to a thickness error from a specified value of the cover layer 14 of the optical disc 11. Yes.

  Since this is not considered in the conventional optical disc system CD and DVD, applying the conventional rules as they are in producing next-generation optical discs will produce erroneous results.

  For this reason, the optical disk 11 described in the first embodiment has a thickness error and refractive index range of the cover layer 14 in consideration of being recorded and reproduced by an optical disk apparatus provided with a spherical aberration correcting means. I have it.

  As a specification of an optical disk apparatus that performs recording / reproduction with respect to a next-generation optical disk, for example, a case where the wavelength of a laser beam is 405 nm and the NA of the objective lens 25 is 0.85 is considered.

  Then, it is assumed that the objective lens 25 is an ideal lens in which the aberration is completely corrected with respect to the optical disc 11 having a lens load with a refractive index of the cover layer 14 of 1.622 and a thickness of 100 μm. To do.

  FIG. 3 shows the result of obtaining the rms value of the wavefront aberration generated when the optical disk 11 having different refractive index n of the cover layer 14 and thickness t of the cover layer 14 is used for the objective lens 25. Yes.

  In FIG. 3, the horizontal axis represents the refractive index n of the cover layer 14, the vertical axis represents the thickness t of the cover layer 14, and the rms value of aberration at each point on the coordinate plane is displayed in contour lines. The step of the contour line is 2/100 of the wavelength of the laser beam (= 405 nm).

  In FIG. 3, the points indicated by ◎ are points of the standard specification, that is, the load specification of the objective lens 25, and the aberration is zero at this point. As a result, when the optical disc 11 having different refractive index n and thickness t of the cover layer 14 is used, in order to make the residual aberration amount constant, for example, when the refractive index n becomes larger than the lens load specification value. It can be seen that it is better to slightly increase the thickness t of the cover layer 14 from the specification value.

  Therefore, as the definition of the cover layer 14 of the optical disc 11 in the next generation DVD, it is possible to specify that the allowable error range of the thickness t of the cover layer 14 is changed as an absolute value according to the refractive index n of the cover layer 14. Necessary.

The range between the refractive index n and the thickness t of the cover layer 14 of the optical disk 11 described in the first embodiment is as shown in FIG. this is,

Shows the area.

  This is almost the same as the range in which the aberration in the contour map shown in FIG. 3 is 0.1λ rms or less. By defining the optical disk 11 in such a range, the thickness t of the cover layer 14 and the refractive index n The aberration due to the deviation from the specification value can be maintained under the condition of approximately 0.1 λrms or less.

  The contour lines of the amount of wavefront aberration in FIG. 3 are arranged substantially parallel to the vertical axis direction, and the curve has a form obtained by giving a constant offset to the above equation (3). For this reason, if the allowable value of the aberration is determined, the range of the thickness t and the refractive index n of the cover layer 14 can be determined by the above equations (1) to (3).

  At this time, the range of the thickness t of the cover layer 14 may be adjusted by changing t1 and t2 according to the allowable value of aberration. For example, when the allowable aberration is 0.04λrms, an appropriate range can be specified by setting t1 and t2 = 0.004 mm in the above formula (see FIG. 5).

  On the other hand, the range of the refractive index n is determined by the material of the cover layer 14 and the wavelength of the laser beam, and defines a range including an effective material for the cover layer 14 of the optical disc 11.

  In this case, the refractive index n in the violet wavelength band of a material effective as the cover layer (light transmission layer) 14 of the optical disk 11 such as polycarbonate can be covered by setting the ratio to about 1.47 to 1.67.

  In the conventional optical disc apparatus, the aberration due to the thickness of the light transmission layer of the optical disc and the refractive index error is limited to 0.02 to 0.03 λrms. Therefore, for example, if the aberration tolerance is 0.02λrms, the error range of the thickness t of the cover layer 14 needs to be t1, t2 = 0.002 mm from FIG.

  In this case, it is easily imagined that the manufacturing margin of the optical disk 11 becomes very narrow. However, the next generation optical disc is premised on introducing spherical aberration correction means as described above.

  For this reason, the allowable aberration amount can be made larger than that of the conventional optical disc apparatus. Therefore, for example, if the allowable aberration amount is set to 0.1λrms as described above, t1 and t2 = 0.010 mm, a margin for manufacturing the optical disk 11 can be increased, and an improvement in yield can be expected.

  On the other hand, when considering a specification in which the thickness error of the cover layer is constant regardless of the refractive index as in the above-described known example, the above equation (3) is a constant (specification value of the cover layer thickness). It corresponds to that.

  In this case, for example, when t1 and t2 = 0.002 mm, the region is as shown in FIG. As can be seen from FIG. 6, such a region does not match the region where the wavefront aberration is below a certain value.

  Considering the case of t1, t2 = 0.010 mm as a comparison with the first embodiment described above, the region is as shown in FIG. Also in this case, the value of the wavefront aberration changes greatly at the boundary between the regions, and for example, when n = 1.47 and t = 110 μm, the aberration reaches 0.12 λrms.

  For this reason, when t1, t2 = 0.010 mm in the first embodiment described above, considering that the wavefront aberration is suppressed to 0.10 λrms or less in the entire region, the allowable aberration amount is increased. Need to be done.

  In other words, the allowable range of the thickness error of the cover layer must be narrowed in the conventional example for a certain allowable aberration amount. That is, the margin for manufacturing the disk is reduced.

  A second embodiment of the present invention will be described. FIG. 8 shows a range of the thickness t and the refractive index n of the cover layer 14 of the optical disk 11 described in the second embodiment. This is almost the same as the range of the optical disk 11 of the first embodiment shown in FIG. 4, but the area is surrounded by a straight line instead of a curve.

  In this case, in FIG. 8, a plurality of (three in the illustrated example) points are sampled from a curve showing an aberration of 0.10 λrms, and the range is set by connecting the sample points with a straight line. Also by this, it is possible to obtain substantially the same effect as the optical disk 11 of the first embodiment.

  Furthermore, a third embodiment of the present invention will be described. FIG. 9 shows a cross section of an optical disc 51 described in the third embodiment. In the optical disc 51, an information recording layer 53 including, for example, a phase change recording film is formed on a substrate 52 made of, for example, polycarbonate.

  A transparent intermediate layer 54 is formed on the information recording layer 53, and another information recording layer 55 is formed thereon. The information recording layers 53 and 55 may both be a read-only layer made of a metal reflection film, or both may be a record / playback layer, or one may be a read-only layer and the other may be a record / playback layer. good.

  Further, a cover layer (light transmission layer) 56 is formed on the information recording layer 55. The cover layer 56 is a sheet made of, for example, a plastic material, and is bonded to the information recording layer 55 via an adhesive or an ultraviolet curable resin.

  The role of the intermediate layer 54 is to optically block leakage of information (crosstalk) from the other information recording layer 55 or 53 when one information recording layer 53 or 55 is reproduced. It is.

  In that sense, the distance between the two information recording layers 53 and 55 should be as large as possible, and the thickness of the intermediate layer 54 should be thick. However, in that case, a burden is imposed on the optical system for recording and reproducing.

  That is, when the thickness from the surface of the cover layer 56 to the center of the intermediate layer 54 is defined as the load of the objective lens 25, the information on the intermediate layer 54 can be recorded or reproduced regardless of which information recording layer 53 or 55 is recorded or reproduced. This is because aberration due to a thickness error of half the thickness occurs.

  Therefore, from the viewpoint of aberration of the recording / reproducing optical system, it is better that the intermediate layer 54 is thinner. That is, the thickness of the intermediate layer 54 is determined as a compromise between the crosstalk between the information recording layers 53 and 55 and the trade-off relationship in the aberration of the recording / reproducing optical system.

  As a specification of an optical disc apparatus for recording / reproducing with respect to a next generation optical disc, for example, when the wavelength of the laser beam is 405 nm and the NA of the objective lens 25 is 0.85, the above-mentioned trade-off is considered. A thickness of about 20 to 30 μm is appropriate.

  The thickness of the light transmission layer of the two-layer type optical disc 51 is defined as the minimum thickness of the cover layer 56, the thickness of the cover layer 56, the thickness of the information recording layer 55 adjacent to the cover layer 56, and the intermediate layer 54. It is good to express with the maximum value of the total value with the thickness of.

  The range of the thickness and refractive index of the light transmission layer of the optical disc 51 at this time is as shown in FIG. As in the embodiments described so far, a lens load is assumed in which the light transmission layer of the optical disc 51 has a refractive index of 1.622 and a thickness of 100 μm.

  In the specified region, the refractive index is 1.47 ≦ n ≦ 1.67, the thickness of the cover layer 56 is f (n) −t1 or more, and the thickness of the cover layer 56 + the information recording layer 55 + the intermediate layer 54 is f. (N) + t2 or less, and t1, t2 = 0.020 mm. In addition, f (n) is as Formula (3). As a result, since the thickness of the intermediate layer 54 is included, the range in the thickness direction is wider than in the case of one layer.

  Furthermore, a fourth embodiment of the present invention will be described. FIG. 11 shows the range of the thickness and refractive index of the cover layer 56 of the optical disk 51 described in the fourth embodiment.

  This is almost the same as the range of the optical disc 51 of the third embodiment shown in FIG. 10, but the range is a range surrounded by a straight line instead of a curve. In this case as well, a plurality of (three in the illustrated example) points are sampled from the curve showing the aberration of 0.20 λrms, and the range is set by connecting the sample points with straight lines.

  Also by this, substantially the same effect as the optical disk 51 of the third embodiment can be obtained. In the third and fourth embodiments described above, the case where the two information recording layers 53 and 55 are provided has been described. However, it goes without saying that the present invention can also be applied to an optical disc having two or more information recording layers. Yes.

  Note that the present invention is not limited to the above-described embodiments as they are, and can be embodied by variously modifying the constituent elements without departing from the scope of the invention in the implementation stage. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above-described embodiments. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements according to different embodiments may be appropriately combined.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side sectional view for explaining a detailed structure of an optical disc according to a first embodiment of the present invention. The block block diagram shown in order to demonstrate the optical disk apparatus which records and reproduces with respect to the optical disk in the said 1st Embodiment. The figure which shows the rms value of the aberration of an optical disk in order to demonstrate the relationship between the refractive index and thickness of a cover layer as a parameter. The figure shown in order to demonstrate the range of the refractive index and thickness of the cover layer of an optical disk in case the allowable aberration is 0.10 (lambda) rms in the said 1st Embodiment. The figure shown in order to demonstrate the range of the refractive index and thickness of the cover layer of an optical disk in case the allowable aberration in the said 1st Embodiment is 0.04 (lambda) rms. The figure shown in order to demonstrate the range of the refractive index and thickness of the cover layer of an optical disk in a well-known example. The figure shown in order to demonstrate the other example of the range of the refractive index and thickness of the cover layer of the optical disk in the same well-known example. The figure shown in order to demonstrate the range of the refractive index and thickness of the cover layer of the optical disk based on 2nd Embodiment of this invention. The sectional side view shown in order to demonstrate the detailed structure of the optical disk based on 3rd Embodiment of this invention. The figure shown in order to demonstrate the range of the refractive index and thickness of the cover layer of the optical disk in the said 3rd Embodiment. The figure shown in order to demonstrate the range of the refractive index and thickness of the cover layer of the optical disk based on 4th Embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Optical disk, 12 ... Board | substrate, 13 ... Information recording layer, 14 ... Cover layer, 20 ... Semiconductor laser light source, 21 ... Collimating lens, 22 ... Polarizing beam splitter, 23 ... Lambda / 4 plate, 24 ... Relay lens system, 25 DESCRIPTION OF SYMBOLS ... Objective lens, 26 ... Photodetection system, 27 ... Photodetector, 28 ... Drive part, 29 ... Drive part, 30 ... Arithmetic circuit, 31 ... Servo driver, 51 ... Optical disk, 52 ... Substrate, 53 ... Information recording layer 54 ... Intermediate layer, 55 ... Information recording layer, 56 ... Cover layer.

Claims (6)

In an optical disc in which two information recording layers are laminated on a substrate with a light-transmitting intermediate layer sandwiched therebetween, and the outer information recording layer is covered with a light-transmitting cover layer.
A function f (n) relating to the refractive index n of the cover layer when the numerical aperture of the lens that irradiates the cover layer with the laser light is 0.85.

When,
Based on the maximum allowable value of the aberration in the light transmission layer composed of the cover layer, the information recording layer, and the intermediate layer, which can be corrected by a spherical aberration correction mechanism included in the optical disc apparatus that records and reproduces the optical disc. Using the determined constants t1 and t2,
The thickness t of the cover layer is set to f (n) -t1 or more, and the total thickness of the cover layer, the intermediate layer, and the information recording layer excluding the information recording layer closest to the substrate is f (n). An optical disc characterized by being set to + t2 or less.   The wavelength of the laser beam incident on the cover layer is 395 to 415 nm, and the refractive index of the cover layer and the intermediate layer is set in a range of 1.47 to 1.67. optical disk.   2. The optical disk according to claim 1, wherein the constants t1 and t2 are set to approximately 0.002 mm or more.   2. The optical disc according to claim 1, wherein the constants t1 and t2 are set to approximately 0.020 mm.   A boundary line obtained by approximating the curve indicated by f (n) -t1 by a plurality of straight lines is set as the minimum value of the thickness t of the cover layer, and the curve indicated by f (n) + t2 is approximated by a plurality of straight lines. 2. The optical disc according to claim 1, wherein the boundary line is the maximum value of the total thickness of the cover layer, the intermediate layer, and the information recording layer excluding the information recording layer closest to the substrate. On the substrate, two information recording layers are laminated with a light-transmitting intermediate layer sandwiched between them, and the outer information recording layer is covered with a light-transmitting cover layer and incident on the cover layer. When the numerical aperture of the lens that irradiates the laser beam is 0.85, the function f (n) relating to the refractive index n of the cover layer

And based on the maximum allowable value of aberration in the light transmission layer composed of the cover layer, the information recording layer, and the intermediate layer, which can be corrected by a spherical aberration correction mechanism included in an apparatus for recording / reproducing with respect to the optical disc. Using the determined constants t1 and t2, the thickness t of the cover layer is set to f (n) −t1 or more, and information recording excluding the information recording layer closest to the cover layer, the intermediate layer, and the substrate Information is recorded / reproduced on / from a predetermined information recording layer by irradiating the optical disk whose total thickness with the layer is set to f (n) + t2 or less from the cover layer side. What is claimed is: 1. An optical disc recording / reproducing apparatus comprising the spherical aberration correction mechanism for correcting spherical aberration in the light transmission layer.

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