JP2005100647A - Optical disk and optical disk device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To specify ranges of the thickness of a light transmission layer and the refractive index of the light transmission layer which are effective in a next generation optical disk, and to provide an optical disk suitable for high-density recording. <P>SOLUTION: The refractive index of the light transmission layer of the optical disk is set so as to be in a range of 1.52-1.72; the numerical aperture of a lens for irradiation with a laser beam incident on the light transmission layer is set at 0.65, and the wavelength of the laser beam is set so as to be in a range of 395-415 nm. The thickness t of the light transmission layer is set so as to be in a range as f(n)-t1≤t≤f(n)+t2, using constants t1 and t2 determined on the basis of the permissive value of aberration and a function f(n) of the refractive index n, so that the aberration is within a specified permissive value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical disc capable of high-density recording.

  As is well known, in recent years, a DVD having a single-layer capacity of 4.7 GB has been put to practical use as an optical disc capable of recording information at high density. There are various types of DVDs, such as a read-only DVD-ROM and a rewritable DVD-RAM. In DVD, an information recording layer is formed on a 0.6 mm thick transparent substrate (hereinafter referred to as a light transmission layer), and laser light is transmitted through the light transmission layer and condensed on the information recording surface. It is configured to write and read information. The numerical aperture (NA) of the objective lens for condensing the beam at this time is based on 0.6. Further, the refractive index n of the light transmission layer is specified in the range of n = 1.45 to 1.65 with respect to the wavelength of 650 nm, and a light transmission layer material that meets this condition is selected. Polycarbonate is generally used as the light transmitting layer material, and the refractive index in this case is n = 1.58.

  As described above, the thickness of the light transmission layer of the DVD is 0.6 mm as a standard, but in practice, it is inevitable that the thickness varies due to the manufacture of the disc. In an optical system for recording / reproducing a DVD, if the light transmission layer is designed with a standard thickness of 0.6 mm, an aberration occurs if the substrate thickness is manufactured with a deviation from 0.6 mm. Since aberrations of the optical system increase the beam spot diameter and adversely affect signal reproduction, it is necessary to suppress it to a certain value or less on the system.

  The aberration of the optical system due to the thickness error of the light transmission layer is determined by both the deviation from the standard value of the light transmission layer and the deviation from the standard value of the refractive index of the light transmission layer. Therefore, in the case of DVD, in order to suppress the aberration of the optical system due to the thickness error of the light transmission layer below a certain value, the range of the light transmission layer thickness is defined as a two-dimensional range with its refractive index. . This range is disclosed, for example, in JP-A-8-273199. That is, for the refractive index n = 1.45 to 1.65, when the error of the light transmission layer thickness with respect to the standard value is ± 0.03 mm, the horizontal axis represents the refractive index and the vertical axis represents the light transmission layer thickness. When the refractive index n is smaller than the lens load specification (standard value), the range of the shape shifted in the direction of increasing the light transmission layer thickness is specified. The thickness of the transmission layer is specified in a range that does not change.

  However, the above-mentioned known examples are not appropriate from the following viewpoints.

  Currently, various companies are developing technologies to further increase the density of DVDs. The spot size of the focused light irradiated on the information recording surface of the optical disc is proportional to the wavelength and inversely proportional to NA indicating the aperture angle of the objective lens for focusing the light. Therefore, in order to reduce the spot size of the focused light in order to improve the recording density, it is necessary to shorten the light source wavelength and increase the NA of the objective lens.

  At this time, since the refractive index of the light transmission layer depends on the light source wavelength, it is necessary to newly define the range of the thickness of the light transmission layer as a two-dimensional range with the refractive index. Examples of the light source wavelength, NA, and light transmission layer thickness of the next generation optical disc include wavelength λ = 405 nm, NA = 0.65, and light transmission layer thickness = 0.6 mm.

  Accordingly, an object of the present invention is to provide an optical disc suitable for high-density recording by defining the range of the light-transmitting layer thickness and the light-transmitting layer refractive index effective in the next-generation optical disc.

In order to achieve the above object, the optical disc of the present invention is formed by covering an information recording layer formed on a substrate with a light transmission layer, and the thickness and refractive index of the light transmission layer are different from the standard values. An optical disc in which the range of the thickness and refractive index of the light transmission layer is set so that the aberration is within a certain allowable value, and the function f (n) of the refractive index n of the light transmission layer and the light Using the constants t1 and t2 determined based on the allowable value of the aberration in the transmission layer, the thickness t of the light transmission layer is set in the range of f (n) −t1 ≦ t ≦ f (n) + t2. The refractive index of the light transmission layer is set in a range of 1.52 to 1.72, and the function f (n) is expressed as follows using constants A1, A2, and A3.

  In one embodiment of the present invention, the constant A1 is 0.26200, A2 is -0.32400, and A3 is 0.00595.

  It is possible to provide an optical disc suitable for high-density recording by defining the range of the light-transmitting layer thickness and the light-transmitting layer refractive index that are effective in the next-generation optical disc.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  An example of a cross-sectional view of an optical disc 1 in the present invention is shown in FIG. An information recording layer 3 including, for example, a phase change recording film is formed on a substrate 2 made of polycarbonate (lower in the figure). When the optical disc 1 is a read-only disc, an information recording layer 3 made of a metal reflection film is formed instead of the phase change recording film. Next, a light transmission layer 4 having a thickness t is formed on the information recording layer 3 (lower in the figure). The light transmission layer 4 is, for example, polycarbonate.

  Next, FIG. 2 shows a configuration example of an optical disc apparatus for recording / reproducing the disc. A short wavelength semiconductor laser 20 is used as the light source. The wavelength of the emitted light is generally in the violet wavelength band in the range of 395 nm to 415 nm (405 ± 10 nm). The emitted light 100 from the semiconductor laser light source 20 is converted into parallel light by the collimator lens 21, passes through the polarization beam splitter 22 and the λ / 4 plate 23, and then enters the objective lens 25. The NA of the objective lens is, for example, in the range of 0.6 to 0.7. Thereafter, the emitted light 100 passes through the light transmission layer 4 of the optical disc 1 and is condensed on the information recording layer 3.

  The reflected light 101 from the information recording layer 3 of the optical disc 1 passes through the light transmission layer 4 of the optical disc 1 again, passes through the objective lens 25 and the λ / 4 plate 23, is reflected by the polarization beam splitter 22, and then detects light. The light passes through the optical system 26 and enters the photodetector 27. The light receiving part of the photodetector 27 is usually divided into a plurality of parts, and a current corresponding to the light intensity is output from each light receiving part. The output current is converted into a voltage by an I / V amplifier (not shown), and then processed by the arithmetic circuit 11 into an RF signal, a focus error signal, a track error signal, and the like.

  Based on these error signals, the servo driver 10 drives the lens drive coil 12 to move the lens 25 in the focus direction (lens optical axis direction) and tracking direction (disk radial direction). As a result, a beam spot is generated on the target track of the information recording layer 3.

  Here, when the thickness of the light transmission layer 4 is a standard value (for example, 0.6 mm), it is designed to enter the objective lens 25 as substantially parallel light. However, when the thickness of the light transmission layer 4 deviates from the standard value, spherical aberration due to the thickness error of the light transmission layer 4 occurs. At this time, the shape of the focused spot on the information recording layer 3 of the optical disc 1 is distorted, so that stable and accurate recording / reproduction becomes difficult.

  Since the next generation optical disc device has a shorter wavelength and higher NA than the conventional optical disc device, applying the DVD and CD rules as they are in producing next-generation optical discs gives an erroneous result. Will be born. Therefore, the optical disc 1 of the present invention is characterized by having a thickness error and a refractive index range of the light transmission layer 4 in consideration of shortening of the wavelength and high NA of the optical disc apparatus.

  As a specification of the next-generation optical disc system, consider a case where, for example, a light source wavelength of 405 nm and an objective lens 25 NA = 0.65 are used. The light beam 100 is directed onto the recording layer 3 using an ideal objective lens that has been completely corrected for aberration with respect to a lens load having a refractive index of 1.622 and a thickness of 0.6 mm. FIG. 3 shows a state in which a beam spot is generated by condensing the light beam. At this time, the wavefront is in order and no wavefront aberration, that is, distortion of the wavefront has occurred. However, if wavefront aberration is caused by the residual spherical aberration of the lens system, an ideal beam spot is not generated as shown by the curve 6 indicated by the dotted line in the figure.

  The result of calculating the rms (root mean square) value of the wavefront aberration generated when the optical disc 1 having various light transmission layer refractive indexes n and light transmission layer thickness t is used for the ideal objective lens as described above. As shown in FIG. In FIG. 4, the refractive index n of the light transmission layer is plotted on the horizontal axis and the light transmission layer thickness t is plotted on the vertical axis, and the rms value of the wavefront aberration at each point on the coordinate plane is indicated by contour lines. The step of the contour line is 1/100 of the light source wavelength (λ = 405 [nm]). In FIG. 4, the center of the double circle is the standard specification, that is, the point of the load specification value of the objective lens 25 (here, the optical standard value for the disk when designing the lens), the thickness t = 600 μm, the refractive index n = 1.622. Here, the aberration is minimized.

  From this result, when using discs with various refractive indexes and thicknesses of the light transmitting layer, in order to make the residual aberration amount constant, the refractive index is shifted in the direction of increasing or decreasing from the lens load specification value. It is understood that it is better to slightly increase the light transmission layer thickness than the standard value. Therefore, as the light transmission layer definition of the next-generation DVD optical disk, the error allowable range of the light transmission layer thickness should be defined so as to change according to the absolute value of the deviation from the standard value 1.622 of the light transmission layer refractive index. Is required.

The range of the refractive index and thickness of the light transmission layer of the optical disc according to the embodiment of the present invention is the range shown in FIG. This shows the following areas.

  The contour lines of the wavefront aberration amount in FIG. 4 are arranged substantially parallel to the vertical axis direction, and the curve can be shown as a curve obtained by giving a constant offset to the above equation (3). Therefore, if the allowable value of aberration is determined, the range of the light transmission layer thickness and the refractive index can be determined by the above formulas (1) to (3) with the allowable value corresponding to the offsets t1 and t2.

  In the case of this embodiment, the range shown in FIG. 5 substantially matches the range in which the aberration in FIG. 4 is 0.03 λrms or less. That is, the range of thickness error ± 13 μm (t1, t2 = 13 μm) corresponds to the range where the aberration is 0.03λrms or less. Therefore, by defining an optical disk in the range shown in FIG. 5, the aberration due to deviation from the standard value (t = 0.6 mm, n = 1.622) of the light transmission layer thickness and the refractive index is approximately 0.03 λrms or less. It can be kept in the condition.

  The allowable value of the aberration is a value determined according to the performance of the optical disk apparatus that performs recording or reproduction on the optical disk or the allowable aberration. At this time, the range of the light transmission layer thickness may be adjusted by changing t1 and t2 according to the allowable value of aberration. For example, when the allowable aberration is 0.04λ, an appropriate range can be specified by setting t1, t2 = 17 μm in the above equation (see FIG. 6). Note that it is difficult to set t1 and t2 to 10 μm or less in the current optical disk manufacturing. Therefore, the minimum value of t1 and t2 is about 10 μm.

  On the other hand, the range of the refractive index is determined by the material of the light transmission layer 2 and the wavelength of the light source, and defines a range including an effective material for the light transmission layer of the optical disk. In this case, the refractive index in the violet wavelength band of a material that is effective as a light transmission layer of an optical disk such as polycarbonate can be covered by setting to about 1.52 to 1.72.

  Furthermore, the range of the thickness and the refractive index of the light transmission layer of the optical disc according to another embodiment of the present invention has the range shown in FIG. This is almost the same as the range of the optical disc of the embodiment shown in FIG. 5, but the range is a range surrounded by a straight line instead of a curve. The effect is equivalent to the optical disc of the above embodiment.

  Next, FIG. 8 shows an example of a cross-sectional view of an optical disc 51 according to another embodiment of the present invention. An information recording layer 53 including, for example, a phase change recording film is formed on a substrate 52 (bottom in the figure) made of polycarbonate. A transparent intermediate layer 54 is formed thereon, and another information recording layer 55 is formed thereon. Both of the information recording layers 53 and 55 may be read-only layers made of metal reflective films, or both may be recordable / reproducible layers, only one of which is a read-only layer and the other is a recordable / reproducible layer. May be. Next, a light transmission layer 56 is formed on the information recording layer 55. The light transmission layer 56 is, for example, polycarbonate. As a manufacturing process, for example, the substrate 52 on which the information recording layer 53 is formed and the light transmission layer 56 on which the information recording layer 54 is formed are bonded via an adhesive such as an ultraviolet curable resin (becomes the intermediate layer 54). Is done.

  The role of the intermediate layer 54 is to optically block leakage of information (crosstalk) from the other information recording layer when one information recording layer is being reproduced. In that sense, the distance between the two information recording layers should be as large as possible, and the intermediate layer 54 should be thick. However, in this case, a burden is imposed on the optical system for recording / reproducing. That is, when the thickness from the surface of the light transmission layer to the center of the intermediate layer is defined as the load of the objective lens, the recording error of either information recording layer is caused by a thickness error that is half the thickness of the intermediate layer. This is because aberration occurs. Therefore, the thinner intermediate layer is better from the viewpoint of aberration of the recording / reproducing optical system. That is, the thickness of the intermediate layer is determined as a trade-off relation between the crosstalk between the information recording layers and the aberration of the recording / reproducing optical system.

  As specifications of the next-generation optical disc system, for example, when the light source wavelength is 405 nm and the objective lens 25 has NA = 0.65, the thickness of the intermediate layer is appropriately about 20 μm to 30 μm in consideration of the above trade-off. The thickness of the light transmissive layer of the dual-layer disc is defined by the minimum thickness of the light transmissive layer 56 and the total thickness of the light transmissive layer 56, the information recording layer 55 in contact with the light transmissive layer, and the intermediate layer 54. It is better to express the maximum value. At this time, the range of the thickness and the refractive index of the light transmission layer of the optical disc is the range shown in FIG. As in the previous embodiments, it is assumed that the light transmission layer of the optical disk has a refractive index of 1.622, a lens load with a thickness of 0.6 mm, and a system allowable aberration of 0.04λ. The defined area is the following range.

Refractive index n: 1.52 ≦ n ≦ 1.72
Light transmission layer thickness: f (n) -t1 or more Light transmission layer + information recording layer 55 + intermediate layer 54 thickness: f (n) + t2 or less
t1, t2 = 17 μm
In this way, f (n) is as shown in Equation (3). Thus, the allowable aberration is set to 0.04λ in consideration of the thickness of the intermediate layer. Become wider.

  Furthermore, the range of the thickness and the refractive index of the light transmission layer of the optical disc of another embodiment of the present invention has the range shown in FIG. This is almost the same as the range of the optical disc of the embodiment shown in FIG. 9, but the range is a range surrounded by a straight line instead of a curve. The effect is equivalent to the optical disc of the above embodiment.

 The above two embodiments have shown the case where the information recording layer is two layers, but it goes without saying that the present invention can also be applied to an optical disc having two or more information recording layers.

Sectional drawing which shows the structure of the 1st optical disk to which this invention is applied. The block diagram which shows the optical disk apparatus which records / reproduces the optical disk of this invention. A state in which a light beam is condensed on a recording layer to generate a beam spot is shown. The figure which shows the relationship between the refractive index and thickness of the light transmission layer of an optical disk as a parameter of wavefront aberration. The range of the refractive index and thickness of the light transmission layer which concerns on one Embodiment of this invention is shown, and the range set when an allowable aberration is 0.03 (lambda) rms is shown. The range of the refractive index and thickness of the light transmission layer which concerns on one Embodiment of this invention is shown, and the range set when an allowable aberration is 0.04 (lambda) rms is shown. The range of the refractive index and thickness of the light transmission layer which concerns on one Embodiment of this invention is shown, and the linear approximation range set when an allowable aberration is 0.03 (lambda) rms vicinity is shown. Sectional drawing which shows the structure of the 2nd optical disk to which this invention is applied. The range of the refractive index and thickness of the light transmission layer which concerns on one Embodiment of this invention is shown, and the range set to a 2nd optical disk is shown when an allowable aberration is 0.04 (lambda) rms. The range of the refractive index and thickness of the light transmission layer which concerns on one Embodiment of this invention is shown, and the linear approximation range set when an allowable aberration is 0.04 (lambda) rms vicinity is shown.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Optical disk, 2 ... Board | substrate, 3 ... Recording layer, 4 ... Light transmission layer, 20 ... Semiconductor laser, 21 ... Collimating lens, 22 ... Beam splitter, 23 ... (lambda) / 4 board, 25 ... Objective lens, 26 ... Light detection Optical system, 27: photodetector, 11: arithmetic circuit, 12: lens driving coil, 10: servo driver

Claims (2)

The information recording layer formed on the substrate is covered with a light transmission layer, and the light transmission layer is configured so that aberrations due to deviations from the standard values of the thickness and refractive index of the light transmission layer are within a certain allowable value. An optical disc in which the thickness and refractive index range of a transmission layer are set,
Using the function f (n) of the refractive index n of the light transmission layer and constants t1 and t2 determined based on the allowable value of aberration in the light transmission layer, the thickness t of the light transmission layer is f. (N) −t1 ≦ t ≦ f (n) + t2 is set,
The refractive index of the light transmission layer is set in a range of 1.52 to 1.72,
The function f (n) uses constants A1, A2, and A3,

The constant A1 is 0.26200, A2 is -0.32400, and A3 is 0.00595. In an optical disc in which a plurality of information recording layers are laminated on a substrate with a light-transmitting intermediate layer sandwiched therebetween, and further covered with a light-transmitting layer,
Using the function f (n) of the refractive index n of the light transmission layer and constants t1 and t2 determined based on the aberration tolerance in the light transmission layer, the information recording layer, and the intermediate layer,
A thickness t of the light transmission layer is set to f (n) −t1 or more;
The total thickness of the light transmission layer, the intermediate layer, and the information recording layer excluding the information recording layer closest to the substrate is set to f (n) + t2 or less,
The refractive index of the light transmission layer and the intermediate layer is set in the range of 1.52 to 1.72,
The function f (n) uses constants A1, A2, and A3,

The constant A1 is 0.26200, A2 is -0.32400, and A3 is 0.00595.

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