JP2005166257A - Optical information recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical information recording medium wherein spherical aberration of a protective layer consisting of a plurality of layers is reduced in comparison with that in the conventional art to enable normal recording and/or reproduction. <P>SOLUTION: In an optical disk 7 having the protective layer 4 comprising a plurality of layers of a sheet layer 1, a coating layer 2, an adhesive layer 3 and the like, the ranges of the refractive index and thickness are newly defined and the refractive index and thickness of the protective layer 4 are determined in these ranges. Thereby, even if variation in thickness of the protective layer 4 is tolerated within ±3 μm, spherical aberration in the optical disk 7 can be substantially reduced to within 30 mλrms. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical information recording medium on which information is recorded and / or reproduced by irradiation with light from an optical head.

  There is an optical disk called DVD (Digital Versatile Disk) which is commercially available as a high-density, large-capacity optical information recording medium.

  Such optical discs have been rapidly spreading recently as recording media for recording images, music, and computer data. An optical disc has a protective layer that can be said to be a feature of the optical disc, and has a strong property against scratches and dirt. However, if there is a thickness error or refractive index error in this protective layer, a third-order spherical aberration component of wavefront aberration is generated, which adversely affects information recording / reproducing characteristics.

  Hereinafter, an example of a conventional optical disk will be described with reference to the drawings. FIG. 12 is a schematic diagram of an optical disc called a DVD. The optical disc 17 includes a protective layer 14, a recording layer 15, and a reinforcing substrate 16, and records and / or records information by irradiating the recording layer 15 with light having a wavelength of 660 nm converged by the objective lens 19 from the protective layer 14 side. Playback is performed.

  The objective lens 19 has a numerical aperture of 0.6, and a third-order spherical aberration component of wavefront aberration generated when light having a wavelength of 660 nm is transmitted through a light-transmitting flat plate having a refractive index of 1.58 and a thickness of 0.6 mm. Is designed to be substantially zero.

  The protective layer 14 is made of polycarbonate, the recording layer 15 is made of a film containing a dielectric, a reflective film, or the like. The reinforcing substrate 16 prevents the optical disk 17 from being warped or broken.

    The protective layer 14 protects the recording layer 15 from the air and separates the surface 18 of the optical disc 17 from the recording layer 15, thereby preventing the recording / reproduction characteristics from being deteriorated due to dust or scratches on the surface 18.

  However, if the protective layer 14 has a thickness error or a refractive index error, spherical aberration occurs in the spot formed on the recording layer 15 and adversely affects information recording / reproduction. Therefore, it is necessary to manage the thickness and refractive index of the protective layer 14.

  FIG. 13 is an example in which the refractive index and thickness of the protective layer in the case of DVD are defined. The horizontal axis represents the refractive index of the protective layer 14, the vertical axis represents the thickness of the protective layer 14, and the refractive index and thickness regions where the spherical aberration is within about 30 mλrms are indicated by broken lines. For example, if the refractive index and thickness design of the protective layer is set at a certain point on the dotted line in FIG. 13 and the variation in thickness is suppressed within the region, a disk capable of normal information recording / reproduction can be obtained.

  FIG. 14 shows a case where the reinforcing substrate 16 having the recording layer 15 and the sheet layer 11 are bonded together by the adhesive layer 13. The protective layer 14 includes a sheet layer 11 such as polycarbonate and an adhesive layer 13 such as an ultraviolet curable resin. The refractive index of each layer at a wavelength of 660 nm is 1.58 for the sheet layer 11 and 1.51 for the adhesive layer 13.

  In such a case, even if there is no error in the thickness of the protective layer 14 of 0.6 mm, spherical aberration occurs due to the difference between these refractive indexes. For example, when the thickness of the sheet layer 11 is 0.56 mm and the thickness of the adhesive layer 13 is 40 μm, the spherical aberration occurs about 0.3 mλrms. But this is small enough.

  As described above, when the numerical aperture is 0.6 and the wavelength is 660 nm, the spherical aberration generated when the refractive indexes of the plurality of layers constituting the protective layer are different is sufficiently small and can be ignored. That is, conventionally, a protective layer composed of a plurality of layers can be handled as one layer, and the refractive index and thickness of the protective layer are within a certain range shown in FIG. 13 as a product standard for optical discs with reduced spherical aberration. By managing to enter, there was no adverse effect on the recording and reproduction of information.

  In recent years, however, research on next-generation optical discs with higher recording density has been conducted in various places. The next generation optical disc is expected as a recording medium replacing the current mainstream VTR (Video Tape Recorder), and is being developed at a rapid pitch.

  As a means for increasing the recording density of the optical disc, there is a method of reducing the spot formed on the recording surface. This is achieved by increasing the numerical aperture of light from the optical head and shortening the wavelength.

  However, this conversely increases the spherical aberration due to the thickness error and refractive index error of the protective layer. Therefore, it is necessary to manage the thickness and refractive index of the protective layer in the same manner as the DVD described above.

  FIG. 15 is a schematic view of an optical disc with increased recording density. The optical disk 27 includes a protective layer 24, a recording layer 25, and a reinforcing substrate 26. Information is recorded and / or recorded by irradiating the recording layer 25 with light having a wavelength of 400 to 410 nm converged by the objective lens 29 from the protective layer 24 side. Or playback is performed.

  Since the objective lens 29 has a high numerical aperture of about 0.85, two lenses are used, and is generated when light having a wavelength of 405 nm is transmitted through a light-transmitting flat plate having a refractive index of 1.62 and a thickness of 100 μm such as polycarbonate. The third-order spherical aberration component of the wavefront aberration is designed to be substantially zero. By increasing the numerical aperture and shortening the wavelength, the spots formed on the recording layer 25 are reduced, and the density is increased.

  The recording layer 25 is a film containing a dielectric, a reflective film, or the like, and the reinforcing substrate 26 prevents the optical disk 27 from being warped or broken.

  The protective layer 24 protects the recording layer 25 from the air and separates the surface 28 of the optical disk 27 from the recording layer 25, thereby preventing deterioration in recording and reproducing characteristics due to dust and scratches on the surface 28.

  Since the reinforcing substrate 26 having the recording layer 25 and the sheet layer 21 are bonded together by the adhesive layer 23, the protective layer 24 is composed of two layers.

  The sheet layer 21 is polycarbonate, and the adhesive layer 23 is an ultraviolet curable resin. The refractive index of each layer at a wavelength of 405 nm is 1.62 for the sheet layer 21 and 1.53 for the adhesive layer 23. In such a case, even if there is no error in the design thickness of 100 μm of the protective layer 24, spherical aberration occurs due to the difference between these refractive indexes.

  For example, even if the thickness of the sheet layer is 60 μm, the thickness of the adhesive layer 23 is 40 μm, and the thickness of the protective layer 24 is 100 μm, the spherical aberration is generated as 5.3 mλrms. This is the spherical aberration that remains initially.

  In addition to this, spherical aberration due to variations in thickness in the manufacture of optical disks is added. Usually, the variation in thickness is about 3 μm, and as a result, the spherical aberration due to the variation in thickness is 30 mλrms.

  Therefore, even if the thickness of the protective layer 24 can be set to 100 μm, the total spherical aberration becomes 35.3 mλrms together with the spherical aberration 5.3 mλrms that remains initially as described above, and normal recording / reproduction is possible. There was a problem that it could not be done.

  FIG. 16 compares spherical aberration generated by the adhesive layer between a numerical aperture of 0.6 and a wavelength of 660 nm (DVD), and a numerical aperture of 0.85 and a wavelength of 405 nm. As can be seen from this figure, at a high numerical aperture and a short wavelength, a large spherical aberration occurs due to a difference in refractive index of a plurality of layers constituting the protective layer. This is more than 15 times larger than that of DVD. Furthermore, if the thickness of the protective layer varies, the spherical aberration exceeds the allowable range, and normal recording / reproduction cannot be performed.

  In consideration of the above-described problems of the conventional optical disk, the present invention provides optical information that can be normally recorded and / or reproduced with a spherical layer that is further suppressed compared to the conventional protective layer composed of a plurality of layers. An object is to provide a recording medium.

The first aspect of the present invention includes a recording layer and a protective layer including at least a first layer to an m-th layer (m ≧ 2), and has a wavelength of 400 to 410 nm converged by an objective lens having a numerical aperture of about 0.85. An optical information recording medium on which information is recorded and / or reproduced by irradiating the recording layer with light from the protective layer side,
i is an integer satisfying 1 ≦ i ≦ m, the refractive index of the i-th layer of the protective layer at a wavelength of 405 nm is n _i , and the thickness of the i-th layer is d _i ,
(A) When the plurality of layers constituting the protective layer are replaced with one substantially equivalent layer, the combined refractive index n of the one layer is expressed as n = ((n ₁ d ₁ + n ₂ d ₂ +... + N _m d _m ) / (d ₁ / n ₁ + d ₂ / n ₂ +... + D _m / n _m )) ^0.5
(B) The composite thickness d of the one layer is d = ((n ₁ d ₁ + n ₂ d ₂ +... + N _m d _m ) × (d ₁ / n ₁ + d ₂ / n ₂ +... + D _m / n _m )) ^0.5 and (c) f (n) = − 109.8n ³ + 577.2n ² −985 as a design criterion for the synthetic thickness of the protective layer. .5n + 648.6,
The refractive index ni of the _i- th layer and the composite refractive index n are 1.45 or more and 1.65 or less, and an absolute value of df (n) which is a difference between the composite thickness and the design standard The optical information recording medium has a value of 3 μm or more and 10 μm or less.

  According to the present invention, in an optical information recording medium having a protective layer composed of a plurality of layers, the spherical aberration is further suppressed as compared with the conventional one, and an advantageous effect that normal recording and / or reproduction can be performed is obtained.

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

(Embodiment 1)
FIG. 1 shows a cross-sectional view of an optical disk as an embodiment of the optical information recording medium of the present invention.

  In FIG. 1, an optical disc 7 has a protective layer 4, a recording layer 5, and a reinforcing substrate 6, and irradiates the recording layer 5 with light having a wavelength of 400 to 410 nm converged by the objective lens 9 from the protective layer 4 side. Information is recorded and / or reproduced.

  Since the objective lens 9 has a high numerical aperture of about 0.85, two lenses are used, and is generated when light having a wavelength of 405 nm is transmitted through a light-transmitting flat plate having a refractive index of 1.62 and a thickness of 100 μm such as polycarbonate. The third-order spherical aberration component of the wavefront aberration is designed to be substantially zero.

  The recording layer 5 is a film containing a dielectric, a reflective film, or the like, and the reinforcing substrate 6 prevents warping or destruction of the optical disk 7. The protective layer 4 protects the recording layer 5 from the air and separates the surface 8 of the optical disk 7 from the recording layer 5, thereby preventing deterioration in recording / reproducing characteristics due to dust and scratches on the surface 8.

  In the optical disk 7, the reinforcing substrate 6 having the recording layer 5 and the sheet layer 1 are bonded to each other with the adhesive layer 3, and further the surface layer is coated with the coat layer 2 to protect it from scratches. Therefore, the protective layer 4 is composed of three layers. .

  The sheet layer 1 is made of polycarbonate, the coat layer 2 is made of an acrylic resin, and the adhesive layer 3 is made of an ultraviolet curable resin. The refractive index of each layer at a wavelength of 405 nm is 1.62 for the sheet layer 1, 1.50 for the coat layer 2, and 1.53 for the adhesive layer 3. Even if the thickness of the protective layer 4 is 100 μm, spherical aberration occurs due to the difference in refractive index. Due to the high numerical aperture and short wavelength, the amount of spherical aberration generated is large and cannot be ignored.

Therefore, in the optical disk of the present invention, the refractive index of the sheet layer 1, the coating layer 2 and the adhesive layer 3 at a wavelength of 405 nm is n ₁ , n ₂ and n ₃ , and the thicknesses are d ₁ , d ₂ and d ₃ (μm), respectively. Then 1.45 ≦ n ₁ ≦ 1.65, 1.45 ≦ n ₂ ≦ 1.65, 1.45 ≦ n ₃ ≦ 1.65, 1.45 ≦ n ≦ 1.65, −3 μm ≦ d -F (n) ≦ 3 μm is satisfied.

  The present inventor newly introduced the concept of the combined refractive index and the combined thickness of one equivalent layer when a plurality of layers constituting the protective layer are replaced with one equivalent layer, thereby introducing a spherical surface. We made it possible to produce optical discs with reduced aberrations.

In the present embodiment, the composite refractive index n can be expressed by the following formula 1.
(Equation 1)
n = ((n ₁ d ₁ + n ₂ d ₂ + n ₃ d ₃ ) / (d ₁ / n ₁ + d ₂ / n ₂ + d ₃ / n ₃ )) ^0.5
The composite thickness d can be expressed by the following formula 2.
(Equation 2)
d = ((n ₁ d ₁ + n ₂ d ₂ + n ₃ d ₃ ) × (d ₁ / n ₁ + d ₂ / n ₂ + d ₃ / n ₃ )) ^0.5
As described above, these n and d are a composite refractive index and a composite thickness derived by replacing a plurality of layers with one equivalent layer. The aberration of light transmitted through a plurality of layers constituting the protective layer is substantially equal to the aberration of light transmitted through one equivalent layer derived as described above.

  The derivation of these expressions 1 and 2 will be described later.

Further, f (n) shown in the following equation 3 is a cubic approximation curve that passes through discrete points where the spherical aberration is substantially zero in the aberration calculation by the ray tracing method. Note that the value of f (n) is the composite thickness d.
(Equation 3)
^{f (n) = - 109.8n 3} + 577.2n 2 -985.5n + 648.6
The thickness of the protective layer is defined to be within ± 3 μm with the curve f (n) as the center.

  The composite refractive index n and the composite thickness d satisfying this condition are in a range surrounded by the curve and straight line in FIG. A dotted line is a curve f (n) in which the spherical aberration is substantially zero, and is a design standard regarding the combined thickness of the protective layer 4.

  From the above formulas 1 and 2, the combined refractive index n and the combined thickness d obtained from the thickness and refractive index of each of the plurality of layers constituting the protective layer are within this range (within the range surrounded by the solid line in FIG. 2). If determined, it becomes possible to produce an optical disc with spherical aberration suppressed substantially within 30 mλrms.

For example, if the design of the disk is n ₁ = 1.62, d ₁ = 50.0 μm, n ₂ = 1.50, d ₂ = 5.0 μm, n ₃ = 1.53, d ₃ = 44.2 μm, From Equations 1 and 2, the combined refractive index n = 1.57 and the combined thickness d = 99.2 μm. The point determined by the combined refractive index and the combined thickness thus obtained is a point Z on the curve f (n) as shown in FIG. 2, and the initially remaining spherical aberration is substantially 0. Become. Therefore, even if the variation in thickness at this time is allowed to be ± 3 μm, an optical disc having a spherical aberration substantially suppressed within 30 mλrms can be manufactured.

  As described above, the optical disk according to the embodiment of the present invention is designed in consideration of the thickness and refractive index of each of the plurality of layers constituting the protective layer, so that an optical disk with reduced generation of spherical aberration can be manufactured. It is.

Here, the meaning of considering the thickness and refractive index of each of the plurality of layers constituting the protective layer will be further described.
That is, as described above, the combined refractive index n and the combined thickness d satisfying Expression 3 are distributed on the dotted line in FIG. Therefore, for example, there are innumerable combinations of the combined refractive index and the combined thickness that satisfy Equation 3. Moreover, in each combination, there are theoretically many combinations of the refractive index n _i and the thickness d _i (i = 1, 2, 3) of each layer satisfying the relationship of the formulas 1 and 2.

Accordingly, if the combination of the refractive index n _i and the thickness d _i of each layer is appropriately selected from a large number of candidates satisfying these equations, the initially remaining spherical aberration is substantially reduced. 0. Therefore, in order to produce an optical disc in which the total spherical aberration is substantially suppressed within 30 mλrms, the variation in the thickness of the protective layer is allowed to ± 3 μm.

  In this way, the present invention is completely different from the conventional optical disc manufactured from the viewpoint of how to suppress the variation by focusing only on the variation in the thickness of the protective layer in order to reduce the spherical aberration, An optical disc based on a new design method is provided.

That is, according to the present invention, the combination of the refractive index n _i and the thickness d _i of each layer is selected from a large number of candidates that satisfy a predetermined relational expression (formula 1 to formula 3 in the present embodiment). Thus, it is possible to allow ± 3 μm, which is the limit value of the variation in the thickness of the protective layer in the current manufacturing technology.

As a result, various parameters such as the refractive index n _i and the thickness d _{i of} each layer can be selected, and the degree of freedom in design can be increased.

Here, the derivation of the above equation will be described with reference to FIGS. 17 (a) and 17 (b). FIG. 17A shows the state of refraction of light that passes through a plurality of layers, and FIG. 17B shows the state of refraction of light that passes through one layer. In order to make the aberration of light transmitted through a plurality of layers equal to the aberration of light transmitted through one layer, conditional expression a that the optical path lengths are the same for rays incident at an angle φ, and the condition that the positions of the emitted light are the same That is, equation b holds simultaneously.
(Conditional expression a)
n ₁ d ₁ / cos θ ₁ + n ₂ d ₂ / cos θ ₂ + n ₃ d ₃ / cos θ ₃ = nd / cos θ
(Conditional expression b)
d ₁ tan θ ₁ + d ₂ tan θ ₂ + d ₃ tan θ ₃ = d tan θ
These two conditional expressions a and b are made simultaneous, and the expression expressing Snell's law, sinφ / sinθ ₁ = n ₁ , sinφ / sinθ ₂ = n ₂ , sinφ / sinθ ₃ = n ₃ , and paraxial If the equation is slightly changed using the following conditional expression c and then solved for n and d, the synthetic refractive index n and the synthetic thickness d are obtained.
(Conditional expression a)
φ = 0
In the present embodiment, the case where the sheet layer 1 is polycarbonate has been described, but an acrylic resin, a norbornene resin, an ultraviolet curable resin, or the like may be used. Moreover, although the case where the protective layer is composed of three layers has been described, the same applies to the case of two layers or four or more layers.

  In the embodiment of the present invention, −3 μm ≦ df (n) ≦ 3 μm, but by setting −10 μm ≦ df (n) ≦ 10 μm, recording is performed with an optical head equipped with a spherical aberration correction element. This is effective for playback. FIG. 9 shows an example of use of a spherical aberration correction element. A spherical aberration correction element 10 as disclosed in Japanese Patent Laid-Open No. 2000-131603 is mounted in the optical path of the optical head.

  If the disc thickness variation is within 10 μm, the disc design margin is widened, yield and mass productivity are improved. In addition, surface coating, which has been difficult to suppress the variation in thickness, can be easily performed. If the thickness fluctuates by 10 μm, spherical aberration is generated by about 100 mλrms, but is corrected by the spherical aberration correction element 10. For example, when the variation in thickness within one circumference of the disk is within 3 μm and the variation in thickness within the surface is within 10 μm, a usage method in which the radial thickness error is DC corrected by the spherical aberration correction element can be used.

(Embodiment 2)
A second embodiment of the present invention will be described with reference to FIG.
The optical disc 7 has protective layers 4, 4 d, two recording layers 5, 5 d, and a reinforcing substrate 6. Light having a wavelength of 400 to 410 nm converged by the objective lens 9 is transferred from the protective layer 4 side to the recording layers 5 and 5 d. Information is recorded and / or reproduced by irradiation. An optical disc having two recording layers is also realized by a DVD, and the amount of information can be increased by a factor of about two.

  Since the objective lens 9 has a high numerical aperture of about 0.85, two lenses are used, and is generated when light having a wavelength of 405 nm is transmitted through a light-transmitting flat plate having a refractive index of 1.62 and a thickness of 100 μm such as polycarbonate. The third-order spherical aberration component of the wavefront aberration is designed to be substantially zero. The spherical aberration correction element 10 is disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-131603, and is mounted in the optical path of the optical head to correct spherical aberration caused by an error of the protective layers 4 and 4d from a thickness of 100 μm. To do.

  The recording layers 5 and 5d are a film containing a dielectric, a reflection film, or the like, and the reinforcing substrate 6 prevents warping or destruction of the optical disk 7. The protective layer 4 protects the recording layer 5 from the air and separates the surface 8 of the optical disk 7 from the recording layer 5, thereby preventing deterioration in recording / reproducing characteristics due to dust and scratches on the surface 8.

  The protective layer 4 related to the recording layer 5 includes the sheet layer 1 and the coat layer 2, and the protective layer 4 d related to the recording layer 5 d includes the sheet layer 1, the coat layer 2, and the adhesive layer 3.

  The sheet layer 1 is made of polycarbonate, the coat layer 2 is made of an acrylic resin, and the adhesive layer 3 is made of an ultraviolet curable resin. The refractive index of each layer at a wavelength of 405 nm is 1.62 for the sheet layer 1, 1.50 for the coat layer 2, and 1.53 for the adhesive layer 3.

In the optical disk of the present invention, the refractive index of the sheet layer 1, the coat layer 2 and the adhesive layer 3 at a wavelength of 405 nm is n ₁ , n ₂ and n ₃ , and the thicknesses are d ₁ , d ₂ and d ₃ (μm), respectively. Then, 1.45 ≦ n ₁ ≦ 1.65, 1.45 ≦ n ₂ ≦ 1.65, 1.45 ≦ n ₃ ≦ 1.65, 1.45 ≦ n ≦ 1.65, f (n) − 10 (μm) ≦ d ≦ g (n) +10 (μm) is satisfied.

However, regarding the protective layer 4, the synthetic refractive index n and the synthetic thickness d can be expressed by the following equations 4 and 5.
(Equation 4)
n = ((n ₁ d ₁ + n ₂ d ₂ ) / (d ₁ / n ₁ + d ₂ / n ₂ )) ^0.5
(Equation 5)
d = ((n ₁ d ₁ + n ₂ d ₂ ) × (d ₁ / n ₁ + d ₂ / n ₂ )) ^0.5
Further, regarding the protective layer 4d, the synthetic refractive index n and the synthetic thickness d can be expressed by the following equations 6 and 7.
(Equation 6)
n = ((n ₁ d ₁ + n ₂ d ₂ + n ₃ d ₃ ) / (d ₁ / n ₁ + d ₂ / n ₂ + d ₃ / n ₃ )) ^0.5
(Equation 7)
d = ((n ₁ d ₁ + n ₂ d ₂ + n ₃ d ₃ ) × (d ₁ / n ₁ + d ₂ / n ₂ + d ₃ / n ₃ )) ^0.5
The composite refractive index n and the composite thickness d are the composite refractive index and composite thickness unique to the present invention derived by replacing a plurality of layers with one equivalent layer. In this case, the aberration of light transmitted through a plurality of layers and the aberration of light transmitted through one equivalent layer are substantially equal.

  Further, the derivation of these equations 4 to 7 can be performed in the same manner as the derivation method described in the first embodiment.

Further, f (n) and g (n) shown in the following equations 8 and 9 are substantially the same as the spherical aberration when the combined thickness errors of -15 μm and 15 μm are corrected by the spherical aberration correcting element by the aberration calculation by the ray tracing method, respectively. It is a cubic approximate curve that passes through discrete points of zero. Equations 8 and 9 are design criteria relating to the combined thickness of the protective layers 4 and 4d, respectively.
(Equation 8)
f (n) = − 105.8n ³ + 551.5n ² −936.9n + 605.2
(Equation 9)
g (n) =-138.7n ³ + 723.7n ² −1228.7n + 796.0
The composite refractive index n and the composite thickness d satisfying this condition are in a range surrounded by the curve and straight line in FIG. Dotted lines are curves f (n) and g (n) in which the spherical aberration is substantially zero by correcting the thickness error of ± 15 μm by the spherical aberration correction element.

  When the combined refractive index n and the combined thickness d obtained from the thickness and refractive index of each of the plurality of layers constituting the protective layer are determined within this range (the range surrounded by the solid line in FIG. 3), the spherical aberration correcting element is obtained. It is possible to manufacture an optical disc having a spherical aberration substantially within 30 mλrms in the mounted optical head.

  If the variation in thickness within one circumference of the disk is within 3 μm and the radial thickness error is DC corrected by the spherical aberration correcting element, the spherical aberration is substantially within 30 mλrms. It should be noted that the correction capability of the spherical aberration correction element can be easily realized with a thickness of about ± 20 μm.

  If the combined thickness of the layers between the first recording layer and the second recording layer is 20 μm or more, this acts as a separation layer, and the focus error signals of the two recording layers can be separated. In addition, an effect of reducing the amount of stray light due to reflection on the other recording layer can be obtained.

  As described above, according to the embodiment of the present invention, since the thickness and refractive index of each of the plurality of layers constituting the protective layer are taken into consideration, an optical disc with reduced generation of spherical aberration can be manufactured.

  Here, the case where the sheet layer 1 is polycarbonate has been described, but an acrylic resin, a norbornene resin, an ultraviolet curable resin, or the like may be used. Moreover, although the case where the protective layer is composed of three layers has been described, the same applies to the case of two layers or four or more layers.

(Embodiment 3)
A third embodiment of the present invention will be described with reference to FIG. The configuration of the optical disc 7 is the same as that of Embodiment 1 of the present invention.

The optical disk 7 of the present invention has a refractive index n ₁ , n _2a and n _2b at a wavelength of 405 nm of the sheet layer 1, the coat layer 2 and the adhesive layer 3, respectively, a thickness of the sheet layer 1 is d ₁ (μm), and a protective layer. 4 is d ₁ + d ₂ (μm), 1.45 ≦ n ₁ ≦ 1.65, 1.50 ≦ n _2a ≦ 1.55, 1.50 ≦ n _2b ≦ 1.55, −3 μm ≦ d ₁ + d ₂ −f (n ₁ ) ≦ 3 μm is satisfied.

However, f (n ₁ ) = A · n ₁ ³ + B · n ₁ ² + C · n ₁ + D, A = 1.280d ₂ −109.8, B = −6.652d ₂ +577.2, C = 1.11. 27d ₂ −985.5, D = −6.257d ₂ +648.6.

Expression f (n ₁ ) is a cubic expression of n ₁ using coefficients A, B, C, and D with d ₂ as a parameter, and a discrete aberration in which the spherical aberration becomes substantially zero in the aberration calculation by the ray tracing method. It represents an approximate curve that passes through a point. The thickness is defined as 3 μm or less around this curve.

The refractive index n ₁ and the thickness d ₁ + d ₂ satisfying this condition are in a range surrounded by the curve and straight line in FIG. The dotted line is a curve f (n ₁ ) where the spherical aberration is substantially zero. Considering the influence of the adhesive layer 3 and the coat layer 2 on the spherical aberration, the characteristic of the embodiment of the present invention is that the curve f (n ₁ ) is changed using the thickness d ₂ as a parameter. When the refractive index n ₁ and the thickness d ₁ + d ₂ are set within this range, an optical disc having a spherical aberration substantially suppressed within 30 mλrms can be manufactured.

FIG. 5 shows a curve d ₁ + d ₂ = f (n ₁ ) when the value of d ₂ is changed. In addition, the points at which the spherical aberration is substantially zero in the aberration calculation by the ray tracing method are plotted. The error between them is 0.1 μm or less, and it can be confirmed that they are in good agreement.
As described above, according to the embodiment of the present invention, since the thickness and refractive index of each of the plurality of layers constituting the protective layer are taken into consideration, an optical disc with reduced generation of spherical aberration can be manufactured.

  Here, the case where the sheet layer 1 is polycarbonate has been described, but an acrylic resin, a norbornene resin, an ultraviolet curable resin, or the like may be used. Moreover, although the case where the protective layer is composed of three layers has been described, the same applies to the case of two layers or four or more layers.

In the embodiment of the present invention, −3 μm ≦ d ₁ + d ₂ −f (n ₁ ) ≦ 3 μm is set, but spherical aberration correction is performed by setting −10 μm ≦ d ₁ + d ₂ −f (n ₁ ) ≦ 10 μm. This is effective when recording / reproducing with an optical head equipped with an element. FIG. 9 shows an example of use of a spherical aberration correction element. A spherical aberration correction element 10 as disclosed in Japanese Patent Laid-Open No. 2000-131603 is mounted in the optical path of the optical head.

  When the disc thickness variation is within 10 μm, the disc design margin is widened, yield and mass productivity are improved. In addition, surface coating, which has been difficult to suppress the variation in thickness, can be easily performed. If the thickness fluctuates by 10 μm, spherical aberration is generated by about 100 mλrms, but is corrected by the spherical aberration correction element 10. For example, when the variation in thickness within one circumference of the disk is within 3 μm and the variation in thickness within the surface is within 10 μm, a usage method in which the radial thickness error is DC corrected by the spherical aberration correction element can be used.

(Embodiment 4)
A fourth embodiment of the present invention will be described with reference to FIG. The configuration of the optical disc 7 is the same as that of Embodiment 1 of the present invention.

The optical disk 7 of the present invention has a refractive index n ₁ , n _2a and n _2b at a wavelength of 405 nm of the sheet layer 1, the coat layer 2 and the adhesive layer 3, respectively, a thickness of the sheet layer 1 is d ₁ (μm), and a protective layer. 4 is d ₁ + d ₂ (μm), 1.61 ≦ n ₁ ≦ 1.63, 1.50 ≦ n _2a ≦ 1.55, 1.50 ≦ n _2b ≦ 1.55, −3 μm ≦ d ₂ −f (d ₁ ) ≦ 3 μm is satisfied. However, f (d ₁ ) = − 0.986d ₁ +98.6, and the expression f (d ₁ ) is an approximate straight line passing through discrete points where the spherical aberration is substantially zero in the aberration calculation by the ray tracing method. is there.

The thicknesses d ₁ and d ₂ that satisfy this condition are hatched regions in FIG. When the thicknesses d ₁ and d ₂ are set within this range, an optical disc having a spherical aberration substantially suppressed within 30 mλrms can be manufactured.

  The embodiment of the present invention is effective when polycarbonate or the like is used for the sheet layer 1. Although the case where the protective layer is composed of three layers has been described, the same applies to the case of two layers or four or more layers.

In the embodiment of the present invention, −3 μm ≦ d ₂ −f (d ₁ ) ≦ 3 μm is set. However, by setting −10 μm ≦ d ₂ −f (d ₁ ) ≦ 10 μm, the spherical aberration correcting element is changed. This is effective when recording / reproducing with an on-board optical head.

(Embodiment 5)
A fifth embodiment of the present invention will be described with reference to FIG. The configuration of the optical disc 7 is the same as that of Embodiment 1 of the present invention.

The optical disk 7 of the present invention has a refractive index n ₁ , n _2a and n _2b at a wavelength of 405 nm of the sheet layer 1, the coat layer 2 and the adhesive layer 3, respectively, a thickness of the sheet layer 1 is d ₁ (μm) and a protective layer 4 is d ₁ + d ₂ (μm), 1.49 ≦ n ₁ ≦ 1.51, 1.50 ≦ n _2a ≦ 1.55, 1.50 ≦ n _2b ≦ 1.55, −3 μm ≦ d ₂ −f (d ₁ ) ≦ 3 μm is satisfied. However, f (d ₁ ) = − 1.002d ₁ +98.6, and the expression f (d ₁ ) is an approximate straight line that passes through discrete points where the spherical aberration is substantially 0 in the aberration calculation by the ray tracing method. is there.

The thicknesses d ₁ and d ₂ that satisfy this condition are hatched areas in FIG. When the thicknesses d ₁ and d ₂ are set within this range, an optical disc having a spherical aberration substantially suppressed within 30 mλrms can be manufactured.

  The embodiment of the present invention is effective when an acrylic resin or the like is used for the sheet layer 1. Although the case where the protective layer is composed of three layers has been described, the same applies to the case of two layers or four or more layers.

In the embodiment of the present invention, −3 μm ≦ d ₂ −f (d ₁ ) ≦ 3 μm is set. However, by setting −10 μm ≦ d ₂ −f (d ₁ ) ≦ 10 μm, the spherical aberration correcting element is changed. This is effective when recording / reproducing with an on-board optical head.

(Embodiment 6)
Embodiment 6 of the present invention will be described with reference to FIG. The configuration of the optical disc 7 is the same as that of Embodiment 1 of the present invention.

The optical disk 7 of the present invention has a refractive index n ₁ , n _2a and n _2b at a wavelength of 405 nm of the sheet layer 1, the coat layer 2 and the adhesive layer 3, respectively, a thickness of the sheet layer 1 is d ₁ (μm) and a protective layer 4 is d ₁ + d ₂ (μm), 1.52 ≦ n ₁ ≦ 1.54, 1.50 ≦ n _2a ≦ 1.55, 1.50 ≦ n _2b ≦ 1.55, −3 μm ≦ d ₂ −f (d ₁ ) ≦ 3 μm is satisfied.

However, f (d ₁ ) = − d ₁ +98.6, and the expression f (d ₁ ) is an approximate straight line that passes through discrete points where the spherical aberration is substantially zero in the aberration calculation by the ray tracing method.

The thicknesses d ₁ and d ₂ that satisfy this condition are hatched regions in FIG. When the thicknesses d ₁ and d ₂ are set within this range, an optical disc having a spherical aberration substantially suppressed within 30 mλrms can be manufactured.

  The embodiment of the present invention is effective when a norbornene resin or the like is used for the sheet layer 1. Although the case where the protective layer is composed of three layers has been described, the same applies to the case of two layers or four or more layers.

In the embodiment of the present invention, −3 μm ≦ d ₂ −f (d ₁ ) ≦ 3 μm is set. However, by setting −10 μm ≦ d ₂ −f (d ₁ ) ≦ 10 μm, the spherical aberration correcting element is changed. This is effective when recording / reproducing with an on-board optical head.
In addition, it cannot be overemphasized that the sheet | seat layer 1 of Embodiment 1-6 of this invention may be an ultraviolet curable resin.

(Embodiment 7)
FIG. 10 shows an example of a method for recording / reproducing one of the optical discs of Embodiments 1 to 6 using an optical head equipped with a spherical aberration correction element.

  Almost half of the light having a wavelength of 405 nm emitted from the semiconductor laser 30 passes through the prism 31 and is collimated by the condenser lens 32 to become a substantially parallel light beam. The collimated light beam is reflected by the mirror 33, passes through the spherical aberration correction element 34, and is converged by the objective lens 35 having a numerical aperture of 0.85 to form a light spot on the recording layer of the optical disk 37 of the present invention.

  The light reflected by the recording layer again passes through the objective lens 35 and the spherical aberration correction element 34, is reflected by the mirror 33, and is narrowed down by the condenser lens 32. Nearly half of the narrowed light is reflected by the prism 31, passes through the cylindrical lens 38, and is detected by the photodetector 39.

  The photodetector 39 detects a reproduction signal, detects a focus control signal for causing the objective lens 35 to follow the recording layer of the optical disc 37 by an astigmatism method, and detects the objective lens 35 by a phase difference method or a push-pull method. Is configured to detect a tracking control signal for following the tracks on the optical disk 37. The objective lens 35 is driven in the focus direction and the tracking direction by the objective lens driving device 36 based on these control signals.

  The spherical aberration correction element 34 is, for example, as disclosed in Japanese Patent Application Laid-Open No. 2000-131603, and corrects spherical aberration caused by the thickness error and refractive index error of the protective layer of the optical disk 37 to The light spot formed on the recording layer is optimized. Note that the spherical aberration correction element 34 can easily correct the thickness error of the protective layer ± 20 μm. If the variation in thickness within one circumference of the disk is within 3 μm, the spherical aberration occurring can be made within 30 mλrms by DC correction of the radial thickness error by the spherical aberration correction element 34.

  Here, the spherical aberration correction element 34 is composed of two lenses, and the distance between these lenses is changed in the optical axis direction so that light incident on the objective lens 35 is weakly diverged or converged. It corrects spherical aberration.

  In addition, all the indications of Unexamined-Japanese-Patent No. 2000-131603 are integrated here by quoting as it is.

  Thus, by recording and reproducing the optical disk of the present invention with an optical head equipped with a spherical aberration correction element, the spherical aberration can be substantially suppressed within 30 mλ rms, and the information density can be increased for the first time.

  According to the present invention, the design margin of the thickness and refractive index of the optical disk is widened, and the yield and mass productivity of the optical disk are improved.

  In addition, surface coating, which has been difficult to suppress the variation in thickness, can be easily performed.

  Furthermore, the effect of further increasing the density can be achieved by using two recording layers.

(Embodiment 8)
FIG. 11 shows a schematic configuration diagram of an embodiment of an optical information recording / reproducing apparatus of the present invention.
The optical disk 37 is any one of the optical disks according to the first to sixth embodiments, and is rotated by the motor 42. The optical head 40 is moved in the radial direction of the optical disc 37 along the shaft 43.

For recording or reproducing information, the light emitted from the semiconductor laser of the optical head 40 is converged on the recording layer of the optical disc 37 by the objective lens 41. The photodetector of the optical head 40 detects a focus control signal for causing the objective lens 41 to follow the surface of the optical disc 37 and a tracking control signal for causing the information track of the optical disc 37 to follow.
The head control circuit 44 performs focus control and tracking control of the optical head 40 based on these control signals.

  The signal processing circuit 45 records information on the optical disk 37 by the optical head 40 at the time of recording, and optical information recorded on the information track of the optical disk 37 from the output signal of the photodetector of the optical head 40 at the time of reproduction. Play.

  With the optical information recording / reproducing apparatus of the present invention, the spherical aberration of the spots formed on the recording layer of the optical disc 37 is sufficiently suppressed, and the information can be densified.

  In addition, by mounting a spherical aberration correcting element as disclosed in Japanese Patent Laid-Open No. 2000-131603 on the optical head 40, the spherical aberration is also obtained in an optical disk having a large variation in the thickness of the protective layer and an optical disk having two recording layers. Is sufficiently suppressed, and normal information recording and reproduction can be performed.

  The optical information recording medium of the present invention can be applied in the same manner to any type of optical information recording and reproduction type or reproduction-only type.

  In the above embodiment, the predetermined wavelength of the present invention has been described for the case of 405 nm. However, the present invention is not limited to this. For example, light having any wavelength within the range of 400 to 410 nm may be used. Good or other wavelengths may be used.

  Further, in the above embodiment, the case where there are two recording layers has been described. However, even if there are two or more recording layers, the above-described invention can be similarly applied by applying the configuration of two layers according to the number of layers. It is applicable to.

  In the above-described embodiment, the case where the approximate curves f (n), g (n), etc. passing through the point at which the spherical aberration becomes substantially zero is used as the design standard regarding the composite thickness of the protective layer. The coefficients and dimensions of the formula representing the approximate curve are not necessarily limited thereto. In short, the relational expression serving as a design criterion may be an approximate curve or approximate line passing through a point at which the spherical aberration is substantially zero in the aberration calculation by the ray tracing method, for example, and the degree of approximation depends on the purpose of use. You can choose freely.

As described above, the method for designing an optical information recording medium having a recording layer and a protective layer including at least a first layer to an m-th layer (m ≧ 2),
i is an integer satisfying 1 ≦ i ≦ m, the refractive index of the i-th layer of the protective layer at a predetermined wavelength is n _i , and the thickness of the i-th layer is d _i , and (a) the protective layer is configured The combined refractive index n of the one layer when replaced with one layer substantially equivalent to the plurality of layers is n = ((n ₁ d ₁ + n ₂ d ₂ +... + N _m d _m ) / (D ₁ / n ₁ + d ₂ / n ₂ +... + D _m / n _m )) ^0.5, and (b) the combined thickness d of the one layer is d = ((n ₁ d ₁ + n ₂ d ₂ +... + n _m d _m ) × (d ₁ / n ₁ + d ₂ / n ₂ +... + d _m / n _m )) ^0.5 and (c) the spherical aberration is substantially 0. A relational expression f (n) representing a relation between the synthetic refractive index n and the synthetic thickness d is defined as a design standard regarding the synthetic thickness of the optical information recording medium,
According to the optical information recording medium design method for determining the refractive index n _i of each layer and the thickness d _i of the layer based on the relational expression f (n), the following effects are exhibited.

That is, according to the above design method, the combination of the refractive index n _i and the thickness d _i of each layer is easily selected from a large number of candidates for n _i and d _i satisfying a predetermined relational expression such as the design criteria. I can do it. As a result, the initially remaining spherical aberration is substantially zero. Therefore, even if the thickness variation of the protective layer is allowed, for example, to ± 3 μm, the spherical aberration of the optical disk can be suppressed to substantially within 30 mλrms.

This also provides an effect that a variety of parameters such as the refractive index n _i and thickness d _{i of} each layer can be selected, and the degree of freedom of design is widened.

As described above, the optical information recording medium has a recording layer and a protective layer including at least a first layer to an m-th layer (m ≧ 2) and is converged by an objective lens having a numerical aperture of about 0.85. An optical information recording medium on which information is recorded and / or reproduced by irradiating the recording layer with light having a wavelength of 400 to 410 nm from the protective layer side,
i is an integer satisfying 1 ≦ i ≦ m, the refractive index of the i-th layer of the protective layer at a wavelength of 405 nm is n _i , and the thickness of the i-th layer is d _i ,
(A) When the plurality of layers constituting the protective layer are replaced with one substantially equivalent layer, the combined refractive index n of the one layer is expressed as n = ((n ₁ d ₁ + n ₂ d ₂ +... + N _m d _m ) / (d ₁ / n ₁ + d ₂ / n ₂ +... + D _m / n _m )) ^0.5
(B) The composite thickness d of the one layer is d = ((n ₁ d ₁ + n ₂ d ₂ +... + N _m d _m ) × (d ₁ / n ₁ + d ₂ / n ₂ +... + D _m / n _m )) ^0.5 and (c) f (n) = − 109.8n ³ + 577.2n ² −985 as a design criterion for the synthetic thickness of the protective layer. .5n + 648.6,
The refractive index _ni and the composite refractive index n of the i-th layer are 1.45 or more and 1.65 or less, and df (n) which is a difference between the composite thickness and the design standard is − The optical information recording medium has a thickness of 10 μm or more and 10 μm or less.

  The optical information recording medium is an optical information recording medium in which the value of df (n) is -3 μm or more and 3 μm or less.

The method for manufacturing an optical information recording medium has a recording layer and a protective layer including at least a first layer to an m-th layer (m ≧ 2) and is converged by an objective lens having a numerical aperture of about 0.85. A method for producing an optical information recording medium in which information is recorded and / or reproduced by irradiating the recording layer with light having a wavelength of 400 to 410 nm from the protective layer side,
i is an integer satisfying 1 ≦ i ≦ m, the refractive index of the i-th layer of the protective layer at a wavelength of 405 nm is n _i , and the thickness of the i-th layer is d _i ,
(A) When the plurality of layers constituting the protective layer are replaced with one substantially equivalent layer, the combined refractive index n of the one layer is expressed as n = ((n ₁ d ₁ + n ₂ d ₂ +... + N _m d _m ) / (d ₁ / n ₁ + d ₂ / n ₂ +... + D _m / n _m )) ^0.5
(B) The composite thickness d of the one layer is d = ((n ₁ d ₁ + n ₂ d ₂ +... + N _m d _m ) × (d ₁ / n ₁ + d ₂ / n ₂ +... + D _m / n _m )) ^0.5 and (c) f (n) = − 109.8n ³ + 577.2n ² −985 as a design criterion for the synthetic thickness of the protective layer. .5n + 648.6,
The refractive index _ni and the composite refractive index n of the i-th layer are 1.45 or more and 1.65 or less, and df (n) which is a difference between the composite thickness and the design standard is − A method for producing an optical information recording medium, wherein the refractive index and thickness of the plurality of layers constituting the protective layer are determined so as to be 10 μm or more and 10 μm or less.

  The method for manufacturing an optical information recording medium is a method for manufacturing an optical information recording medium in which the value of df (n) is -3 μm or more and 3 μm or less.

  Thereby, it is possible to suppress the occurrence of spherical aberration and perform more optimal recording and reproduction.

  The optical information recording medium according to the present invention is advantageous in that, in an optical information recording medium having a protective layer composed of a plurality of layers, spherical aberration is further suppressed as compared with the conventional one, and normal recording and / or reproduction can be performed. And is useful as an optical information recording medium or the like on which information is recorded and / or reproduced by irradiation with light from an optical head.

Sectional drawing showing the structure of the optical disk of Embodiment 1 of this invention The figure showing the relationship between the synthetic refractive index n and synthetic thickness d of Embodiment 1 of this invention The figure showing the relationship between the synthetic refractive index n and synthetic thickness d of Embodiment 2 of this invention Diagram showing the relationship between the refractive index n ₁ and the thickness d ₁ + d ₂ of the third embodiment of the present invention Diagram showing the relationship between the refractive index n ₁ and the thickness d ₁ + d ₂ of the third embodiment of the present invention Diagram showing the relationship between the thickness d ₁ and the thickness d ₂ of the fourth embodiment of the present invention Diagram showing the relationship between the thickness d ₁ and the thickness d ₂ of the fifth embodiment of the present invention Diagram showing the relationship between the thickness d ₁ and the thickness d ₂ of the sixth embodiment of the present invention Sectional view showing an example of a spherical aberration correction element mounted on an optical head Explanatory drawing of recording / reproducing optical disk with optical head equipped with spherical aberration correction element Explanatory drawing of the optical information recording / reproducing apparatus of the present invention Sectional drawing showing the structure of the conventional optical disk A diagram showing the relationship between the refractive index and thickness of a conventional optical disc Sectional drawing showing the structure of the conventional optical disk Sectional view showing the structure of a high-density optical disc Diagram showing the relationship between adhesive layer thickness and spherical aberration (A), (b): Explanatory drawing for explaining derivation of expressions 1 and 2 of the first embodiment of the present invention Sectional drawing showing the structure of the optical disk of Embodiment 2 of this invention

Explanation of symbols

1, 11, 21 Sheet layer 2 Coat layer 3, 13, 23 Adhesive layer 4, 14, 24 Protective layer 5, 15, 25 Recording layer 6, 16, 26 Reinforcing substrate 7, 17, 27 Optical disk 8, 18, 28 Surface 9, 19, 29 Objective lens 10 Spherical aberration correction element 30 Semiconductor laser 31 Prism 32 Condensing lens 33 Mirror 34 Spherical aberration correction element 35 Objective lens 36 Objective lens driving device 37 Optical disk 38 Cylindrical lens 39 Optical detector 40 Optical head 41 Objective Lens 42 Motor 43 Shaft 44 Head control circuit 45 Signal processing circuit

Claims (1)

A recording layer and a protective layer including at least a first layer to an m-th layer (m ≧ 2), and the light having a wavelength of 400 to 410 nm converged by an objective lens having a numerical aperture of about 0.85 is provided on the protective layer side An optical information recording medium on which information is recorded and / or reproduced by irradiating the recording layer from
i is an integer satisfying 1 ≦ i ≦ m, the refractive index of the i-th layer of the protective layer at a wavelength of 405 nm is n _i , and the thickness of the i-th layer is d _i ,
(A) When the plurality of layers constituting the protective layer are replaced with one substantially equivalent layer, the combined refractive index n of the one layer is expressed as n = ((n ₁ d ₁ + n ₂ d ₂ +... + N _m d _m ) / (d ₁ / n ₁ + d ₂ / n ₂ +... + D _m / n _m )) ^0.5
(B) The composite thickness d of the one layer is d = ((n ₁ d ₁ + n ₂ d ₂ +... + N _m d _m ) × (d ₁ / n ₁ + d ₂ / n ₂ +... + D _m / n _m )) ^0.5 and (c) f (n) = − 109.8n ³ + 577.2n ² −985 as a design criterion for the synthetic thickness of the protective layer. .5n + 648.6,
The refractive index ni of the _i- th layer and the composite refractive index n are 1.45 or more and 1.65 or less, and an absolute value of df (n) which is a difference between the composite thickness and the design standard An optical information recording medium having a value of 3 μm or more and 10 μm or less.

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