JP4313743B2 - Optical recording medium - Google Patents

Description

  The present invention relates to an optical recording medium, and more particularly to an optical recording medium having three or more recording layers.

  As recording media for recording digital data, optical recording media such as CDs (compact discs) and DVDs (digital versatile discs) are widely used. In such an optical recording medium, there is a multilayer optical recording medium having a plurality of information recording layers in order to increase the storage capacity. In such a multilayer optical recording medium, each recording layer has a structure in which a light transmissive spacer layer is interposed therebetween.

  In the optical recording medium having the multilayer structure as described above, when a recording mark of a certain recording layer is reproduced, a recording mark of another recording layer is not reproduced, but reflected light from the other recording layer is reproduced. When the amount of reflected light or the distance between recording layers varies for some reason, the variation (hereinafter referred to as crosstalk variation) is superimposed on the reproduced signal as noise.

  As a countermeasure for reducing the influence of such interlayer crosstalk, for example, as described in Patent Document 1, an optical recording medium in which the recording interlayer distance is nonuniform for each recording layer has been proposed.

JP 2004-213720 A

  An object of the present invention is to provide an optical recording medium in which interlayer crosstalk is more efficiently reduced from the relationship between the configuration of each recording layer and the crosstalk in a multilayer optical recording medium.

  As a result of diligent research, the present inventor has found that third-order crosstalk (described later) constitutes most of interlayer crosstalk when the total thickness of the light-transmitting spacer layer is 5 μm or more, and almost all of them are It was found that this was due to confocal crosstalk (explained later). In addition, the actual four-layer optical recording medium has a light-transmitting spacer layer of about 50 μm, and it has been found that almost all the influence due to interlayer crosstalk is due to confocal third-order crosstalk. .

Here, a description the "3 Tsugiku cross talk" and the "confocal crosstalk".

  As shown in FIG. 3A, for example, an optical recording including four recording layers of L0 layer 16, L1 layer 18, L2 layer 20, and L3 layer 22 between the substrate 12 and the light-transmitting cover layer 14. When reproducing, for example, the L0 layer 16 of the medium 10, the component of the reproduction light incident on the light-transmitting cover layer 14 that is reflected only by the L0 layer 16 and is emitted to the outside of the optical recording medium is signal light. On the other hand, there is also a component that is reflected only once by any of the L1 layer 18, L2 layer 20, and L3 layer 22 and is emitted to the outside of the optical recording medium. This is so-called crosstalk light. In order to distinguish, it will be referred to as “first order crosstalk (light)”. Since the multilayer optical recording medium has a structure in which recording layers having a finite reflectance are laminated, there is a multiple reflection component that repeatedly reflects between these recording layers.

  The only multiple reflection component that actually exits the optical recording medium is only the light that has experienced an odd number of reflections by the recording layer, and the one that has the smallest number of reflections experiences three reflections. This is called “third-order crosstalk (light)”.

  FIG. 3A shows an example of third-order crosstalk, which is a component that reflects in the order of L2 layer → L3 layer → L1 layer. In addition, there are components that experience more reflections, such as 5 times, 7 times, etc., but the intensity of light is attenuated at each reflection, so multiple reflection components that are reflected more than 5 times are ignored. It's okay. In FIG. 3A, the optical path of the signal light is shown by a dotted line, but the third-order crosstalk light shown by the thick line is emitted to the same optical path as the signal light. In general, first-order and third-order crosstalk light is emitted to a different optical path from the signal light. However, when the interval between recording layers is special, the crosstalk light is emitted to the same optical path as the signal light as shown in FIG. Exists. Such a component is particularly called “confocal crosstalk (light)”.

  FIG. 3 shows a single ray having a specific incident angle, but in reality, there are an infinite number of rays in a certain incident angle range, and the point where these are concentrated at one point is the focus. It is. Although the confocal crosstalk light is condensed inside the optical recording medium at a different point from the signal light, it behaves as a divergent spherical wave having the same focal point as the signal light outside the optical recording medium.

  The present invention achieves the above object by an optical recording medium capable of effectively reducing the overall crosstalk by reducing the confocal third-order crosstalk.

  That is, the following object can be achieved by the present invention described below.

  (1) Optical recording in which at least three recording layers of L0, L1 and L2 layers and a light-transmitting spacer layer between the recording layers are laminated from the substrate side between the substrate and the light-transmitting cover layer. The tilt angle α of the medium, at least one of the recording layers excluding the L0 layer, with respect to a plane parallel to the L0 layer is 0.3 ° ≦ α ≦ 0.6 °, Further, the tilt period P is formed in a range of 0.64 μm ≦ P ≦ 800 μm by periodically varying the tilt angle α in at least one direction of the track direction and the direction orthogonal thereto. A characteristic optical recording medium.

  (2) The optical recording medium according to (1), wherein the recording layer in which the tilt angle α varies is provided with a gradient in which the tilt direction changes periodically.

  (3) The optical recording medium according to (1) or (2), wherein the tilt angle is changed only in the L1 layer.

  (4) Optical recording in which at least three recording layers of L0, L1 and L2 layers and a light-transmitting spacer layer between the recording layers are laminated from the substrate side between the substrate and the light-transmitting cover layer. The medium is a medium, and at least one of the recording layers excluding the L0 layer has a thickness direction distance to the L0 layer, a variation amount h is 1,7 nm ≦ h ≦ 2 μm, and a variation period Q Is formed in a range of 0.64 μm ≦ Q ≦ 800 μm and periodically varies in at least one of a track direction and a direction orthogonal thereto.

  (5) The variation amount h is given so that the distance of the recording layer from the L0 layer sequentially increases and decreases periodically, and the gradient direction changes periodically as a whole. The optical recording medium according to (4).

  (6) The recording layer is provided at least in a groove, and the groove is formed with different depths in a groove group every other track and another groove group every other track in between. The optical recording medium according to (4).

  (7) The optical recording medium according to any one of (4) to (6), wherein only the L1 layer is changed in the thickness direction.

  In the present invention, light is formed by laminating at least three recording layers of L0, L1, and L2 layers and a light-transmitting spacer layer between the recording layers from the substrate side between the substrate and the light-transmitting cover layer. The third-order crosstalk light is shifted from the light receiving element by periodically varying the tilt angle with respect to the L0 layer or the distance to the L0 layer in at least one of the recording layers excluding the L0 layer in the recording medium. As a result, the effect of interlayer crosstalk can be greatly reduced.

  The optical recording medium includes an L0 layer, an L1 layer, an L2 layer, an L3 layer recording layer and a light transmitting spacer layer between the recording layers (between the substrate and the light transmitting cover layer (hereinafter referred to as a cover layer)). The L1 layer has a tilt angle α of 0.3 ° ≦ α ≦ 0.6 ° and a tilt period P of 0. 0 with respect to a plane parallel to the L0 layer. In the range of 64 μm ≦ P ≦ 800 μm, the tilt angle α is periodically changed in the direction perpendicular to the track direction.

  Next, Embodiment 1 of the present invention shown in FIGS. 1 and 2 will be described.

  The optical recording medium 10 according to Example 1 includes four recording layers of an L0 layer 16, an L1 layer 18, an L2 layer 20, and an L3 layer 22 between a substrate 12 and a cover layer 14, and a spacer layer between the recording layers. 17, 19, and 21 are laminated.

  The L1 layer 18 has a tilt angle α of 0.3 ° ≦ α ≦ 0.6 ° and a tilt period P of 0.64 μm ≦ P ≦ with respect to a virtual plane 24 parallel to the L0 layer 16. In the range of 800 μm, it is formed periodically varying in the direction perpendicular to the track direction (disk diameter direction).

  Next, an action mechanism for reducing the third-order confocal crosstalk by the optical recording medium 10 of the first embodiment will be described with reference to FIGS. FIG. 3 shows the optical path of confocal crosstalk light when reproducing the L0 layer 16 in a four-layer optical recording medium.

  Although explanation is omitted, it is understood that if the confocal crosstalk light of the three types of patterns shown in FIGS. 3A, 3B, and 3C is reduced, the overall recording / reproducing characteristics are improved. Yes. As shown in FIGS. 3A to 3C, it can be seen that the confocal crosstalk light of the three types of patterns takes exactly the same optical path as the signal light reflected only once by the L0 layer 16. When this crosstalk light is reduced, the overall recording / reproducing characteristics are improved.

  FIG. 4 is an enlarged view of the case of FIG. 3A, and the action mechanism for reducing the influence of confocal crosstalk light will be described in more detail.

  In FIG. 4, among the infinite number of incident light rays, a light beam corresponding to the optical axis is shown as incident light I. The crosstalk light C also indicates a light beam corresponding to the optical axis. Further, the signal light S indicates a light beam corresponding to the edge of the beam spot diameter.

  Further, for the sake of clarity, in FIG. 4, the light reflected by the L2 layer 20 and the L3 layer 22 and reaching the L1 layer 18 omits the optical path between them, and is reflected by the L1 layer 18 before being reflected by the optical recording medium 10. The optical path until the light is emitted to the outside is indicated by a thick line. Further, it is assumed that the L1 layer 18 is inclined by the tilt angle α on the optical axis of the incident light I.

  As shown in FIG. 4, the incident light I reaches the L0 layer 16 through the L3 to L1 layers 22, 20, and 18, and the reflected light becomes the signal light S. The reflected light of the incident light I at the L1 layer 18 is emitted as crosstalk light C as shown in FIG.

  The crosstalk light C travels in a direction different from the signal light S by 2α, which is twice the tilt angle α of the L1 layer 18 when reflected by the L1 layer 18. Further, the light is deflected by refraction at the surface of the cover layer 14 and emitted in a direction having an angle φ with respect to the normal of the surface, that is, a straight line parallel to the incident light I.

  Here, by geometric consideration (Snell's law), when the refractive index of the cover layer 14 is n, the relationship between φ, n, and α is given by the following equation (1).

φ = sin ⁻¹ {nsin (2α)} (1)

The distance Δ between the intersection of the optical axis extension line C _{1 of the} crosstalk light C emitted to the outside and the optical axis extension line S ₁ of the signal light S and the optical axis of the incident light I is L0 When the depth of the layer 16 from the surface of the cover layer 14 is T, and the interlayer distance between the L0 layer 16 and the L1 layer 18 is t, the following equation (2) is given.

Δ = tan 2α / tan α · (T−t) −tan θ / tan θ ₀ · T (2)

  Next, with reference to FIG. 5, the shift of the signal light S and the crosstalk light C on the light receiving element 26 in the optical head (not shown) will be described. If this deviation is large enough that the light receiving element 26 does not receive crosstalk light, the influence of the crosstalk light is eliminated.

In FIG. 5, symbol L ₁ is the distance between the L1 layer 18 and the main plane of the objective lens 28 of the optical head in the central optical axis _CL of the light receiving element 26, and L ₂ is the main plane of the objective lens 28 and the light reception. The distance L ₃ between the light receiving surface of the element 26 and the light receiving surface of the light receiving element 26 and the intersection of the central optical axis C _L and the optical axis of the crosstalk light C between the objective lens 28 and the light receiving element 26. Each distance.

  As shown in FIG. 4, the incident light I condensed on the L0 layer 16 is reflected here to become signal light S, and is condensed on the light receiving element 26 by the objective lens 28 as shown in FIG. Is done. On the other hand, as shown in FIG. 5, the crosstalk light C (optical axis) behaves as a light beam emitted at an angle φ from a point separated from the condensing point of the signal light S by Δ. Thus, it reaches the position shifted by X in the direction orthogonal to the optical axis from the condensing point of the signal light.

  The interrelationship between the above variables is expressed by the following equations (3) to (6) by geometric analysis calculation. Here, β represents the magnification of the objective lens 28, and f represents the focal length.

L ₁ = (β + 1) / β · f (3)
L ₂ = (β + 1) · f (4)
L ₃ = β ² · f · Δ / (f + β · Δ) (5)
X (Δ, φ) = (L ₁ + Δ) / (L ₁ −L ₃ ) · L ₃ · tan φ (6)

  Further, the shift amount X depends on α via Δ and φ, and this relationship is as shown in FIG. However, in this case, NA of the objective lens was 0.85, magnification β = 10, light source wavelength λ = 405 nm, T = 100 μm, and t = 10 μm. In the optical system represented by these parameters, the spot diameter of the signal light on the light receiving element 26 is about 5.81 μm.

  Therefore, if the size of the light receiving element 26 is about 5.81 μm, a good signal can be reproduced, and the influence can be reduced if the center of the crosstalk light C is shifted to the end of the light receiving element 26. Become. This means that the influence of the crosstalk light C can be eliminated if the shift amount X of the crosstalk light C with respect to the signal light S is 1/2 or more on the light receiving element 26.

  In this condition, the tilt angle α when the shift amount X is 2.9 μm or more, which is indicated by a one-dot chain line in FIG. 6, may be 0.3 ° or more from FIG. 6.

  Next, the tilt period P will be described. From FIG. 2, the tilt period P of the L1 layer 18 assumes that the maximum value of the thickness fluctuation h due to the periodic tilt is H. From geometrical considerations, the relationship between X, P and α is expressed by the following equation (7). Given. When the relationship of the equation (7) is graphed, it is as shown in FIG.

h _max = H = (P / 2) · tan α (7)

  Further, regarding the amount of variation h, the distance between the L0 layer and the LM layer (N ≧ 3, 1 ≦ M ≦ N−1) subject to crosstalk varies by h, which means that the flat L0 layer On the other hand, it means that the LM layer has a maximum of irregularities of h. However, if the concavo-convex amount h of the LM layer is too large, there arises a problem that good reproduction is hindered because spherical aberration cannot be corrected during reproduction of the LM layer and focus servo cannot follow. For these reasons, the thickness variation h is preferably 2 μm or less.

  On the other hand, from the viewpoint of good recording / reproduction in the L1 layer 18, the tilt angle α is preferably 0.6 ° or less. Further, when recording / reproducing to / from the optical recording medium, it is preferable that the recording / reproducing laser beam is perpendicularly incident on the recording layer. If a finite angle is generated between the optical axis of the laser beam and the normal line of the recording layer due to the warp of the disk or the vibration of the drive, coma aberration occurs and good recording / reproduction is hindered. In order to keep such coma aberration within an allowable range, the tilt angle α is preferably 0.6 ° or less.

  Therefore, FIG. 7 shows a case where α = 0.3 to 0.6 °. Further, as described above, since the maximum value H of the thickness fluctuation h due to the periodic tilt is 2 μm, the upper limit of the tilt period P is determined. From FIG. From the viewpoint, the upper limit is preferably about 800 μm.

  On the other hand, the lower limit of the tilt period P is determined by restrictions on the optical recording medium manufacturing process and the minimum unit of the groove shape. According to the recent optical recording medium manufacturing process technology, it is easy to form a groove of about 0.3 μm and a finer pit structure. Therefore, the lower limit of the tilt period P is determined by the minimum unit of the groove shape. Become.

  This corresponds to the configuration indicated by the two-dot chain line in FIG. 10, but if the tilt period and the recording track period are equal, the intended effect of the present invention cannot be obtained. Therefore, the minimum value of the tilt period is two periods of the recording track, and when the same recording parameter as described above is used (for example, wavelength λ = 405 nm, NA = 0.85...), The appropriate recording track period is about 0. Since it is .32 μm, the minimum value of the tilt period is twice that of 0.64 μm.

  As a result, the tilt period P of the periodic tilt structure is preferably 0.64 μm ≦ P ≦ 800 μm.

  Furthermore, the minimum value of the fluctuation amount h is hmin = 1.7 nm when the minimum value of P is 0.64 μm and α = 0.3 ° from the equation (7).

  In the first embodiment, only the confocal crosstalk light pattern shown in FIG. 3 (A) is analyzed, but two types of FIGS. 3 (B) and 3 (C) are analyzed by the same geometrical consideration. With respect to the pattern, according to the embodiment of the present invention, a larger shift amount X is expected. Therefore, analysis is unnecessary and description thereof is omitted.

  Next, a second embodiment of the present invention shown in FIG. 8 will be described.

  In the optical recording medium 30 of Example 2, the L1 layer 32 is formed with a tilt angle α with respect to the virtual plane 24, + α or −α, and a gradient in which the inclination direction changes periodically. is there. In FIG. 8, reference numeral 34 denotes a groove, and 36 denotes a land portion. Here, in the concave and convex groove shape formed to define the track, a region that is near (shallow) as viewed from the light incident surface is referred to as a groove, and a region that is far (deep) is referred to as a land.

  In order to manufacture the optical recording medium 30 as in Example 2, an L0 layer is formed on the substrate 12 by magnetron sputtering, and a radiation curable resin that is cured by radiation such as ultraviolet rays is applied thereon, This is an uncured state, irradiated with radiation in a state of pressing a stamper having groove shapes or prepit irregularities, transferred the stamper shape to a radiation curable resin, and a spacer between the L0 layer and the L1 layer Form a layer. Here, the L1 layer 32 can be formed without sacrificing the productivity of the optical recording medium 30 by providing the stamper with a periodic gradient as shown in FIG.

  Next, a third embodiment of the present invention shown in FIG. 9 will be described.

  The L1 layer 42 of the optical recording medium 40 (not shown) of Example 3 is formed so as to have an overall inclination by changing the step of the inclined portion of the groove 44 for each adjacent inclined surface.

  In the optical recording medium 40 of the third embodiment, since the individual grooves 44 or land portions 46 forming the recording marks can be made flat (without being inclined), recording is performed on the L1 layer 42 inclined as a whole. When performing, it is not necessary to adjust the tilt on the optical head side.

  Further, when a beam having a large spot diameter such as crosstalk light is reflected by the L1 layer 42, a large number of tracks, grooves, or lands are included in the beam spot, so that these function as a diffraction grating. The beam can be deflected. This utilizes the fact that the 0th-order reflected light of the diffraction grating matches the exit angle of specular reflection.

  Next, an optical recording medium 50 (entire illustration is omitted) according to Embodiment 4 of the present invention will be described with reference to FIG.

  The L1 layer 52 in the optical recording medium 50 has a distance in the thickness direction with respect to the L0 layer 51, the fluctuation amount h is 1.7 nm ≦ h ≦ 2 μm, and the fluctuation period Q (corresponding to the tilt period P) is 0. In the range of .64 .mu.m.ltoreq.Q.ltoreq.800 .mu.m, it is formed by periodically varying in the direction perpendicular to the track direction.

  More specifically, a groove 54B having a large height (depth) is provided for every other track with respect to the conventional groove 54A to modulate the groove step.

  In the optical recording medium 50 of the fourth embodiment, when a beam having a large spot diameter such as crosstalk light is reflected, a large number of grooves 54A and 54B in the L1 layer 51 act as diffraction gratings. As described above, since the height (depth) of the group of the groove 54A and the group of the groove 54B alternately change, a diffraction angle that gives a peak of diffraction intensity is newly added, and crosstalk reflected light (diffraction) The 0th-order light) intensity is reduced.

  In the optical recording media of the above Examples 1 to 4, the tilt angle of the L1 layer and the variation in the distance to the L0 layer are set in the direction orthogonal to the track, but the present invention is not limited to this. The variation may be given both in the track direction or in the direction orthogonal to the track.

  The optical recording medium 60 of the fifth embodiment (not shown) is subjected to modulation as shown in the first to fourth embodiments in the track direction, as schematically shown in FIG.

  In each of the above embodiments, the optical recording medium has a four-layer structure of the L0 layer to the L3 layer, and the L1 layer is given a change in tilt angle and distance from the L0 layer. The invention is not limited to this, and the recording layer that is applied to an optical recording medium having three or more recording layers and that gives fluctuations in tilt angle, etc., is provided in at least one of the recording layers other than the L0 layer. It may be applied.

  However, when a change in tilt angle or the like is given to only one layer, it is effective to provide it in the L1 layer.

1 is a partial cross-sectional perspective view showing an optical recording medium according to Embodiment 1 of the present invention. Sectional drawing which expands and schematically shows the A part in FIG. Sectional drawing which shows typically the optical path pattern of the crosstalk light at the time of reproduction | regeneration of L0 layer in the Example 1 The expanded sectional view which shows typically the relationship between the structure of the crosstalk light, signal light, and L1 layer which are generated at the time of reproduction | regeneration of L0 layer in the optical recording medium of Example 1. Sectional drawing which shows typically the state in which the signal light and crosstalk light which were radiate | emitted from the optical recording medium of Example 1 arrive at a light receiving element FIG. 5 is a diagram showing the relationship between the shift of the imaging position of the crosstalk light on the light receiving element shown in FIG. 5 and the tilt angle of the L1 layer. Diagram showing relationship between tilt angle, tilt period and maximum thickness variation in L1 layer of Example 1 Sectional drawing which shows typically the state of L1 layer of the optical recording medium which concerns on Example 2. FIG. Sectional drawing which shows typically the state of L1 layer of the optical recording medium which concerns on Example 3. FIG. Sectional drawing which shows typically the state of L1 layer in the optical recording medium which concerns on Example 4. FIG. Sectional drawing which shows typically the state of L1 layer in the optical recording medium which concerns on Example 5

Explanation of symbols

10, 30, 40, 50, 60 ... optical recording medium 12 ... substrate 14 ... light transmissive cover layer 16 ... L0 layer 17, 19, 21 ... light transmissive spacer layer 18, 32, 42, 52, 51 ... L1 layer 20 ... L2 layer 22 ... L3 layer 24 ... virtual plane 26 ... light receiving element 28 ... objective lens 34, 44, 54A, 54B ... groove 36, 46 ... land portion

Claims (7)

An optical recording medium in which at least three recording layers of L0 layer, L1 layer, and L2 layer and a light transmissive spacer layer between the recording layers are laminated from the substrate side between the substrate and the light transmissive cover layer. And
At least one of the recording layers excluding the L0 layer has a tilt angle α of 0.3 ° ≦ α ≦ 0.6 ° and a tilt period P with respect to a plane parallel to the L0 layer. However, in the range of 0.64 μm ≦ P ≦ 800 μm, the tilt angle α is periodically varied in at least one of the track direction and the direction perpendicular thereto. Medium. In claim 1,
The optical recording medium according to claim 1, wherein the recording layer in which the tilt angle α varies is provided with a gradient in which the direction of the inclination changes periodically. In claim 1 or 2,
An optical recording medium, wherein the tilt angle is changed only in the L1 layer. An optical recording medium in which at least three recording layers of L0 layer, L1 layer, and L2 layer and a light transmissive spacer layer between the recording layers are laminated from the substrate side between the substrate and the light transmissive cover layer. And
At least one of the recording layers excluding the L0 layer has a distance in the thickness direction with respect to the L0 layer, the fluctuation amount h is 1.7 nm ≦ h ≦ 2 μm, and the fluctuation period Q is 0.64 μm ≦ An optical recording medium characterized by being periodically varied in at least one of a track direction and a direction orthogonal to the track direction in a range of Q ≦ 800 μm. In claim 4,
The light having the fluctuation amount h given so that the distance between the recording layer and the L0 layer increases and decreases sequentially and periodically, and the direction of the gradient changes periodically as a whole. recoding media. In claim 4,
The recording layer is provided in at least a groove, and the groove is formed with different depths in a groove group for every other track and another groove group for every other track in between. recoding media. In any one of Claims 4 thru | or 6.
An optical recording medium, wherein only the L1 layer is changed in the thickness direction.

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