JP2002170278A - Optical recording medium, reproducing head and reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To equalize qualities of reproduced signals when the respective signal surfaces of an optical disk are reproduced in a one-surface reading two-layer optical disk of high density. SOLUTION: First signal layers 2 and 42, translucent films 4 and 44, intermediate layers 6 and 46, reflection films 10 and 50 and second signal layers 8 and 48 are successively formed to produce the one-surface reading two-layer optical disk which is provided with signal pits respectively at the translucent film sides of the first signal layers 2 and 42 and at the reflection film sides of the second signal layers 8 and 48. The thicknesses of the first signal layers 2 and 42, the NA of the reproduction head H or the wave length of the laser beam are properly selected so as to se a focus position FP where the reading laser beam is most converged to be inside the intermediate layers when a reading laser beam from a reproduction head H is made incident nearly vertically on the one-surface reading two-layer optical disk from the side of the first signal layer. Then jitters are nearly equalized when reproducing the respective signal pigs of the first and second signal layers 2, 42, 8 and 48.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a two-layer optical disk for realizing high-density and single-sided reading, and a design of a focal position of a reproducing head for reading and reproducing the optical disk.

[0002]

2. Description of the Related Art In recent years, digital video discs (DVs) have been developed.
As one of the standards of D), a two-layer optical disk capable of reproducing two signal surfaces from one side is regarded as promising as a large-capacity medium in the future.

[0003] The structure of a conventional single-sided reading dual-layer optical disk will be described below. (A) a signal layer (the signal surface is on the second layer side) as the first layer (the read laser beam incident side); (b) a semi-transparent film as the second layer; and (c) a two-layer signal layer as the third layer. Intermediate layer for separation (d) Reflective film as fourth layer (e) Signal layer with signal pits formed on fourth layer side as fifth layer

[0004] In the case of reproducing such a single-sided reading type two-layer type optical disk, a reading head arranged on the first layer side of the optical disk is used to focus a laser beam read out from the reproducing head onto the optical disk, and the laser beam is incident on the optical disk. For example, the signal surface of the first layer is provided at the focal position where the read laser light is most narrowed, or the focal position of the reproducing head is designed so that the laser light is most narrowed at the signal surface of the first layer.

[0005]

However, in the above prior art, the focus position where the read laser light from the reproducing head is most narrowed is located on the signal surface of the first layer (read laser light incident side signal layer). When the focal position where the read laser beam is most narrowed is shifted to the signal surface of the fifth layer, which is the other signal layer, an aberration occurs due to the thickness of the intermediate layer (the optical path length becomes longer), and the difference between when the first layer is reproduced and The aperture of the laser beam becomes larger in comparison. As a result, the signal pits at the front, rear, left, and right are reproduced at the time of reproducing the signal of the fifth layer rather than at the time of reproducing the signal of the first layer, and the jitter (signal read variation rate) of the reproduced signal is deteriorated.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art. By setting the position where the reading laser beam is most narrowed to an intermediate position between two signal layers, the jitter of the reproduced signal is reduced. It is an object of the present invention to provide a high-density optical recording medium, a reproducing head, and a reproducing apparatus capable of equalizing the data.

[0007]

In order to achieve the above-mentioned object, the invention according to claim 1 of the present invention reproduces data by using a reproducing head which has an optimum focus at a specific specified base material thickness. An optical recording medium comprising: a first signal layer, a translucent film, an intermediate layer, a reflective film, and a second signal layer, which are sequentially formed;
In an optical recording medium in which a signal surface is formed on the signal layer on the translucent film side and a signal surface is formed on the reflection film side of the second signal layer, the laser beam read from the reproducing head is applied to the first signal layer. An optical recording medium, wherein the thickness of the first signal layer is adjusted so that a focal position where read laser light is most focused when the light is incident substantially perpendicularly from the signal layer side is set in the intermediate layer. Wherein the focal position is set substantially between the center of the intermediate layer in the optical axis direction and the translucent film, and return light when reading each signal surface of the first signal layer and the second signal layer. The intensity is such that (intensity of return light from the translucent film) <(intensity of return light from the reflection film).

According to a second aspect of the present invention, the wavelength of the laser beam in the reproducing head, which focuses optimally at a specific specified substrate thickness, is approximately 650 nm or less, and the numerical aperture of the objective lens is approximately 0.6 or more. It is characterized by being.

Further, a third aspect of the present invention is a reproducing head for reproducing the optical recording medium according to the first or second aspect.

The invention described in claim 4 is characterized in that the wavelength of the laser beam is approximately 650 nm or less, and the numerical aperture of the objective lens is approximately 0.6 or more.

Further, the invention according to claim 5 is characterized in that an optimum focus is formed when the thickness of the base material is approximately 0.6 mm.

Further, the invention according to claim 6 is a reproducing apparatus for reading out a signal using a reproducing head for reproducing the optical recording medium according to claim 1 or 2.

Further, the invention according to claim 7 is characterized in that the wavelength of the laser beam is approximately 650 nm or less, and the numerical aperture of the objective lens is approximately 0.6 or more.

The invention described in claim 8 is characterized in that an optimum focus is achieved when the thickness of the substrate is approximately 0.6 mm.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. Embodiment 1 As Embodiment 1, polycarbonate is used as the substrate material of the signal layer, Au is used as the translucent film, and UV is used as the material of the intermediate layer.
Cured resin, Al as reflective film, laser wavelength of approximately 650 nm for reproduction system, aperture NA of objective lens of reproduction head
Will be described as an example in the case of using about 0.6.

FIG. 1 shows a single-sided reading type 2 according to the present invention.
FIG. 3 shows a sectional view of a layer disc. In FIG. 1, reference numeral 2 denotes a first signal layer on the incident side of the read laser beam, which is substantially transparent (substantially transmits) to the read laser beam and has a refractive index of about 1.45 to 1.65, such as resin compression molding. The signal pits are formed spirally on one side with a thickness of 0.1.
It consists of a disc of about 56-0.58 mm. Reference numeral 4 denotes a film formed on the signal surface side of the first signal layer 2 by sputtering or the like, and has a reflectance of about 20 to 40% with respect to the amount of incident light when laser light is incident from the first signal layer 2. The translucent film has a property of partially reflecting incident laser light and transmitting the rest, and can be realized by using a material having a low light absorptance such as a dielectric film. Numeral 6 is substantially transparent (substantially transmitted) to the reading laser beam and has a refractive index of 1.35.
Focus position F designed so that the laser beam read from the reproducing head is most focused on the intermediate layer of about 1.75.
From P, each has a thickness of about 20 to 30 μm in the optical axis OA direction of the read laser beam placed substantially perpendicular to the disk surface. Reference numeral 8 denotes a second signal layer having a signal pit formed on the intermediate layer 6 side and having the same structure as the first signal layer 2, and is formed of a disk having signal pits formed spirally on one surface. Reference numeral 10 denotes a reflection film having a reflectance of 70% or more formed on the second signal layer 8 by a method such as sputtering, and RA denotes a rotation center axis when the disk rotates. The above-mentioned reflectance is a value measured using a spectrophotometer, and the reflectance described below is also a value measured using a spectrophotometer.

When the first signal surface on the read laser beam incident side is reproduced from the single-sided read type two-layer optical disk having the above-described structure, the read laser beam incident on the lens of the read head is centered on the rotation center axis RA. The focus control of the reproducing head is performed so that it can be narrowed down most on the signal surface of the disk rotating at a constant linear velocity, and then the tracking control is performed to follow the signal train, and the reflected light from the signal surface is received. Detected by a detector and read out as an analog signal. When reproducing the other signal surface, that is, the second signal surface on the far side of the read laser beam incident side, the focus control of the reproducing head is performed so that the laser beam can be narrowed down most on this signal surface. Tracking control is performed to follow the column, and signal detection is performed in the same manner as when reproducing the first signal surface on the reproduction light incident side.

Table 1 shows a single-sided reading type 2 according to the present invention.
It shows the reproduction signal jitter (signal read variation rate) when both signal surfaces of a layered optical disk are reproduced and when both signal surfaces of a conventional single-sided read-type two-layer optical disk are reproduced.

[Table 1]

As is apparent from Table 1, the single-sided read-type dual-layer optical disk according to the present invention has the same reproduction signal jitter on each signal surface as compared with the conventional single-side read-type dual-layer optical disk, It is possible to reproduce the surface under almost the same conditions.

It should be noted that the second side of the read laser beam
The signal layer 8 has a conventionally used substrate thickness of about 0.6.
It is also possible to prepare and use a substrate material in mm, for example, close to the sum of the thickness of the first signal layer 2 on the read laser beam incident side and approximately half the thickness of the intermediate layer 6. The signal layer may have a thickness. Further, it is also possible to use a material instead of the material used in the first embodiment.

Embodiment 2 As Embodiment 2, a case where a conventional single-sided reading type double-layer optical disc is read using a reproducing head according to the present invention will be described as an example. As in the first embodiment, the material of the optical disk is polycarbonate as the substrate material of the signal layer, Au as the translucent film, and UV as the intermediate layer.
Al is used as the cured resin and the reflection film.

As shown in FIG. 2, the reproducing head H according to the present invention has a semiconductor laser 12 as a laser light source for emitting a reading laser beam, and transmits a laser beam emitted from the semiconductor laser 12 while transmitting the laser beam. A beam splitter 14 for reflecting the reflected light from the optical disc 90 degrees in the horizontal direction, a quarter-wave plate 16 arranged on the optical disc side of the beam splitter 14, and a quarter-wave plate 16 further arranged on the optical disc side At least one reproduction lens 18, a stop lens 20 for stopping the laser beam reflected by the beam splitter 14, and a stop lens 20
Detector 2 on which the laser beam narrowed down by the laser enters
2 is provided.

In the reproducing head H having the above-described structure, the wavelength of the emitted laser beam is selected to be shorter than that of the conventional (approximately 650 nm) by appropriately selecting the semiconductor laser 12, or the reproducing lens 18 is appropriately selected to form the aperture NA. Is made larger than the conventional one (approximately 0.6), so that the focus position of the laser beam most narrowed down by the reproducing head H is shifted between the first signal surface on the laser light incident side and the second signal surface on the laser light incident deep side. Thus, for example, the intermediate layer 6 can be positioned on a center line that divides the thickness of the intermediate layer 6 into approximately half (see FIG. 3).

Next, a description will be given of a method of reproducing the first signal surface on the laser beam incident side of the conventional single-sided reading type two-layer optical disk using the reproducing head H.

The optical disk clamped at the center is rotated at a constant linear velocity by a spindle. When the optical disk has a warp, the signal surface as seen from the focal position of the head in a stationary state swings up and down due to this rotation operation. At this time, as shown in FIG. 2, the height of the head or the disk is adjusted so that the signal surface goes up and down about the focal position. If there is no warp in the disk, the height of the head or the disk is adjusted so that the focal position is on the signal surface.

The laser light emitted from the semiconductor laser 12 passes through the beam splitter 14 and the quarter-wave plate 16 and is irradiated on the disk by the reproducing lens 18. Of the laser light applied to the disc, the laser light reflected on the first signal surface re-enters the reproducing lens 18 and
The light passes through the four-wavelength plate 16, is reflected by the beam splitter 14, is stopped down by the stop lens 20, and is incident on the light receiving detector 22. The laser light detected by the light receiving detector 22 is detected by the focus error detection circuit 24 as a focus error signal, which is a shift amount from the focal position of the laser light to the signal surface, and is converted from an optical signal to an electric signal. The electric signal is input to the coil driver 26, and a current for driving the head up and down so as to cancel the amount of deviation between the focal position and the signal surface is supplied to the up / down driving coil 28 to perform head focus control. . Subsequently, a tracking error signal for following the signal pit row is detected by the tracking error detection circuit 30, and is converted from an optical signal to an electric signal as in the case of the focus control. The electric signal is input to the coil driver 32, and a tracking control is performed by supplying a current for driving the head left and right to the left and right driving coils 34 so as to cancel the deviation amount of the reproduction laser spot from the signal pit row, Read the reproduced signal as an analog signal.

On the other hand, when reproducing the second signal surface on the inner side of the laser beam incident side, focus control of the reproducing head H is performed so that the laser beam can be narrowed down most on the second signal surface, and subsequently the signal sequence is followed. Is performed, and signal detection is performed as in the case of reproducing the first signal surface on the reproduction light incident side.

Table 2 shows reproduction signal jitters (signals) when the reproduction head H according to the present invention reproduces both signal surfaces of a single-sided reading type double-layer optical disk and when the conventional reproduction head reproduces both signal surfaces. (Reading variation rate).

[Table 2]

As is clear from Table 2, the reproducing head H according to the present invention has the same reproduction signal jitter as that of the conventional reproducing head when reproducing the respective signal surfaces of the single-sided reading type double-layer optical disk. Both signal surfaces can be reproduced under substantially the same conditions.

Embodiment 3 As a single-sided reading type double-layer optical disc according to Embodiment 3,
Single-sided reading dual-layer optical disc of Embodiment 1 (FIG. 1)
This will be described in comparison with.

As a prototype of the single-sided read-type two-layer optical disk of Embodiment 1, polycarbonate was used as a substrate material for the signal layers 2 and 8, and a semi-transparent film 4 was used.
Is used as the first signal layer 2 on the laser beam incident side.
Laser light is incident from the
% Reflectance. Further, by using a UV curable resin as the intermediate layer 6 and using Al as the reflection film 10 and setting the reflectance to 70% or more, the light intensity reflected by the reflection film 10 and returning to the reproducing head can be reduced. The reflected light intensity of the transparent film 4 was made substantially equal. The laser wavelength of the reproducing system is approximately 650 nm, and the aperture NA of the objective lens of the reproducing head is approximately 0.6 (the substrate material of the signal layer and the thickness of the UV curable resin are approximately 0.6 mm).
At which the aberration is minimized).
In this structure, the light intensity when the reproducing light is incident on the disk by the reproducing head and the reproducing light reflected by the reflection film 10 returns to the reproducing head again can be expressed by the following equation. (Return light intensity from reflective film [%]) = (Transmittance of translucent film [%]) × (Transmittance of resin material [%]) × (Reflectivity of reflective film [%])

Further, as a prototype of the single-sided reading type dual-layer optical disc of Embodiment 3, polycarbonate was used as the substrate material of the signal layer, Au was used as the semi-transparent film, and the first laser on the laser beam incident side was used. Laser light was incident from the signal layer, and the reflectance was approximately 20% with respect to the incident light amount. In addition, a UV curable resin was used for the intermediate layer, Al was used for the reflective film, and the reflectance was 70% or more. As the laser wavelength of the reproducing system and the aperture NA of the objective lens of the reproducing head, the same ones as those in the first embodiment were selected, and those having a wavelength of approximately 650 nm and a wavelength of approximately 0.6, respectively, were used.

In this structure, the light intensity when the reproduction light is incident on the disk by the reproduction head and the reproduction light reflected by the reflection film returns to the reproduction head again is the same as that of the first embodiment as the material of the substrate. When a resin material is used, the light intensity reflected by the reflective film and returning to the reproducing head becomes higher than the reflected light intensity of the translucent film due to an increase in the transmittance of the translucent film.

FIG. 4 is a cross-sectional view of a single-sided reading type dual-layer optical disk according to the third embodiment. In FIG. 4, reference numeral 42 denotes a readout in which spiral signal pits having a track pitch of about 0.74 μm and a shortest pit length of about 0.44 μm are formed on one side by resin compression molding or the like, which is substantially transparent to the read laser beam. In the first signal layer on the laser beam incident side,
Refractive index is about 1.45 to 1.65, thickness is 0.56 to 0.58
It consists of a disc of about mm. Reference numeral 44 denotes a film formed on the signal surface side of the first signal layer 42 by a method such as sputtering, and has a reflectance of about 20 to 40% with respect to the incident light amount when a laser beam is incident from the first signal layer 42. The translucent film has the property of partially reflecting incident laser light and transmitting the rest. Numeral 46 is transparent to the read laser beam, and each of the reference numerals 46 is in the direction of the optical axis OA of the read laser beam placed substantially perpendicularly to the disk surface from the reproducing head focal position FP.
An intermediate layer having a thickness of about 0 to 30 μm. 48
Indicate that signal pits are formed on the intermediate layer 46 side and the first signal layer 4
In the second signal layer on the inner side of the laser beam incident side having the same structure as 2,
Track pitch 0.74μ on one side by resin compression molding
m, spiral signal pits having a minimum pit length of about 0.44 μm are formed, and the refractive index is 1.45 to 1.45.
It is made of a disc having a size of about 65 and a thickness of about 0.56 to 0.58 mm. Reference numeral 50 denotes a reflection film having a reflectance of 70% or more formed on the second signal layer 48 by a method such as sputtering, and RA denotes a central axis when the disk rotates.

When the first signal surface on the read laser beam incident side is reproduced from the single-sided read type two-layer type optical disk having the above-described structure, the read laser beam incident on the lens of the reproducing head is centered on the rotation center axis RA. The focus control of the reproducing head is performed so that it can be narrowed down on the signal surface of the rotating disk, the tracking control is performed to follow the signal sequence, and the reflected light from the signal surface is detected by the light receiving detector. , As an analog signal. When reproducing the other signal surface, that is, the second signal surface on the far side of the read laser beam incident side, the focus control of the reproducing head is performed so that the laser beam can be focused on this signal surface, and then the signal train The tracking control is performed in order to follow this, and a signal is detected in the same manner as when reproducing the first signal surface on the reproduction light incident side.

FIG. 5A shows the results of trial production of a single-sided read-type two-layer optical disk of Embodiment 1 and measurement of reproduction.
(B) and (c) showing the single-sided reading type 2 of the third embodiment.
Fig. 6 shows the results of trial production of a layered optical disk and measurement of the reproduction.
(A), (b) and (c) of FIG. In each of the figures, the vertical axis represents the reproduction signal jitter (signal read variation rate), and the horizontal axis represents the distance (optical path length) from the time when the reproduction laser light enters the disk to the signal surface to be reproduced.

FIG. 5A shows a case where the reproducing light incident side substrate 2 has a thickness of about 0.54 mm, 0.56 mm and 0.58 mm, the thickness of the intermediate layer 6 is approximately 30 μm, and 4 is a laser beam which is incident from the first signal layer 2 and has a reflectivity of about 25% with respect to the amount of incident light.
The total thickness of the disc as the signal layer substrate 8 is 1.14 to 1.5.
This is a result of reproducing and measuring three kinds of prototype disks, which were adjusted in thickness to about mm. Reference numeral 52 denotes a result of reproducing and measuring the signal surface of the reproducing light incident side substrate 2 of the disk, and reference numeral 54 denotes a result of reproducing and measuring the signal surface of the reproducing light incident rear substrate 8. FIG. 5B shows the thickness of the reproduction light incident side substrate 2 as 0.54 mm, 0.56 mm, 0.58 mm,
The thickness of the intermediate layer 6 should be approximately 4 mm
0 μm, a semi-transparent film 4 having a reflectivity of approximately 25% with respect to the amount of incident laser light from the first signal layer 2 when a laser beam is incident on the first signal layer 2. The thickness is adjusted so that the total thickness of the disc as the second signal layer substrate 8 on the back side is about 1.14 to 1.5 mm,
It is a result of reproducing and measuring four kinds of prototype disks. 5
Reference numeral 6 denotes a result of reproduction measurement of the signal surface of the reproduction light incident side substrate 2 of the disk, and reference numeral 58 denotes a result of reproduction measurement of the signal surface of the reproduction light incident side substrate 8. FIG. 5C shows the thickness of the reproduction light incident side substrate 2 as 0.61 mm and 0.62 m.
m, about 0.63 mm, the thickness of the intermediate layer 6 is approximately 50 μm, and a laser beam is incident from the first signal layer 2 as the translucent film 4 and the reflectance is approximately 25% with respect to the incident light amount. The thickness is adjusted so that the reflectance of the reflection film 10 is 70% or more, and the total thickness of the disc as the signal layer substrate 8 on the rear side of the reproduction light is about 1.14 to 1.5 mm. It is a result of reproducing and measuring three kinds of prototype disks. Reference numeral 60 denotes a result of reproduction measurement of the signal surface of the reproduction light incident side substrate 2 of the disk, and reference numeral 62 denotes a result of reproduction measurement of the signal surface of the reproduction light incident side substrate 8.

Here, when reproducing the signal surface of a disk, manufacturing variations such as warpage, eccentricity, and birefringence occur among the individual disks, and manufacturing variations also occur in the reproducing head. Taking this into account, the reproduced signal jitter must be kept within 10%.

According to FIG. 5A, when the thickness of the intermediate layer 6 is approximately 30 μm, the limit of the reproduction signal jitter of 10%
Not satisfied within. This is presumably because the thickness of the intermediate layer 6 is thin, so that when one of the signal layers is reproduced, the other signal layer is reproduced, and the reproduced signal jitter is deteriorated as crosstalk between the signal layers.

As shown in FIG. 5B, the thickness of the intermediate layer 6 is approximately 40.
In μm, the reproduction light incident side substrate 2 must be 0.54 mm or more in order to satisfy the limit of the reproduction signal jitter of 10% or less. In addition, the signal surface is located at a position equidistant from the position in the optical axis direction of the reproduction light with respect to the thickness of the substrate material and the resin capable of narrowing the reproduction light by the reproduction head to about 0.6 mm. The reproduction signal jitter is made substantially the same on both signal surfaces.

From FIG. 5C, it can be seen that the thickness of the intermediate layer 6 is approximately 50.
In μm, in order to satisfy the limit of the reproduction signal jitter of 10% or less, the length to the rear substrate 2 on which the reproduction light is incident is set to be equal to 0.
It must be less than 66 mm.

However, with respect to the single-sided reading type two-layer type optical disk of the first embodiment, the thicknesses of the translucent film 4 and the reflective film 10 formed on the signal surface, and the thicknesses of the substrates 2, 8 and the intermediate layer 6 A high level of control is required for manufacturing variations. In the actual manufacturing process, the control for forming the translucent film 4 requires a margin of about 15% in reflectance. FIG. 6 shows the condition of the limit of the reflectance of the translucent film 4 and the reflective film 10 that can be recognized by the disc reproducing apparatus due to the manufacturing variation, that is, the single-sided reading type two-layer type optical disc of the third embodiment is prototyped and measured. is there.

FIG. 6A shows that the reproducing light incident side substrate 42 has a thickness of about 0.56 mm, 0.58 mm and 0.61 mm, the intermediate layer 46 has a thickness of about 30 μm, and the translucent film has a thickness of about 30 μm. 44, a laser beam is incident from the first signal layer 42 and has a reflectivity of about 20% with respect to the incident light amount. The reflectivity of the reflective film 50 is 70% or more, The thickness is adjusted so that the total thickness of the disc as the 2 signal layer substrate 48 is about 1.14 to 1.5 mm,
It is a result of reproducing and measuring three kinds of prototype disks. 6
4 is the result of reproduction measurement of the signal surface of the reproduction light incident side substrate 42 of the disk, and 66 is the reproduction light incident back side substrate 48
Is a result of reproduction measurement of the signal surface of FIG. FIG. 6B shows the reproduction light incident side substrate 42 having a thickness of 0.54 mm and 0.5 mm.
The thickness of the intermediate layer 46 is about 40 μm, and the thickness of the intermediate layer 46 is about 40 μm.
Laser light is incident from the light source and the light amount is approximately 20%
Is used, and the reflectance of the reflection film 50 is set to 70
%, The thickness of the signal layer substrate 48 on the rear side of the reproduction light incident side was adjusted so that the total thickness of the disks was about 1.14 to 1.5 mm, and the results were reproduced and measured for three types of prototype disks. Reference numeral 68 denotes a substrate 42 of the disk on which the reproduction light
Is the result of reproduction measurement of the signal surface of FIG. 7, and 70 is the result of reproduction measurement of the signal surface of the reproduction light incident back side substrate 48. FIG.
(C) shows the thickness of the reproduction light incident side substrate 42 as 0.6.
Use the one of about 1 mm and 0.63 mm, and
Of the first signal layer 4 as a semi-transparent film 44 having a thickness of about 50 μm.
2 and the laser light is incident on it, and
%, And the reflectance of the reflective film 50 is 7%.
This is a result of reproducing and measuring two kinds of prototype disks by adjusting the thickness so that the total thickness of the disk as the signal layer substrate 48 on the rear side of the reproduction light incidence of 0% or more is about 1.14 to 1.5 mm. . Reference numeral 72 denotes a substrate 4 on the side of the reproduction light incident side of the disc.
2 is a result of reproduction measurement of the signal surface of No. 2, and 74 is a result of reproduction measurement of the signal surface of the reproduction light incident back side substrate 48.

From the result of reproducing the single-sided read-type dual-layer optical disc of the third embodiment, the reproduction signal on the reproduction light incident side signal surface is more reproducible than the reproduction signal on the rear side. Large jitter value. This is because there is a difference in the amount of return light from the reflective film 50 and the translucent film 44 on each signal surface to the reproducing head, and the light returns to the light receiving detector when reproducing the signal surface on the reproduction light incident side where the amount of reflected light is small. This is because the noise component affects the incoming optical signal and the conversion of the optical signal into an electrical signal before measurement.

According to FIG. 6A, the reproduction signal jitter when reproducing the reproduction light incident side signal layer having a thickness of about 30 μm of the intermediate layer 46 does not satisfy the limit of 10% or less. This is similar to the case where the thickness of the intermediate layer 6 of the single-sided reading dual-layer optical disc of Embodiment 1 is approximately 30 μm.
It is considered that when one of the signal layers is reproduced due to the small thickness of the intermediate layer, reproduction is performed up to the other signal layer, and the reproduced signal jitter is deteriorated as crosstalk between the signal layers.

From FIG. 6B, it can be seen that the thickness of the
At 0 μm, the reproduction light incident side substrate 42 must be 0.56 mm or more in order to satisfy the limit of the reproduction signal jitter of 10% or less. Also, here, the reproduction light incident side substrate 4
2 is about 0.6 mm, the jitter when reproducing the reproduction light incident side signal surface is 8.1%, and the jitter when reproducing the reproduction light incident side signal surface is 8.0%. Are equally divided. This is because the amount of return light from the translucent film on the signal surface when reproducing the signal surface on the reproduction light incident side is small, and the signal surface is easily affected by external noise. 6
The aberration is minimized by setting the position in mm. Conversely, when reproducing the signal surface on the far side of the reproduction light incident side, jitter degradation due to aberration due to the distance of approximately 40 μm, which is the thickness of the intermediate layer, from the position where the reproduction head is most narrowed, ie, the position of 0.6 mm, is reduced. The effect of noise is reduced by increasing the amount of return light from the reflective film on the side signal surface.

That is, the focal position where the laser light is most focused is set between the substantially center of the intermediate layer 46 in the optical axis OA direction and the translucent film 44, and the reflectivity of the translucent film 44 is appropriately selected. By making the intensity of the return light from the reflective film larger than the intensity of the return light from the translucent film, the jitter between the reproduction of the signal surface on the reproduction light incident side and the reproduction of the signal surface on the back side of the reproduction light is approximately equalized. can do.

According to the present embodiment, in consideration of manufacturing variations of the disk, the reflectivity margins of the translucent film and the reflective film are each about 20 to 40%, and the optical path length, that is, 0.56 mm or less for the reproduction light incident side substrate 42. About 0.58mm,
It is considered appropriate that the thickness of the intermediate layer is about 40 to 60 μm (within an optical path length of 0.64 mm to the signal surface of the substrate 48 on the rear side of the reproduction light).

In the third embodiment, the intensity of light reflected by the reflective film and returning to the reproducing head is larger than the amount of light reflected by the translucent film. Conversely, the light intensity is reflected by the reflective film and returns to the reproducing head. The light intensity can be smaller than the amount of reflected light from the translucent film.

That is, the focal position where the laser light is most focused is set between the substantially center of the intermediate layer 46 in the direction of the optical axis OA and the reflective film 50, and the reflectivity of the translucent film 44 is appropriately selected. By making the intensity of the return light from the reflective film smaller than the intensity of the return light from the transparent film, the jitter when reproducing the signal surface on the reproduction light incident side and the jitter when reproducing the signal surface on the rear side of the reproduction light are approximately equalized. be able to.

As described above, by using the third embodiment, even if the signal surface is provided at a position unequal distance from the focal position of the reproducing head, the amount of return light can be made different between the translucent film and the reflective film. This makes it possible to substantially equalize the jitter when each signal layer is reproduced.

As a condition of the single-sided reading type two-layer type optical disk, even if the reflectance of the translucent film is 30% and the reflectance of the reflective film is 70% or more, the same disk structure margin result as in the above embodiment can be obtained.

Further, a reproducing head having an aperture NA capable of setting a focal position on the signal surface side where the amount of return light is small when reproducing each signal surface is used, or the wavelength of laser light emitted from the semiconductor laser is appropriately selected. This makes it possible to substantially equalize the jitter when each signal layer is reproduced.

[0054]

Since the present invention is configured as described above, it has the following effects.

According to the present invention, the focal position at which the read laser beam is most focused is located between the two signal planes, so that the jitter at the time of reproducing each signal plane can be substantially equalized. , The quality of the reproduced signal can be made substantially equal.

Further, since each signal surface is located at a position substantially equidistant from the focal position in the optical axis direction, the aperture diameter of the light spot on each signal surface when reproducing each signal surface is inferior due to aberration. Under the conditions, the return light intensities are substantially the same, and the degree of influence of noise on the reproduced signal is also substantially equal, so that both signal surfaces can be reproduced under substantially the same conditions.

Further, since the two signal layers have the same thickness, the same substrate material can be used.

Further, since the thickness of the incident-side signal layer is set to a thickness obtained by adding approximately 1/2 of the thickness of the intermediate layer to the thickness of the incident-side signal layer, the thickness of the conventionally used substrate material is adjusted. It can be manufactured and used as a signal layer on the rear side without incident.

Further, the signal surface far from the focal position designed so that the readout laser beam is most narrowed by the reproducing head, and the signal surface which is greatly affected by the aberration, is not reflected by the reflection film or the translucent film on the signal surface. By increasing the intensity of the return light to the read head, the influence of noise is reduced, and when reproducing the other signal surface, the focus position is designed so that the read laser beam is most focused by the read head. The signal side is near,
Since the influence of aberration is small, the quality of the reproduced signal when each signal surface is reproduced can be made substantially equal by lowering the intensity of the returning light from the reflective film or translucent film on the signal surface to the reproducing head. .

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a single-sided reading type dual-layer optical disc according to the present invention.

FIG. 2 is a schematic configuration diagram of a reproducing head according to the present invention.

FIG. 3 is a cross-sectional view in a case where a focal position at which a laser beam read from the reproducing head of FIG. 2 is most narrowed is set substantially at the center in the optical axis direction of an intermediate layer of a single-sided reading type two-layer optical disc.

FIG. 4 is a cross-sectional view showing a modified example of the single-sided reading type dual-layer optical disc of FIG.

FIG. 5 shows a result of measuring jitter using the single-sided read-type two-layer optical disc of FIG. 1;
(A) shows a case where the thickness of the intermediate layer is set to approximately 30 μm, (b) shows a case where the thickness of the intermediate layer is set to approximately 40 μm, and (c) shows a case where the thickness of the intermediate layer is set to approximately 50 μm.

FIG. 6 shows the results of measuring jitter using the single-sided read-type dual-layer optical disc of FIG. 4;
(A) shows a case where the thickness of the intermediate layer is set to approximately 30 μm, (b) shows a case where the thickness of the intermediate layer is set to approximately 40 μm, and (c) shows a case where the thickness of the intermediate layer is set to approximately 50 μm.

[Explanation of symbols]

 2,42 First signal layer 4,44 Translucent film 6,46 Intermediate layer 8,48 Second signal layer 10,50 Reflective film 12 Semiconductor laser 14 Beam splitter 16 Quarter wave plate 18 Reproduction lens 20 Focusing lens 22 Light reception Detector 24 Focus error detection circuit 26, 32 Coil driver 28, 34 Coil 30 Tracking error detection circuit H Playback head FP Focus position OA Optical axis RA Rotation center axis

Claims (8)

[Claims] 1. An optical recording medium which is reproduced by using a reproducing head which has an optimum focus at a specified thickness of a base material, comprising: a first signal layer, a translucent film, an intermediate layer, a reflective film, and a second signal. An optical recording medium in which a signal surface is formed on the semitransparent film side of the first signal layer and a signal surface is formed on the reflection film side of the second signal layer. The thickness of the first signal layer is set such that when the read laser light from the head is made substantially perpendicularly incident from the first signal layer side, the focal position where the read laser light is most focused is set in the intermediate layer. An optical recording medium, wherein the focal position is set between substantially the center of the intermediate layer in the optical axis direction and the translucent film, and the first signal layer and the second signal layer are adjusted. The intensity of the return light when reading each signal surface of Return light intensity from the film) <
(Intensity of return light from the reflective film). 2. The wavelength of a laser beam in the reproducing head, which achieves an optimum focus at a specific specification base material thickness, is approximately 650 n.
2. The optical recording medium according to claim 1, wherein the numerical aperture of the objective lens is approximately 0.6 or less. 3. A reproducing head for reproducing the optical recording medium according to claim 1. 4. A laser beam having a wavelength of about 650 nm or less,
The reproducing head according to claim 3, wherein the numerical aperture of the objective lens is approximately 0.6 or more. 5. The reproducing head according to claim 4, wherein an optimum focal point is formed when the thickness of the substrate is approximately 0.6 mm. 6. A reproducing apparatus for reading out a signal using the reproducing head for reproducing the optical recording medium according to claim 1. 7. A laser beam having a wavelength of about 650 nm or less,
7. The reproducing apparatus according to claim 6, wherein the numerical aperture of the objective lens is about 0.6 or more. 8. The reproducing apparatus according to claim 6, wherein an optimum focus is achieved when the thickness of the base material is approximately 0.6 mm.