JP2001014729A - Optical disk and optical disk device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain high density recording by setting a track pitch in a specific range and satisfying a prescribed relation among the wavelength of an optical beam, numerical aperture of an objective lens, thickness of an optical disk, and an allowable inclined angle at a specific value or less. SOLUTION: An optical disk records information as lines of pit with a prescribed track pitch, the information being reproduced by irradiating the disk with an optical beam through an objective lens. In this optical disk assuming the wavelength of an optical beam is λ nm and the numerical aperture is NA, the track pitch is set within a range as shown in the expression I. In this case, assuming the thickness of the optical disk is ds μm and the allowable inclined angle is θA, a relation is fulfilled as shown in the expression II. In addition, the allowable inclined angle θA is set to be 7 mrad or less. Even if a track pitch or a pit pitch is set small compared with the beam spot diameter of a reproducing optical beam, a cosstalk between adjacent tracks can be reduced to the extent causing no problem practically.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical disk on which information is recorded at a high density in the form of pits, and an optical disk device including the optical disk and a reproduction optical system.

[0002]

2. Description of the Related Art In recent years, digital signal processing techniques for images and MPEG (Moving Picture Image Coding Experts) have been developed.
Group), a video compression technology that is being promoted by a standardization organization, etc., replaces VTRs and laser discs and reproduces movie-like moving picture information in the same size as a CD (compact disc) for 2 hours. Expectations for possible optical discs are increasing. The recording capacity required for recording two hours of moving picture information in the form of an analog video signal of a standard TV system such as a NTSC like a laser disk is as large as 80 Gbytes including audio. If a moving image compression technology defined by a standardization method called “S-VHS” is used, a high-quality VT such as S-VHS can be used.
Even if the image quality is the same as R, the required capacity is only about 4 GB. This 4GB disk is
A write-once optical disc with a diameter of 300 mm has already been put to practical use. However, considering the future widespread use for general households, it is required that a CD-size of 120 mm, which is easy to handle, has the same capacity.

At present, the capacity of a CD format, which is widely used as a music CD or CD-ROM, is 790 Mbytes at the maximum (when the linear velocity is 1.2 m / s). 2 can store compressed video information for only 24 minutes. For this reason, in order to store compressed moving image information of MPEG-2 for 2 hours in a CD size, the recording density must be increased to 5 times as compared with a CD. By the way, the current C
In the D format, the substrate thickness is 1.2 mm, the track pitch is 1.6 μm, and the pit pitch is 1 when the linear speed (the relative speed between the light beam and the optical disk = the peripheral speed of the optical disk) is 1.2 m / s. .66 μm, pit length 0.59
μm, modulation method is EFM (Eight to Fourteen Modulati
on). On the other hand, in a reproducing optical system, a wavelength of a reproducing semiconductor laser, that is, a laser diode (LD) is 780.
nm, the NA (numerical aperture) of the objective lens is 0.45, and the beam spot diameter is approximately 1.4 μm. The value of the beam spot diameter is selected mainly from the viewpoint of avoiding the influence of crosstalk between adjacent tracks.

In order to increase the recording density of an optical disc, a processing technique for forming small pits on the optical disc and a technique for reducing the beam spot size on the optical disc in a reproducing optical system are required. Regarding pit processing technology, for example, an optical disk master recording technology using a Kr ion laser beam (ultraviolet light) having a wavelength of 351 nm has been proposed (Autumn 1993, National Convention of the Japan Society of Applied Physics, 28-SF-2). It is possible to process smaller pits as compared with the Ar ion laser. Regarding the reproduction optical system, shortening the wavelength of the reproduction laser beam and NA
Can increase the beam spot diameter. However, in actuality, in a conventional technique such as a CD player, even if a short-wavelength light source such as a red laser diode is used, the effect of increasing the capacitance by wavelength is at most about 1.5 times, so that it takes only 2 hours. It is not possible to increase the capacity by five times the capacity of a normal CD required for recording compressed moving image information for one minute.

[0005]

As described above, in the conventional optical disk technology, in order to avoid the problem of crosstalk between adjacent tracks, the track pitch and pit pitch are made larger than the beam spot diameter of the reproduction light beam. Therefore, if only the wavelength of the reproduction light beam is shortened and the NA of the objective lens is increased, for example, MPEG-
However, there is a problem that the recording density cannot be increased to such an extent that a capacity necessary for storing the compressed moving image information for 2 hours is obtained.

[0006]

According to the present invention, even if the track pitch or pit pitch is set smaller than the beam spot diameter of the reproducing light beam, the crosstalk between adjacent tracks is reduced to a practically acceptable level. It is an object of the present invention to provide an optical disk and an optical disk device that can be made higher in density and capacity as compared with the related art.

Another object of the present invention is to provide an optical disk and an optical disk device capable of high-density recording when the tilt angle of the optical disk is 10 mrad or less.

In order to achieve the present invention, the present inventors approximated the pit shape on the optical disk to a soccer stadium shape, and optimized the pit shape in the track width direction (optical disk radial direction) and the track direction (tangential direction), that is, adjacent shapes. We have found a pit shape that suppresses crosstalk between tracks to a level that can be used practically and that can obtain a sufficient signal level of a reproduction signal and a push-pull signal.

The present invention has a substrate on which information reproduced by irradiation with a light beam through an objective lens is recorded as a row of pits at a predetermined track pitch, and a reflective layer formed on the substrate. On an optical disc, when the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is set in the range of (0.72-0.8) α × (λ / NA) /1.14 μm, Assuming that the thickness of the optical disk is ds μm and the allowable tilt angle of the optical disk is θA, α = 3.617 × 10 ⁻¹⁴ × (θ _A ds / λ) ³ +1.248
The present invention provides an optical disk characterized by the formula: × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865, wherein the allowable inclination angle θA is 7 mrad or less.

Further, the present invention provides an optical disk comprising a substrate on which information is recorded as a row of pits at a predetermined track pitch and a reflective layer formed on the substrate, and an objective lens provided opposite the optical disk. An optical disc comprising: means for irradiating an optical disc with a light beam through an objective lens; and means for detecting reflected light of the light beam applied to the optical disc by the irradiation means and reproducing information recorded on the optical disc. In the apparatus, when the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is set in the range of (0.72-0.8) α × (λ / NA) /1.14 μm, Assuming that the thickness of the optical disk is ds μm and the allowable tilt angle of the optical disk is θA, α = 3.617 × 10 ⁻¹⁴ × (θ _A ds / λ) ³ +1.248
The present invention provides an optical disk device represented by × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865, wherein the allowable inclination angle θA is 7 mrad or less.

Further, the present invention relates to a pair of transparent substrates which face each other and on which information reproduced by irradiation with a light beam through an objective lens is recorded as a row of pits at a predetermined track pitch. In an optical disc having a pair of reflective layers formed on opposing surfaces of a transparent substrate, the wavelength of the light beam is λ nm, and the numerical aperture of the objective lens is N.
When A is set, the track pitch is set in the range of (0.72 to 0.8) α × (λ / NA) /1.14 μm, the thickness of the optical disk is ds μm, and the allowable tilt angle of the optical disk is θA. α = 3.617 × 10 ⁻¹⁴ × (θ _A ds / λ) ³ +1.248
X is provided, and an allowable tilt angle θA is 7 mrad or less.

Further, the present invention provides a substrate on which information reproduced by irradiating a light beam through an objective lens is recorded as a row of pits at a predetermined track pitch, and a reflection layer formed on the substrate. In an optical disc having
When the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is (0.72-0.8) α ×
(Λ / NA) /1.14 μm, and each of the pits is (0.3-0.50) α × (λ / NA) / 1.
Top width in the range of 14 μm and (0.2-0.32) α
× (λ / NA) /1.14 μm in the trapezoidal cross section having a lower width in the range, and the thickness of the optical disk is dsμ.
m, and the allowable tilt angle of the optical disk is θA, α = 3.617 × 10 ⁻¹⁴ × (θ _A ds / λ) ³ +1.248
The present invention provides an optical disc characterized by being determined by the formula: × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865.

Further, the present invention provides an optical disk having a substrate on which information is recorded as a row of pits at a predetermined track pitch and a reflective layer formed on the substrate; an objective lens provided to face the optical disk; An optical disk device comprising: means for irradiating an optical disk with a light beam through an objective lens; and means for detecting reflected light of the light beam applied to the optical disk by the irradiating means and reproducing information recorded on the optical disk. In the above, when the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is set in the range of (0.72-0.8) α × (λ / NA) /1.14 μm, Of each (0.3-0.5
0) Top width in the range α × (λ / NA) /1.14 μm and (0.2-0.32) α × (λ / NA) /1.14
a trapezoidal pit having a lower width in the range of μm,
alpha: the magnification to ensure an acceptable optical disk inclination angle at a wavelength of ^{0.65μm, α = 3.617 × 10 -14} × (θ A ds / λ) 3 +1.248
Provided is an optical disk device characterized by being obtained by: × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865.

Further, the present invention provides a substrate on which information reproduced by irradiation with a light beam through an objective lens is recorded as a row of pits at a predetermined track pitch;
In an optical disc having a reflective layer formed on a substrate, the wavelength of the light beam is λ nm, and the numerical aperture of the objective lens is N.
When A, the track pitch is (0.72-0.8)
α × (λ / NA) /1.14 μm, and each of the pits is (0.3 to 0.50) × (λ / NA) /
Top width in the range of 1.14 μm and (0.2-0.3
2) A pit having a trapezoidal cross section having a lower width in the range of × (λ / NA) /1.14 μm, where α: wavelength is 0.65
This is a magnification for guaranteeing an allowable disk tilt angle in μm, and α = 3.617 × 10 ⁻¹⁴ × (θ _A ds / λ) ³ +1.248
The present invention provides an optical disc characterized by being determined by the formula: × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865.

Further, the present invention provides an optical disk having a substrate on which information is recorded as a row of pits at a predetermined track pitch, a reflective layer formed on the substrate, an objective lens provided to face the optical disk, An optical disk device comprising: means for irradiating an optical disk with a light beam through an objective lens; and means for detecting reflected light of the light beam applied to the optical disk by the irradiating means and reproducing information recorded on the optical disk. In the above, when the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is set in the range of (0.72-0.8) α × (λ / NA) /1.14 μm, Of each (0.3-0.5
0) × (λ / NA) /1.14 μm top width and (0.2-0.32) × (λ / NA) /1.14 μm
A trapezoidal pit having a lower width in the range of α:
The magnification to ensure an acceptable disc tilt angle at the wavelength ^{0.65μm, α = 3.617 × 10 -14} × (θ A ds / λ) 3 +1.248
Provided is an optical disk device characterized by being obtained by: × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the embodiments, the basic concept of the present invention will be described.

In order to increase the density of an optical disk, it is necessary to reduce the beam spot diameter of a reproduction light beam.
For that purpose, it is essential to shorten the wavelength of the reproducing laser diode and increase the NA of the objective lens. The wavelength is already 685n
m, low noise type laser diode with output of several mW (Self-pulsation type laser diode)
Has been put to practical use, and a laser diode having a wavelength of 650 nm has come to a level close to practical use.

On the other hand, the NA of the objective lens is limited by the ease of making the lens and the tilt angle between the lens and the optical disk. An objective lens is easier to make as the lens load is smaller (the smaller the substrate of the optical disc is thinner) and the NA is smaller, but if the objective lens has an NA of about 0.6, even a single aspherical lens has a smaller beam spot diameter. Reduction can be realized. However, in an objective lens used in a reproduction optical system of an optical disk, coma is generated due to a tilt (tilt) between the optical disk and the reproduction light beam caused by an inclination of the optical disk or an optical axis of the objective lens.

That is, when the spot size of the reproducing light beam is reduced by increasing the NA of the objective lens, the aberration of the objective lens rapidly increases due to the tilt between the optical disk and the reproducing light beam. If the aberration of the objective lens increases, the amount of crosstalk between adjacent tracks increases, and the reproduction resolution decreases. The effect of this tilt is smaller as the substrate of the optical disk is thinner. Reference: Jp
nJAppl.Phsys.Vol.32 (1993), pp.5402-5405, the wavelength is 690 nm, NA = 0.6, and the thickness of the substrate is 1.2 mm which is the same as the CD, and 0.6 mm which is half that. At this time, how the beam spot shape of the reproducing light beam changes due to the tilt is shown. According to this, when the substrate thickness is 1.2 mm, if there is a tilt of 5 mrad, the center intensity of the beam spot is reduced by as much as 10%, and a side lobe bulge or aberration which causes crosstalk occurs. . On the other hand, when the substrate thickness is 0.6 mm, it can withstand a tilt up to 10 mrad.

FIGS. 11 and 12 show the case where the substrate thickness (t) is 1.2 mm and 0.6 mm
The results of calculating the tilt characteristics for the case (1) are shown. The horizontal axis indicates the tilt angle, and the vertical axis indicates the normalized peak intensity of the reproduced signal. The wavelength (λ) of the reproducing light beam was 690 nm. Substrate thickness 0.6mm, NA = 0.6
In the case of (1), the peak intensity of the reproduced signal decreases by 10% when the tilt is 9.5 mrad. When the substrate thickness is 1.2m
Looking at m, NA = 0.49. That is,
By setting the substrate thickness from 1.2 mm to 0.6 mm of the conventional CD, the NA can be increased from 0.49 to 0.6, and the areal recording density can be increased about 1.5 times.
That is, the spot size is proportional to λ / NA, and the areal recording density is inversely proportional to the square of the spot size.
6 / 0.49) ² , the areal recording density is about 1.5 times higher than the areal recording density of the conventional CD.

However, if the thickness of the substrate is simply reduced, the warpage of the substrate due to temperature and humidity may become significant. The warpage of the substrate is a major cause of tilt. As a countermeasure, double-sided optical disks as with laser disks,
That is, it is most effective to make the optical disk a symmetrical structure. In that case, information can be recorded on both sides. In the case of a conventional optical disk having a single-plate structure such as a CD, the injection conditions during the production of the substrate and the formation of a reflective film or a protective film made of aluminum on one surface of the substrate make the moisture absorption of the substrate asymmetrical from the front to the back. Tends to occur. When the optical disk is double-sided, the distortion of the substrate due to such moisture absorption is canceled, and a large tilt can be prevented.

From the above examination results, a laser diode having a wavelength of 685 nm, a substrate having a thickness of 0.6 mm, and NA = 0.
If the objective lens combination of No. 6 is used, the wavelength is reduced from 780 nm to 685 nm, and N
As A increased from 0.45 to 0.6,
Even under the conventional CD design method, it is possible to achieve a recording density approximately 2.3 times higher than that of the conventional CD format. That is, since the spot size is proportional to λ / NA and the surface recording density is inversely proportional to the square of the spot size,
From {(780 / 0.45) / (685 / 0.6)} ² , the recording density increases to about 2.3 times that of the conventional CD. However, as described above, the recording density (capacity) must be about the same as that of the conventional CD format in order to obtain the capacity required for recording compressed moving image information by MPEG-2 for two hours in the CD size. It must be increased by a factor of 5, which is not sufficient.

The present invention relates to an optical disk having a higher density
By optimizing the pit shape under the same beam spot size as described above to achieve a large capacity, the track is secured while ensuring low crosstalk characteristics and a sufficient signal level for the reproduction signal and push-pull signal. An object of the present invention is to provide an optical disk capable of further reducing the pitch. Hereinafter, the pit shape according to the present invention will be described in detail.

(First Embodiment) FIG. 1 is a diagram for explaining a pit shape on an optical disk according to the present invention. As shown in the figure, the shape of the pit 10 is similar to a so-called soccer stadium having a trapezoidal cross section. The inner wall 11 of the pit 10 has a downward slope.
The bottom 12 is substantially flat. 13 is pit 1
Numeral 0 indicates a cross section in the radial direction of the optical disk (track width direction), and reference numeral 14 indicates a cross section in the circumferential direction of the optical disk (track direction). , Wi are the dimensions of the bottom of the pit 10 in the track width direction (hereinafter referred to as bottom width), h
m is the depth of the pit 10, Zm is the length of the pit 10 in the track direction, and θ is the angle of the inner wall 11 of the pit 10 (the angle formed by the inner wall 11 with respect to the optical disk surface).

FIG. 2 shows a model of the reproducing optical system of the optical disk apparatus used for the analysis. The incident light distribution 20 (V1 (x, y)) of the reproducing light beam, the incident light 21, and the incident light 21 are shown. A polarization beam splitter (or half mirror) 22 for separating the reflected light 26, an objective lens 23 having a numerical aperture NA, and a distribution 24 (V2 (x) of focused light (beam spot) on the optical disk recording surface (pit surface) by the objective lens 23. , y)), the optical disk 25 having the complex reflectance r2 (x, y), the reflected light 26, and the distribution 27 (V3 (x, y)) of the reflected light 26 on the photodetector.

FIG. 3 is a diagram schematically showing a pit arrangement on the optical disk for calculating the levels of the reproduction signal and the push-pull signal. The track pitch (pit pitch in the track width direction) is Pt. The pitch (pit pitch in the track direction) is Pmy. Beam spots 30 and 31 of reproduction light beam
Indicates a spot A at the center of the pit and a spot B at an intermediate position between the pits. The amplitude of the reproduced signal is represented by | S (A) -S (B) |.
Here, S (A) or S (B) represents an output signal of the photodetector when the beam spot is at the position of (A) or (B). Lines 32 and 33 are push-pull signals (a difference signal between output signals of at least two detection regions arranged in the track direction of the split photodetector) in a region (C) having pits and a region (D) having no pits. Is shown. These push-pull signals are average pp values in the regions C and D, respectively.

FIG. 4 shows a reproduction laser beam wavelength of 685 n.
m, NA = 0.6, Zm = 0.5 μm, Pmy = 1 μm
The results of calculating the levels of the reproduced signal and the push-pull signal using the size of the pit in the track width direction when m and Pt = 0.72 μm and the pit depth hm as parameters are shown. Beam filling rate A / W of the reproduction light beam in the track width direction (X) and the track direction (Y)
The values of (X) and A / W (Y) are as shown in the figure. As shown in the figure, the levels of the reproduction signal and the push-pull signal do not largely depend on the pit shape except for the case where Wm = 0.3 and Wi = 0.2. Also, there is no pit depth that maximizes the level of the playback signal and the push-pull signal at the same time, but it is desirable to obtain the maximum playback signal level while minimizing the drop in the push-pull signal level. For example, it can be seen from FIG. 4 that the depth of the pit is approximately λ / 5, preferably in the range of λ / 4.5 to λ / 6.

FIG. 5 is a diagram schematically showing an MTF (modulation transfer function) of a reproducing optical system and a pit arrangement on an optical disk used for evaluating crosstalk between adjacent tracks. In the figure, the beam spot 5 of the reproducing light beam
0 and 51 indicate pits passing through the center of the pit (A) and the position (B) separated from the center of the pit by the distance td. MTF is represented by the power of the fundamental frequency component of the output signal of the photodetector obtained when the beam spot passes through the center of the pit. Crosstalk is represented by the power of the fundamental frequency component of the output signal of the photodetector obtained when the beam spot passes through position B.

FIG. 6 shows that the track pitch Pt is 0.72 μm.
m and the crosstalk characteristic when the upper width Wm and the lower width Wi of the pit 10 are variously changed while the depth hm of the pit 10 is fixed at 0.2 μm. The spatial frequency and the vertical axis represent MTF and crosstalk, respectively. The values of the beam filling factors A / W (X) and A / W (Y) in the track width direction (X) and the track direction (Y) of the reproducing light beam are as shown in the figure. As shown in the figure, the MTF has a difference of about 1 to 2 dB depending on the pit shape, but is not a very large value. On the other hand, it can be seen that the crosstalk greatly changes depending on the pit shape.

FIG. 7 shows the same conditions as in FIG.
FIG. 6 is a diagram showing MTF and crosstalk characteristics when is set to 0.650 μm.

FIG. 8 shows that the track pitch Pt is 0.72 μm.
FIG. 7 is a diagram showing MTF and crosstalk characteristics when the upper width Wm of the pit 10 and the angle θ of the inner wall 11 of the pit 10 are variously changed while the depth hm of the pit 10 is kept constant at 0.2 μm while the depth hm of the pit 10 is kept constant. The horizontal axis represents spatial frequency, and the vertical axis represents MTF and crosstalk. Beam filling ratios A / W (X) and A / W of the reproducing light beam in the track width direction (X) and the track direction (Y).
The value of (Y) is as shown in the figure.

FIG. 9 shows the pit 10 under the same conditions as FIG.
Of the pit 10 with a constant upper width Wm of 0.35 μm.
MTF when only the angle θ of the inner wall 11 is variously changed
FIG. 3 is a diagram illustrating a crosstalk characteristic.

Here, it is assumed that an RLL (Run-Length Limited) method is used as a modulation method of information recorded on an optical disk. In this method, it is necessary to pay attention to crosstalk due to low frequency components when the longest pit is detected.

The crosstalk characteristics shown in FIGS. 6 to 9 are for the case where there is no tilt, but it is actually necessary to consider the tilt. FIG. 10 shows MTF and crosstalk characteristics when tilt is considered. As shown in the figure, when the tilt is considered, the MTF hardly changes, but the amount of crosstalk increases, and the conditions for determining the pit parameters become more severe.

In the system design of the optical disk device, if a tilt of 5 mrad is given by the warp of the optical disk itself and 3 mrad is given as a tilt caused by the device, it is necessary to allow a tilt of about 8 mrad as a whole. According to the simulation of FIG. 10, the crosstalk amount can be suppressed to a value of −20 dB or less, which is practically required, up to a tilt of ± 10 mrad for the same spatial frequency. From now on, the wavelength is 685 nm and the track pitch is 0.7
It turns out that a value of 2 μm is appropriate.

Considering the characteristics of FIGS. 6 to 9 from the above viewpoints, first, in the case where the track pitch Pt is 0.72 μm shown in FIGS. 6 and 8, Wm = 0.45
Up to μm, the crosstalk amount (the difference between the MTF values of the MTF characteristics and the crosstalk characteristics) is -20 dB or less. On the other hand, when Wm = 0.5 μm, the amount of crosstalk at a low frequency sharply increases, and exceeds -20 dB. The MTF characteristics are relatively good up to Wm = 0.3 μm, but deteriorate sharply when Wm is less than 0.3 μm. Therefore, Wm = 0.3 μm to 0.4
It can be said that 5 μm is an appropriate range.

On the other hand, as for the angle θ of the inner wall 11 of the pit 10, as shown in FIG. 9, when θ = 80 ° or more, the crosstalk amount exceeds −20 dB, and when θ = 40 ° or less, the MTF characteristic sharply increases. Deteriorates. Therefore, θ = 50
It can be said that an angle of 70 to 70 is an appropriate range.

From the above results, when the pit shape in the track width direction is normalized at a wavelength of 685 nm and NA = 0.6 (that is, λ / NA = 1.14), the upper width of the pit becomes (0.3 to 0.3). 0.45) × (λ / NA) /1.14
μm, and the lower width of the pit is (0.2 to 0.25) × (λ /
NA) /1.14 μm or the upper width Wm is (0.3 to
0.45) × (λ / NA) /1.14 μm, and the angle θ of the inner wall is preferably in the range of 50 ° to 70 °. That is, the track pitch Pt is set to (0.72 to 0.8) ×
When the track pitch is smaller than the beam spot diameter of the reproducing light beam by selecting the range of (λ / NA) /1.14 μm, the upper width Wm and lower width Wi of the pit or the upper width Wm of the pit may be reduced. By selecting the angle θ of the inner wall within the above range, ± 1 assumed in an actual optical disc device can be obtained.
Up to a tilt of 0 mrad, the amount of crosstalk can be suppressed to a value of -20 dB or less, which is required for practical use, and a dramatic improvement in recording density can be achieved. As a result, the track pitch and the pit shape, the laser diode having a wavelength of 685 nm, for example, and the.
With the combination of the 6 mm thick substrate and the objective lens with NA = 0.6, it is possible to easily achieve the original problem of recording compressed moving image information of MPEG-2 for 2 hours in CD size. .

The parameters used in the description of the present invention are obtained by calculation assuming that the pits have an ideal trapezoidal shape. However, in practice, the pits are not exactly trapezoidal, but are curved at their corners as shown in FIG. Therefore, the parameters of an ideal trapezoidal pit, ie, a model pit, are different from the actual pit shape. FIG. 14 shows the difference between the bottom groove width wi and the top groove width wg for the model pit and the actual pit. As is apparent from FIG. 14, the value of the bottom groove width wi of the model pit is in the range of 0.2 μm to 0.32 μm,
The value of the upper groove width wg is in the range of 0.3 to 0.45 μm. On the other hand, the actual value of the bottom groove width wi of the pit is 0.
It is in the range of 2 μm to 0.32 μm, and the value of the upper groove width wg changes in the range of 0.3 to 0.45 μm. Further, the inclination angle θ of the pit is as follows. That is, as shown in FIG. 15, the inclination angle of the model pit is in the range of 50 ° to 70 °. On the other hand, the actual pit inclination angle is 30
In the range of ° to 60 °.

Next, the structure of the optical disk according to the present invention will be described. FIGS. 16A and 16B are a perspective view and a cross-sectional view, respectively, of an optical disk 100 having two sides, and show transparent substrates 101 and 102 made of a translucent resin such as polycarbonate or acrylic having embossed pits. On one surface, a reflective film 103 such as aluminum
And 104 are deposited, on which protective films 105 and 106 are formed. Transparent substrates 101 and 1
02 has a thickness of 0.6 mm. With the transparent substrates 101 and 102 facing the protective films 105 and 106 side,
Adhesive layer 107 made of thermosetting adhesive and having a thickness of several tens μm
Are bonded together. A hole 108 for clamping is formed in the center of the optical disc 100, and a clamping zone 109 is provided around the hole. A reproducing light beam 110 emitted from a laser diode (not shown) and entering through a reproducing optical system is
1 to the optical disc 100 via the transparent substrates 101 and 1
The light enters from the side 02 and is focused on the reflection films 103 and 104 as a minute beam spot.

FIG. 17 shows an embodiment of an optical disk apparatus for reproducing compressed moving image information using the optical disk 100 described above. In FIG. 17, the optical disk 100 is housed in a cartridge 200. By housing the optical disc 100 in the cartridge 200, it is not necessary to care about how to hold the optical disc, dust, fingerprints, and the like like a CD, and it is advantageous in handling and carrying. When the optical disk is exposed, such as a CD, it is necessary to determine the error correction capability in consideration of an unexpected situation such as a scratch. However, if the cartridge 200 is used, such consideration is unnecessary. Therefore, the LDC Reed-Solomon error correction method can be used in sector units as used in a recording / reproducing optical disk. Thus, for example, when formatting an optical disk in units of 2 k to 4 k bytes, the recording efficiency can be improved by 10% or more compared to a CD.

For example, a 4/9 modulation method is used as a modulation method of information to be recorded on the optical disc 100.
Assuming that the track pitch on the track No. 00 is 0.72 μm and the pit pitch is 0.96 μm, the pit density ratio is 3.84 times higher than that of the conventional CD format, and the modulation method is 20 times.
% And the format efficiency are expected to increase by 10%, so that a total capacity increase of about 5.1 times can be expected. As described above, moving image information such as a movie is stored in the S-VH
4.5M including sound when playing back with S-level high image quality
Since the rate is bps, the capacity required for 2 hours of reproduction is 4 GB. With the 5.1 times increase in the capacity, the capacity of 4 Gbytes can be realized on one side of the optical disk. Further, if the optical disk is double-sided as shown in FIGS. 16A and 16B, it is possible to record for a maximum of 4 hours on one optical disk.

Returning to the description of FIG.
Is chucked by the tapered cone 220 and rotated by the spindle motor 201. Spindle motor 20
1 is driven by a spindle motor drive circuit 202. On the other hand, the reproducing optical system is configured as follows.

The objective lens 2 faces the optical disc 100.
The objective lens 203 is movable in the optical axis direction by a focus coil 204 and is movable in the track width direction by a tracking coil 205. The oscillation wavelength of the laser diode 207 driven by the laser diode (LD) driver 206 is 685
The light beam emitted from the laser diode 207 is converted into a parallel light beam by the collimator lens 208 and then enters the polarization beam splitter 209. Since the light beam emitted from the laser diode 207 generally has an elliptic far field pattern, a beam shaping prism may be arranged after the collimator lens 208 when a circular pattern is required. The light beam that has passed through the polarizing beam splitter 209 is converged by the objective lens 203 and enters the optical disc 100.

The light reflected by the reflection film of the optical disc 100 returns through the objective lens 203 in the direction opposite to the incident light beam,
The light is reflected by the polarization beam splitter 209 and is
The light enters a photodetector 212 via a detection optical system such as 10 and a cylindrical lens 211. The photodetector 212 is, for example, a quadrant photodetector, and its four detection outputs are input to an amplifier array 213 including an amplifier and an adder / subtractor.
Here, a focus error signal, a tracking error signal, and a reproduction signal are generated. The tracking error signal is obtained as the above-described push-pull signal by, for example, a method called a push-pull method. The focus error signal and the tracking error signal are supplied to the focus coil 204 and the tracking coil 205 via the servo controller 214, respectively. Thereby, the objective lens 203 is moved in the optical axis direction and the track width direction, and the focusing of the light beam on the surface of the reflective film, which is the recording surface of the optical disc 100, and the tracking on the target track are performed.

The reproduced signal from the amplifier array 213 is input to a signal processing circuit 215, where the signal is binarized and data pulses are detected. The detected data pulse is input to the optical disk controller 216, where the format is decoded, an error is corrected, and the like, and then input to the MPEG-2 decoder / controller 217 as a bit stream of moving image information. On the optical disc 100,
Since data obtained by compressing (encoding) moving image information in accordance with the MPEG-2 standard is recorded, the MPEG-2 decoder / controller 217 decompresses (decodes) the input bit stream to obtain the original moving image. Play information. The reproduced moving image information is supplied to a video signal generation circuit 218.
And a blanking signal or the like is added to form a video signal of a predetermined television format. In addition,
Techniques related to MPEG-2 are described in U.S. Pat. 5,31
7,397 and U.S. Pat. 08 / 197,8
62.

As described above, the optical disc according to the present embodiment can reduce the crosstalk between adjacent tracks to a practically required level while setting the track pitch to a value smaller than the spot diameter of the reproducing light beam. Pit shape (pit top width and bottom width or pit top width and pit inner wall angle)
The track density can be increased about 1.5 times as compared with the conventional CD, and the level of the reproduction signal and the push-pull signal used for tracking can be sufficiently secured.

As a result, as shown in the above-mentioned embodiment, for example, the capacity of about 5 times as large as that of the conventional CD can be realized even in the case of the CD size, and the quality including the voice is as high as that of the high quality VTR compressed to 4 Mbps. Moving image information can be stored for two hours, and its practical effect is extremely large.

In the above-described embodiment, the values of the track pitch, the top width and the bottom width of the pit are λ / NA
(Λ: wavelength (μm), NA: set as a value obtained by multiplying the numerical aperture of the objective lens by a proportional coefficient in a predetermined range. Therefore, the wavelength of the light to be used or the aperture of the objective lens is set. It is possible to determine parameters effective for increasing the recording density without depending on the number.

However, the conditions in the above-described embodiment are conditions determined based on the condition that the aberration generated by the tilt of the disk is equivalent with respect to the wavelength. This will be described in detail with reference to FIG.

FIG. 18 shows the results of calculating aberrations caused by the tilt of the disk for several wavelengths. The horizontal axis is the disk tilt angle (mrad) in the radial direction, and the vertical axis is the rms (root mean square) of the generated aberration.
The values are shown in units of wavelength. The rms value of the aberration is substantially proportional to the disk tilt angle and inversely proportional to the wavelength. When the tilt angle is less than 20 mrad, the rms of the aberration
The value, Wrms, is given as:

Wrms = 3.58 × 10 ⁻³ × θ (mrad) / λ (μm) For example, when the wavelength is 0.65 μm and the disk tilt angle is 10
In the case of mrad, Wrms = 0.0551λ.

The condition of the above embodiment is that the wavelength is 0.65 μm.
This is a value obtained under the condition that an allowable inclination angle of 10 mrad is given in the vicinity. Therefore, an inclination angle of 10 mrad is allowed for a wavelength in the vicinity of this, and the allowable inclination angle becomes smaller when light of a shorter wavelength is used. This relationship is expressed by the following equation.

.Theta. (Mrad) = 15.4.times..lambda. (.Mu.m) Referring to FIG. 18, when there is a disk tilt of 10 mrad at a wavelength of 0.65 .mu.m, the rms value of the aberration is 0.055.
It is 1λ. For example, when using light generated by SHG (Second Harmonic Generation) by a combination of YVO ₄ and KTP, the wavelength of the light is 0.532 μm. At this time, the aberration is reduced to an rms value equivalent to the above condition, that is, 0. In order to suppress the angle to 0551λ, the allowable disk tilt angle is 8.2 mrad. Further, for example, a wavelength of 0.42 μm or 0.36
When it is reduced to μm, the allowable inclination angles are 6.5 mr each.
ad or 5.5 mrad.

When an attempt is made to improve the recording density at such short wavelengths, the allowable disk tilt angle becomes smaller accordingly. Therefore, the accuracy of the shape of the optical disk, the accuracy of the spindle motor and the turntable, the chucking accuracy of the optical disk, and the like are reduced. The demands on precision have become strict, making it difficult to provide inexpensive devices.

This embodiment has been made in view of such circumstances, and realizes an optical disk device having a recording density as high as possible without strict requirements for mechanical accuracy. In the present embodiment, an optical disk device with a high recording density is realized while the allowable disk tilt angle is kept constant, for example, at 10 mrad. In this case, 10mr at each wavelength
Since the amount of aberration generated by ad becomes large, it is necessary to set a relatively large value of a parameter such as a track pitch and a detection window width that determines the recording density in order to allow such aberration. In FIG. 18, for example, 0.532 μm
m, 0.42 μm and 0.36 μm, when the light wavelength is set to be small, the amount of aberration generated at a disk tilt of 10 mrad is 0.0673λ, 0.0852λ and 0.0
It gradually increases to 994λ. Tolerating such large aberrations is equivalent to a 0.65 μm optical system.
Allowable values of inclination angle are 12.2mrad and 15.5mr, respectively.
This would correspond to large values such as ad and 18.1 mrad. This conversion is expressed by the following equation based on the above proportional relationship between the amount of aberration and the allowable tilt angle.

Θeq (mrad) = 6.5 / λ (μm) where θeq is an aberration equivalent to the aberration generated when there is a 10 mrad inclination at each wavelength, at a wavelength of 0.65 μm.
Represents the angle when it occurs in the optical system of FIG. This angle θeq increases in inverse proportion to the wavelength.

In order to allow such a large inclination angle, it is necessary to set parameters such as a track pitch and a detection window width that determine recording density to be larger than those set in proportion to (λ / NA). .

FIG. 19 shows the relationship between the disk tilt angle and the window occupancy. As typical values of the above embodiment, the wavelength is 0.65 μm, the numerical aperture is 0.6, the track pitch is 0.725 μm, the pit top width is 0.35 μm,
Assuming that a pit bottom width is 0.2 μm and a detection window width is d = 2, a modulation code system is adopted. The pit depth is 0.134 μm, and the pit depth is 1/5 of a value obtained by dividing the wavelength λ by the substrate refractive index n. did.
In this case, the relationship between the disk tilt angle and the window occupancy is a curve shown as (1.0 times). (1.1 times)
The curve shown as and (1.2 times) is the track pitch,
Pit upper width, pit bottom width, the detection window is a curve when expanded to all 1.1-fold and 1.2-fold the respective widths, respectively recording density 1 / 1.1 ² and 1 / 1.2 ²
However, the allowable tilt angle of the optical disk increases instead.

The window occupancy on the vertical axis in FIG. 19 is a value obtained by calculating a reproduced signal based on the scalar analysis theory, and indicates the inter-code interference caused by various bit patterns generated under the restriction of the modulation code. 2 shows the lower limit of the window occupancy that can be achieved when the jitter is reduced by using an optimal equalizing circuit in consideration of crosstalk from an adjacent track. Such a calculation of the window occupancy requires a considerably enormous amount of calculation time, and is possible only by developing a calculation program based on a high-speed algorithm. In addition, since the NA is large, a more accurate evaluation method based on ray tracing is used in the calculation of the aberration caused by the substrate tilt, instead of the approximation formula that is often used.

FIG. 20 is a diagram derived from FIG. 19 and obtained by plotting the required magnification with respect to the allowable disk tilt angle based on the window occupancy of 80%.

According to the above-described embodiment, the design procedure of the optical disk device when using the short-wavelength light beam is as follows. First, referring to FIG. 18, it is determined whether or not it is possible to set the allowable value of the disk tilt angle small so that the aberration falls below the aberration corresponding to the case where the wavelength is 0.65 μm and the disk tilt angle is about 10 mrad. . If possible, parameters such as track pitch, pit top width, pit bottom width, and detection window width may be determined based on the previous embodiment. However, when the allowable disk tilt angle cannot be set so small, the wavelength used (for example, 0.42 μm) and the allowable disk tilt angle (for example, 10.2 μm) are used.
0mrad), the value of an allowable aberration amount (0.08) is read from FIG. 18, and this aberration amount (0.08) is the disk tilt angle corresponding to the aberration amount obtained by the optical system having a wavelength of 0.65 μm. (15.0 mrad) is considered to be an allowable disc tilt angle converted to an optical system of 0.65 μm. From FIG. 20, the magnification (1.2) required to realize the allowable disk tilt angle (15.0 mrad) is read. Finally, each parameter value in the case of following the previous embodiment is assigned to the magnification (1.
By setting by multiplying by 2), it is possible to realize an optical disc capable of securing a practical allowable disc tilt angle even at a short wavelength.

In the description of this embodiment, the thickness of the substrate is 0.6 mm. However, since the aberration is proportional to the thickness of the substrate, the parameters are calculated by the proportional calculation in the case of a substrate having another thickness. Can be determined. For example, the thickness of the substrate is 0.4 m
In the case of m, the value of the aberration is 2/3 times that of FIG. 18, so that the aberration may be set according to this magnification. That is, since the amount of aberration is inversely proportional to the wavelength as described above, using this condition means that the substrate thickness becomes γ times, which means that the wavelength becomes 1 / γ times from the viewpoint of the amount of aberration. Equivalent to. Therefore, by converting under these conditions, it is possible to determine the magnification for the parameters of the track pitch, the top width and the bottom width of the pits in the previous embodiment, as in the above-described design procedure.

When the substrate thickness is 600 μm and the wavelength is λ μm, θeq (mrad) = 6.5 / λ as described above.
(Μm) holds, but when the substrate thickness is d μm, the following equation holds.

Θ eq (mrad) = 6.5 / λ × (d / 60
0) (μm) = 1.083 × 10 ⁻² × (d / λ) In accordance with this relationship, the scale on the horizontal axis is d / λ at the top of the graph of FIG. Although the graph of FIG. 20 is created based on the calculation using the wavelength of 0.65 μm as an example, if the scale at the top of FIG.
What is necessary is just to calculate the ratio between the substrate actually used and the wavelength and read the necessary magnification without being conscious of the conversion using the parameter of the μm system.

More specifically, in the above embodiment, the ranges of the track pitch, the upper width, the lower width, and the depth of the pits are set as follows.

Track pitch: (0.72-0.8) × (λ / NA) 1.14 μm Upper width: (0.3-0.5) × (λ / NA) /1.14 μm Lower width: ( 0.2 to 0.32) × (λ / NA) /1.14 μm Depth: (1 / 4.2 × λ / n) to (1 / 5.2 × λ / n) In the above formula, n is refraction Each parameter is a value when the wavelength is 0.65 μm, and when the wavelength of the used laser beam is shorter than this wavelength, each parameter is multiplied by a multiple α. That is, each coefficient is multiplied by α as in the following equation.

Track pitch: (0.72-0.8) α × (λ / NA) 1.14 μm Upper width: (0.3-0.5) α × (λ / NA) /1.14 μm Lower width : (0.2 to 0.32) α × (λ / NA) /1.14 μm Depth: (1 / 4.2 × λ / n) α to (1 / 5.2 × λ / n) α The multiple α can be read from the graph of FIG. 20, but is expressed by an approximate mathematical expression for convenience in application such as design, as follows. That is, if the horizontal axis is the allowable disk tilt angle θeq (mrad) converted to 0.65 μm and the vertical axis is the dimensional magnification α, α is expressed by the following equation.

Α = 0.002236 × θeq2-0.01
575 × θeq + 0.934 When the substrate thickness ds and the wavelength λ are used, the above equation, that is, θeq (mrad) = 1.083 × 10−2 × (ds / λ) is substituted into the above equation of α. Is the value of the following equation.

Α = 2.6226 × 10 ⁻⁷ × (d / λ) ²
According to ^{-1.7057 × 10 -4 (ds / λ} ) +0.934 above formula, the optimum ratio in the case of changing both the substrate thickness and wavelength, be determined as a function of substrate thickness and wavelength it can.

In the above embodiment, the allowable disk tilt angle is set to 1
Although set to 0 mrad, the present invention can be applied to other disk inclination angles. In this case, θeq (mrad) and α are represented by the following equations.

Θ eq (mrad) = 1.083 × 10 ⁻³ × (θ _A d / λ) where θ _A (mrad) is an allowable disk tilt angle.

[0073] ^{α = 2.623 × 10 -9 × (} θ A d /
λ) ² −1.706 × 10 ⁻⁵ (θ _A d / λ) +0.93
4 Recently, the tilt has been reduced to 0mr by improving the tilt servo technology.
You can now get closer to ad. Obviously, the recording density can be improved as the tilt becomes smaller. Therefore, next, an embodiment in which α is determined when the tilt is a small value of 10 mrad or less will be described.

For example, when the system is designed with a residual tilt of about 7 mrad or less in terms of a wavelength λ = 0.65 (μm) and a substrate thickness ds = 600 (μm),
A new formula is desired. More precisely, in a system in which the tilt is suppressed to a value of about 6500 or less (θ _A ds / λ),
It is desirable to design the system using the new formula shown below. However, the unit is a value when θA _is mrad and _ds and λ are both μm.

Α = 3.617 × 10 ⁻¹⁴ × (θ _A ds / λ) <sup>3
+ 1.248 × 10 -9 × (θ</sup> A ds / λ) 2 +0.865 magnification relationship to the allowable tilt angle based on the new formula FIG. 21 (a), the shown in (b). However, in the figure, the horizontal axis represents the wavelength λ = 0.65 (μm) and the substrate thickness d.
Effective allowable tilt angle θ when s = 600 (μm)
A (mrad), and the vertical axis indicates the value of α. Horizontal axis (θ _A )
Is 0.865, α = 0.865 is the graph obtained by the above new equation.

According to FIG. 21A, the tilt angle θA is 1
Up to 0 (mrad), the α value follows almost the same locus as in FIG.
When the tilt angle θA becomes 10 (mrad) or more, FIG.
As shown in (b), the α value is small.

By the way, the target specifications of HD-DVD are wavelength λ = 0.410 (μm), numerical aperture NA = 0.6,
Track pitch Pt = 0.4 (μm), substrate thickness ds = 6
00 (μm). Considering this specification, tilt angle θ
When A is 0, α = 0.
When Pt is obtained based on 865, it is as follows.

Pt = (0.72-0.8) × α × (λ / NA) /1.14 (μm) = (0.72-0.8) × 0.865 × (0.410 / 0) .6) /1.14 (μm) = 0.37-0.41 That is, the target of HD-DVD, that is, Pt = 0.4 (μm)
Can be achieved.

Next, it is assumed that there is a residual tilt of, for example, about 4 mrad. In this case, θ _A = 4 mrad, λ = 0.
41 μm and ds = 600 μm.

(ΘAds / λ) = 4 × 600 / 0.41 = 5853 α = 3.617 × 10 ⁻¹⁴ × (5853) ² + 1.248 × 10 ⁻⁹ × (5853) ² + 0.865 = 0. 915 θ _A = 4 mrad, that is, when α = 0.915, Pt = (0.72-0.8) × α × (λ / NA) /1.14 (μm) = (0.72-0.8 ) × 0.915 × (0.410 / 0.6) /1.14 (μm) = 0.39-0.44, and the target of the HD-DVD specification, that is, Pt = 0.4
(Μm) can be achieved.

The following device can be used as the tilt servo mechanism for performing the above-described tilt correction.

As shown in FIG. 22, the objective lens 203 is
The inner peripheral portion 301a is a normal lens, and the outer peripheral portion 301b is an annular prism. Here, the objective lens 203
The inner peripheral portion 301a is used for focusing a light beam (hereinafter, referred to as a recording / reproducing light beam) for performing recording and / or reproducing, and the outer peripheral portion 301b is used for tilting the optical disk 10 with respect to the optical head (hereinafter, referred to as an inclination). It is used for irradiation of a tilt detection light beam for detecting optical disc tilt.

A part of the light beam emitted from the semiconductor laser light source 207 is focused and irradiated as a recording / reproducing light beam on the information recording surface 321 by the inner peripheral portion 301a of the objective lens 203, and the other part, that is, A beam in a region which has not been used as a recording / reproducing light beam and which has conventionally lost a light amount is irradiated on the information recording surface 321 as a parallel light tilt detecting light beam by the outer peripheral portion 301b of the objective lens 203. The light beam applied to the information recording surface 321 is reflected by the information recording surface 321 and the reflected light is reflected by the objective lens 20.
3 is guided to the half mirror 209 after passing through the inner peripheral portion 301a in the direction opposite to the irradiation light, and is reflected by the half mirror 209, and then through a detection optical system such as the condenser lens lens 210 and the cylindrical lens 211. The light enters a photodetector 315 having a disc tilt detection function.

The photodetector 315 has five detection areas d0, d1 to d4 as shown in FIG. 23 (a) or (b), and each of the recording / reproducing light beam and the tilt detecting light beam. Detect reflected light. That is, the central main detection area d0 detects the reflected light of the recording / reproducing light beam, and the four sub-detection areas d1 to d arranged at 90-degree intervals around.
Reference numeral 4 detects reflected light of the tilt detection light beam. FIG.
In FIG. 23A, each of the sub-detection areas d1 to d4 is composed of one detection surface, but in FIG. Main detection area d0 of photodetector 315
The output signals respectively corresponding to the sub-detection areas d1 to d4 are input to an arithmetic circuit (not shown), and output to the main detection area d0.
From the output signal corresponding to the sub-detection areas d1 to d4.
The disc tilt is detected by the operation on the four output signals corresponding to. The servo controller 21 according to the tilt direction and the tilt amount obtained by the tilt detection.
4 drives the gokas coil 204 to perform tilt correction.

The tilt servo mechanism as described above is described in, for example, ““ Fast Disk Skew Servo for Optical Disk Pickup ”.
up ”SYMPOSIUM OPTICAL MEMORY 1994, Higher Density
Optical Recording Technologies and Their Applican
t, July 11-13, 1994, International Conference Cent
er, pp.83-84 ”, a servo device that controls the lens of an optical pickup using the actuator disclosed in“ Tilt Servo using a Liquid Crystal Device ”INTE.
RNATIONAL SYMPOSIUM ON OPTICAL MEMORY AND OPTICAL
DATA STORAGE, 1996 Technical Digest Series Vol.12,
pp. 351-353 ", a tilt servo mechanism using liquid crystal may be used.

It is conceivable to use a crosstalk canceller as an auxiliary technique for improving the recording density. This crosstalk canceller is a device that takes in information of pit rows arranged on both sides of a pit row to be reproduced and cancels crosstalk using this information. Specifically, as shown in FIG. 25, three beams scan the main track, the inner track and the outer track, and the resulting main signal, inner track signal and outer track signal are shown in FIG. Three FIR (Finite Impulse Response) filters 40
1, 402 and 403 respectively. FIR
The filters 401, 402, and 403 shift the signals of the inner and outer tracks by a predetermined time, and apply a damping coefficient to the resulting signal to generate a pseudo crosstalk signal. The generated pseudo crosstalk signal is subtracted from the main signal in the subtractor 404. Thereby, the reproduction signal from which the crosstalk has been removed is output from the crosstalk canceller.

When the above-described crosstalk canceller is used, the interval between pit rows is narrow on a recording medium having a high recording density, and even if crosstalk occurs, the crosstalk is canceled. Therefore, if a crosstalk canceller is combined with the present invention, the recording density can be further improved.

[0088]

According to the present invention, even if the track pitch and pit pitch are set smaller than the beam spot diameter of the reproducing light beam, the crosstalk between adjacent tracks can be reduced to a practically acceptable level. It is possible to provide an optical disk and an optical disk device capable of increasing the density and capacity in comparison.

Further, it is possible to provide an optical disk and an optical disk device capable of high-density recording when the disk has a tilt angle of 10 mrad or less.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining a pit shape on an optical disc according to one embodiment of the present invention.

FIG. 2 is a schematic diagram showing an outline of a reproduction optical system in the optical disk device.

FIG. 3 is a schematic diagram showing a pit array on an optical disc for calculating a level of a reproduction signal and a push-pull signal detected from the optical disc.

FIG. 4 is a diagram showing a relationship between a reproduction signal level and a push-pull signal level obtained by calculation using a pit size in a track width direction and a pit depth as parameters.

FIG. 5 is a schematic diagram showing a pit arrangement on an optical disk used for evaluating MTF of a reproduction optical system and crosstalk between adjacent tracks.

FIG. 6 is a diagram showing the pit shape dependency of the MTF of the reproducing optical system and the crosstalk characteristics between adjacent tracks when the upper width Wm and the lower width Wi of the pit are variously changed.

FIG. 7 shows the same conditions as in FIG. 6, except that λ is 0.650 μm.
FIG. 9 is a diagram showing the pit shape dependency of the MTF of the reproduction optical system and the crosstalk characteristics between adjacent tracks when the above condition is satisfied.

FIG. 8 is a diagram showing the pit shape dependency of the MTF of the reproducing optical system and the crosstalk characteristics between adjacent tracks when the upper width Wm of the pit and the angle θ of the inner wall of the pit are variously changed.

9 is a diagram showing the pit shape dependence of the MTF of the reproducing optical system and the crosstalk characteristics between adjacent tracks when the upper width Wm of the pit 10 is fixed at 0.35 μm under the same conditions as in FIG.

FIG. 10 is a diagram illustrating tilt dependency of crosstalk characteristics between an MTF of a reproduction optical system and an adjacent track.

FIG. 11 is a view showing tilt dependence of the NA of an objective lens when an optical disk having a substrate thickness of 1.2 mm is used.

FIG. 12 is a diagram showing the tilt dependence of the NA of an objective lens when an optical disk having a substrate thickness of 0.6 mm is used.

FIG. 13 shows a cross-sectional profile of an actual pit.

FIG. 14 shows a relationship between an actual pit groove width and a model pit groove width.

FIG. 15 shows a relationship between an actual pit inclination angle and a model pit inclination angle.

FIG. 16 is a perspective view and a sectional view showing the structure of an optical disc according to one embodiment of the present invention.

FIG. 17 is a block diagram showing a configuration of an optical disc device according to one embodiment of the present invention.

FIG. 18 is a graph showing the relationship between the tilt of the optical disc and aberration.

FIG. 19 is a graph showing the relationship between the disk tilt angle and the window occupancy.

FIG. 20 is a graph showing a relationship between an allowable disk tilt angle, a substrate / wavelength, and a magnification.

FIG. 21 is a graph showing a relationship between an allowable disk tilt angle, a substrate / wavelength, and a magnification.

FIG. 22 is a diagram showing a configuration of an objective lens when a tilt detection mechanism is provided.

FIG. 23 illustrates a configuration of a photodetector having a tilt detection function.

FIG. 24 is a diagram showing a state where a track is scanned by three beams.

FIG. 25 is a block diagram of a crosstalk canceller.

[Explanation of symbols]

 Reference Signs List 10 pit 11 pit inner wall 12 pit bottom 13 pit cross section in radial direction 14 pit cross section in track direction 22 polarizing beam splitter 23 objective lens 25 optical disk 30, 31 beam spot

Claims (7)

[Claims] 1. An optical disk having a substrate on which information reproduced by irradiating a light beam through an objective lens is recorded as a row of pits at a predetermined track pitch, and a reflective layer formed on the substrate. In the above, when the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is set in a range of (0.72 to 0.8) α × (λ / NA) /1.14 μm. When the thickness of the optical disk is ds μm and the allowable tilt angle of the optical disk is θA, α = 3.617 × 10 ⁻¹⁴ × (θ _A ds / λ) ³ +1.248
An optical disk, represented by × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865, wherein an allowable inclination angle θA is 7 mrad or less. 2. An optical disk comprising a substrate on which information is recorded as a row of pits at a predetermined track pitch and a reflective layer formed on the substrate, an objective lens provided to face the optical disk, and the objective Means for irradiating the optical disc with a light beam via a lens, and means for detecting reflected light of the light beam illuminated on the optical disc by the irradiation means and reproducing information recorded on the optical disc. When the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is (0.72-0.8) α × (λ / NA) /1.14 μm. Assuming that the thickness of the optical disk is ds μm and the allowable tilt angle of the optical disk is θA, α = 3.617 × 10 ⁻¹⁴ × (θ _A ds / λ) ³ +1.248
An optical disc device represented by × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865, and having an allowable inclination angle θA of 7 mrad or less. 3. A pair of transparent substrates which face each other, and on which information reproduced by irradiation with a light beam via an objective lens is recorded as a row of pits at a predetermined track pitch. An optical disc having a pair of reflective layers formed on the opposing surfaces respectively, wherein the track pitch is (0.72 to 0.8) when the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA. ) Is set in the range of α × (λ / NA) /1.14 μm, where the thickness of the optical disk is ds μm and the allowable tilt angle of the optical disk is θA, α = 3.617 × 10 ⁻¹⁴ × (θ _A ds / λ ) ³ +1.248
An optical disk, represented by × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865, wherein the allowable inclination angle θA is 7 mrad or less. 4. An optical disk having a substrate on which information reproduced by irradiating a light beam through an objective lens is recorded as a row of pits at a predetermined track pitch, and a reflective layer formed on the substrate. In the above, when the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is (0.72 to 0.72).
0.8) α × (λ / NA) /1.14 μm, and each of the pits is (0.3-0.50) α ×
Upper width in the range (λ / NA) /1.14 μm and (0.2-0.32) α × (λ / NA) /1.14 μm
Is a trapezoidal pit having a lower width in the range of, the thickness of the optical disk is dsμm, and the allowable tilt angle of the optical disk is θA.
Then, α = 3.617 × 10 ⁻¹⁴ × (θ _A ds / λ) ³ +1.248
An optical disc characterized by the following formula: × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865. 5. An optical disk having a substrate on which information is recorded as a row of pits at a predetermined track pitch and a reflective layer formed on the substrate, an objective lens provided to face the optical disk, and the objective lens Means for irradiating the optical disc with a light beam through the optical disc, and means for detecting reflected light of the light beam applied to the optical disc by the irradiation means and reproducing information recorded on the optical disc. In the optical disk device, when the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is in the range of (0.72 to 0.8) α × (λ / NA) /1.14 μm. And each of the pits is (0.3-0.50) α × (λ / N
A) An upper width in the range of 1.14 μm and (0.2 to
0.32) These are pits having a trapezoidal cross section having a lower width in the range of α × (λ / NA) /1.14 μm. If the thickness of the optical disk is ds μm and the allowable tilt angle of the optical disk is θA, α = 3.617. × 10 ^-14 × (θ _A ds / λ) ³ +1.248
An optical disk device characterized by being determined by the formula: × 10 ^-9 × (θ _A ds / λ) ² +0.865. 6. An optical disc having a substrate on which information reproduced by irradiating a light beam via an objective lens is recorded as a row of pits at a predetermined track pitch, and a reflective layer formed on the substrate. In the above, when the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is (0.72 to 0.72).
0.8) α × (λ / NA) /1.14 μm, and each of the pits is (0.3 to 0.50) × (λ
/NA)/1.14 μm top width and (0.2
.About.0.32) .times. (. Lambda./NA)/1.14 .mu.m, and is a trapezoidal section pit having a lower width. When the thickness of the optical disk is ds .mu.m and the allowable tilt angle of the optical disk is .theta. × 10 ^-14 × (θ _A ds / λ) ³ +1.248
An optical disc characterized by the following formula: × 10 ⁻⁹ × (θ _A ds / λ) ² +0.865. 7. An optical disk having a substrate on which information is recorded as a row of pits at a predetermined track pitch, a reflective layer formed on the substrate, an objective lens provided to face the optical disk, and the objective Means for irradiating the optical disc with a light beam via a lens, and means for detecting reflected light of the light beam illuminated on the optical disc by the irradiation means and reproducing information recorded on the optical disc. When the wavelength of the light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is (0.72-0.8) α × (λ / NA) /1.14 μm. And each of the pits is (0.3 to 0.50) × (λ / N
A) An upper width in the range of 1.14 μm and (0.2 to
0.32) × (λ / NA) /1.14 μm, which is a trapezoidal pit having a lower width in the range. 10 ^-14 × (θ _A ds / λ) ³ +1.248
An optical disc device characterized by being obtained by the formula: × 10 ^-9 × (θ _A ds / λ) ² +0.865.