JP2006079825A - Optical disk medium and drive system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem in a conventional high density recording, wherein a transparent substrate is made thin and resistance to adhesion of a fingerprint and dust is poor and a cartridge is needed. <P>SOLUTION: A change in tilt of an optical disk medium over one circuit of each of tracks is set equal to or less than an allowable value, a change in tilt over the entire surface of the optical disk medium is permitted to be larger than the allowable value and only low frequency band tilt control is carried out by the drive system to realize recording and play back of the high density optical disk medium. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to an optical disc medium and a drive device that perform recording or reproduction by condensing laser light into a minute spot by a lens.

Optical disks such as CDs (Compact Disks) and DVDs (Digital Versatile Disks) have come to be widely used, but higher recording density is expected for high-definition video recording applications. In the current recording / reproducing configuration in which a minute spot of laser light is formed on an optical disk medium using an objective lens, the recording density, particularly the recording linear density in the track direction, is governed by the formed beam spot diameter. For this reason, the specification of the optical disk medium always specifies the wavelength of the light source laser that determines the beam spot diameter and the lens numerical aperture of the objective lens.

When the wavelength of the light source laser is λ [μm] and the numerical aperture of the objective lens is NA, the beam spot diameter is λ / NA (1)
Is proportional to The signal amplitude when reproduced with this beam spot diameter is recorded on the optical disk and varies depending on the period of the pits. FIG. 1 shows the relationship between the pit period and the reproduction signal amplitude. As the pit period decreases toward λ / (2 · NA), which is the wavelength of the optical cutoff of the objective lens, the reproduction signal amplitude decreases monotonously. Since the minimum pit length or space length that can be reproduced is considered to be half the optical cutoff wavelength,
0.25 · λ / NA (2)
It becomes. Assuming that the wavelength λ = 0.65 μm and the lens numerical aperture NA = 0.6 in the DVD standard conditions, about 0.27 μm is the cut-off pit length from the equation (2).

A conventional optical disc uses a pit length that is sufficiently longer than the pit length of this optical cut-off, and a DVD has a minimum pit length Lp of 0.4 μm, a minimum pit length Lp, a wavelength λ, and a lens numerical aperture. The relationship of NA is
Lp = 0.37 · λ / NA (3)
It has become.

In an optical disc system that records or reproduces digital data using an optical disc, generally, the original digital data signal sequence is not recorded as it is, but some encoding is performed. Since it is necessary to simultaneously detect the clock signal from the reproduction signal series, the low frequency component included in the reproduction signal is suppressed by limiting the continuation of 0 or 1, and the SNR is reduced by limiting the reproduction signal band. Improvements are being made.

As practical encoding, DVD standard 8/16 variable code or 1-7 code is used. In the 8/16 variable code, the original 8-bit data is converted into 16-bit code data, and the shortest code data bit length is limited to 3 bits. In the 1-7 code, the original 2-bit data is converted into 3-bit code data, and the shortest code data bit length is limited to 2 bits. This shortest code data bit length corresponds to the minimum pit length Lp.

In the 8/16 variable code, the shortest code data bit length is 1.5 with respect to the original 1-bit data length, and 1.33 in the 1-7 code. At first glance, since the shortest code data bit length, in other words, the minimum pit length Lp is longer in the 8/16 variable code, it seems that the 8/16 variable code is suitable for higher density in the encoding. However, the clock length of the code data is 0.5 with respect to the original 1-bit data length in the 8/16 variable code and is narrower than 0.67 of the 1-7 code, so that a determination error is likely to occur. As a result, in an actual optical disc system, it has been confirmed that the 1-7 code is more suitable for higher density. In recent proposals for higher density of optical disc media, the encoding is close to 1-7 code. The method is adopted. Therefore, in order to increase the density, the minimum pit length Lp is closer to the pit length that becomes the optical cutoff.

In order to increase the density of the optical disk medium, in general, it is very effective to shorten the wavelength λ of the laser light of the light source and to increase the lens numerical aperture NA of the objective lens. An optical disc specification (hereinafter, also simply referred to as “specification 1”) of about 23 GB with the same disc size is proposed (for example, see Non-Patent Document 1). In this specification 1, since the wavelength λ = 0.405 μm, the lens numerical aperture NA = 0.85, and the minimum pit length Lp = 0.16 μm, the minimum pit length Lp is
Lp = 0.336 · λ / NA (4)
It has become. Although the minimum pit length Lp determines the density in the track direction, the track pitch is also reduced by reducing the beam spot diameter in the same way as the recording linear density, so the recording capacity increases in the order of the square of the beam spot diameter. Can be expected.

However, what becomes a problem at this time is a margin reduction of a deviation in inclination between the lens optical axis and the optical disc vertical axis. In the case of an optical disk medium, a transparent substrate or a transparent cover layer (hereinafter, “transparent substrate” and “transparent cover layer” are simply referred to as “transparent substrate”) in order to protect the recording surface having the information recording track. Thus, the laser beam is condensed on the recording surface. If the vertical axis of the optical disk is tilted with respect to the optical axis of the lens, an aberration is generated by the transparent substrate, the beam spot diameter is enlarged on the optical disk medium, and the reproduction signal characteristics are deteriorated.

It is known that coma aberration is dominant in a region where the inclination is small. The amount of coma aberration generated is defined as follows: d [μm] is the thickness of the transparent substrate, n is the refractive index, and φ is the inclination amount.
d / 2 · (n ² −1) · n ² · sin φ · cos φ / (n ² −sin ² φ) ^5/2 · NA ³ / λ (5)
Is proportional to Multiplying this equation by an appropriate constant represents the mean square deviation (hereinafter also referred to as “rms value”) of wavefront aberration in the lens aperture. Hereinafter, in this specification, “coma aberration” is evaluated by “wavefront aberration”.

In the small range where the tilt amount φ is 1 deg or less, the following equation is obtained by approximation of a trigonometric function as the amount of wavefront aberration generated.
(N ² -1) / (2n ³ ) · d · NA ³ / λ · φ (6)
From this equation, it can be seen that the amount of wavefront aberration generated is proportional to the amount of inclination φ. FIG. 2 shows the rms value of the amount of wavefront aberration generated in the lens aperture by ray tracing. According to this figure, it can be seen that there is a proportional relationship between the rms value of the wavefront aberration amount and the inclination amount φ, and it is confirmed that the equation (6) obtained by the approximate analysis has a correlation with the rms value of the wavefront aberration amount. The

The result shown by the broken line in this figure is the characteristic under the conditions in the current DVD standard. For example, in the DVD-ROM standard, the inclination in the track orthogonal direction is defined as 0.4 deg or less, and the wavefront aberration amount is It can be seen that an rms value of 0.04 rmsλ or less is adopted as a practical allowable value.

According to this equation (6), when the wavelength λ is changed from 0.65 to 0.405 μm and the lens numerical aperture NA is changed from 0.6 to 0.85, the tilt amount φ that gives the same wavefront aberration amount is about 0. It can be seen that it decreases by 22 times. This indicates that the slope margin during recording / reproduction is reduced by that amount.

It can also be seen from equation (6) that the amount of wavefront aberration is proportional to the thickness d of the transparent substrate. Therefore, in the proposal for increasing the density of the optical disc medium of specification 1, in order to compensate for the decrease in the margin, the thickness d of the transparent substrate is reduced from 600 μm of the DVD standard to 100 μm, which is similar to the DVD standard. An inclination margin is secured.

FIG. 3 shows the relationship between the amount of tilt and the error rate. When the error rate of the reproduction signal is taken on the vertical axis and the amount of inclination is taken on the horizontal axis, the error rate draws a curve as shown in FIG. Aberrations occur in the laser light transmitted through the transparent substrate due to the tilt, and the beam spot diameter is enlarged or deformed. For this reason, a read error occurs due to a decrease in reproduction signal amplitude or waveform distortion, and the error rate increases. In general, since the occurrence of errors increases at an acceleration from a region exceeding a certain amount of aberration, the characteristic of the curve as shown in this figure is shown.

In an actual apparatus, the inclination margin is defined with an allowable predetermined error rate such as 1 × 10 ⁻⁴ and a width as indicated by an arrow.

When the thickness d decreases, the change in the curve becomes loose as indicated by the broken line, and the inclination margin increases. In the optical disk specification of about 23 GB using the blue laser as the light source proposed in the specification 1, the thickness d of the transparent substrate is as thin as 100 μm, and the improved characteristic is the allowable maximum value of the inclination of about 0.7 deg. That is, it has been confirmed that the allowable width of the inclination is about ± 0.7 deg. This is almost equivalent to the allowable range of inclination ± 0.6 deg obtained from the error rate characteristics of the DVD standard. In the DVD standard, a standard value of ± 0.4 deg, which is a value having a margin, is determined based on actual measurement in consideration of simultaneous occurrence with other error rate increase factors such as defocus as an allowable range of tilt. Therefore, even in this specification 1, the same standard value can be set.

If the overlap with other factors that increase the error rate is estimated to be small, it is assumed that the tilt amount is 0.5 deg and the rms value of wavefront aberration is about 0.05 rmsλ or less, which is practically acceptable under DVD conditions. When this is applied to the equation (6), the limit condition uses a predetermined value K,
K = (n ² −1) / (2n ³ ) · d · NA ³ /λ·(0.5) (7)
It is expressed. If the conditions of the DVD standard are applied and n = 1.58, NA = 0.6, λ = 0.65 μm, and d = 600 μm, K = 19. Therefore, when this value of K is used, the allowable condition for the inclination θ of the optical disk is
19 ≧ (n ² −1) / (2n ³ ) · d · NA ³ / λ · θ (8)
θ ≦ 38 · λ / (NA ³ · d) · n ³ / (n ² −1) [deg] (9)
Can also be expressed. Even if NA = 0.85, λ = 0.405 μm, d = 100 μm, and φ = 0.6 deg, K is almost the same value, so it can be seen that the allowable value of inclination is governed by the amount of aberration generated, Equation (9) is an equation that gives a generally acceptable measure of inclination.

Keith Shep and 7 others, “22 GB DVR Disc Format Overview and Evaluation of THE 22.5 GB DVR DISC”, International Conference on Optical Memory 2000 Technical Symposium 2000, Technical Digest), Japan Society of Applied Physics, September 2000, p. 210-211

However, when such a thin transparent substrate is used, the distance between the recording surface and the transparent substrate surface becomes short, and there is a problem that data errors are likely to occur due to fingerprints and dust.

FIG. 4 shows the relationship between the beam diameter a of the laser beam on the transparent substrate surface and the convergent beam shape. When a condensing spot is formed on the recording surface 1, the laser beam converges in the transparent substrate 2 having a thickness d along an optical path such as a solid line. The lens numerical aperture NA is a value indicating the convergence of the laser light in the air, and is related to the optical path in the transparent substrate 2 having the refractive index n as follows.
NA = sin β (10)
n · sin β ′ = sin β (11)
a = 2 · d · tan β ′ (12)
From these equations, the beam diameter a on the surface of the transparent substrate 2 is obtained by the following equation.
a = 2 · d · tan (sin ⁻¹ (NA / n)) (13)
FIG. 5 shows the values of the thickness d of the transparent substrate and the beam diameter a of the surface of the transparent substrate, obtained when the refractive index n = 1.5 and the lens numerical aperture NA = 0.6 and 0.85.

In the conventional DVD, since the thickness d of the transparent substrate is 600 μm and the lens numerical aperture NA is 0.60, the beam diameter on the surface of the transparent substrate is 500 μm or more as shown by black stars in the figure. On the other hand, under the high density condition proposed in the specification 1, since the transparent substrate has a thickness of 100 μm and the lens numerical aperture NA is 0.85, the beam diameter on the surface of the transparent substrate is about 150 μm as shown by the white star in the figure. It turns out that it becomes small.

For example, in consideration of fingerprint attachment that is most likely to occur in practice, the width of one stripe of a fingerprint having a stripe shape is often around 300 μm. Therefore, when the beam diameter on the surface of the transparent substrate is smaller than this, it can be easily imagined that the influence becomes large.

Therefore, if the beam diameter is as small as 150 μm on the surface of the transparent substrate, it is difficult to put it into practical use unless a cartridge or the like that protects the optical disk medium is easily affected by fingerprints and dust.

Experiments have shown that the beam diameter on the surface of the transparent substrate must be at least 400 μm in order to obtain practical fingerprint resistance and dust resistance comparable to those of conventional DVDs. If this is applied to equation (13),
d ≧ 200 / tan (sin ⁻¹ (NA / n)) (14)
Thus, it can be seen that when the lens numerical aperture NA is 0.85, a transparent substrate thickness d of 290 μm or more is required.

However, when the thickness d of the transparent substrate is increased from the equation (9), the allowable amount of inclination is reduced, and a contradiction arises in that a material that satisfies both characteristics cannot be obtained.

The present invention has been made in view of such a situation, and the object of the present invention is to use a transparent substrate having the same thickness as the conventional one without causing the problems of fingerprint resistance and dust resistance as described above. Another object of the present invention is to provide an optical disk medium and a drive device that enable high-density recording.

One embodiment of the present invention relates to an optical disc medium. In this optical disk medium, concentric or spiral information recording tracks are formed, and a disk-shaped optical disk medium for recording or reproducing information by irradiating the information recording tracks with a laser beam via a transparent substrate or a transparent cover layer The numerical aperture of the objective lens through which the laser beam to be irradiated onto the optical disk medium is transmitted is NA, the wavelength of the laser beam is λ [μm], the refractive index of the transparent substrate or the transparent cover layer is n, the transparent substrate or When the thickness of the transparent cover layer is d [μm] and the shortest pit length of the pit formed on the information recording track is Lp [μm], d ≧ 200 / tan (sin ⁻¹ (NA / n)) and Lp ≦ 0.35 · λ / NA is satisfied, the change in the inclination of the surface of the optical disk medium in one round of each information recording track is set to a first predetermined value or less, and the optical disk The slope changes in body entire set lower than a second predetermined value, to increase the second predetermined value than the first predetermined value. The shortest pit length of the pits formed on the information recording track is more preferably Lp ≦ 0.33 · λ / NA.

Another aspect of the present invention relates to an optical disc medium. In this optical disk medium, concentric or spiral information recording tracks are formed, and a disk-shaped optical disk medium for recording or reproducing information by irradiating the information recording tracks with a laser beam via a transparent substrate or a transparent cover layer The numerical aperture of the objective lens through which the laser beam to be irradiated onto the optical disk medium is transmitted is NA, the wavelength of the laser beam is λ [μm], the refractive index of the transparent substrate or the transparent cover layer is n, the transparent substrate or When the thickness of the transparent cover layer is d [μm] and the shortest pit length of pits formed on the information recording track is Lp [μm], d ≧ 200 / tan (sin ⁻¹ (NA / n)) and , Lp ≦ 0.35 · λ / NA, the change in the inclination of the surface of the optical disk medium in one round of each information recording track is set to be equal to or less than a first predetermined value, and the optical disk The slope changes in the medium over the entire surface to a first predetermined value or more.

Another aspect of the present invention relates to an optical disc medium. In this optical disk medium, concentric or spiral information recording tracks are formed, and a disk-shaped optical disk medium for recording or reproducing information by irradiating the information recording tracks with a laser beam via a transparent substrate or a transparent cover layer The numerical aperture of the objective lens through which the laser beam to be irradiated onto the optical disk medium is transmitted is NA, the wavelength of the laser beam is λ [μm], the refractive index of the transparent substrate or the transparent cover layer is n, the transparent substrate or When the thickness of the transparent cover layer is d [μm] and the shortest pit length of the pit formed on the information recording track is Lp [μm], d ≧ 200 / tan (sin ⁻¹ (NA / n)) and The relationship of Lp ≦ 0.35 · λ / NA is satisfied, and the change in the inclination of the surface of the optical disc medium in the track orthogonal direction in one round of each information recording track is equal to or less than the first predetermined value. The inclination change in the track direction over the entire surface of the optical disc medium is set to a second predetermined value or less, and the change in the inclination in the track orthogonal direction over the entire surface of the optical disk medium is set to a third predetermined value or less. The value is increased, and the third predetermined value is set larger than the second predetermined value.

Another aspect of the present invention relates to an optical disc medium. This optical disk medium is a disk-shaped optical disk medium on which concentric or spiral information recording tracks are formed, and information recording tracks are irradiated with laser light via a transparent substrate or transparent cover layer to record or reproduce information. The numerical aperture of the objective lens through which the laser beam to be irradiated onto the optical disk medium is transmitted is NA, the wavelength of the laser beam is λ [μm], the refractive index of the transparent substrate or the transparent cover layer is n, the transparent substrate or When the thickness of the transparent cover layer is d [μm] and the shortest pit length of the pit formed on the information recording track is Lp [μm], d ≧ 200 / tan (sin ⁻¹ (NA / n)) Yes, the relationship of Lp ≦ 0.35 · λ / NA is satisfied, and the change in inclination in the track orthogonal direction of the surface of the optical disk medium in one round of each information recording track is less than or equal to the first predetermined value. And then, a change in inclination of the direction perpendicular to the track in the optical disk medium over the entire surface as a first predetermined value or more, the change in slope in the track direction equal to or less than a second predetermined value in the optical disk medium over the entire surface.

Here, examples of the material of the transparent substrate or the transparent cover layer include polymer plastic materials such as polycarbonate and polymethyl methacrylate, and the refractive index of these materials is generally about 1.5 to 1.6. As the structure of the optical disk medium, for example, a structure in which a phase change recording material is laminated as a recording layer on the above-described transparent substrate can be adopted. Examples of the recording layer include a structure in which ZnS—SiO ₂ , Ge ₂ Sb ₂ Te ₅ , ZnS—SiO ₂ , and Al—Ti are stacked in this order from the transparent substrate.

By satisfying these conditions, the reproduction error rate can be suppressed to an allowable range. In particular, this is effective for increasing the density of the optical disk medium. The first predetermined value may be 38 · λ / (NA ³ · d) · n ³ / (n ² -1) [deg] or less. By changing the inclination change of the optical disk medium surface in one round of each information recording track to 38 · λ / (NA ³ · d) · n ³ / (n ² -1) [deg], the reproduction error rate can be further increased. Reduction can be achieved.

Another aspect of the present invention also relates to an optical disc medium. This optical disk medium is a disk-shaped optical disk medium in which concentric or spiral information recording tracks are formed, and information recording tracks are irradiated with laser light via a transparent substrate or transparent cover layer to record and reproduce information. The laser to be irradiated to the optical disk medium with a change in the inclination of the surface of the optical disk medium in one round of each information recording track being set to a first predetermined value or less and a change in inclination over the entire surface of the optical disk medium being set to a second predetermined value or less. The numerical aperture of the objective lens through which light passes is NA, the wavelength of the laser light is λ [μm], the thickness of the transparent substrate or transparent cover layer is d [μm], its refractive index is n, and the shortest pit of the information recording track When the length is Lp [μm], the first predetermined value is θa [deg], and the second predetermined value is θb [deg], Lp ≦ 0.35 · λ / NA, θa ≦ 38 · λ / ( ^{A 3 · d) · n 3} / (n 2 -1), d ≧ 200 / tan (sin -1 (NA / n)), satisfies the relationship .theta.a <.theta.b.

Another aspect of the present invention also relates to an optical disc medium. This optical disc medium is a disc-shaped optical disc medium in which concentric or spiral information recording tracks are formed, and information recording tracks are irradiated with laser light through a transparent substrate or transparent cover layer to record and reproduce information. The change in the inclination of the surface of the optical disk medium in one round of each information recording track is set to a first predetermined value or less, the wavelength of the laser beam for recording / reproducing is λ [μm], and the laser light to be irradiated to the optical disk medium is transmitted The numerical aperture of the objective lens is NA, the thickness of the transparent substrate or transparent cover layer is d [μm], its refractive index is n, the shortest pit length of the information recording track is Lp [μm], and the first predetermined value is When θa [deg], Lp ≦ 0.35 · λ / NA, θa ≦ 38 · λ / (NA ³ · d) · n ³ / (n ² −1), d ≧ 200 / tan (sin ^{− 1} (N / N)), satisfying the relationship. In addition, the change in inclination over the entire surface of the optical disk medium may be set to a first predetermined value or more.

Another aspect of the present invention also relates to an optical disc medium. This optical disk medium is a disk-shaped optical disk medium in which concentric or spiral information recording tracks are formed, and information recording tracks are irradiated with laser light via a transparent substrate or transparent cover layer to record and reproduce information. The change in the inclination of the surface of the optical disk medium in the track orthogonal direction in one round of each information recording track is set to a first predetermined value or less, and the change in the track direction over the entire surface of the optical disk medium is set to a second predetermined value or less. The change in tilt in the direction perpendicular to the track over the entire surface of the medium is set to a third predetermined value or less, the wavelength of the laser beam for recording / reproduction is λ [μm], and the lens numerical aperture of the objective lens through which the laser beam to be irradiated to the optical disk medium NA, the thickness of the transparent substrate or the transparent cover layer is d [μm], its refractive index is n, and the shortest pit length of the information recording track is Lp [ m], when the first predetermined value is θa [deg], the second predetermined value is θb [deg], and the third predetermined value is θc [deg], Lp ≦ 0.35 · λ / NA, θa ≦ 38 · λ / (NA ³ · d) · n ³ / (n ² −1), θb ≦ 38 · λ / (NA ³ · d) · n ³ / (n ² −1), θc ≦ 38 · The relationship of λ / (NA ³ · d) · n ³ / (n ² −1), θa ≦ θc, θb ≦ θc, d ≧ 200 / tan (sin ⁻¹ (NA / n)) is satisfied.

For example, for an optical disk medium deformed into a parabolic shape, an increase in manufacturing cost of the optical disk medium can be suppressed by allowing the deformation. This is particularly effective when a value equal to or less than 38 · λ / (NA ³ · d) · n ³ / (n ² -1) [deg] is required as the first predetermined value.

The wavelength of the recording / reproducing laser beam may be 0.40 μm or more and 0.42 μm or less, and the lens numerical aperture may be 0.6 or more. Even when the wavelength of the optical laser is changed from the conventional 0.65 μm to the above range, it is not necessary to increase the lens numerical aperture of the objective lens, and the degree of freedom of design on the drive device side is increased. Further, the first predetermined value θa may be not less than 0.1 deg and less than 0.25 deg. As a result, the reproduction error rate can be further reduced.

The second predetermined value may be 0.1 deg or more and less than 0.25 deg. By setting the inclination change in this range, it is possible to keep the reproduction error rate within an allowable range while suppressing the manufacturing cost of the optical disk medium.

Another embodiment of the present invention relates to a drive apparatus. This drive device is a drive device for recording or reproducing the optical disk medium, and has a function of compensating for and controlling the aberration generated by the inclination between the objective lens included in the drive apparatus and the optical disk medium. . Further, the control band of the function may be set lower than the rotation period of the optical disk medium.

In a drive device that records or reproduces an optical disk medium, a PRML (Partial Response Maximum Likelihood) method may be used for signal reproduction. This PRML method combines a waveform shaping technique related to multi-level equalization called partial response and a maximum likelihood decoding method that Viterbi decodes the most probable sequence using correlation between signal sequences due to waveform interference. Yes, it is effective for stably reproducing high-density recording signals.

By configuring the drive device as described above, it is possible to reduce the reproduction error rate while suppressing an increase in the manufacturing cost of the drive device.

As described above, according to the present invention, the thickness of the transparent substrate or transparent cover layer provided on the optical disk medium is set to a thickness that is not affected by fingerprints or dust. Therefore, it is possible to eliminate the factor that increases the manufacturing cost of the optical disk medium.

It should be noted that any combination of the above-described constituent elements and a conversion of the expression of the present invention between a method, an apparatus, and the like are also effective as an aspect of the present invention.

ADVANTAGE OF THE INVENTION According to this invention, the optical disk medium and drive apparatus which can perform a high-density recording by the same thickness as the conventional which is hard to receive to a transparent substrate or a transparent cover by a fingerprint, dust, etc. are realizable.

Hereinafter, preferred embodiments of the present invention will be described. When the thickness d of the transparent substrate is not reduced, it is difficult to increase the lens numerical aperture NA unlimitedly due to the inclination limitation determined by the above-described equation (5). In particular, when the wavelength of the laser beam is changed from 0.65 μm for red to 0.4 to 0.42 μm, which is put to practical use as a blue laser, the tilt margin is reduced by itself. This wavelength band is the only wavelength band currently in practical use for direct oscillation of a semiconductor laser, and is practically effective for increasing the density of optical disk media.

For example, when the wavelength λ = 0.405 μm, the lens numerical aperture NA = 0.65, and the thickness d = 600 μm, the amount of wavefront aberration with respect to the tilt exhibits the characteristics shown by the solid line in FIG. Compared with the broken line of the DVD condition, the value of the wavefront aberration is approximately doubled at the same tilt angle.

When the wavelength λ and the lens numerical aperture NA are changed in this way, the decrease in the minimum pit length Lp represented by the expression (3) is about 1.7 times, and even when the track density is increased, the recording capacity is converted. Thus, the capacity increases about 3 times the DVD, and there is a capacity of less than 14 GB. As a practically meaningful capacity, 15 GB is said to be one standard. Therefore, if the expression (3) is set to about Lp = 0.34 · λ / NA at some sacrifice of the recording / reproducing margin as compared with the DVD, the recording linear density is about 10% from the ratio of 0.37: 0.34. Therefore, 15 GB can be realized as the capacity.

Furthermore, with the aim of securing a recording margin and realizing a high recording linear density, an attempt was made to reduce the error rate using the PRML method described above.

FIG. 6 shows an example of an equalized waveform of a partial response when there is an impulse input. When a simple binary slice is performed, binary waveform equalization is performed such that the value at a specific data clock point is 1 and 0 at other clock points. However, when the recording density increases and the optical cutoff condition is approached, and the difference between the amplitude of the reproduction signal from the short mark and the amplitude of the reproduction signal from the long mark becomes large, simple binary equalization becomes difficult. Therefore, it becomes easier to equalize to a waveform having values at several data clock points. In this figure, it is a waveform in the case of performing PR (1, 2, 2, 1) equalization, and is equalized to have a value of “1, 2, 2, 1” at four consecutive data clock points. The

FIG. 7 shows an example in which this PR (1, 2, 2, 1) equalization is performed with a random signal waveform. It can be seen that one clock point has seven values from level 0 to level 6 as indicated by the level of the arrow. Therefore, the original signal sequence is reproduced by performing 7-value determination instead of simple binary determination of 0 and 1.

Viterbi decoding is the most reliable by checking the data series in multiple clock periods using the limit of level transitions at adjacent clock points such as “no change from level 0 to level 6”. This is a method of reproducing the original signal series.

FIG. 9 shows an example of an apparatus configuration for recording / reproducing the optical disk medium 3. A signal is reproduced by the optical head 4 from the optical disk medium 3 set on the spindle 5. The reproduction signal is decoded and output by a PR (Partial Response) equalization circuit 13 that performs signal processing using a partial response equalization algorithm and an ML (Maximum Likelihood) circuit 14 that performs signal processing using a maximum likelihood decoding algorithm. The reproduction signal circuits such as the PR equalization circuit 13 and the ML circuit 14 can be easily created by LSI (Large Scale Integration).

Using this technology, the error rate degradation of the conventional method of discriminating the reproduced signal with binary values exceeds the limit at which it begins rapidly,
Lp ≦ 0.35 · λ / NA (15)
In this case, it was confirmed that the error rate can satisfy the practical characteristics comparable to the conventional one.

FIG. 10 shows the relationship between the shortest pit length and the reproduction error rate when the 1-7 encoding method is used. The vertical axis assumes a log scale. The permissible value for the bit error rate of the raw signal is about 1 × 10 ^−4, but in the conventional method, the error rate is rapidly increased when the shortest pit length Lp exceeds 0.35 · (λ / NA) μm. Begins to deteriorate and crosses the allowable value in the vicinity of 0.30 · (λ / NA) μm.

In the case of wavelength λ = 0.405 μm and lens numerical aperture NA = 0.65, error rate deterioration starts at the shortest pit length Lp = 0.22 μm and reaches the limit at 0.19 μm.

On the other hand, when the PRML method is used, it can be seen that an error rate exceeding the limit and below the allowable value can be realized. The shortest pit length Lp crossing the allowable value is about 0.23 · (λ / NA) μm. In FIG. 10, the partial response equalization when the PRML method is used is optimized under each density condition. When the shortest pit length Lp is around 0.3 · (λ / NA) μm, PR (1, 2,2,1) equalization, PR (1,2,2,2,1) equalization was used around 0.25 · (λ / NA) μm.

In FIG. 10, it is understood that when the PRML method is used, an error rate that sufficiently satisfies the allowable value can be realized even when the shortest pit length Lp is 0.25 · (λ / NA) μm, which is the optical cutoff condition. This is due to the characteristic of PR (1, 2, 2, 2, 1) equalization. FIG. 8 shows eye pattern waveforms when PR (1, 2, 2, 2, 1) equalization is performed. As indicated by the arrows, the equalization level is 9 levels from level 0 to level 8. At this time, it can be seen that in the waveform near level 4, which is the center, the eye is hardly open. In other words, when PR (1, 2, 2, 2, 1) equalization is used, the signal from the pit with the minimum period has an amplitude of 0 and has an intermediate reflectance between “with pit” and “without pit”. We are only assuming that. Therefore, unlike the conventional method, the S / N of the signal from the long pit is the dominant factor without depending on the S / N of the signal from the short pit, so the shortest pit length exceeds the optical cutoff condition. Even in this case, an error rate below the allowable value can be realized.

By increasing the recording linear density by the PRML method, a capacity of 15 GB or more can be realized even with a lens numerical aperture NA = 0.65. Further, by improving the S / N characteristics of the optical disk medium, it is possible to realize a capacity of 20 GB or more and the same capacity as the optical disk standard proposal using the lens numerical aperture NA = 0.85 of the specification 1.

On the other hand, unless the recording linear density is improved to the range satisfying the formula (15), a sufficient recording capacity cannot be realized, and the practical value is lowered.

Here, the problem is the deterioration of the tilt characteristic that occurs when a thick transparent substrate is used. FIG. 11 shows a comparison of slope characteristics under three types of conditions. Compared to the characteristics of signal reproduction of the conventional method at low density indicated by the broken line, the characteristics when the wavelength λ indicated by the alternate long and short dash line is shortened and the lens numerical aperture NA is increased and the density is increased are optimum slopes. The bottom value of the error rate rises and the characteristic slope becomes tighter. In this state, when the PRML method is applied, the optimum value of the error rate can be reduced to the same level as the conventional method as shown by the characteristic indicated by the solid line. However, the slope characteristic at this time does not change much.

That is, from this characteristic, it has been clarified that the characteristic with respect to the tilt is mainly governed by the optical aberration expressed by the equation (6), and does not depend much on the reproduction method. By saying that “the tilt characteristic is determined due to optical aberration”, this tilt characteristic is the same even if the encoding method is an 8/16 variable code used in a DVD other than the 1-7 code. It is easy to imagine a trend.

This is the same result as that the allowable value of the inclination is governed by the amount of aberration as shown in the previously obtained equation (8). Therefore, if NA = 0.65, λ = 0.405 μm, and d = 600 μm, the allowable value is 0.245 deg or less from Equation (8).

In an actual drive apparatus, the tilt characteristic becomes a problem when the optical disk medium is set on a turntable provided in the drive apparatus and rotated, the optical disk medium is perpendicular to the optical axis of the objective lens for recording and reproduction. This is because the inclination partially changes due to warpage of the optical disk substrate or the like.

In the conventional optical disc medium standard, sufficient characteristics are ensured by defining the amount of change below the margin value of the slope. For example, in the DVD-ROM, the tilt margin can be ensured to be 0.5 deg or more, whereas the change in the tilt of the optical disk medium is specified to be 0.4 deg or less, and the wavefront aberration is specified to be 0.04 rmsλ or less. Even if included, sufficient characteristics were secured.

On the other hand, in order to change the wavelength λ targeted by the present invention from 0.65 μm to 0.405 μm and the lens numerical aperture NA from 0.6 to 0.65, and to secure a practical capacity, the equation (15) When the density is increased as in (1), the tilt margin is reduced to about half that of the conventional DVD, so that the tilt of the same optical disk medium as that of the DVD cannot be allowed. The amount of inclination at which the wavefront aberration becomes 0.04 rmsλ is 0.2 deg or less from FIG. Even if the allowable value from the equation (8), where the allowable aberration value is estimated to be 0.05 rmsλ, is 0.25 deg or less, which is stricter than DVD.

As one method for solving the above-mentioned problem, it may be possible to tighten the regulation of the tilt change, but the improvement in the manufacturing accuracy of the optical disc medium and the increase in defective products due to inspection cause the cost increase. It's not a good idea.

As another method, there is a method in which the tilt of the objective lens is changed on the drive device side in accordance with the change of the tilt of the optical disk medium, thereby reducing the relative tilt and preventing the deterioration of the signal reproduction characteristics.

However, in order to follow the tilt that changes due to the rotation of the optical disk medium, servo control with a high band equal to or higher than the rotation speed is required for the tilt of the objective lens. Some devices have a high rotational speed of 100 Hz or more. If a servo that covers such a band is realized, the cost of the drive device will be greatly increased.

Therefore, in the present invention, as a method in which neither the cost of the optical disk medium nor the cost of the drive device is changed so much, the lens inclination is made to follow only a slow inclination change during recording or reproduction of the optical disk medium. We propose a method that can achieve high-density characteristics.

That is, the change in the inclination of the spiral or concentric circular track provided on the disk-shaped optical disk medium is defined to be a predetermined margin value or less, and the inclination change over the entire surface of the optical disk medium is a predetermined margin value or more. However, on the drive side, the tilt of the lens is compensated by controlling it in accordance with a component with a slow tilt change of the optical disk medium.

Normally, when a recording signal on an optical disk medium is reproduced, signal reading is performed by causing a light beam spot to follow the track direction. Since the information recording track is formed in a spiral shape, in the track following operation, the circumferential position changes according to the rotation speed, but the radial position changes only by the track pitch in one rotation. This is a slight change. For example, when a tracking operation is performed with a linear velocity v = 5 m / s on a track having a track pitch P = 0.5 μm at a position where the radius r = 30 mm, the moving speed v ′ in the radial direction is v ′ = v × P. / (2πr), which is only 13 μm / s. Of course, in the access operation, the spot may be moved in the radial direction at a high speed, but at this time, no signal is read, so no problem occurs even if the inclination is slightly increased.

Therefore, a realistic optical disc system is realized by limiting only the amount of change in tilt in one round of the optical disc and compensating for the tilt change when moving in the radial direction by tilt control in a relatively slow operating band. It becomes possible.

As a method of tilt control, for example, a band pass filter that transmits only a low frequency component of a detected tilt change signal is exemplified.

Since the amount of change in the inclination of one rotation is limited to an allowable value or less, the cut-off frequency on the high frequency side of the bandpass filter can be set to the rotation frequency or less.

As can be seen from the previous calculation example, the moving speed ratio in the track direction and the radial direction perpendicular to the track are different by 5 digits or more. Therefore, the cut-off frequency on the high frequency side of the bandpass filter can sufficiently follow the speed of change in the radial direction as long as a characteristic that follows the average value in one rotation of the linear velocity is obtained. Therefore, sufficient characteristics can be obtained with one digit or less of the rotation component. Therefore, even if the rotation speed is 100 Hz, the cut-off frequency on the high frequency side of the bandpass filter can be 10 Hz or less, which is sufficiently realized at low cost. Possible value.

In the present invention, like the optical disk medium 3 shown in FIG. 12, in the disk-shaped optical disk medium 3, recording tracks are formed spirally or concentrically with respect to the center of rotation. The change in the inclination of the recording track when it makes one round is defined to be equal to or less than a predetermined margin value as indicated by arrows indicated by solid lines or broken lines. That is, an allowable inclination margin value with a margin is obtained from the inclination characteristic of the error rate, and a predetermined value θ of the inclination change is determined. When viewed over the entire surface of the optical disc medium 3, a change in the inclination between the inner radius side track having a small radius and the outer radius side track having a large radius can be allowed even if it is equal to or larger than θ.

Although it is possible to limit the tilt tolerance on the entire surface of the optical disc medium 3, the tilt compensation range on the drive device side cannot be made infinite in practice, so the tilt amount on the entire surface of the optical disc medium 3 is also constant. It is desirable to limit to below the value.

In this case, it is a feature of the present invention that θa <θb is set, where the change in the inclination of one round of the track is the first allowable predetermined value θa and the change in the inclination of the entire surface of the optical disk medium is the second allowable predetermined value θb.

In practice, the second allowable predetermined value θb may be set to a value of about 0.4 deg, which is the same as that of a conventional DVD.

FIG. 13 shows the relationship between the amount of inclination and the inclination characteristic of the reproduction error rate. The fluctuation range θ of one round is limited to a range in which the error rate is less than or equal to an allowable value, and allows the fluctuation of the entire surface of the optical disk medium to be large. In the drive device, by controlling the lens optical axis to the average value of the inclination of the circumference of each track, an error rate below the allowable value can always be realized. The fluctuation range of the entire surface of the optical disk medium shown in FIG. 13 indicates a change in the inclination of the lens optical axis and the vertical axis of the optical disk medium when the lens optical axis is not controlled. Will be limited.

Even in the conventional drive device, there has been an example in which the lens tilt is controlled in a control band lower than the rotational speed. However, in this case, the purpose is to cope with a poor optical disc medium exceeding the original standard to some extent, and there is no provision for how to change the tilt on the optical disc medium side. For this reason, there is an optical disk medium in which the inclination greatly changes in one turn, and it has not been able to cope with it sufficiently.

When the change in the inclination in one track round is smaller than the change in the inclination of the surface of the optical disk medium, the shape of the optical disk medium becomes a parabolic shape as shown in FIG. In this structure, the amount of change in the circumferential direction is small, but the amount of change in the radial direction is large.

On the other hand, when the amount of change in the opposite circumferential direction is large but the amount of change in the radial direction is small, the shape of the optical disk medium is a potato chip type as shown in FIG. In the present embodiment, the parabolic type deformation is allowed, so that a high-density optical disk medium can be realized without causing a large cost increase in the production of the optical disk medium.

In manufacturing method 1 used in conventional DVDs and the like, when two optical disk substrates are bonded together, an adhesive is applied to the bonded surfaces, and then flat plates are pressed from both sides to eliminate warping and bond I was trying to do it. In this way, the tilt amount on the entire surface of the optical disk medium can be suppressed to a certain extent, but the parabolic type of distortion remains after the escape field such as distortion that the optical disk substrate has becomes indefinite and separated from the flat plate. And potato chip type deformations had occurred.

In the present embodiment that allows a certain degree of parabolic deformation to the manufacturing method 1, two optical disk substrates may be simply brought into close contact with each other without being pressed with a flat plate or the like at the time of bonding. A manufacturing method in which two optical disk substrates are simply brought into close contact with each other is also referred to as manufacturing method 2. As a result of actually manufacturing the optical disk medium by the manufacturing method 2, non-uniform distortion does not remain in the optical disk substrate, so that the distortion is uniform and the deformation generated as the optical disk medium is only a parabolic type. Was confirmed.

Assuming that the predetermined value θ of the tilt change is θ <0.25 deg, the predetermined value θ is an allowable value of about 60% with respect to the conventional DVD tilt standard, and the current optical disk medium manufacturing technology has a very high cost. It is feasible enough without incurring a rise.

A smaller value that can realize the predetermined value θ of the inclination change is desirable, but considering the cost of the optical disk medium, a number of 0.1 deg or more is appropriate. As a result of actually making a prototype of the optical disk medium, the possibility of mass production in the range of 0.1 to 0.25 deg as the predetermined value of the tilt change was confirmed.

By the way, generally speaking, the inclination is considered to be divided into the inclination characteristics of the track direction and the track orthogonal direction. However, in the above description, the inclination direction is not particularly defined. This is because the allowable value of tilt is determined by the occurrence of wavefront aberration, so that the amount of wavefront aberration generated is a constant value regardless of the tilt direction, and both tilt characteristics are substantially equal.

However, it is somewhat complicated to control the tilt in the two directions with the drive device. Therefore, in the following description, an embodiment will be described in which the tilt regulation of the optical disk medium is defined separately for the track direction and the track orthogonal direction.

The inclination in the track direction is the inclination in the circumferential direction of the optical disk, and since it is easier to make the inclination of the optical disk medium smaller than the radial direction, which is generally the track orthogonal direction, 0.15 deg in the conventional DVD standard and 0.4 deg in the track orthogonal direction. The standard value is less than half of the standard value.

Therefore, only in the track orthogonal direction, the inclination change in one round is set to be equal to or less than the first predetermined value θ1 within the margin range, the change in inclination of the entire surface of the optical disk medium is set to be equal to or less than the second predetermined value θ2, and the track direction is the third over the entire surface of the optical disk medium. The predetermined value θ3 or less. θ1 <θ2 and θ3 <θ2 are characteristic features.

In this case, since there is no need to control the tilt in the track direction on the drive device side, there is an advantage that only the tilt control in the track orthogonal direction is sufficient. Of course, it is also possible to define the optical disk medium under the conditions of only the first predetermined value θ1 and the third predetermined value θ3 without defining the second predetermined value θ2. In this case, a wide range of tilt amount correction is required on the apparatus side.

If the inclination margins in the track direction and the track orthogonal direction are substantially equal, the first predetermined value θ1 and the third predetermined value θ3 may be the same, or the third predetermined value θ3 in accordance with the capability of manufacturing the optical disc medium. The first predetermined value θ1 may be set loosely. In any case, in order to satisfy the reproduction characteristics, it is desirable that the predetermined value θ satisfies the equation (8) that gives the limit amount of wavefront aberration.

The same applies to the optical disk medium assumed in the above description of the embodiment, whether it is a ROM medium in which embossed pits are formed in advance or a recordable medium that can be additionally written or rewritten. Also, as a track structure, one recorded on the groove and one recorded on both the land and the groove are conceivable.

Further, the invention described in this embodiment can also be applied to a recording layer having a multilayer structure of two or more layers. In the case of multiple layers, only the assumed thickness d of the transparent substrate changes depending on the layer.

FIG. 15 shows an outline of a configuration diagram of the drive device 6 for reproducing or recording the above-described optical disk medium. An optical disk medium 3 is installed on a spindle 5 having a turntable 11 and rotates. An objective lens 12 is installed in the optical head 4 and a minute spot is formed on the recording surface. In FIG. 15, the tilt signal detected from the optical head 4 is controlled by controlling the tilt drive mechanism 7 via the bandpass filter 15 and the tilt drive circuit 16 and changing the tilt as indicated by the arrow. Yes.

As a mechanism for compensating for the tilt, the carriage that accesses the optical head 4 in the radial direction can be tilted, or only the optical head 4 installed on the carriage can be tilted. Further, optical control such as tilting only the objective lens 12 or generating wavefront aberration to compensate for tilt with a liquid crystal element installed in the optical system can be used. In addition, instead of optical control, it is also possible to compensate for tilt characteristics by signal processing, such as detecting signals from adjacent tracks at the same time, reducing the amount of crosstalk to the track itself, and controlling the characteristics of the crosstalk canceller, etc. .

As an inclination detecting means for control, a method of providing an optical reflective medium inclination sensor on the optical head 4 or searching for an optimum inclination point so that the reproduction signal characteristic is the best can be considered. .

In this way, by causing the tilt of the objective lens 12 to follow only a slow tilt change during recording or reproduction of the optical disc medium 3, the optical disc medium 3 and the drive device 6 can be reduced in cost while suppressing an increase in cost. High density characteristics can be realized.

As described above, according to the present embodiment, the transparent substrate or the transparent cover layer provided as a protective layer on the entire surface of the optical disk medium can be set to a thickness that is not affected by fingerprints, dust, etc., and the manufacturing cost of the optical disk medium is increased. Can be removed.

It is a figure which shows a reproduction signal amplitude characteristic. It is a figure which shows the relationship between the amount of inclination, and a wavefront aberration. It is a figure which shows the inclination characteristic of the inclination amount of a prior art, and the inclination characteristic of a reproduction error rate. It is a figure explaining the beam spot diameter of the optical disk medium whole surface. It is a figure which shows the relationship between a surface beam spot diameter and transparent substrate thickness. It is a figure which shows the example of the equalization waveform of a partial response when there exists an impulse input. It is a figure which shows the example of the eye pattern of PR (1, 2, 2, 1) equalization. It is a figure which shows the example of the eye pattern of PR (1, 2, 2, 2, 1) equalization. It is a figure which shows the apparatus structural example of a PRML system. It is a figure which shows the recording linear density characteristic of the reproducing method of a prior art, and the reproducing method by PRML. It is a figure which shows the inclination characteristic of a prior art and this Embodiment. It is a figure which shows the optical disk medium based on this Embodiment. It is a figure which shows the inclination characteristic of the amount of inclination based on this Embodiment, and the inclination characteristic of a reproduction error rate. It is a figure which shows the example of an optical disk external shape. It is a figure which shows the schematic structure of the drive device which concerns on this Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Recording surface 2 Transparent substrate 3 Optical disk medium 4 Optical head 5 Spindle 6 Drive apparatus 7 Tilt drive mechanism 11 Turntable 12 Objective lens 13 PR equalization circuit 14 ML circuit 15 Band pass filter 16 Tilt drive circuit

Claims (12)

A disc-shaped optical disk medium on which concentric or spiral information recording tracks are formed and for recording or reproducing information by irradiating the information recording tracks with a laser beam via a transparent substrate or a transparent cover layer,
The numerical aperture of the objective lens through which the laser beam to be irradiated onto the optical disk medium is transmitted is NA, the wavelength of the laser beam is λ [μm], the refractive index of the transparent substrate or transparent cover layer is n, the transparent substrate or When the thickness of the transparent cover layer is d [μm] and the shortest pit length of the pits formed on the information recording track is Lp [μm],
d ≧ 200 / (tan (sin ⁻¹ (NA / n)))
Lp ≦ 0.35 · (λ / NA),
Satisfy a relationship that is
The change in the amount of inclination of the optical disk medium in one round of each information recording track is set to a first predetermined value or less,
The amount of inclination over the entire surface of the optical disk medium is set to a second predetermined value or less,
An optical disc medium, wherein the second predetermined value is made larger than the first predetermined value. A disc-shaped optical disk medium on which concentric or spiral information recording tracks are formed and for recording or reproducing information by irradiating the information recording tracks with a laser beam via a transparent substrate or a transparent cover layer,
The numerical aperture of the objective lens through which the laser beam to be irradiated onto the optical disk medium is transmitted is NA, the wavelength of the laser beam is λ [μm], the refractive index of the transparent substrate or transparent cover layer is n, the transparent substrate or When the thickness of the transparent cover layer is d [μm] and the shortest pit length of the pits formed on the information recording track is Lp [μm],
d ≧ 200 / (tan (sin ⁻¹ (NA / n)))
Lp ≦ 0.35 · (λ / NA),
Satisfy a relationship that is
The change in the amount of inclination of the optical disk medium in one round of each information recording track is set to a first predetermined value or less,
An optical disc medium characterized in that an amount of inclination over the entire surface of the optical disc medium is equal to or greater than the first predetermined value. A disc-shaped optical disk medium on which concentric or spiral information recording tracks are formed and for recording or reproducing information by irradiating the information recording tracks with a laser beam via a transparent substrate or a transparent cover layer,
The numerical aperture of the objective lens through which the laser beam to be irradiated onto the optical disk medium is transmitted is NA, the wavelength of the laser beam is λ [μm], the refractive index of the transparent substrate or transparent cover layer is n, the transparent substrate or When the thickness of the transparent cover layer is d [μm] and the shortest pit length of the pits formed on the information recording track is Lp [μm],
d ≧ 200 / (tan (sin ⁻¹ (NA / n)))
Lp ≦ 0.35 · (λ / NA)
Satisfy a relationship that is
The change in the amount of inclination in the track orthogonal direction of the optical disc medium in one round of each information recording track is set to be equal to or less than a first predetermined value,
The amount of inclination in the track orthogonal direction on the entire surface of the optical disk medium is not less than the first predetermined value,
An optical disc medium characterized in that the amount of inclination in the track direction on the entire surface of the optical disc medium is set to a second predetermined value or less. The first predetermined value is 38 · (λ / (NA ³ · d)) · (n ³ / (n ² -1)) [deg] or less. An optical disk medium according to any one of the above. A disc-shaped optical disc medium in which concentric or spiral information recording tracks are formed, and the information recording tracks are irradiated with laser light through a transparent substrate or a transparent cover layer to record and reproduce information,
The change in the amount of inclination of the optical disk medium in one round of each information recording track is set to a first predetermined value or less,
The amount of inclination over the entire surface of the optical disk medium is set to a second predetermined value or less,
The numerical aperture of the objective lens through which the laser beam to be irradiated onto the optical disk medium is transmitted is NA, the wavelength of the laser beam is λ [μm], the thickness of the transparent substrate or transparent cover layer is d [μm], When the refractive index is n, the shortest pit length of the information recording track is Lp [μm], the first predetermined value is θa [deg], and the second predetermined value is θb [deg],
Lp ≦ 0.35 · (λ / NA)
θa ≦ 38 · (λ / (NA ³ · d)) · (n ³ / (n ² −1))
d ≧ 200 / (tan (sin ⁻¹ (NA / n)))
θa <θb
An optical disc medium satisfying the relationship: A disc-shaped optical disc medium in which concentric or spiral information recording tracks are formed, and the information recording tracks are irradiated with laser light through a transparent substrate or a transparent cover layer to record and reproduce information,
The change in the amount of inclination of the optical disk medium in one round of each information recording track is set to a first predetermined value or less,
The wavelength of the laser beam for recording / reproduction is λ [μm], the numerical aperture of the objective lens through which the laser beam to be irradiated on the optical disk medium is transmitted, NA, and the thickness of the transparent substrate or the transparent cover layer is d [μm]. When the refractive index is n, the shortest pit length of the information recording track is Lp [μm], and the first predetermined value is θa [deg],
Lp ≦ 0.35 · (λ / NA)
θa ≦ 38 · (λ / (NA ³ · d)) · (n ³ / (n ² −1))
d ≧ 200 / (tan (sin ⁻¹ (NA / n)))
An optical disc medium satisfying the relationship: 7. The optical disc medium according to claim 1, wherein a wavelength of the laser beam for recording / reproducing is 0.40 μm or more and 0.42 μm or less, and the lens numerical aperture is 0.6 or more. . The optical disc medium according to any one of claims 1 to 7, wherein the first predetermined value is 0.1 deg or more and 0.25 deg or less. 8. The optical disk medium according to claim 1, wherein the second predetermined value is 0.1 deg or more and 0.25 deg or less. A drive device for recording or reproducing the optical disk medium according to any one of claims 1 to 9,
A drive device having a function of compensating and controlling aberration generated by an inclination between an objective lens provided in the drive device and the optical disk medium. A drive device for recording or reproducing the optical disk medium according to any one of claims 1 to 9,
Having a function to compensate and control aberrations caused by the inclination between the objective lens provided in the drive device and the optical disk medium, and the control band of the function is set lower than the rotation period of the optical disk medium A drive device characterized by this. In the drive apparatus which records or reproduces | regenerates the optical disk medium in any one of Claim 1 to 9,
A drive device using a PRML system for signal reproduction.

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