JP2005196831A - Optical disk and optical disk system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide various kinds of optical disks in which information recordings and reproductions are possible in common without enlarging or thickening an optical disk system and without performing the troublesome exchange of an objective lens. <P>SOLUTION: In an optical disk 1 which can perform the recording or reproducing of information by being irradiated with an optical beam through an objective lens, a groove 103 is formed on the surface of a substrate 101 with a prescribed track pitch TP, and at least a recording film and a reflective film are stacked one by one on the surface of the substrate 101. When the wavelength of the optical beam and a numerical aperture of the objective lens are set to λ [nm] and NA, respectively, the track pitch TP is TP=(0.45-0.50)λ/0.623 NA [μm], and the thickness t of the substrate 101 is t=600±30×λ/650 [μm], and information is recorded on a groove track by the groove 103 by a multi-value recording method. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention at least reproduces information recorded on an optical disc such as a compact disc (CD), a digital versatile disc (DVD), and a large-capacity optical disc capable of recording information by a multi-level recording method, and the information recorded on the optical disc. The present invention relates to an optical disc apparatus capable of performing the above.

Optical discs include compact discs called CDs, which are mainly used as media for music information, and digital versatile discs called DVDs, which are rapidly spreading as video and computer software media. Furthermore, although the standard and name are not yet unified, a next-generation large-capacity optical disk (hereinafter simply referred to as “large-capacity optical disk”) having a storage capacity of about 20 GB or more has also been realized.
In addition, CD-DVD and DVD-R, which are write-once media called “write once”, which can be written only once, and CD-RW, DVD-RW, which are rewritable media up to a certain number of times, are available for CD and DVD. And DVD + RW.

For example, as seen in Patent Document 1, a light emitting element that emits light at a single wavelength of about 400 nm and has a noise component with RIN (Relative Intensity Noise) of −115 to −135 dB / Hz, and a numerical aperture of 0 An optical disc capable of recording or reproducing information by irradiating a convergent light beam from an optical pickup having an objective lens of .7 to 0.8 is disclosed. In this optical disc, at least a recording layer and a waveguide layer are sequentially laminated on a substrate, and a fine track pattern used for recording or reproducing information is formed on an information recording surface including the recording layer. Yes. The thickness of the waveguide layer (cover layer) is 80 μm to 200 μm based on the tilt margin and the size of the defect.
JP 2001-23243 A

  By the way, as mentioned above, there are various standards such as CDs such as CD-R / RW, DVDs such as DVD + R / RW, and large-capacity optical disks. Accordingly, it is desired that an optical disc apparatus that records information on an optical disc and reproduces the recorded information can be applied to various optical discs. However, when the numerical aperture of the objective lens is 0.8, the working distance of the optical pickup is relatively short. Therefore, the optical pickup is not suitable for an optical disc having a relatively large thickness such as a CD-R / RW. In some cases, the light beam emitted from the optical disk cannot be focused on the information recording surface of the optical disk.

  Therefore, in order to allow a single optical disc device to focus the light beam on the information recording surface of various optical discs, a plurality of objective lenses having different numerical apertures are mounted in the optical disc device. In addition, a technique such as exchanging them according to the type of the optical disc is required. However, doing so increases the size of the optical disk device and makes it difficult to replace the objective lens.

  Further, when the working distance of the optical pickup is relatively short, the distance between the surface of the objective lens and the surface of the optical disk is relatively short. For this reason, dust tends to be caught between the surface of the objective lens and the surface of the optical disk. If the optical disk is rotated while dust is caught, the objective lens and the optical disk may be damaged. Then, information cannot be recorded on the optical disc or information recorded on the optical disc cannot be reproduced, and replacement of the optical pickup including the objective lens and the optical disc is forced.

In order not to replace the optical pickup, for example, it may be possible to record / reproduce information in a state where the optical disk is stored in the cartridge. However, in this case, there is a problem that the manufacturing cost of the optical disk increases and the optical disk apparatus becomes thicker by the amount of the cartridge.
Furthermore, although the invention described in Patent Document 1 reduces the noise component superimposed on the reproduction signal by reducing the noise component of the light emitting element, the substrate (support) included in the optical disk is reduced. There are no measures to reduce the noise component superimposed on the playback signal due to the noise level, mechanical properties of the substrate, optical non-uniformity of the recording layer and the reflective layer, etc., and sufficient noise measures are taken It was not.

  The present invention has been made to solve these problems, and does not require a troublesome replacement work of the objective lens without increasing the size or thickness of the optical disk device, and is sufficient for low-cost and sufficient media noise countermeasures. It is an object of the present invention to provide an optical disc that can perform reproduction of at least information recorded on the optical disc.

In the optical disk according to the present invention, grooves are formed at a predetermined track pitch on the surface of the substrate, and at least a recording film and a reflective film are sequentially stacked on the surface of the substrate, and a light beam is irradiated through the objective lens. In order to achieve the above object, the present invention is characterized in that it is configured as follows.
That is, when the wavelength of the light beam is λ [nm] and the numerical aperture of the objective lens is NA, the track pitch is (0.45 to 0.50) λ / 0.623 NA [μm], and The thickness was set to 600 ± 30 × λ / 650 [μm], and information was recorded on the above-described crew track by the multi-value recording method.

In an optical disk configured in the same manner as described above, grooves and lands are alternately formed at a predetermined track pitch on the surface of the substrate, and information is recorded on the crushed track and the land land track by the multi-value recording method. You may do it.
In these optical discs, the recording signal with the smallest reflected light amount was recorded continuously with Nmax as the noise level of the reproduction signal obtained when the track on which the recorded mark with the largest reflected light amount was continuously recorded was reproduced. It is desirable that the difference ΔN between Nmax and Nmin is 3.9 dB or less (ΔN ≦ 3.9 dB), where Nmin is the noise level of the reproduction signal obtained when the track is reproduced.

Further, when a track in which a recording mark with the maximum reflected light amount and a recording mark with the second largest reflected light amount are alternately recorded as a pattern is reproduced, the carrier level at the frequency corresponding to the repeated pattern and the The difference (CNR) from the noise level in frequency is preferably 30 dB or more.
In this case, it is desirable that the reproduction light power when reproducing the track on which the repeated pattern is recorded is a reproduction light power that maximizes the difference (CNR).

In these optical disks, when the two substrates are bonded together with the surfaces on which the grooves are formed facing each other, the angle between the normal of the surface of the optical disk and the light beam is within ± 0.2 degrees. It is desirable that
The recording film in these optical discs may be a phase change recording film.

  The optical disk apparatus according to the present invention is an optical disk apparatus for reproducing the information recorded on the optical disk, and means for irradiating the recording surface of the optical disk with a light beam having the wavelength λ through an objective lens having a numerical aperture of NA. And means for detecting reflected light of the light beam applied to the optical disk by the means and reproducing information recorded on the optical disk in a multi-valued manner.

According to the optical disk and the optical disk apparatus according to the invention, it is possible to record or reproduce information with a common optical disk apparatus without increasing or increasing the thickness of the optical disk apparatus and without performing troublesome replacement work of the objective lens. Become. In addition, it is possible to accurately demodulate multilevel data with less influence of media noise.
The present invention is applicable to all three generation optical discs such as CDs, DVDs, and large-capacity optical discs.

Embodiments of the present invention will be described below with reference to the drawings.
[Example of optical disc apparatus]
FIG. 1 is a block diagram showing a schematic configuration of an embodiment of an optical disk apparatus according to the present invention. This optical disc apparatus includes a motor (spindle motor) 5 that rotates an optical disc 1 that is an information recording medium by a motor shaft 5a, and a pickup head 6 for recording and reproducing information with respect to the optical disc 1. The pickup head 6 includes a semiconductor laser that generates laser light, an objective lens 7 that converges the laser light and irradiates the information recording surface of the optical disk 1, and reflected light from the optical disk 1 as an electrical signal. An optical pickup equipped with a light receiver for conversion is mounted.

  Further, in this optical disc apparatus, a laser drive circuit 10 for driving a semiconductor laser provided in the optical pickup of the pickup head 6 and a recording pulse based on information recorded on the optical disc 1 are generated for the laser drive circuit 10. A recording pulse generation circuit 11, a modulation signal generator 12 for sending a modulation signal to the recording pulse generation circuit 11, a reproduction signal amplifier 13 for amplifying an electric signal (reproduction signal) output from the pickup head 6, A demodulation circuit 14 for demodulating the amplified reproduction signal is provided.

  This optical disk apparatus also includes a tracking motor for tracking the optical beam of the pickup head 6 so that the light beam L is accurately applied to the track of the optical disk 1, and a servo for servo-controlling the tracking motor and the spindle motor 5 described above. A controller including a system and a microcomputer that performs overall control of each part of the apparatus is also provided, but these are not shown in FIG.

  The optical pickup of the pickup head 6 includes, for example, three semiconductor lasers, and these are selectively used according to the type of the optical disc 1. Each semiconductor laser emits a laser beam having a wavelength λ of 780 nm, 650 nm, and 405 nm, and when a semiconductor laser that emits a laser beam having a wavelength λ of 780 nm or 650 nm is used, the semiconductor laser on the incident surface side of the objective lens 7 is used. By restricting the aperture, a light spot having a required size can be formed on the information recording surface of the optical disc 1.

Actually, multilevel recording of information was performed on the so-called three-generation optical disc (CD-RW disc, DVD + RW disc, large-capacity optical disc) by each of the semiconductor lasers and the information was reproduced. As a result, symbol error rate (SER [%] ), The cell length [μm] of the recording cell was as shown in Table 1.

As shown in this table, the symbol error rate SER is 3% or less for any of the three generations of optical disks, which is a practically acceptable level, and the demodulation circuit 14 accurately demodulates the multilevel data. Can do.
The numerical aperture of the objective lens 7 is set to 0.65, for example. Furthermore, in this embodiment, for example, 8-level (3-bit) multi-value data is recorded on the optical disc 1. However, this number of bits is an example, and the present invention is not limited to this.

[Example of optical disc]
FIG. 2 is a schematic sectional view showing an example of an optical disk according to the present invention. In the optical disc 1 shown in FIG. 2, a groove 103 is formed in a continuous spiral shape on the surface of a substrate 101 having a diameter of 120 cm and a thickness of 0.6 mm (600 μm). The remaining part of the surface becomes the land 104. The groove 103 has a width W of 0.2 μm, a depth d of 30 nm, and a track pitch (groove pitch) TP of 0.4 μm to 0.5 μm.
Further, at least a recording film and a reflection film are sequentially stacked on the surface of the substrate 101. For example, as shown in a partially enlarged cross section in FIG. 3, a dielectric film 111, a phase change recording film 112 made of Ag—In—Sb—Te, a dielectric film 113, and a reflective film 114 are formed on the surface of the substrate 101. A phase change type optical disk such as DVD-RW or DVD + RW is manufactured by sequentially stacking.

The substrate 101 is made of synthetic resin, metal, or the like containing glass, ceramic, synthetic resin, metal powder or ceramic powder (silicon, carbon, glass, etc.). In the case of the phase change recording film 112, a phase change material is used as the recording film, but in the case of another type of optical disk, a low roughness metal material, an optical interference material, a magneto-optical material, a dye material, or the like is used. You can also The reflective film 114 is formed of a material such as Al, AlCr, AlTi, or AlTa. In addition, the reflective film may be formed of ZnS-SiO ₂ or the like.

  The groove 103 is formed by, for example, an ultraviolet curable resin (2P) method. In order to be compatible with the ROM disk, the information may be recorded only on the groove track, which is a track formed by the groove 103, so that tracking at the time of recording or reproducing information can be performed stably. Therefore, recording may be performed on both the groove track and the land track which is a track formed by the land 104 formed between the grooves 103.

[Track pitch]
As will be described later, the definition of the track pitch differs depending on whether the information recording destination is only the groove 103 or both the groove 103 and the land 104.
In the present invention, the track pitch is defined as the interval between recording tracks. That is, when the information recording destination is only the groove 103, it is an interval between adjacent grooves, and when information is recorded on both the groove 103 and the land 104, it is an interval between the groove and the adjacent land. The value of the track pitch is the case where only the groove 103 is the information recording destination.
The value of the track pitch depends on the wavelength of the light beam L emitted from the pickup head 6 on the optical disk apparatus side shown in FIG. 1 and the numerical aperture of the objective lens 7. When the wavelength of the light beam L is λ [nm] and the numerical aperture of the objective lens 7 is NA, the track pitch (groove pitch) value TP is set to a value calculated by the following equation (1).

  The denominator coefficient “0.623” in Equation 1 is a value corresponding to λ / NA when the wavelength λ of the light beam is 405 nm (0.405 μm) and the numerical aperture NA of the objective lens is 0.65. The track pitch value TP is “0.623” which is the value of λ / NA in the optical disk apparatus actually used and the value of λ / NA when λ = 0.405 μm and NA = 0.65. It depends on the ratio. That is, the coefficient for normalizing with the track pitch when λ = 0.405 μm and NA = 0.65 is “0.623”.

Here, the grounds for the upper and lower limits of “0.45 to 0.57” in the numerator of Equation 1 will be described. The upper limit value of the track pitch needs to be determined based on the recording capacity of information required for the optical disc. The information recording capacity is calculated as follows.
Recording capacity [bytes] = (area of groove land formation area [mm ² ] × number of bits in multi-value recording method [bit] × format efficiency) / (track pitch [μm] × recording cell length [μm] × 8 [bit / byte]

Here, there is a demand for a recording capacity of 20 GB, the diameter of the optical disk 10 is 58 mm, the diameter of the central area of the optical disk where the grooves and lands are not formed is 22 mm, and the number of bits in the multilevel recording method is 3 bits. Assuming (8 values), the format efficiency is 0.77, and the cell length of the recording cell is 0.23 μm, the following results.
20 × 10 ⁹ [bytes] = π (58 ² −22 ² ) [mm ² ] × 3 [bits] × 0.77 /
TP [μm] x 0.23 [μm] x 8 [bit / byte]

The track pitch TP can be calculated by converting this equation as follows.
TP [μm] = π (58 ² −22 ² ) [mm ² ] × 3 [bit] × 0.77 /
20 × 10 ⁹ [bytes] × 0.23 [μm] × 8 [bit / byte]
= (3.14 × 2880 × 3 × 0.77) / (20 × 10 ⁹ × 0.23 × 8)
= 20890 / 36.8 × 10 ⁹ = 0.57 × 10 ⁻⁶ [m] = 0.57 [μm]
If this value (0.57 [μm]) is not set as the upper limit value of the track pitch, the information recording capacity of the optical disk will be less than 20 GB. Therefore, the upper limit of the track pitch when λ = 0.405 μm and NA = 0.65 is 0.57 μm.

  On the other hand, the lower limit value of the track pitch needs to be determined under conditions that allow stable tracking during information recording or reproduction. Various optical disks in which grooves were formed by changing the track pitch and the optical disk apparatus shown in FIG. 1 were used to calculate the lower limit of the track pitch by experiments.

  FIG. 4 is a diagram showing the results of tracking experiments for determining the lower limit value of the track pitch. The horizontal axis in FIG. 4 indicates the track pitch (TP) [μm], and the vertical axis indicates the value of PPb. PPb is the level of the groove signal (a signal detected only in the groove side area) that is twice the amplitude of the push-pull signal, which is the sum of the signals detected in the two areas of the light receiver in the pickup head of the optical disk apparatus. The value standardized by. If the PPb value is 0.25 or more, stable tracking can be performed.

Therefore, it can be seen from FIG. 3 that the track pitch in this case needs to be 0.45 μm or more.
Therefore, the lower limit of the track pitch when λ = 0.405 μm and NA = 0.65 is 0.45 μm.
In this way, as a result of obtaining the upper limit value and the lower limit value of the track pitch, a mathematical expression defining the track pitch TP shown in Equation 1 was derived.

5A and 5B are diagrams for explaining the track pitch of the optical disk, in which FIG. 5A is a partial plan view of the optical disk, and FIG. 5B is a cross-sectional view taken along line XX. In addition, each film | membrane shown in FIG. 3 is abbreviate | omitting illustration.
FIG. 5A shows a track pitch TPa when the information recording destination on the optical disc 1 is only the groove 103, and a track pitch TPb when the information recording destination is both the groove 103 and the land 104. It shows. The track pitch TPa can be indicated by the distance between the center lines in the width direction of the lands 104 adjacent to both sides of the groove 103. The track pitch TPb can be indicated by the distance between the center lines in the width direction of the adjacent grooves 103 and lands 104.

[Thickness of substrate]
When the thickness t of the substrate 101 of the optical disc 1 shown in FIG. 2 is deviated from the design value, the spot shape of the light beam on the recording surface of the substrate 101 changes, the light intensity at the center of the spot decreases, the spot diameter As a result, an increase in the strength of the side lobe and the like occur, so that good recording / reproducing characteristics cannot be obtained. Therefore, it is necessary to define an error in the substrate thickness.

  FIG. 6 is a diagram showing the relationship between the thickness error of the substrate 101 and the wavefront aberration at the light spot. The horizontal axis of FIG. 6 indicates the substrate thickness error [μm], and the vertical axis indicates the wavefront aberration [λrms]. As the optical disc, a DVD + RW disc and a large capacity optical disc (optical disc having a recording capacity of about 20 GB) are used. A triangle plot point indicates a DVD + RW disc and a square plot point indicates a large capacity optical disc.

The optical disk apparatus shown in FIG. 1 was used. The semiconductor laser used was different for each disk. The DVD + RW disk had a wavelength of the light beam L of 650 nm, and the large capacity optical disk had a wavelength of the light beam L of 405 nm. did.
As shown in FIG. 6, as the substrate thickness error increases, the wavefront aberration at the light spot also increases substantially proportionally. In addition, in the case of a large-capacity optical disk, that is, in the case where the wavelength of the light beam is 405 nm, the rate of increase of the wavefront aberration at the light spot with respect to the increase in the substrate thickness error is large. Therefore, considering the case of using a large-capacity optical disc, the thickness of the optical disc substrate is quantified based on the content of the substrate thickness specification defined for the DVD + RW disc.

In the substrate thickness specification defined for DVD + RW discs, the thickness t of the substrate 101 shown in FIG. 2 is [0.60 ± 0.03 mm]. When the substrate thickness error of the DVD + RW disc is ± 0.03 mm (= 30 μm), the wavefront aberration at the corresponding light spot is 0.042λ from FIG.
Therefore, when the wavefront aberration at the light spot is 0.042λ, the substrate thickness error of the corresponding large-capacity optical disk is found to be approximately ± 0.02 mm (= 20 μm) or less.

The substrate thickness error may be obtained from the wavelength ratio of the semiconductor laser. In this case,
Substrate thickness error = 30 [μm] × 405 [nm] / 650 [nm]
Therefore, 18.7 [μm]. This was the same result as about 20 [μm] in the above case. When stationary, the substrate thickness error at the wavelength λ can be expressed as ± 30λ / 650 μm.
Therefore, the thickness of the substrate should be 600 ± 30 × λ / 650 [μm]. If the thickness of the substrate is set, the light spot can be focused on the recording surface of the optical disc, so that information can be recorded or reproduced satisfactorily. This can prevent problems such as a decrease in the center light intensity of the light spot, an increase in the light spot diameter, and an increase in side lobe intensity.

[Media noise]
7A and 7B are diagrams for explaining the principle of the multi-value recording method, in which FIG. 7A is a schematic diagram of a recording mark, and FIG. 7B is a diagram showing a change in reflected light amount corresponding to FIG.
As shown in FIG. 7A, a plurality of recording cells 207 are formed on the recording track 200 of the optical disc. In each recording cell 207, amorphous phase recording marks 201-205 are formed. A crystal phase 206 is formed around each recording mark 201 to 205 in each recording cell 207. The recording marks 201 to 205 are classified into several sizes.

  In this example, information is recorded in 3 bits, and the contents of the information are indicated by 8 types of recording marks having different sizes (areas). That is, information is written in any of the eight types according to the content of information to be recorded. Here, for example, the recording mark 202 indicates the multi-value data 1 having the second smallest area, and the recording mark 203 indicates the multi-value data 7 having the maximum area. Note that each of the eight types of multi-value data is referred to as multi-value data 0 to multi-value data 7, and the number attached to the data increases in proportion to the area. When reproducing the information recorded on the recording track 200, the reproducing light spot 208 and the recording track 200 are relatively moved while the reproducing light spot 208 is irradiated on the recording track 200. Note that D in the figure indicates the diameter of the reproduction light spot 208.

FIG. 7B shows the amount of reflected light when the recording track 200 is irradiated with the light spot 208. Here, time is shown on the horizontal axis, and the amount of reflected light is shown on the vertical axis.
As shown in this figure, the amount of light reflected from the optical disk decreases in inverse proportion to the size of the recording marks 201-205. In this example, the point in time at which the center point of the light spot 208 is located at the center point in the longitudinal direction of each recording cell 207 is a sampling point (indicated by ●) of the reflected light amount.
As described above, the amount of reflected light changes in accordance with the size of the recording marks 201 to 205, and the content of information recorded on the optical disc can be determined from the difference in the amount of reflected light.

However, since the noise of the reproduction signal is superimposed on the sampling result, the content of the read information may be erroneously determined if the noise level of the reproduction signal is high. Therefore, it is necessary to reduce noise caused by the optical disk. Therefore, the following experiment was conducted.
That is, three types of sample stampers (hereinafter referred to as stampers) having different surface roughness were produced. A stamper is a mold for producing an optical disk substrate.
Normally, as shown in FIG. 14, the stamper forms a photoresist layer on a glass master and exposes and develops it to form grooves and lands. Next, the glass master is electroformed with nickel (Ni), and then peeled off from the glass master to obtain a stamper having grooves and lands transferred onto the nickel surface. In this method, the surface roughness of the stamper is equivalent to the surface roughness of the photoresist layer.

  On the other hand, in order to improve the surface roughness of the stamper, as shown in the manufacturing process in FIG. 15, the glass master is dry-etched using the photoresist layer formed after development as a mask, and then the resist layer is removed. There is a dry etching method in which grooves and lands are formed on a glass master, and a stamper is manufactured by electroforming the glass master. The surface roughness of the stamper by this method is equivalent to the surface roughness of the glass master, and can be made smaller than the surface roughness of the stamper manufactured by the ordinary method shown in FIG.

  As the above-described three types of sample stampers having different surface roughnesses, the first sample stamper was manufactured by a dry etching method using a high resolution type photoresist. The second sample stamper was manufactured by a normal method using a high resolution type photoresist. The fourth sample stamper was manufactured by a normal method using a high sensitivity type photoresist. The high resolution type photoresist has finer resist particles than the high sensitivity type photoresist. Therefore, the surface roughness of the photoresist layer is also small. The surface roughness of the stamper is the smallest for the first sample stamper and the largest (rough) for the third sample stamper.

Optical disk substrates A, B, and C are prepared from the first to third sample stampers, respectively, and a dielectric film, a phase change recording film, a dielectric film, and a reflective film are sequentially stacked on these substrates to form sample disks SD1, SD2. And SD3 was made.
Then, the same multivalued data was continuously recorded on each of the sample disks SD1 to SD3, and the noise level at the cell frequency of the reproduced signal was measured with a spectrum analyzer.

  FIG. 8 is a diagram showing the relationship between the multilevel data 0 to 7 and the noise level of each sample disk SD1 to SD3. In FIG. 8, the horizontal axis represents multivalued data 0 to 7, and the vertical axis represents the noise level N [dB]. In each of the sample disks SD1 to SD3, the noise level increases as the multi-value data decreases (the area of the recording mark decreases). However, in the sample disk SD3, a noise level difference ΔN = N0−N7 between the noise level N0 when the multi-value data 0 is continuously recorded and the noise level N7 when the multi-value data 7 is continuously recorded is obtained. I was able to make it smaller.

FIG. 9 is a diagram showing the relationship between the noise level difference ΔN and the symbol error rate (SER), the horizontal axis showing the noise level difference ΔN [dB], and the vertical axis showing the SER [%].
Here, multi-value random data was recorded on each of the sample disks SD1 to SD3, and after performing waveform equalization processing with a 7-tap transversal filter, the SER was measured.
A disk with a large noise level difference ΔN is considered to increase the SER because the distribution deviation of the multi-value data 0 to 2 is larger than the deviation of other multi-value level distributions. If the SER is 3% or less, the required demodulation of the multilevel data can be performed after the error correction, and it can be seen from FIG. 9 that the noise level difference ΔN should be 3.9 dB or less.

The results of measurement for each SER of the sample disks SD to SD3 are as shown in Table 2, and FIG. 9 is a graph in which this is plotted with the noise level difference ΔN as the horizontal axis. Therefore, the measurement results of the three sample disks SD to SD3 are shown, but there is only one line in FIG.

As described with reference to FIG. 8, when the multi-value data is small, the noise level of the reproduction signal is relatively high as compared to the case where the multi-value data is large. In particular, the noise level of the reproduction signal in the case of multilevel data 0 to 2 is higher than the noise level of the reproduction signal in the case of multilevel data 3 to 7. In order to prevent erroneous determination of the content of reproduced information, it is necessary to improve the carrier level / noise level (C / N) ratio. In order to improve the C / N ratio, the difference between the carrier level and the noise level (hereinafter referred to as “CNR”) may be increased.
Here, multi-value data 0 and multi-value data 1 were alternately recorded on each of the sample disks SD1 to SD3, and the dependency of the CNR on the reproduction light power at the cell frequency of the reproduction signal was examined.

FIG. 10 is a diagram showing the relationship between the reproduction light power and the CNR. The horizontal axis represents the reproduction light power Pr [mW], and the vertical axis represents the CNR [dB].
As shown in FIG. 10, when the reproduction light power is increased from about 0.4 mW, the CNR also increases. The CNR is leveled off from around 0.7 mW to around 0.9 mW. Further, when the reproduction light power is increased, the CNR decreases. This phenomenon is caused by deterioration of the reproduction light, and is caused by recrystallization of the recording mark by the reproduction light.
As described above, in order to improve the C / N ratio, the CNR may be increased. Therefore, the reproduction light power Pr may be selected within 0.7 to 0.9 [mW]. In view of the above, a lower value is better, so a value closer to 0.7 [mW] is preferable.

[Tilt margin]
FIG. 11 is a schematic cross-sectional view of an optical disc created for measuring the reproduction tilt margin. In this optical disc 100, a phase-change optical disc 1a having a substrate 101 similar to the optical disc 1 shown in FIGS. 2 and 3 and a phase-change optical disc 1b having a substrate 102 are recorded with grooves 103 and lands 104 formed thereon. The surface sides are made to face each other and bonded by an adhesive layer 105. The substrates 101 and 102 each have a diameter of 120 mm and a thickness of 0.6 mm, and the total thickness is 1.2 mm.
The groove has a width of about 0.20 μm, a depth of about 30 nm, and a track pitch of 0.4 μm to 0.5 μm. The length of the recording cell in the circumferential direction is 0.23 μm.

  FIG. 12 is a timing chart showing the relationship among the clock pulse CLK, the recording data pulse Lv2, and the recording pulse used in the optical disc apparatus when information is recorded on the optical disc 100 shown in FIG. The recording pulse in FIG. 13 shows the recording pulse waveform in the case of multi-value data 2. The clock pulse CLK is composed of pulses a to e, and the recording data pulse Lv2 is composed of a pulse f. The time during which the pulse f is at the high level is a time corresponding to four pulses of the clock pulse CLK. The recording pulse consists of a pulse g. The time when the pulse g is at the high level is a time corresponding to 0.5 period of the clock pulse CLK, and the time when it is at the low level is the time corresponding to 1.5 periods of the clock pulse CLK. It is.

In addition, the recording pulse is set to the erasing power Pe until the time Ta elapses, the recording power Pw until the time Tb elapses, and then biased until the time Tc elapses with respect to the rising point of the recording data pulse Lv2. The power is set to Pb, and thereafter the erasing power Pe is set again.
Here, when recording or reproducing data, for example, a semiconductor laser that emits a light beam having a wavelength of 405 nm is used, the numerical aperture of the objective lens is 0.65, and the recording linear velocity is 6.0 m / s. . The time Ta is, for example, a time corresponding to one clock cycle, the time Tb is, for example, a time corresponding to 1.5 clocks, and the time Tc is, for example, a time corresponding to 3 clocks. As typical power conditions at a linear velocity of 6.0 m / s, for example, the erase power Pe is 4.5 mW, the recording power Pw is 9.0 mW, and the bias power Pb is 0.1 mW.

  The reproduction power Pr (not shown) is set to 0.7 mW, for example. The area of the recording mark is controlled mainly by adjusting the pulse width that can be represented by time Tc−time Tb. That is, the area of the recording mark is controlled by changing the time during which the recording power Pw at which the recording pulse is at a high level is changed.

FIG. 13 is a diagram showing the relationship between the angle (rad.til) [deg] of the objective lens 7 with respect to the radial direction of the optical disc 100 and SER [%]. Here, the horizontal axis indicates the angle, and the vertical axis indicates the SER. As shown in FIG. 13, the SER is minimum in the vicinity of 0 deg, and increases when it deviates in the plus direction or in the minus direction.
As described above, if the SER is 3% or less, the multi-value data can be demodulated after error correction. Therefore, under the conditions shown in this example, the radial tilt margin, that is, the normal in the surface direction of the optical disk and the disk device It was found that the angle formed with the optical axis of the light beam irradiated from the side should be within ± 2 deg.

  The present invention is applicable to optical discs of various standards such as CDs such as CD-R / RW, DVDs such as DVD + R / RW, and large capacity optical discs (so-called third generation optical discs). In addition, information can be recorded on or reproduced from the exchanged optical disk by a common optical disk device, and the recorded information can be reproduced, the optical disk device can be downsized, and troublesome switching operations depending on the type of optical disk are not required. become.

It is a block diagram which shows the principal part of one Example of the optical disk apparatus using the optical disk by this invention. It is typical sectional drawing of the optical disk shown in FIG. It is a partially expanded sectional view which similarly shows the film | membrane structure of the recording surface. It is a diagram which shows the experimental result of the tracking for determining the lower limit of a track pitch. FIG. 2 is a plan view of a part of an optical disc for explaining a track pitch and a cross-sectional view taken along line XX.

It is a diagram which shows the relationship between the substrate thickness error of an optical disk, and the wavefront aberration by a light spot. It is a figure which shows the relationship between the recording mark for demonstrating the principle of a multi-value recording system, and its reflected light. It is a diagram showing the relationship between multilevel data 0 to 7 recorded on three types of sample discs and the noise level of the reproduction signal when it is reproduced. It is a diagram which shows the relationship between noise level difference (DELTA) N and a symbol error rate (SER).

It is a diagram which shows the relationship between reproduction optical power and CNR. It is a typical sectional view of an optical disc produced in order to measure reproduction tilt margin. FIG. 12 is a timing chart showing a relationship among a clock pulse CLK, a recording data pulse Lv2, and a recording pulse in the optical disc apparatus when information is recorded on the optical disc shown in FIG. It is a diagram which shows the relationship between the angle of the objective lens with respect to the radial direction of an optical disk, and SER. It is process drawing for demonstrating the normal stamper manufacturing method. It is process drawing for demonstrating the stamper manufacturing method by the dry etching method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,100: Optical disk 5: Motor (spindle motor) 6: Pickup head 7: Objective lens 10: Laser drive circuit 11: Recording pulse generation circuit 12: Modulation signal generator 13: Reproduction signal amplifier 14: Demodulation circuits 101, 102: Substrate 103: Club 104: Land 105: Adhesive layer 200: Recording track 201-205: Recording mark 206: Crystal layer 207: Recording cell 208: Reproducing light spot

Claims (8)

Grooves are formed on the surface of the substrate at a predetermined track pitch, and at least a recording film and a reflective film are sequentially laminated on the surface of the substrate, and information recording or recording is performed by irradiating a light beam through an objective lens. An optical disc that can be played,
When the wavelength of the light beam is λ [nm] and the numerical aperture of the objective lens is NA, the track pitch is (0.45 to 0.50) λ / 0.623 NA [μm],
The thickness of the substrate is 600 ± 30 × λ / 650 [μm],
An optical disc, wherein information is recorded by a multi-level recording method on a club track by the club. Grooves and lands are alternately formed at a predetermined track pitch on the surface of the substrate, and at least a recording film and a reflective film are sequentially stacked on the surface of the substrate, and a light beam is irradiated through an objective lens. An optical disc capable of recording or reproducing information,
When the wavelength of the light beam is λ [nm] and the numerical aperture of the objective lens is NA, the track pitch is (0.45 to 0.50) λ / 0.623 NA [μm],
The thickness of the substrate is 600 ± 30 × λ / 650 [μm],
An optical disc, wherein information is recorded by a multi-value recording method on a crew track by the crew and a land track by the land.   3. The optical disc according to claim 1 or 2, wherein a noise level of a reproduction signal obtained when reproducing a track in which a recording mark having the maximum amount of reflected light is continuously recorded is Nmax, and the recording mark having the minimum amount of reflected light is recorded. An optical disc, wherein a difference ΔN between Nmax and Nmin is 3.9 dB or less, where Nmin is a noise level of a reproduction signal obtained when a continuously recorded track is reproduced.   The optical disk according to any one of claims 1 to 3, wherein a track in which a recording mark having the maximum reflected light quantity and a recording mark having the second largest reflected light quantity are alternately recorded as a pattern is reproduced. An optical disc, wherein a difference (CNR) between a carrier level at a frequency corresponding to the repetitive pattern and a noise level at the frequency is 30 dB or more.   5. The optical disk according to claim 4, wherein the reproduction optical power when reproducing the track on which the repeated pattern is recorded is a reproduction optical power that maximizes the difference (CNR).   The optical disk according to any one of claims 1 to 5, wherein when the two substrates are bonded together with the surfaces facing each other, an angle formed between a normal of the surface of the optical disk and the light beam is An optical disc characterized by being within ± 0.2 degrees.   7. The optical disk according to claim 1, wherein the recording film is a phase change recording film. 8. An optical disc apparatus for reproducing information recorded on an optical disc according to claim 1, wherein a light having a wavelength of [lambda] is applied to a recording surface of the optical disc through an objective lens having a numerical aperture of NA. An optical disk apparatus comprising: means for irradiating a beam; and means for detecting reflected light of the light beam irradiated on the optical disk by the means and reproducing information recorded on the optical disk .

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