JP3010798U - Optical disc and optical disc device - Google Patents

Abstract

(57) [Abstract] [Purpose] Even if the track pitch and pit pitch are set smaller than the beam spot diameter of the reproduction light beam, the crosstalk between adjacent tracks can be reduced to a level where there is no practical problem, and the density is dramatically increased.・ Providing optical discs with high capacity. When the wavelength of the reproducing light beam is λ nm and the numerical aperture of the objective lens is NA, the track pitch is (0.7
2 to 0.8) × λ / NA / 1.14 μm, and the upper width of the pit 10 is Wm = (0.3 to 0.45) × λ / NA / 1.
14 μm, the angle of the pit inner wall 11 is θ = 50 ° to 70 °
An optical disc characterized by:

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to an optical disc in which information is recorded in high density in the form of pits and an optical disc device including the optical disc and a reproduction optical system.

[0002]

[Prior art]

 In recent years, due to the progress of digital signal processing technology for images and moving picture compression technology being promoted by standardization organizations called MPEG (Moving Picture Image Coding Experts Group), CDs (compact discs) have been replaced by VTRs and laser discs. ), There is an increasing expectation for an optical disc that is the same size as) and that can reproduce moving image information such as movies for 2 hours. The capacity required for recording 2 hours of moving image information in the form of an analog video signal of a standard TV system such as NTSC such as a laser disc is 80 Gbytes including audio. For example, MPEG-2 This is because, if a moving image compression technique defined by a standardization method called "S" is used, the required capacity is about 4 Gbytes even if the image quality is similar to that of a high quality VTR such as S-VHS. . The capacity of 4 GB has already been put to practical use in a write-once optical disk with a diameter of 300 mm, but in consideration of future popularization for general households, the same capacity can be achieved with a CD size of 120 mm, which is easy to handle. Is required.

At present, the capacity of a CD format which is widely used as a music CD or a CD-ROM is 790 MB at the maximum (when the linear velocity is 1.2 m / s), and the capacity is about this. In, it is possible to store the compressed moving image information according to MPEG-2 for only 24 minutes. Therefore, in order to store the compressed moving image information of MPEG-2 for 2 hours in a CD size, it is necessary to increase the recording density to 5 times that of a CD. By the way, in the current CD format, the substrate thickness is 1.2 mm, the track pitch is 1.6 μm, and the pit pitch is a linear velocity (relative velocity between the light beam and the disc = peripheral velocity of the disc) of 1.2 m / When s is 1.66 μm, the pit length is 0.59 μm, and the modulation method is EFM (eight to fourteen modulation). On the other hand, in the reproducing optical system, the wavelength of the reproducing semiconductor laser (LD) is 780 nm, the NA (numerical aperture) of the objective lens is 0.45, and the beam spot diameter is approximately 1.4 μm. The value of the beam spot diameter is selected mainly from the viewpoint of avoiding the influence of crosstalk between adjacent tracks.

By the way, in order to increase the recording density of an optical disc, a processing technique for forming small pits on the disc and a technique for reducing the beam spot size on the optical disc in the reproducing optical system are required. As a pit processing technique, for example, an optical disc master recording technique using a Kr ion laser beam (ultraviolet light) having a wavelength of 351 nm has been proposed (Autumn 1993, National Congress of Applied Physics, 28-SF-2). It is possible to process smaller pits than the conventional Ar ion laser. Regarding the reproduction optical system, the beam spot diameter can be made smaller by shortening the wavelength of the reproducing laser beam and increasing the NA. However, in reality, with conventional CD players and other methods, even if a short-wavelength light source such as a red LD is used, the effect of increasing the capacity by wavelength is at most about 1.5 times. It cannot be expected that the capacity will increase five times as much as that required to record the compressed image information for the time.

[0005]

[Problems to be solved by the device]

 As described above, in the conventional optical disc technology, in order to avoid the problem of crosstalk between adjacent tracks, the track pitch and the pit pitch are set larger than the beam spot diameter of the reproduction light beam. It is said that it is not possible to increase the recording density to such an extent that the capacity required to store, for example, 2 hours of compressed moving image information by MPEG2 in a CD size can be obtained only by shortening the wavelength of the beam and increasing the NA of the objective lens. There was a problem.

Therefore, the present invention can reduce the crosstalk between adjacent tracks to a practically unproblematic level even if the track pitch and the pit pitch are set smaller than the beam spot diameter of the reproducing light beam. It is an object of the present invention to provide an optical disk and an optical disk device capable of dramatically increasing the density and capacity as compared with the above.

[0007]

[Means for Solving the Problems]

 In order to solve the above-mentioned problems, the present inventors approximated the pit shape on the optical disk with a circular stadium shape, and optimized the shape in the track width direction (disk radial direction) and the track direction (tangential direction), that is, the adjacent shape. We have found a pit shape that can suppress crosstalk between tracks to a practical level and can obtain a sufficient signal level for playback signals and push-pull signals.

That is, the optical disc according to the present invention has a track pitch of (0.72 to 0.8) × λ / NA / 1, where λnm is the wavelength of the reproduction light beam and NA is the numerical aperture of the objective lens. .14 μm, the upper width of the pit is (0.3 to 0.45) × λ / NA / 1. The inner wall of the pit has an angle of 50 ° to 70 °. Here, λ / NA / 1.14 μm means that the ratio of λ / NA is standardized at λ = 685 nm and NA = 0.6. In other words, if λ = 685 nm and NA = 0.6, the track pitch, the upper width of the pit, and the angle of the inner wall of the pit are the values shown in (), but if the values of λ and NA change, The optimum track pitch, pit top width and pit inner wall angle will change accordingly.

The optical disk according to the present invention is further characterized in that the pit depth on the optical disk is λ / 4.5 to λ / 6. Further, the optical disk device according to the present invention uses an optical disk having such a track pitch and a pit shape, irradiates the optical beam with an optical beam through an objective lens, detects the reflected light, and detects the reflected light. It is characterized in that it is configured to reproduce the information recorded in. The optical disc preferably records data obtained by compressing moving image information in accordance with MPEG2, and the optical disc device is provided with a decoder compatible with MPEG2 and has a function of expanding the compressed data to reproduce the original moving image information. .

[0010]

In a conventional optical disc, such as a CD, the track pitch is selected to be larger than the spot diameter of the reproduction light beam. On the other hand, in the present invention, the track pitch is (0.72 to 0.8) × λ / NA / 1.14 μm in order to increase the density and capacity of the optical disc, that is, from the spot diameter of the reproducing light beam. Use a small value. Then, under the track pitch in this range, the upper width of the pit is set to (0.3 to 0.45) × λ / NA / 1.14 μm, and the angle of the inner wall of the pit is set to a range of 50 ° to 70 °. More preferably, the pit depth is set in the range of λ / 4.5 to λ / 6. By selecting the pit shape in this way, the crosstalk amount between adjacent tracks can be suppressed to the crosstalk amount (-20 dB) or less required to reproduce the original information from the reproduced signal, and the reproduced signal can be suppressed. The level and push-pull signal level for tracking are sufficiently secured.

[0011]

【Example】

 First, before explaining the embodiments, the basic idea of the present invention will be explained. In order to increase the density of optical discs, it is necessary to reduce the beam spot diameter of the reproduction light beam, and for this purpose it is essential to shorten the wavelength of the reproduction LD and increase the NA of the objective lens. A low-noise LD (self-pulsation) with a wavelength of 685 nm and an output of about several mW has already been put to practical use, and an LD with a wavelength of 650 nm is nearing the level of practical use.

On the other hand, the NA of the objective lens is limited by the ease of making the lens and the tilt angle between the lens and the disk. The smaller the lens load (the smaller the optical disk substrate is) and the smaller the NA, the easier it is to make an objective lens, but any objective lens with an NA of about 0.6 can be realized with a single aspherical lens. However, in the objective lens used for the reproduction optical system of the optical disc, coma aberration occurs due to the inclination (tilt) between the optical disk and the reproduction light beam caused by the inclination of the optical disk or the inclination of the optical axis of the objective lens. .

That is, when the NA of the objective lens is increased to reduce the spot size of the reproduction light beam, the aberration of the objective lens sharply increases due to the tilt between the optical disc and the reproduction light beam. As the aberration of the objective lens increases, the amount of crosstalk between adjacent tracks naturally increases, and the reproduction resolution also decreases. The influence of this tilt is smaller as the substrate of the optical disc is thinner. Reference: Jpn.J.Appl.Phsys.Vol.32 (1993), pp.5402-5405, wavelength 690 nm, NA = 0.6, and substrate thickness 1.2 mm, which is the same as CD. It is shown how the beam spot shape of the reproduction light beam changes due to the tilt when the half of the value is 0.6 mm. According to this, when the substrate thickness is 1.2 mm, the tilt of 5 mrad reduces the center intensity of the beam spot by 10%, and the side lobe swelling and the aberration causing the crosstalk occur. . On the other hand, when the substrate thickness is 0.6 mm, it can withstand a tilt of up to 10 mrad.

FIG. 9 and FIG. 10 show the results of calculating the tilt characteristics when the substrate thickness (t) is 1.2 mm and 0.6 mm with NA as a parameter. The horizontal axis shows the tilt angle, and the vertical axis shows the normalized peak intensity of the reproduced signal. The wavelength (λ) of the reproducing light beam was set to 690 nm. When the substrate thickness is 0.6 mm and NA = 0.6, the peak intensity of the reproduced signal decreases by 10% when the tilt is 9.5 mrad. When this is observed for a substrate thickness of 1.2 mm, NA = 0.49. That is, the NA can be increased from 0.49 to 0.6 by increasing the substrate thickness from 1.2 mm of the conventional CD to 0.6 mm, and the areal recording density can be increased by about 1.5 times. . That is, since the spot size is proportional to λ / NA and the areal recording density is proportional to 1/2 ² of the spot size, the areal recording density is about 1.5 times higher than (0.6 / 0.49) ^2. .

However, if the thickness of the substrate is simply reduced, the warpage of the substrate due to temperature and humidity may become significant. The warp of the substrate is a major factor in tilt. The most effective way to deal with this is to double-side the optical disk as with the laser disk, that is, to make the optical disk have a symmetrical structure. In that case, information can be recorded on both the front and back sides. In a conventional single-disc optical disc such as a CD, the injection conditions at the time of manufacturing a substrate and the reflection film and the protective film made of aluminum are formed on one surface of the substrate, so that the moisture absorption of the substrate becomes asymmetrical. Warping is apt to occur. When the optical disc is double-sided, the distortion of the substrate due to such moisture absorption can be canceled and a large tilt can be prevented.

From the above examination results, if a combination of an LD having a wavelength of 685 nm, a substrate having a thickness of 0.6 mm, and an objective lens having NA = 0.6 is used, the wavelength is shortened from 780 nm to 685 nm. Due to the fact that the NA is increased from 0.45 to 0.6, it is possible to achieve a recording density about 2.3 times that of the conventional CD format even under the conventional CD design method. it can. That is, since the spot size is proportional to λ / NA, the recording density is about 2.3 times higher than (685 / 0.6) / (780 / 0.45). However, as described above, in order to obtain the capacity required to record the compressed moving image information by MPEG2 for 2 hours in the CD size, the recording density (capacity) should be about 5 times that of the conventional CD format. It needs to be doubled, which is not enough.

The present invention optimizes the pit shape under the same beam spot size as described above in order to achieve higher density and larger capacity of the optical disc, thereby achieving a low crosstalk characteristic and a reproduction signal and The track pitch can be further reduced while ensuring sufficient signal levels such as push-pull signals. Hereinafter, the pit shape according to the present proposal will be described in detail.

FIG. 1 is a diagram for explaining a pit shape in an optical disc according to the present invention. As shown in the figure, the shape of the pit 10 approximates a so-called soccer stadium shape with a trapezoidal cross section. The inner wall 11 of the pit 10 is a downward slope, and the bottom 12 is substantially flat. 13 is a cross section of the pit 10 in the radial direction of the optical disc (track width direction), 14 is a cross section in the circumferential direction of the optical disc (track direction), and Wm is the dimension of the upper part of the pit 10 in the track width direction (hereinafter referred to as upper width). Wi is the dimension of the bottom of the pit 10 in the track width direction (hereinafter referred to as the bottom width), hm is the depth of the pit 10, Zm is the length of the pit 10 in the track direction, and θ is the inner wall of the pit 10. 11 (angle formed by the inner wall 11 with respect to the surface of the optical disk).

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

FIG. 3 is a diagram schematically showing a pit arrangement on the optical disc for calculating the levels of the reproduction signal and the push-pull signal, where Pt is the track pitch (pit pitch in the track width direction). , The pit pitch (pit pitch in the track direction) is Pmy. Numerals 30 and 31 are beam spots of the reproducing light beam, which are respectively shown at the center of the pit (A) and at the middle of the pit (B). The amplitude of the reproduction signal is represented by S (A) -S (B). However, S (X) represents the output signal of the photodetector when the beam spot is at the X position. Also, 32 and 33 are push-pull signals (difference signal between output signals of at least two detection areas arranged in the track direction of the split photodetector) in the area (C) with pits and the area (D) without pits. Represent These push-pull signals are average pp values in the areas C and D, respectively.

FIG. 4 shows the size of the pit in the track width direction when the reproducing laser beam wavelength is 6785 nm, NA = 0.6, Zm = 0.5 μm, Pmy = 1 μm, and Pt = 0.72 μm. The results of calculating the levels of the reproduction signal and the push-pull signal using the pit depth hm as a parameter are shown. The values of the beam filling rates A / W (X) and A / W (Y) in the track width direction (X) and the track direction (Y) of the reproducing light beam are as shown in the figure. As shown in the figure, the levels of the reproduction signal and the push-pull signal do not largely depend on the pit shape except when Wm = 0.3 and Wi = 0.2. Also, there is no pit depth that maximizes the levels of the playback signal and push-pull signal at the same time, but trying to obtain the maximum playback signal level while minimizing the level drop of the push-pull signal. For example, it can be seen from FIG. 4 that the depth of the pit is preferably around λ / 5, preferably in the range of λ / 4.5 to λ / 6.

FIG. 5 is a diagram schematically showing the MTF (modulation transfer function) of the reproduction optical system and the pit arrangement on the optical disc used for evaluation of the crosstalk between adjacent tracks. In the figure, reference numerals 50 and 51 denote beam spots of the reproducing light beam, respectively, when passing through the center (A) of the pit and the position (B) separated by a distance td from the center of the pit. The MTF is represented by the power of the fundamental frequency component of the output signal of the photodetector obtained when the beam spot passes through the center of the pit. The crosstalk is represented by the power of the fundamental frequency component of the output signal of the photodetector obtained when the beam spot passes the position B.

In FIG. 6, the track pitch Pt is kept constant at 0.72 μm, the depth hm of the pit 10 is kept constant at 0.2 μm, and the upper width Wm of the pit 10 and the angle θ of the inner wall 11 of the pit 10 are variously changed. FIG. 6 is a diagram showing MTF and crosstalk characteristics in the case, where the horizontal axis represents the spatial frequency and the vertical axis represents the MTF and crosstalk. The values of the beam filling rates A / W (X) and A / W (Y) in the track width direction (X) and the track direction (Y) of the reproducing light beam are as shown in the figure.

Further, FIG. 7 shows the MTF and crosstalk characteristics when the upper width Wm of the pit 10 is kept constant at 0.35 μm under the same conditions as in FIG. 6 and only the angle θ of the inner wall 11 of the pit 10 is variously changed. FIG.

Here, as a modulation method of information recorded on the optical disc, a 4/9 modulation method, which is more efficient than the EFM used in the conventional CD, and (3,17) RLL (Run-Length Rimited) is used. ) Method shall be adopted. This method is an encoding method that converts 4 bits of original information into 9 bits and limits the continuous length of 0 to a range of 3 to 17, and the density ratio is improved by 20% when compared with EFM including DCC. To do. In this case, if the shortest pit length is 0.48 μm, the longest pit length is 2.16 μm. Therefore, it is necessary to pay attention to the crosstalk due to the low frequency component when the longest pit is detected.

The crosstalk characteristics shown in FIGS. 6 and 7 are for the case where there is no tilt, but in practice it is necessary to consider tilt. FIG. 8 shows MTF and crosstalk characteristics when tilt is taken into consideration. As shown in the figure, it can be seen that the MTF hardly changes in consideration of the tilt, but the crosstalk amount increases and becomes more severe.

In the system design of the optical disc device, if the tilt caused by the warp of the optical disc itself is 5 mrad and the tilt caused by the device is 3 mrad, it is necessary to allow a tilt of about 8 mrad as a whole. According to the simulation of FIG. 8, the crosstalk amount can be suppressed to a value of −20 dB or less required for practical use up to a tilt of ± 10 mrad with respect to the same spatial frequency. From this, it can be seen that the values of the wavelength of 685 nm and the track pitch of 0.72 μm are appropriate.

Considering the characteristics of FIGS. 6 and 7 from the above viewpoints, first, in the example in which the track pitch Pt is 0.72 μm shown in FIG. 6, the crosstalk amount (MTF characteristic) is up to Wm = 0.45 μm. And the MTF value difference between the crosstalk characteristic) is −20 dB or less. On the other hand, when Wm = 0.5 μm, the amount of crosstalk at a low frequency suddenly increases and exceeds −20 dB. In addition, the MTF characteristics are relatively good up to Wm = 0.3 μm, but deteriorate rapidly when Wm is less than 0.3 μm. Therefore, it can be said that Wm = 0.3 μm to 0.45 μm is an appropriate range.

On the other hand, regarding the angle θ of the inner wall 11 of the pit 10, as shown in FIG. 7, when θ = 80 ° or more, the crosstalk amount exceeds −20 dB, and when θ = 40 ° or less, the MTF characteristic is It deteriorates rapidly. Therefore, it can be said that θ = 50 ° C. to 70 ° C. is an appropriate range.

From the above results, the pit shape in the track width direction is normalized with a wavelength of 685 nm and NA = 0.6 (that is, λ / NA = 1.14), and the upper width Wm is (0.3 to It is understood that 0.45) × λ / NA / 1.14 μm, and the angle θ of the inner wall is preferably in the range of 50 ° to 70 °. That is, when the track pitch Pt is selected in the range of (0.72 to 0.8) × λ / NA / 1.14 μm and the track pitch is made smaller than the beam spot diameter of the reproduction light beam, By selecting the upper width Wm and the angle θ of the inner wall within the above range, the crosstalk amount can be suppressed to a value of −20 dB or less required for practical use up to a tilt of ± 10 mrad expected in an actual optical disk device. Therefore, it is possible to achieve a dramatic improvement in recording density. As a result, by combining these track pitches and pit shapes, the LD with a wavelength of 685 nm, the substrate with a thickness of 0.6 mm, and the objective lens with NA = 0.6, for example, a compressed video by MPEG2 with a CD size is obtained. The original task of recording image information for 2 hours can be easily achieved.

Next, the structure of the optical disc according to the present invention will be described. 11A and 11B are a perspective view and a cross-sectional view of the double-sided optical disc 100, showing one side of transparent substrates 101 and 102 made of a translucent resin such as polycarbonate or acrylic having embossed pits. Reflective films 103 and 104 made of aluminum or the like are deposited on the surfaces, and protective films 105 and 106 are formed thereon. The transparent substrates 101 and 102 have a thickness of 0.6 mm. Then, the transparent substrates 101 and 102 are bonded to each other with the protective films 105 and 106 facing each other, and an adhesive layer 107 made of a thermosetting adhesive and having a thickness of several tens of μm. A hole 108 for clamping is formed in the center of the optical disc 100, and a clamping zone 109 is provided around the hole 108. Then, the reproducing optical beam 110 emitted from the LD (not shown) and incident through the reproducing optical system is incident on the optical disc 100 from the transparent substrate 101, 102 side via the objective lens 111, and then on the reflecting films 103, 104. It is focused as a minute beam spot.

FIG. 12 shows an embodiment of an optical disk device for reproducing compressed moving image information using the above-mentioned optical disk 100. In FIG. 12, the optical disc 100 uses thin substrates 101 and 102 having a thickness of 0.6 mm, and thus is more susceptible to dust and dirt attached to the surface than a CD using a substrate having a thickness of 1.2 mm. , Cartridge 200. By housing the optical disc 100 in the cartridge 200, it becomes possible to live without worrying about how to hold the disc, dust, fingerprints, etc. like a CD, and it is advantageous in terms of handling and carrying. When a disc such as a CD is exposed, it is necessary to determine the error correction capability in consideration of an unexpected situation such as a scratch, but if the cartridge 200 is used, such consideration is unnecessary. Therefore, the LDC Read Solomon error correction method can be used in units of sectors as used in the recording / reproducing optical disc. As a result, when the optical disc is formatted in units of 2 k to 4 k bytes, the recording efficiency can be improved by 10% or more as compared with the CD.

As described above, if the 4/9 modulation method is used as the modulation method of the information recorded on the optical disc 100, and the track pitch on the optical disc 100 is 0.72 μm and the pit pitch is 0.96 μm, the conventional method is used. It is expected that the pit density ratio will increase by 3.84 times, the modulation method will increase by 20%, and the format efficiency will increase by 10% compared to the CD format of the above, so that a total capacity increase of about 5.1 times can be expected. Become. As mentioned above, when playing back moving image information such as movies with high image quality equivalent to S-VHS, the rate including the audio is 4.5 Mbps, so the capacity required for 2 hours of playback is 4G bytes. is there. By increasing the capacity by 5.1 times as described above, this capacity of 4 GB can be realized on one side of the disk. Furthermore, if the optical disc is double-sided as shown in FIG. 11, recording can be performed for up to 4 hours on a single optical disc.

Returning to FIG. 12, the optical disc 100 is chucked by the taper cone 220 and rotated by the spindle motor 201. The spindle motor 201 is driven by a spindle motor drive circuit 202. On the other hand, the reproduction optical system is constructed as follows.

An objective lens 203 is arranged so as to face the optical disc 100, and the objective lens 203 can be moved in the optical axis direction by a focus coil 204 and in the track width direction by a tracking coil 205. The oscillation wavelength of the LD (semiconductor laser) 207 driven by the LD driver 206 is 685 nm, and the light beam emitted from this LD 207 is collimated by the collimator lens 208 and then incident on the polarization beam splitter 209. To do. Since the light beam emitted from the LD 207 generally has an elliptic far-field pattern, if a circular pattern is required, a beam shaping prism may be arranged after the collimating lens 208. The light beam that has passed through the polarization beam splitter 209 is focused by the objective lens 203 and enters the optical disc 100.

The light reflected by the reflection film of the optical disc 100 returns to the objective lens 203 in the opposite direction to the incident light beam, is reflected by the polarization beam splitter 209, and is detected by the detection optical system such as the condenser lens 210 and the cylindrical lens 211. Then, the light is incident on the photodetector 212. The photodetector 212 is, for example, a 4-division photodetector, and its four detection outputs are input to an amplifier array 213 including an amplifier and an adder / subtractor, where a focus error signal, a tracking error signal and a reproduction signal are generated. The tracking error signal is obtained as the push-pull signal described above by a method called the push-pull method, for example. The focus error signal and the tracking error signal are supplied to the focus coil 204 and the tracking coil 205 via the servo controller 214, respectively. As a result, the objective lens 203 is moved in the optical axis direction and the track width direction, and the focusing of the light beam on the surface of the reflection film which is the recording surface of the optical disc 100 and the tracking on the target track are performed.

The reproduced signal from the amplifier array 213 is input to the signal processing circuit 215, where it is binarized, and then the data pulse is detected. The detected data pulse is input to the disk controller 216, subjected to format decoding and error correction, and then input to the MPEG2 decoder / controller 217 as a bit stream of moving image information. Data obtained by compressing (encoding) moving image information in accordance with the MPEG2 standard is recorded on the optical disc 100. Therefore, the MPEG2 decoder / controller 217 decompresses (decodes) the input bit stream to reproduce the original moving image information. The reproduced moving image information is input to the video signal generation circuit 218, and a blanking signal or the like is added to form a video signal of a predetermined television format.

[0038]

[Effect of device]

 As described above, the optical disc according to the present invention is an optimum pit that can reduce the crosstalk between adjacent tracks to a level that is practically required while setting the track pitch to a value smaller than the spot diameter of the reproducing light beam. Since it has a shape (pit upper width and pit inner wall angle), the track density can be increased by about 1.5 times compared to conventional CDs, and the level of the push-pull signal used for playback and tracking can be increased. Can be sufficiently secured.

As a result, according to the present invention, as shown in the above-described embodiment, it is possible to realize a capacity of about 5 times that of a conventional CD even with a CD size, for example, and a high image quality of 4 Mbps including audio is compressed. It is possible to store moving image information of VTR quality for two hours, and its practical effect is extremely large.

[Brief description of drawings]

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

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

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

FIG. 4 is a diagram showing the results of calculating the levels of a reproduction signal and a push-pull signal using the size of the pit in the track width direction and the depth of the pit as parameters.

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

FIG. 6 is a diagram showing pit shape dependence of crosstalk characteristics between an MTF of a reproduction optical system and an adjacent track.

FIG. 7 is a diagram showing pit shape dependence of crosstalk characteristics between the MTF of the reproduction optical system and adjacent tracks.

FIG. 8 is a diagram showing tilt dependence of crosstalk characteristics between the MTF of the reproducing optical system and adjacent tracks.

FIG. 9 is a diagram showing tilt dependence due to NA of an objective lens when an optical disc having a substrate thickness of 1.2 mm is used.

FIG. 10 is a diagram showing tilt dependence due to NA of an objective lens when an optical disc having a substrate thickness of 0.6 mm is used.

FIG. 11 is a perspective view and a sectional view showing the structure of an optical disc according to an embodiment of the present invention.

FIG. 12 is a block diagram showing a configuration of an optical disc device according to an embodiment of the present invention.

[Explanation of symbols]

 10 ... Pit 11 ... Pit inner wall 12 ... Pit bottom Wm ... Pit upper width θ ... Pit inner wall angle Zm ... Pit length hm ... Pit depth

Claims (4)

[Scope of utility model registration request] 1. An optical disc in which information is recorded on a substrate as a row of pits at a predetermined track pitch, and information is reproduced by irradiating a light beam through an objective lens. When the numerical aperture of the objective lens is NA and the track pitch is (0.72
0.8) × λ / NA / 1.14 μm, the upper width of the pit is (0.3 to 0.45) × λ / NA / 1.14 μm,
An optical disc, wherein the angle of the inner wall of the pit is 50 ° to 70 °. 2. An optical disc in which information is recorded on a substrate as a row of pits at a predetermined track pitch, and information is reproduced by irradiating a light beam through an objective lens. When the numerical aperture of the objective lens is NA and the track pitch is (0.72
0.8) × λ / NA / 1.14 μm, the upper width of the pit is (0.3 to 0.45) × λ / NA / 1.14 μm,
An optical disk characterized in that an angle of an inner wall of the pit is 50 ° to 70 ° and a depth of the pit is λ / 4.5 to λ / 6. 3. An optical disc having information recorded on a substrate as a row of pits at a predetermined track pitch, a means for irradiating the optical disc with a light beam through an objective lens, and the optical disc irradiated by the means. A means for reproducing information recorded on the optical disc by detecting reflected light of the light beam, wherein the optical disc has a wavelength of the light beam of λ nm and a numerical aperture of the objective lens of NA, The track pitch is (0.72 to 0.8) × λ / NA / 1.14μ
m, the upper width of the pit is (0.3 to 0.45) × λ /
NA / 1.14 μm, the angle of the inner wall of the pit is 50 °
An optical disk device characterized by an angle of up to 70 °. 4. An optical disk having information recorded on a substrate as a row of pits at a predetermined track pitch, a means for irradiating the optical disk with a light beam through an objective lens, and the optical disk irradiated by the means. A means for reproducing information recorded on the optical disc by detecting reflected light of the light beam, wherein the optical disc has a wavelength of the light beam of λ nm and a numerical aperture of the objective lens of NA, The track pitch is (0.72 to 0.8) × λ / NA / 1.14μ
m, the upper width of the pit is (0.3 to 0.45) × λ /
NA / 1.14 μm, the angle of the inner wall of the pit is 50 °
˜70 °, and the depth of the pit is λ / 4.5 to λ / 6.