HK1055503B - Optical disc and information reproducing apparatus for same - Google Patents

Description

Optical disc and information reproducing apparatus thereof

Technical Field

The present invention relates to an optical disc on which information is recorded in the form of a pit train, and an information reproducing apparatus for mounting such an optical disc and reproducing information thereon using an optical beam.

Background

Conventionally, a DVD (digital versatile disc) has been developed as an optical disc on which information is densely recorded. The DVD can record 2.6MB of data from one surface of one optical disc by irradiating the optical disc with a light beam having a wavelength of 650nm through an optical system having a numerical aperture NA of 0.6. A DVD is also capable of recording nearly one hour of video signal on one surface thereof.

Meanwhile, the basic recording time of a home video tape recorder is approximately two hours. In order to guarantee the operation of the video tape recorder on the optical disk and the player or information reproducing apparatus, it is necessary to record more data on the optical disk. Meanwhile, in order to realize processing such as editing by making full use of the random access function of the optical disc, it is necessary to record video signals for nearly three hours.

Also, a special optical disc for high density reproduction is required in the market. The transmission rate of digital HDTV (digital high definition television) that can be provided by digital television broadcasting of BS (broadcast satellite) is expected to be 20 to 24 mbps. A digital HDTV video signal as much as one movie, that is, about two and a half hours or 150 minutes, needs 22.5 to 27GB ═ 20-25Mbps)/(8(bits)/1000 × 150(min.) × 60 (sec.). basically, the reproduction capability of an information reproducing apparatus is determined by NA/λ obtained by one objective lens NA and one readout beam having a wavelength λ, so that it is possible to improve the recording/reproducing density of an optical disc by increasing NA and decreasing λ.

For example, japanese patent No. 2,704,107 discloses an optical disc having a track pitch of (0.72 to 0.8) × λ/NA/1.14 μm, a pit width or upper portion width of (0.3 to 0.45) × λ/NA/1.14 μm, a pit bottom width or lower portion width of (0.2 to 0.25) × λ/NA/1.14 μm, and assuming that the wavelength of a reproduction light beam is λ [ μm ], the numerical aperture of an objective lens is NA. In such an optical disc system, by selecting an appropriate pit pattern within the range of the track pitch, crosstalk between adjacent tracks can be suppressed to within a certain value.

With this related technique, when determining the track pitch and the pit width, as shown in fig. 1, a pit train of a single frequency is assumed to estimate (as crosstalk) the signal amplitude of a fundamental frequency component in a photodetector output signal obtained when the reproducing spot scans a spot deviated from the track center by a certain amount.

In order to record data at a larger density, for example, it can be assumed that λ is 405nm and NA is 0.85, and an optical disc using this technique can provide pits in a specific form with a track pitch TP of 0.301 to 0.334 μm, a pit upper width Wm of 125 to 188nm, and a pit bottom width Wi of 80 to 100 nm. However, when an optical disc using this technique is reproduced, the relationship between the track pitch and the pit width deviates from the optimum value. As a result, the signal-to-noise ratio, which represents the crosstalk signal amplitude and the main signal amplitude in the RF signal performance, exceeds-9 dB, so that it is impossible to secure sufficient system redundancy.

Disclosure of Invention

Accordingly, the present invention has been made in view of the above problems. An object of the present invention is to provide a new generation of optical disc and information reproducing apparatus capable of realizing recording of data at a higher density than a conventional DVD by using a reproduction light beam of a shorter wavelength and an optical system of a larger numerical aperture.

An optical disk according to the present invention comprises an information recording layer and a light transmitting layerA layer which records information in a pit train having a predetermined track pitch, a light-transmitting layer which is formed above the information recording layer so that the information is reproduced by a light beam which passes through an objective lens and passes through the light-transmitting layer toward the information recording layer, characterized in that the relation 0.194(λ/NA) is satisfied²≤TP×Tmin≤0.264(λ/NA)²Wherein TP is a track pitch, Tmin is a shortest pit length, λ is a beam wavelength, NA is a numerical aperture of the objective lens, and an upper pit width is 120nm or less in a range of the track pitch of 0.280 to 0.325 μm.

According to an aspect of the recording medium of the present invention, the width of the bottom of the pit is 40nm or more.

According to another aspect of the recording medium of the present invention, the wavelength λ of the light beam is 400 to 415nm, and the numerical aperture NA of the objective lens is 0.78 to 0.86.

An optical disc information reproducing apparatus according to the present invention is an information reproducing apparatus including: means for rotatably supporting an optical disc having an information recording layer for recording information in a pit train of a predetermined track pitch, and a light-transmitting layer formed above the information recording layer; a light source for emitting a light beam; an objective lens for focusing the light beam on the information recording layer through the light-transmitting layer of the optical disc; an illumination optical system for directing the light beam toward the objective lens; and a detection optical system including a photodetection means for guiding reflected light from the information recording layer through the objective lens toward the photodetection means; means for reproducing information recorded on the optical disc based on an output of the light detecting means; the light source is a blue semiconductor laser, the wavelength lambda of the blue semiconductor laser is 400-415 nm, and the numerical aperture NA of the objective lens is 0.78-0.86; the optical disk satisfies the relation 0.194 (lambda/NA)²≤TP×Tmin≤0.264(λ/NA)²Wherein TP is a track pitch, Tmin is a shortest pit length, and an upper pit width is 120nm or more in the range of 0.280 to 0.325 μm in the track pitchIs small.

In one aspect of the information reproducing apparatus according to the present invention, a bottom width of the pit is 40nm or more.

In order to solve the foregoing problems, the present inventors have devised an appropriate estimation method as a system adjacent at λ 405nm and NA 0.85, and found that it is further stabilized by using the same range of track pitch and pit width as an optical disc system. As a result, it was found that the track pitch should be set in the range of 0.280 to 0.325 μm and the pit width should be 120nm or less. Such an optical disc provides a value of crosstalk signal amplitude/main signal amplitude (which represents a good/bad in RF signal characteristics) of-9 dB or less upon reproduction, which is not a practical problem, thereby realizing a stable system.

Drawings

Fig. 1 is a diagram showing a pit arrangement on an optical disc for estimating crosstalk between adjacent tracks in the related art;

fig. 2 is a graph of a change in a comet aberration coefficient (comma aberration coefficient) due to a tilt of an optical disc;

FIG. 3 is a graph of a change in wavefront aberration coefficient according to an amount of defocus in an optical disc;

fig. 4 is an exemplary diagram of a pit arrangement according to the present invention, a pit having the shortest pit length is arranged in a single period on a track of an optical disc for calculating a main signal and a crosstalk signal in a crosstalk signal amplitude/estimation of a main signal amplitude;

fig. 5 is an exemplary diagram of a pit arrangement according to the present invention, a pit having the longest pit length is arranged in a single period on a track of an optical disc for calculating a main signal and a crosstalk signal in a crosstalk signal amplitude/estimation of a main signal amplitude;

FIG. 6 shows the minimum pit length at the track pitch TPThe product of Tmin is 0.044 μm²A three-dimensional graph in the case of showing a change in the value of crosstalk signal amplitude/main signal amplitude when the track pitch and pit width are changed;

FIG. 7 is a two-dimensional graph corresponding to FIG. 6;

FIG. 8 shows a diagram in which the product of the track pitch TP and the shortest pit length Tmin is 0.050 μm²A three-dimensional graph in the case of showing a change in the value of crosstalk signal amplitude/main signal amplitude when the track pitch and pit width are changed;

FIG. 9 shows a diagram in which the product of the track pitch TP and the shortest pit length Tmin is 0.060 μm²A three-dimensional graph in the case of showing a change in the value of crosstalk signal amplitude/main signal amplitude when the track pitch and pit width are changed;

FIG. 10 is a two-dimensional graph corresponding to FIG. 8;

FIG. 11 is a two-dimensional graph corresponding to FIG. 9;

FIG. 12 shows a relationship between the product of the track pitch TP and the shortest pit length Tmin of 0.194(λ/NA)²In this case, a three-dimensional graph of the change in the ratio of the crosstalk signal amplitude/main signal amplitude when the numerical apertures are respectively given as 0.78;

FIG. 13 shows a relationship between the product of the track pitch TP and the shortest pit length Tmin of 0.194(λ/NA)²In this case, a three-dimensional graph of the change in the ratio of the crosstalk signal amplitude/main signal amplitude when the numerical apertures are respectively given as 0.86;

FIG. 14 shows a relationship between the product of the track pitch TP and the shortest pit length Tmin of 0.194(λ/NA)²In this case, a three-dimensional graph of changes in values of crosstalk signal amplitude/main signal amplitude when the beam wavelength is given as 415nm, respectively;

FIG. 15 shows a relationship between the product of the track pitch TP and the shortest pit length Tmin of 0.194(λ/NA)²In this case, when the light beam wavelengths are respectively given 400nm, the crosstalk signal amplitude/main signalA three-dimensional plot of the variation in the value of the amplitude;

FIGS. 16 to 19 are two-dimensional graphs corresponding to FIGS. 12 to 15, respectively;

FIG. 20 is an exemplary diagram of pit patterns on an optical disc according to one embodiment of the present invention;

fig. 21 is a graph showing variations of Main signal amplitude Main (tmin), CrossTalk signal amplitude Cross Talk (Tmax) and CrossTalk signal amplitude/Main signal amplitude (Cross Talk/Main) in an optical disc of an embodiment of the present invention, in which the pit upper width is fixed at 100nm and the pit lower width is shifted from 0 to 100 nm;

fig. 22 to 24 respectively show typical sectional views of a main disc of an optical disc of an embodiment of the present invention;

fig. 25 is a typical schematic diagram of a model of a reproducing optical system of an optical disc information reproducing apparatus, showing a structure of an information recording/reproducing apparatus having an optical reading device, as an example of an embodiment.

Detailed Description

The basic idea considered by the present invention is first explained. The present inventors produced an optical disc by using an electron beam recorder (hereinafter abbreviated as EBR) so as to make pits with a narrow width (y.kojima, h.kitahara, o.kasono, m.katsumura and y.wada: "high density electron beam", jpn.j.appl.phys.37(1998) p 2137-2143). Also, the optimum track pitch and pit width of the optical disc system of the present invention are determined by optical simulation.

Coma aberration W occurring when an optical disc is tilted₃₁Is obtained by the following formula:

here, n is a refractive index of the optical disc substrate, Dth is a thickness of the optical disc substrate, NA is a numerical aperture, Dti is an optical disc tilt angle, and λ is a wavelength of the light beam.

In the conventional DVD, the substrate through which the reproduction beam is transmitted has a thickness of 0.6 mm. Accordingly, the system reproduction margin has been determined by the tilt of the optical disc with respect to the beam axis, particularly the coma aberration caused by the tilt in the radial direction. For a system, the most stringent of the radial tilt characteristics is that the track pitch and pit width must be sufficiently optimized to reduce crosstalk.

In the optical disc system of the present invention, if the coma aberration is considered to be caused by the tilt of the optical disc, when the thickness of the light-transmitting layer is 0.1mm as shown in fig. 2, the coma aberration W caused by the tilt of the optical disc is compared between the optical disc (NA, λ) of the present example (0.85, 405nm) and the conventional DVD (NA, λ) of the conventional DVD (0.60, 650nm)₃₁Is reduced.

On the other hand, the wavefront aberration in defocus is obtained by the following equation:

as can be seen from the above equation, when the optical disc is (NA, λ) — (0.85, 405nm), the same defocus amount Ddef causes the optical disc to be out of focusWavefront aberration amount W₂₀Is increased to about 3.8 times (0.60, 650nm) (see fig. 3) as compared with the conventional DVD (NA, λ). That is, in the optical disc system of the present invention, the defocus characteristic determines the system reproduction margin. At defocus, the crosstalk signal increases, as does the inter-symbol interference (which is interpreted as phase-cut crosstalk). Thus, for optical disc systems with stringent defocus characteristics, the track pitch and pit width need to be optimized to substantially reduce crosstalk while reducing inter-symbol interference.

Generally, inter-symbol interference (isi) can be suppressed by increasing the amplitude of the main signal.

Thus, the present invention builds a computational model for determining track pitch and pit width as follows:

(1) increasing the case where the product of the track pitch TP and the shortest pit length Tmin is constant (i.e., the recording density is constant) (hereinafter abbreviated as TP × Tmin);

(2) the estimated value is given not as a signal amplitude obtained by the reproduction light spot deviating from the center of the track to a point between the tracks but as a crosstalk signal amplitude/main signal amplitude;

(3) for calculating the main signal, the criterion used is that the pits with the shortest pit length are arranged in a single period on the track, as shown in fig. 4. This is because the inter-symbol interference does not easily occur under this condition. Meanwhile, in order to calculate a crosstalk signal, it is necessary that pits having the longest pit length are arranged on the track in a single period, as shown in fig. 5. This is because crosstalk does not easily occur under this condition. The crosstalk signal amplitude is given as a signal amplitude when the reproduction beam scans a spot exactly one track pitch from a spot on the track. Further, the sectional form of the dimple is an inclined surface, one side surface of which has a width of 0, i.e., an inclination angle of 90 degrees from the top surface to the bottom surface. The case of having an inclined surface angle in the cross section of the dimple will be described below.

Using this calculation model, the average value of (NA, λ) ═ 0.85, 405nm and TP × Tmin ═ 0.044 μm²In the case of (2), the track pitch (μm) and the pit width (nm) were changed, the variation of the crosstalk signal amplitude/main signal amplitude (dB) was simulated, and then the ranges of the determined track pitch and pit width were estimated. TP × Tmin ═ 0.044 μm²The value of (2) is recorded, which is about 27GB, under modulation (1, 7) RLL, with a 12cm diameter disc. This density is sufficient to record HDTV video images in an amount of 2 hours and 30 minutes, as described above.

Fig. 6 shows a three-dimensional graph of the simulation results. As can be seen from the figure, in the range of the track pitch of 0.24 to 0.36 μm and the pit width of 10 to 190nm, it can be determined that the distribution representing the bottom value is around the track pitch of 0.3 μm and the pit width of 10 to 40 nm.

Similarly, fig. 7 shows a contour plot of the simulation results. It is considered that there is almost no practical problem in the case where the crosstalk signal amplitude/main signal amplitude is-8 to-9 dB or less (the crosstalk signal is one third or less of the main signal). Larger pit widths generally favor suppression of the S/N because the ability to obtain the absolute amplitude of the host signal becomes greater. The track pitch, if wider, will be advantageous for tracking servo systems. That is, in fig. 7, it is desirable to provide a crosstalk signal amplitude/main signal amplitude of-9 dB or less, and in an area as far to the upper right as possible.

As can be seen from the above discussion, if the track pitch and pit width are selected without consideration of practical problems, the rectangular area a indicated by a thick line in fig. 7 is obtained, i.e., the track pitch is in the range of 0.27 to 0.325 μm and the pit width is 120nm or less.

Then, the verification calculation is performed with increasing the value of TP × Tmin (i.e., decreasing the recording capacity).

The three-dimensional graphs of FIGS. 8 and 9 respectively show simulation results in which values of TP x Tmin are 0.050 μm, respectively²And 0.060 μm². From these three-dimensional graphs, it can be seen that the distribution range is the range of the track pitch and the pit width at low values of the crosstalk signal amplitude/main signal amplitudeDistribution in the enclosure. Similarly, fig. 10 and 11 are contour graphs corresponding to fig. 8 and 9, respectively, from which it can be seen that even if the value of TP × Tmin is increased, the value of crosstalk signal amplitude/main signal amplitude can be kept small, lying in the bold line region a in fig. 10 and 11, i.e. within the range of 0.27 to 0.325 μm in track pitch and 120nm or less in pit width. That is, it should be understood that when TP × Tmin is 0.044 μm²(corresponding to a recording capacity of 27 GB), if the selection range is limited, increasing TP × Tmin (i.e., decreasing the recording capacity) is completely unproblematic.

Here, the value of TP. times.Tmin is expressed by (lambda/NA)²Is expressed by from 0.44 μm²To 0.060 μm²Is given a value ranging from 0.194 (lambda/NA)²To 0.264 (lambda/NA)²The range of (1).

As can be seen from the above discussion, for the signals from 0.194(λ/NA)²To 0.264 (lambda/NA)²With the value of TP × Tmin, crosstalk and inter-symbol interference can be suppressed to a small extent, so that there is no practical problem in the track pitch range of 0.270 to 0.325 μm and the pit width range of 120nm or less. If the (1, 7) RLL modulation is used, the above range corresponds to about 27.1 to 19.9GB of data recording in an optical disc having a diameter of 12 cm.

Then, the influence of changing the numerical aperture NA and the beam wavelength λ is considered. In this case, TP × Tmin is given as 0.194(λ/NA)²As the most stringent conditions in the above consideration.

Fig. 12 and 13 show three-dimensional graphs of simulation results for numerical apertures NA of 0.78 and 0.86, i.e., (NA, λ) ═ 0.78, 405nm and (NA, λ) ═ 0.86, 405 nm. Meanwhile, fig. 14 and 15 show three-dimensional graphs of simulation results for beam wavelengths λ of 415nm and 400nm, i.e., (NA, λ) ═ 0.85, 415nm and (NA, λ) ═ 0.85, 400 nm. As can be seen from these three-dimensional graphs, even if the numerical aperture NA and the beam wavelength λ are changed, the distribution in the range of the track pitch and the pit width is substantially desirable with a small value of the crosstalk signal amplitude/main signal amplitude. Similarly, fig. 16 to 19 are contour plots corresponding to fig. 12 to 15, respectively, from which it can be seen that, particularly when NA is given at 0.78, it is necessary to reduce the lower limit of the track pitch from 0.27 μm to 0.28 μm, as shown in fig. 16. In addition to this, it can be seen that the range of track pitches and pit widths is unproblematic.

By summarizing the above results, the following should be understood.

In an optical disk system, TP x Tmin of 0.194 (lambda/NA) is used²To 0.264 (lambda/NA)²In the range of NA from 0.78 to 0.86 and in the condition of wavelength λ from 400 to 415nm, by setting the track pitch in the range of 0.280 to 0.325 μm and the pit width in the range of 120nm or less, crosstalk and intersymbol interference can be suppressed to a small extent so that there is no practical problem.

On the other hand, in manufacturing a special reproduction optical disc, a laser beam recorder (hereinafter, abbreviated as LBR) has been used to form pits. However, LBR is limited to forming pits with a pit width of about 200 nm.

The use of EBR however makes it possible to achieve the pit width described above. The pits formed using EBR are much narrower in the pit-inclined portions than the pits formed using LBR. It is not possible, however, to conform the upper and lower widths (bottom widths) of the pits to the model used in the above considerations.

Therefore, when the pit upper width is fixed, the influence on reducing the pit lower width has been considered. Fig. 20 is a schematic diagram illustrating a pit form in the optical disc of the present invention. In this case, the inclination of the peripheral edge of the pit 10 is gradually decreased, and the bottom thereof is substantially flat. Reference numeral 11 is a cross-sectional view of the pit 10 in the radial direction of the disc (track width direction), 12 is a cross-sectional view in the circumferential direction of the disc (track direction), Wm is the length of the upper opening of the pit 10 in the track width direction (upper width), and Wi is the length of the bottom of the pit 10 in the track width direction (lower width). Assume that (NA, λ) ═ 0.85, 405nm and TP × Tmin ═ 0.194(λ/NA)²By fixing the upper width Wm of the pit to 100 μm, the lower width Wi of the pit changes from more than 0 to 100nm, as shown in fig. 20. For this optical disc, an optical disc having a track pitch of 0.30 μm and a shortest pit length of 0.147 μm corresponds thereto.

Fig. 21 shows a graph of the results of such a simulation. As can be seen from the figure, if the pit upper width is fixed, the variations of the main signal amplitude main (tmin) and the crosstalk signal amplitude Cross Talk (Tmax) are similar. Therefore, it can be understood that the value of the crosstalk signal amplitude/Main signal amplitude (Cross-Talk/Main) hardly changes even if the pit lower width is narrowed.

That is, it should be understood that the above-described ranges of the track pitch and the pit width are valid, independent of the value of the width of the lower portion of the pit.

However, if the width of the lower portion of the pit is too narrow, the pit will be changed when the pit is formed in a special reproduction optical disc, and it is difficult to obtain a pit pattern having good reproduction capability. That is, in manufacturing the master, the tilt angle of the concave side is difficult to control due to the type of the protective material, the distribution of the film thickness, and the change of the forming conditions, thereby causing a case where the concave surface of the pit is not formed as a bottom.

The reason why the formation of the pit is difficult to stabilize if the width of the lower portion of the pit is narrowed will be described in detail below.

In the manufacture of optical discs by EBR, a protective layer is applied over the master disc. It is rotated and exposed by irradiating an electron beam to the main disk. Then, the protective layer is developed, thereby forming recesses corresponding to the pits in the exposed areas.

Fig. 22 and 23 show typical schematic views of a cross section of a main disc, and the recessed surface of one pit is formed at an angle generally to the depth direction. This angle change depends not only on the type of the protective layer but also on the exposure intensity distribution, the development process, and the like, so that it is difficult to make precise control. Variations are to some extent unavoidable.

Fig. 22 and 23 show the case where the inclination of the recessed surface is changed, respectively, assuming that the pit depth is 64nm and the pit upper width is 110 nm.

Fig. 22 shows the manner in which the side slope angle has changed by ± 10 degrees from the center of the 75 degree angle. The width of the lower part of the pit varies depending on the slope, i.e., 76nm for a side slope of 75 degrees, 50nm for 65 degrees, and 99nm for 85 degrees.

Meanwhile, fig. 23 shows a manner in which the inclination angle of the side face has changed by ± 10 degrees from the angle of 50 degrees as a center. When the inclination angle of the side face is 50 degrees, the width of the lower portion of the pit is 2.6nm, almost providing a triangular section. Therefore, when the inclination angle is 60 degrees, the pit lower width is 36 nm. However, in the case of 40 degrees, pits cannot be formed to a required depth of 64nm (λ/6.25). As can be seen from this, if the width of the lower portion of the pit is too narrow in the manufacture of the master disc, it is difficult to stably form the pit having a desired depth.

As the pit width becomes narrower, injection molding of resin using a disc substrate (stumper) made of a main disc becomes difficult. The result of the limit test obtained by the experiment was that the minimum width was 40nm for a pit having a depth of 60nm (λ/6.66) and a tilt angle of about 90 degrees. Therefore, it should be considered that this shaping is possible, in which the pit lower width is 40nm or more and the pit upper width is larger than this width. Based on this assumption, the cross section of the main disc is assumed assuming that the width of the lower portion of the pit is 40 nm. In this case 61 degrees. It is assumed that the inclination angle is deviated from the point as the center by ± 10 degrees as shown in fig. 24. Thus, when the pit lower width is 40nm, the bottom of the recess cannot be made shallow even if the tilt angle is reduced by 10 degrees due to variations in manufacturing.

From the results of the repetition of the above experiments, it was found that the pit-bottom width of the optical disc should be greater than or equal to 40 nm.

Combining all the considerations described above, the following can be found.

In a lightIn the disk system, the amount of the catalyst used is 0.194(λ/NA) in TP × Tmin²To 0.264 (lambda/NA)²The conditions of NA from 0.78 to 0.86 and wavelength λ from 400 to 415nm, by setting the track pitch to 0.280 to 0.325 μm, the pit width to 120nm or less, and the pit lower width to 40nm or more, a special reproduction optical disc having crosstalk and inter-symbol interference suppressed to a small extent without practical problems can be stably manufactured. The pit width is a pit maximum width.

Fig. 25 is a schematic diagram of a reproducing optical system of an optical disc information reproducing apparatus according to the present invention, showing a schematic structure of an optical recording/reproducing apparatus provided with an optical reading device. An optical reader has a blue semiconductor laser LD1 for emitting a short wavelength blue light portion having a wavelength of about 400nm to 415nm, preferably about 405 nm.

The optical reader has a polarization beam splitter 13, a collimating lens 14, an 1/4 wavelength plate 15 and two sets of lens cells 16. Through the above illumination optical system, the laser beam emitted from the semiconductor laser LD1 is converted into a collimated beam by the collimating lens 14. The light beam then passes through the polarization beam splitter 13 and is transmitted through the 1/4 wavelength plate 15, and is converged by the objective lens unit 16 onto the optical disc 5 placed around the focal point thereof. Thus, a light spot is formed on the pit train on the information recording surface of the optical disc 5.

In addition to the illumination optics described above, the optical reader also includes a light detection optics, such as a detection lens 17. The objective lens unit 16, 1/4 wavelength plate 15 and polarization beam splitter 13 are also used in the light detection optical system. The light reflected on the optical disk 5 is condensed by the objective lens unit 16, passes through the 1/4 wavelength plate 15, and is then directed toward the detection focusing lens 17 by the polarizing beam splitter 13. The condensed light condensed by the detection lens 17 passes through an astigmatism generating element (not shown), such as a cylindrical lens or a multiple lens, to form a spot at the center of or around the light receiving surface 19 of the photodetector.

Meanwhile, the light receiving surface 19 of the photodetector is connected with the demodulator 30 and the error detection circuit 31. The error detection circuit 31 is connected to a drive circuit 33 for driving a mechanism including the actuator 26 for tracking control and focus control of the objective lens unit.

The photodetector supplies an electric signal, which is related to the image of the light spot formed in the center of or around the light receiving surface 19, to the demodulation circuit 30 and the error detection circuit 31. The demodulation circuit 30 generates a recording signal based on the electric signal. The error detection circuit 31 generates a focus error signal, a tracking error signal, and other servo signals from the electric signals, and supplies the respective drive signals to the respective actuators via the drive circuits 33 with respect to the actuators. These actuators drive the objective lens unit 16 and the like under servo control in accordance with respective drive signals.

The structure of the optical disc according to the present invention is explained below. A circular substrate made of a light-transmitting resin (e.g., polycarbonate or acrylic resin) on one surface of which a reflective film of aluminum or the like is deposited has the above-described embossed pits formed thereon with a light-transmitting layer having a thickness of 0.1 mm. The reading is performed on the light-transmitting layer side. Meanwhile, the two substrates are opposed and bonded together by a thermosetting type adhesive layer or the like, and a double-sided disk can be formed. The center of the optical disk is provided with a clamping hole, and a clamping area is arranged around the clamping hole.

Industrial applicability

As described above, the optical disc according to the present invention has a pit form in which the track pitch and the pit width are optimized, thereby reducing the crosstalk signal amplitude/main signal amplitude representing superiority and inferiority in the reproduced RF signal characteristics to-9 dB or less, thereby ensuring sufficient system margin and greatly increasing the information recording density as compared to the DVD.

Claims (5)

1. An optical disc comprising an information recording layer for recording information in a pit train having a predetermined track pitch and a light transmitting layer formed above said information recording layer so that said information is reproduced by a light beam directed to said information recording layer through an objective lens and through said light transmitting layer, characterized in that the relation 0.194(λ/NA) is satisfied²≤TP×Tmin≤0.264(λ/NA)²Wherein TP is a track pitch, Tmin is a shortest pit length, λ is a beam wavelength, NA is a numerical aperture of the objective lens, and at a track pitch of 0.280 to 0Upper pit width in the range of 325 μm of 120nm or less.

2. The optical disc of claim 1, wherein: the width of the bottom of the pit is 40nm or more.

3. Optical disc according to claim 1 or 2, characterized in that: the wavelength lambda of the light beam is 400 to 415nm, and the numerical aperture NA of the objective lens is 0.78 to 0.86.

4. An information reproducing apparatus comprising: means for rotatably supporting an optical disc having an information recording layer for recording information in a pit train of a predetermined track pitch, and a light-transmitting layer formed above the information recording layer; a light source for emitting a light beam; an objective lens for focusing the light beam on the information recording layer through the light-transmitting layer of the optical disc; an illumination optical system for directing the light beam toward the objective lens; and a detection optical system including a photodetection means for guiding reflected light from the information recording layer through the objective lens toward the photodetection means; means for reproducing information recorded on the optical disc based on an output of the light detecting means;

the light source is a blue semiconductor laser, the wavelength lambda of the blue semiconductor laser is 400-415 nm, and the numerical aperture NA of the objective lens is 0.78-0.86; the optical disk satisfies the relation 0.194 (lambda/NA)²≤TP×Tmin≤0.264(λ/NA)²Wherein TP is a track pitch, Tmin is a pit shortest length, and an upper pit width is 120nm or less in a range of the track pitch of 0.280 to 0.325. mu.m.

5. The information reproducing apparatus according to claim 4, characterized in that: the width of the bottom of the pit is 40nm or more.