JP2000030299A - Recording medium and optical head device applicable to device for recording and reproducing information suitable therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a numerical aperture of an objective lens and a material for a transparent layer unaffected by the thickness of a transparent layer protecting a recording material. SOLUTION: An optical head device 3 has a laser element to emit a laser beam and an objective lens 36 to converge laser beams from the laser element at a prescribed position and a prescribed depth of a recording surface of an optical disk D, and a mumerical aperture NA of the objective lens is set to satisfy NA4×d<=0.0744 according to a thickness d of a transparent protective layer for protecting the recording layer 103 (113) of the optical disk, i.e., transparent substrates 101 and 111. As the result, without being affected by spherical aberration derived from the transparent substrate which is the protective layer, the recording density is enhanced. By specifying a material for the protective layer in accordance with the numerical aperture of the subjective lens, the recording density is enhanced without adding specified structure to the optical disk device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a recording medium for recording information using a light beam and an information recording / reproducing apparatus for recording information on the recording medium or reproducing information from the recording medium.

[0002]

2. Description of the Related Art At present, a high-density DVD (Digital Versatile Disk) type optical disk capable of reproducing moving images for about 2 hours has been put to practical use, and the technology has been applied to an auxiliary storage device used in a personal computer and the like. I have.

The above-mentioned DVD-type optical disk, which is capable of recording and rewriting information, is a DVD-R.
There are many proposals for AM disks. In addition, there is a demand for further densification, and development of various elemental technologies for realizing this is also in progress.

In order to increase the recording density, it is effective to form finer recording marks on the recording surface of the disk. The size of the recording mark is obtained by condensing a light beam (coherent light, mainly using a laser beam) applied to the recording surface of the disk by an objective lens (lens device). It is defined by the size of the focused spot.

[0005] However, the size of the focused spot is
As is well known, since the wavelength is proportional to the wavelength of the laser beam emitted from the light source and inversely proportional to the numerical aperture NA of the objective lens, the wavelength of the light emitted from the light source must be increased in order to increase the recording density. Must be shorter. The wavelength is 780 to 830 nm for a compact disk, which is an initial product of an optical disk, and the wavelength of 685 to 635 nm belonging to the red region is currently in practical use.
At present, many semiconductor laser devices capable of emitting a laser beam in a blue-violet or blue wavelength range have been proposed.

On the other hand, in order to increase the recording density, it is necessary to increase the numerical aperture NA of the objective lens.
Symposium on Optical Memory and Optical Data Stora
ge, 345-347, (OFA2-1) 1996, a method has been proposed in which an objective lens is constructed using two lenses to obtain a high numerical aperture.

[0007]

However, in the method of increasing the numerical aperture NA described above, the optimum value of the thickness of the transparent layer, which is the protective layer of the disk, is not defined, and the method is widely used for mass-production disks. When a resin material typified by polycarbonate, acrylic, or the like is processed into a disk by injection molding, there is a problem that a certain manufacturing error is included in the thickness of the transparent layer.

[0008] The thickness error gives a spherical aberration to the condensed spot, and thus deteriorates the reproduction and recording characteristics.
Since the spherical aberration increases in proportion to the thickness of the transparent layer and in proportion to the fourth power of the numerical aperture NA, in an optical disc apparatus using an objective lens having a large numerical aperture NA,
There is a problem that reproduction and recording characteristics are largely changed by a slight change in thickness of the transparent layer.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems. In a system for increasing the recording density by increasing the numerical aperture NA of an objective lens and decreasing the spot size, a transparent material for protecting a recording material is used. Manufacturing conditions of optical discs that can easily set the relationship between the numerical aperture of the objective lens, the thickness of the transparent layer and the refractive index that are not affected by errors in the thickness of the layer, and the requirements of an optical head device for an information recording / reproducing device Is to provide.

[0010]

SUMMARY OF THE INVENTION The present invention has been made based on the above-mentioned problems. A recording material is provided on one surface of a substrate, and the recording material is supplied to the recording material via a lens device. By irradiating coherent light, in a recording medium capable of reproducing and recording information, a distance between a surface of the transparent protective layer that protects the recording material on the side where the light is incident and the recording material Where d is the numerical aperture of the lens device and NA is the numerical aperture of the lens device, wherein NA ⁴ × d ≦ 0.0744 is satisfied.

Further, according to the present invention, a recording material is provided on one surface of a substrate, and the recording material is irradiated with coherent light supplied through a lens device to reproduce and record information. A distance d between the surface of the transparent protective layer that protects the recording material on the light incident side and the recording material, and a refractive index n of the material used for the protective layer. Is expressed as: (n ² −1) / 8n ³ × NA ⁴ × d ≦ where NA is the numerical aperture of the lens device.
A recording medium characterized by satisfying 3.52 × 10 ⁻³ is provided.

Further, according to the present invention, a recording material is provided on one surface of a substrate, and the recording material is irradiated with coherent light supplied through a lens device to reproduce and record information. In a recording medium capable of recording, the numerical aperture of the lens device is NA, the distance between the light incident surface of the transparent protective layer for protecting the recording material and the recording material is d, When the refractive index of the material used for the layer is n, (n ² -1) / 8n ³ × NA ⁴ × d
≦ 3.52 × 10 ⁻³ is provided.

Still further, according to the present invention, there is provided a recording material in which a state is changed by irradiating a light beam, and information is selectively recorded according to a difference in light intensity of the light beam;
In an optical head device for irradiating a recording medium having a transparent protective layer provided so as to cover the recording material with respect to a support supporting the recording material, a numerical aperture NA of an objective lens of the optical head device is provided. Is a distance d between the surface of the protective layer of the recording medium on the side where the light is incident and the recording material.
The optical head device is set to satisfy NA ⁴ × d ≦ 0.0744.

Still further, according to the present invention, there is provided a recording material in which a state is changed by irradiating a light beam, and information is selectively recorded according to a difference in light intensity of the light beam;
In an optical head device for irradiating a recording medium having a transparent protective layer provided so as to cover the recording material with respect to a support supporting the recording material, a numerical aperture NA of an objective lens of the optical head device is provided. Is the distance between the recording material and the surface of the transparent protective layer that protects the recording material, on the side where the light is incident, where d is the refractive index of the material used for the protective layer, (N ² -1) / 8n ³ × NA ⁴ ×
An optical head device characterized by being set so as to satisfy d ≦ 3.52 × 10 ⁻³ .

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 schematically shows an optical disk device to which an embodiment of the present invention is applied. An optical disk device 1 includes an optical disk D as a recording medium.
An optical head device 3 for writing information by reading coherent light, for example, a laser beam, or reading information already recorded from the optical disk D, and information read from the optical disk D by the optical head device 3 , A control signal for controlling the position of the optical head device 3 and the positional relationship between the objective lens of the optical head device 3 and the optical disc D, and information to be recorded for writing information on the optical disc D are recorded signals. A signal processing unit 5 for conversion; a disk motor 7 for rotating the optical disk D at a predetermined speed; a position of the optical head device 3; a positional relationship between the objective lens of the optical head device 3 and the optical disk D; And a control unit 9 for controlling

The optical head device 3 controls the irradiation position of the laser beam irradiating the optical disk D, that is, the position of the objective lens described later in detail, under the control of the control unit 9, and communicates with the signal processing unit 5 in a predetermined manner. By exchanging signals, information is recorded on the optical disc D or the optical disc D
Extract information from

The signal processing unit 5 is electrically connected to the optical head device 3, the control unit 9, and an external device 99 represented by, for example, a host computer. A laser beam emitted from the optical head device 3 to the optical disk D and a laser beam reflected by the optical disk D for taking out recorded information or writing and recording information on the optical disk D are transmitted to the subsequent stage of the optical head device 3. The control unit 9 outputs a position control for controlling the position of the optical head device 3 to a photodetector (photodetector) to be described in detail so that the position of the optical head device 3 can be controlled, and the control unit 9 can control the rotation speed of the optical disk D rotated by the disk motor 7. The instruction signal is transmitted to the control unit 9. The signal processing unit 5 also instructs the control unit 9 to further control the irradiation position of the laser beam and the rotation of the optical disk D by the disk motor 7 based on the information signal obtained from the optical disk D by the optical head device 3. At the same time, the information signal from the optical disc D is subjected to predetermined signal processing such as decoding, and then transmitted to the external device 99.

The control unit 9 is electrically connected to each of the optical head device 3, the signal processing unit 5 and the disk motor 7, and based on an instruction signal from the signal processing unit 5, Control signal is transmitted to the optical disk D, and the irradiation position of the laser beam emitted from the optical head device 3 to the optical disk D and the rotation speed of the optical disk D by the disk motor 7 are controlled.

The disk motor 7 is electrically connected to the control unit 9 and rotates the optical disk D at a predetermined rotation speed based on a control signal from the control unit 9. In the optical disk device 1 described above, the signal processing unit 5 receives a command signal for reproducing or recording information on the optical disk D from the external device 99. Based on the command signal, the signal processing unit 5 exchanges an electric signal with the optical head device 3 and transmits a control signal to the control unit 9. Based on the transmitted control signal, the control unit 9 controls the position of the optical head device 3 to irradiate a predetermined position of the optical disk D with a laser beam and rotates the disk motor 7 at a predetermined speed. .

The optical head device 3 irradiates the optical disk D with a laser beam at the above-mentioned predetermined position based on a control signal exchanged with the signal processing unit 5, and records information on the optical disk D. A pit row, which is recording information, is formed at a predetermined position. On the other hand, in reproducing (reading) information, the laser beam reflected by the optical disk D is received, and an electric signal corresponding to the light intensity of the received laser beam is output to the signal processing unit 5.

The signal processing unit 5 converts the information recorded on the optical disc D and the laser beam from the information obtained from the optical disc D via the optical head device 3 and the output signal corresponding to the laser beam reflected by the optical disc D. In order to receive information on the beam irradiation position, and to optimally set the position of the optical head device 3 and the position of the objective lens, the rotation speed of the optical disk D, and the like, the control unit 9 sends a predetermined control signal to the control unit 9. The electric signal corresponding to the information recorded in the electronic device is subjected to processing such as decoding, and the processed electric signal is output to the external device 99.

An external device 99 receiving an electric signal corresponding to the information recorded on the optical disc D from the signal processing unit 5
Transmits an instruction signal to the signal processing unit 5 again so as to prompt the optical disk apparatus 1 to operate based on the output signal.

By repeating a series of operations as described above, the optical disc apparatus 1 records information on the optical disc D,
Alternatively, information is reproduced from the optical disc D. Next, FIG.
The structure of the optical head device 3 will be described with reference to FIG.

The optical head device 3 is fixed on a base 31 and has a laser light emitting / receiving unit (hereinafter, referred to as a fixed optical system) provided with a structure described in detail later with reference to FIG.
31a and an actuator 31 shown below with reference to FIG.
b.

The actuator 31b includes a slider base 33 and a slider base 3 which are movable along a pair of guide rails 32 arranged in parallel with the base 31.
3, a rising mirror 35 that reflects the laser beam from the fixed optical system 31a toward the recording surface of the optical disk D, and reflects the laser beam reflected by the rising mirror 35 on the recording surface of the optical disk D. And a lens holder 37 for holding the objective lens 36 movably in a direction parallel to the recording surface of the optical disc D and across a guide groove (or pit row) formed on the recording surface. have. In addition,
The numerical aperture NA of the objective lens 36 is determined by the thickness of the protective layer of the optical disk D (a transparent substrate provided between the recording layer of the optical disk and the objective lens shown in FIG. 6), which will be described later with reference to FIG. The predetermined numerical aperture is set based on the distance d between the incident surface and the recording layer. The slider base 33 is, as shown in FIG.
It is configured to be movable in the radial direction of the optical disk D along the guide rail 32 by a propulsive force provided by a coil 34 and a yoke (not shown).

The fixed optical system 31a, as shown in FIG.
It has a housing 10 made of, for example, aluminum (Al). A laser element (semiconductor laser) 11 that generates a laser beam L having a predetermined wavelength, for example, about 650 nm is fixed to one end of the housing 10.

In the direction in which the laser beam L emitted from the semiconductor laser 11 travels, a collimator lens 12 for collimating the divergent laser beam L is arranged. In the direction in which the laser beam L collimated by the collimator lens 12 is guided, an ellipse for correcting the cross-sectional beam shape of the laser beam L emitted in an ellipse from an ellipse to a circle in relation to an aspect ratio specific to the laser beam L The laser beam L formed integrally with the correction prism 13a and having a cross-sectional shape corrected to be substantially circular
b, that is, the light is passed through the first beam splitter 13 and the beam splitter 13 for passing the laser beam L ′ reflected by the recording surface (not shown) of the optical disk D from the laser beam L traveling toward the optical disk D while passing the laser beam L ′ toward the optical disk D. The direction of the plane of polarization of the laser beam L directed to the actuator 31b is changed from linearly polarized light to circularly polarized light, and the direction of the plane of polarization of the reflected laser beam L 'reflected by the optical disk D is directed from the circularly polarized light to the actuator 31b. A λ / 4 plate (retarder) 14 for converting linearly polarized light whose polarization direction is rotated by 90 ° with respect to the direction of the polarization plane of the laser beam L is arranged in order.
The beam splitter 13 is a known polarization beam splitter.

The optical disk D by the beam splitter 13
In the direction in which the reflected laser beam L 'separated from the laser beam L traveling toward
And two reflected laser beams L′ a and L′ b
A second beam splitter 15 of a half mirror type for splitting into two is arranged.

In the direction in which one of the two reflected laser beams L'a split by the beam splitter 15 is guided, a converging lens 16 for giving the reflected laser beam L'a predetermined image forming characteristics and convergence. Is arranged.

In the direction in which the reflected laser beam L'a, which has been given convergence and predetermined imaging characteristics by the converging lens 16, travels, the converging lens 16 reflects the reflected laser beam L'a.
Lens 1 for improving aberration due to convergence given to lens
7. Reflected laser beam L'a passed through concave lens 17
In addition, a cylindrical lens 18 for providing a predetermined imaging characteristic for detecting a focus shift described later, and a reflected laser beam L′ a having a predetermined imaging characteristic given by the cylindrical lens 18 are received, and the reflected laser beam A first photodetector 19 that outputs an output signal corresponding to the light intensity of the beam L'a is arranged in order.

In a direction in which the reflected laser beam L'b split into two by the beam splitter 15 is guided, a mirror (right-angle prism) 20 for guiding the reflected laser beam L'b reflected on the optical disk D in a predetermined direction. Is arranged.

In the direction in which the reflected laser beam L'b bent by the mirror 20 travels, a converging lens 21 for giving a predetermined convergence to the reflected laser beam L'b is arranged.

In the direction in which the reflected laser beam L'b having a predetermined convergence given by the converging lens 21 is guided, a photodetector 22 used for detecting a track shift and an offset amount described later is arranged. Have been.

FIG. 5 shows the actuator 31 shown in FIG.
The lens holder 37 holding the objective lens 36 has a bearing portion 37a substantially at the center, and the objective lens 36 is positioned on the circumference of a concentric circle having a predetermined radius centered on the bearing portion 37a. The lens holding surface 37b holding the lens 36 and a direction perpendicular to the lens holding surface 37b are formed in a cylindrical shape with a part cut away. The bearing 37a
Is formed rotatably around the shaft 39 by engaging a bearing portion 37a with a shaft 39 extending from a substantially center of a lens holder base 38 fixed to a predetermined position of the carriage 33. Have been. Further, two sets of coils 40 and 40 are provided on the cylindrical portion of the lens holder 37, that is, the cylindrical surface 37c, at positions defined so as to divide the outer periphery of the cylindrical surface 37c into approximately four equal parts along an axis passing through the bearing portion 37a. 40 and 4
1, 41 are provided.

The lens holder base 38 also has a shaft 39.
A yoke 4 having a shape in which a part of a cylinder is cut off at a position corresponding to the circumference of a concentric circle with an arbitrary radius whose radius is increased as compared with the cylindrical surface 37c of the lens holder 37 with the center axis as a central axis.
2 are formed. At a predetermined position on the inner wall of the yoke 42, two sets of magnets 43, 43 and 4 for providing a magnetic field in a predetermined direction toward the cylindrical surface 37 c of the lens holder 37.
4, 44 are provided. The magnets 43, 43
The N and S poles are magnetized in such a manner that the N and S poles are divided into two parts by a plane orthogonal to the optical axis of the objective lens 36.
N is divided into two parts by a plane parallel to the optical axis of the objective lens 36.
The pole and the S pole are magnetized.

The objective lens 36 is connected to the lens holder 3.
7 and reciprocation in the axial direction, the optical disc D
Are movably held in a direction parallel to the recording surface of the optical disk D and in a tracking direction orthogonal to a guide groove (not shown) formed in advance on the recording surface of the optical disk D and a focus direction orthogonal to the recording surface of the optical disk D. I have.

The objective lens 36 is a semiconductor laser 1
1 has an effective diameter that gives an aperture ratio higher than 0.6 with respect to the wavelength of 650 nm of the laser beam L emitted from the laser beam L.
Fo = Fo = 3.3mm, effective diameter is about 4m
m. In addition, the wavelength of the usable laser beam L is
For example, 635 to 685 nm.

As an optical disk D that can be used in the optical disk apparatus 1 shown in FIGS.
As described below with reference to FIG. 6A or FIG. 6B, the transparent substrate 1 having a predetermined thickness d (approximately d = 1.2 mm) is used.
A pit row or a guide groove 102 is formed on one surface of the recording layer 103, a recording layer 103 is formed on the pit row (guide groove) 102 with a predetermined thickness, and the recording layer 103 is formed on a resin layer 104
And the thickness t of the entire disk is set to approximately 1.2 mm (FIG. 6A), or a predetermined thickness d, for example, d = 0.58 mm, and the transparent thickness of less than 0.6 mm A guide groove 112 is formed on one surface of the substrate 111, and a single-sided recording medium 114 having a recording layer 113 formed on the guide groove 112 with a predetermined thickness is provided with an adhesive layer 115 having a predetermined thickness. Similarly, a single-sided recording medium 114 composed of a transparent substrate 111, a guide groove 112, and a recording layer 113 is attached to each other so that the thickness t of the entire disc is approximately 1.2 mm (FIG. 6B). Proposed. In addition,
The transparent substrates 101 and 111 are respectively
4 and also functions as a protective layer for the recording layers 103 and 113 formed on the adhesive layer 115.

The thickness d of each of the transparent substrates 101 and 111 in each optical disc D is closely related to the aperture ratio NA of the objective lens 36 of the actuator 31b of the optical head device 3, and when the aperture ratio NA is high. The beam diameter of the laser beam transmitted through the transparent substrate 101 or 111 and converged on the recording layers 103 and 113 when the thickness d of the transparent substrate 101 or 111 fluctuates from the reference thickness d ₀ during manufacturing. For
There is a problem of causing spherical aberration. Also, in many cases,
For the transparent substrates 101 and 111, a resin material typified by, for example, PMMA (acryl) or PC (polycarbonate) is used. Therefore, the difference in the refractive index n depending on the material is such that the objective lens 36 is set to an arbitrary value. There is a problem that the beam diameter of the laser beam converged on the recording layers 103 and 113 fluctuates with respect to the aperture ratio NA.

Under the above conditions, the size of the recording mark formed on the optical disc D is 0.4 to 0.44 in the mark width (the direction orthogonal to the tangential direction of the guide groove).
μm, the mark length (in the direction along the guide groove) is 0.63 μm to 2.31 μm, and the depth of the mark is approximately 0.7 μm.
The interval between the marks in the tangential direction of the guide groove is approximately 0.74 μm.

Incidentally, there is a document, for example, “Optical Disc Technology” between the thickness error Δd of the transparent substrates 101 and 111, the refractive index n, and the numerical aperture NA of the objective lens 36.
(Onoe et al., Radio Technology Company, 1989), page 62 and others, That is, it can be regarded as a spherical aberration coefficient unique to the disk D.

As one of the techniques for increasing the recording density, it is recognized that the recording density can be increased by increasing the numerical aperture NA of the objective lens. However, as shown in the equation (1), the recording density is converged on the recording layer of the optical disc D. The effect of the spherical aberration exerted by the transparent substrate on the beam diameter of the laser beam to be emitted increases in proportion to the fourth power of the numerical aperture NA of the objective lens.

That is, in order to increase the recording density,
When the aperture ratio NA of the objective lens 36 is increased, high precision is required for the management width of the thickness d of the transparent substrates 101 and 111. As a requirement for the thickness d, when the thickness d of the transparent substrates 101 and 111 deviates from the reference thickness d ₀ , the light passes through the transparent substrates 101 and 111 and is converged on the recording layers 103 and 113. Since the beam quality of the laser beam is adversely affected (especially, spherical aberration is increased), the upper limit value of the thickness d of the transparent substrates 101 and 111 must be set as a function of the numerical aperture NA of the objective lens 36. .

By the way, in an optical disk device having a number of product records so far, the numerical aperture NA of an objective lens is large.
Is about 0.5 or less, and the refractive index n is 1.5740 in a polycarbonate resin, for example.

For a disc which has been used as an optical disc D such as a compact disc for music (CD), the thickness d of the transparent substrate (protective layer) is 1.2 m.
m, the thickness tolerance at this time (allowable range of fluctuation)
Is about 0.05 mm. If this is simply calculated based on this, the allowable thickness error per 1 mm of the total thickness of the disc is 0.05 × 1.0 / 1.2 = 0.042 mm.

From the above requirements, when the recording density is increased by increasing the numerical aperture NA of the objective lens 36, the disk device 1 can be used without using any special measures unnecessarily.
Transparent substrates (protective layers) 101 and 111 of the optical disc D capable of securing the same or better performance than the conventional ones
Consider the relationship between the thickness d (see FIG. 6) and the numerical aperture NA of the objective lens.

This is extremely important from the viewpoint that the manufacturing cost and the manufacturing difficulty are not increased and the reliability is not reduced. Here, assuming that the same material as the compact disk (CD) described above is used as the material of the transparent substrates 101 and 111, the refractive index n
Are equal and the numerical aperture NA of the objective lens 36 is 0.5. Becomes

From the equation (2), a relationship of NA ⁴ × d ≦ 0.0744 (3) is derived.

FIG. 7 shows the range of the numerical aperture NA and the thickness d of the transparent substrate (protective layer) with respect to the equation (3). The numerical aperture NA of the objective lens 36 expected to be used in the future and the transparent NA are shown in FIG. The combination with the thickness d of the substrate is shown below.

That is, when the numerical aperture NA of the objective lens 36 is increased in order to increase the recording density, the thickness d of the transparent substrate of the optical disc D is determined by the fact that the numerical aperture NA of the objective lens 36 is, for example, NA = 0 from FIG. 0.995 mm or less at 0.95, 0.113 mm or less at NA = 0.90, 0.143 mm or less at NA = 0.85, 0.182 mm or less at NA = 0.80, NA = 0.75 Is set to 0.235 mm or less, 0.310 mm or less when NA = 0.70, and 0.410 mm or less when NA = 0.65.
When the laser beam is converged on each of the laser beams (see FIG. 6), it is possible to reduce the occurrence of the spherical aberration due to the influence of the thickness d of the transparent substrate on the beam diameter of the converged laser beam, and to reduce the degree thereof. As is clear from FIG. 7, in the method of increasing the recording density by increasing the numerical aperture NA, the thickness d of the transparent substrate becomes smaller (thinner) as the NA becomes larger.

As a specific method for increasing the numerical aperture NA of the objective lens 36, in addition to the method disclosed in the above-mentioned document, for example, an objective lens used in a microscope (a practical lens up to about NA = 0.9) is used. It can be easily achieved by using the design method of ().

On the other hand, as for the method of reducing the thickness of the transparent substrate (protective layer), injection molding used today can be used up to a certain extent. There is a method such as attaching a high polycarbonate resin film (to the recording layer).

If a new material having a different refractive index n is used as the material used for the transparent substrate, (n ² -1) / 8n ³ × NA ⁴ × d ≦ 3 from (1). The relationship of 52 × 10 ^-3 (4) holds.

Here, the correspondence to the spherical aberration will be described again. When spherical aberration occurs due to an error in the thickness d of the transparent substrate (protective layer), even if the relationship between the numerical aperture and the thickness d described above is not always satisfied, for example, the spherical aberration depends on the amount of the generated spherical aberration. By changing the laser beam incident on the objective lens from a parallel beam to a convergent light or a divergent light, the influence can be reduced. However, in this case, a mechanism for accurately detecting the amount of spherical aberration and a mechanism for converting the parallel beam into a convergent light or a divergent light by a predetermined amount are required, and the configuration of the optical head device 3 becomes extremely complicated. There is a problem that the cost and the manufacturing difficulty increase, and the reliability decreases. In particular, it is not suitable for practical use in terms of cost.

It is also effective to reduce the error in the thickness d of the transparent substrate (protective layer) in order to keep the amount of spherical aberration low, but in this case, it is necessary to strictly control the process during manufacturing. Due to increased manufacturing costs,
There is a problem that the yield is reduced by selecting only products having a small error as products.

Next, the flow of the laser beam in the optical head device 3 described with reference to FIGS. 2 to 5 will be described. Laser beam L emitted from semiconductor laser 11
Is converted into a parallel light beam by the collimator lens 12,
The cross-sectional shape is corrected to a substantially circular shape by the elliptical correction prism 13a, and the light passes through the polarization beam splitter 13.

The laser beam L transmitted through the beam splitter 13 passes through the quarter-wave plate 14 so that the polarization direction is changed from linearly polarized light to circularly polarized light, and is emitted toward the rising mirror 35 of the actuator 31b. Is done.

The laser beam L guided to the rising mirror 35 is reflected by the rising mirror 35 in a direction perpendicular to the recording surface of the optical disk D, and is converged by the objective lens 36 held in the lens holder 37 to a predetermined degree. The optical disc D is irradiated with light at a predetermined position on the recording surface of the optical disk D and converged to a predetermined depth on the recording surface.

The reflected laser beam L ′ guided to the recording surface of the optical disk D and reflected by the recording surface
Then, the rising mirror 35 is sequentially returned, and the polarization state is converted from circularly polarized light to linearly polarized light again by the 波長 wavelength plate 14 and guided to the beam splitter 13. At this time, since the polarization direction of the reflected laser beam L 'is rotated by exactly 90 ° with respect to the polarization direction of the original laser beam L emitted from the semiconductor laser 11, the reflected laser beam L' is This time, the light is reflected by the polarization plane of the beam splitter 13.

The reflected laser beam L 'separated from the laser beam L from the semiconductor laser 11 toward the objective lens 36 by the beam splitter 13 is converted into two reflected laser beams having substantially the same light intensity by the second beam splitter 15. It is divided into L'a and L'b.

The reflected laser beam L′ a transmitted through the beam splitter 15 is given a predetermined image forming characteristic and convergence by the converging lens 16, then the aberration characteristic is improved by the concave lens 17, and is further focused by the cylindrical lens 18. Astigmatism for displacement detection is given, and the light is irradiated on a light receiving surface (not shown) of the first photodetector 19.

The reflected laser beam L'a applied to the photodetector 19 is converted by the photodetector 19 into an electric signal having a magnitude corresponding to the light intensity, and is used as a focus error signal and a reproduction signal. In this example, the detection of the focus error signal is a well-known astigmatism method, and a detailed description thereof will be omitted.

On the basis of the focus error signal generated by the photodetector 19, focus control, that is, focusing, for eliminating a deviation between the focal point of the spot converged by the objective lens 36 and the optical axis direction of the recording surface of the optical disk D is performed. You.

At this time, by supplying a current in a predetermined direction to the coils 40, 40 based on the focus error signal, attraction or repulsion due to an electromagnetic interaction with the magnetic field provided by the magnets 43, 43 is generated. As a result, the lens holder 37 (objective lens 36) is moved either in a direction approaching or away from the recording surface of the optical disc D. The method of detecting the focus shift is not limited to the above-described astigmatism method, and various methods such as a knife edge method can be used.

The remaining reflected laser beam L'b reflected by the beam splitter 15 is reflected in a predetermined direction by a mirror (oblique side of a right-angle prism) 20 and given a predetermined convergence by a converging lens 21 so that a track The light is guided to a light receiving surface (not shown) of the photodetector 22 used for detecting the displacement.

The output signal photoelectrically converted by a light-receiving region (not shown) of the photodetector 22 is described in detail in, for example, a well-known push-pull method (for example, a document “Optical Disk Technology” (by Noboru Murayama et al., Radio Technology Company, 1989)). ) Is used for the track shift signal. Less than,
Based on the track shift signal generated by the photodetector 22, track control, that is, tracking is performed to eliminate the shift between the focus of the spot converged by the objective lens 36 and the center of the guide groove on the recording surface of the optical disc D. Is done.

At this time, the output of the light receiving area (not shown) of the photodetector 22 is combined with a predetermined combination to obtain a difference signal to obtain a coil shift signal based on the track shift signal.
When a current is supplied to the optical disc D in a predetermined direction, the magnetic field provided by the magnets 44 causes attraction or repulsion due to an electromagnetic interaction with the magnetic field. The lens holder 37 (that is, the objective lens 3)
6) is moved along the recording surface of the optical disc D.
As a method for obtaining a track shift signal, for example, a three-beam method widely used mainly in CDs (compact disks) can be used.

As described above, according to the optical disk device 1 shown in FIGS. 1 to 5, the transparent substrates (protective layers) 101 and 11 having the thickness d shown in FIG.
The recording density can be increased without being affected by the spherical aberration caused by (1). Further, by specifying the material of the protective layer in accordance with the numerical aperture of the objective lens 36, the recording density can be increased without adding a special configuration to the optical disc device 1 as a recording / reproducing device.

The optical disk and the optical disk device may be of the read-only type or of the recording / reproducing type, and if they are of the recording / reproducing type, the type may be a magneto-optical type or a phase change type. The disc may be of a groove type provided with a guide groove or a pit type such as a music disc.

[0070]

As described above, according to the present invention,
A transparent protective layer (transparent substrate or resin layer) located between the recording layer of the optical disk and the objective lens of the optical head device
Is set in the range of NA ⁴ × d ≦ 0.0744 with respect to the numerical aperture NA of the objective lens, so that the recording density can be increased without adding a special configuration to an optical disk device used today. Can be enhanced.

According to the present invention, when the thickness of the protective layer (transparent substrate) is d, the refractive index of the protective layer is n, and the numerical aperture of the objective lens is NA, (n ² -1) / 8n ³ x NA ⁴ x d ≤ 3.52 x 1
0 ^-3 so as to satisfy, by setting the thickness d and the refractive index n of the transparent substrate (protective layer), without adding a special configuration to the optical disc apparatus in use today, to increase the recording density Can be.

Further, according to the present invention, when the thickness of the transparent substrate is d, the refractive index is n, and the numerical aperture of the objective lens is NA, (n ² -1) / 8n ³ × NA ⁴ × d ≤
By setting the numerical aperture NA of the objective lens so as to satisfy 3.52 × 10 ⁻³ , it is possible to increase the recording density without largely changing the optical disk device used today.

[Brief description of the drawings]

FIG. 1 is an exemplary diagram showing an example of an optical disk device to which an embodiment of the present invention can be applied;

FIG. 2 is a schematic diagram showing an example of an optical head device applicable to the optical disk device shown in FIG.

FIG. 3 is a schematic diagram showing an example of an actuator of the optical head device shown in FIG. 2;

FIG. 4 is a schematic diagram showing an example of a fixed optical system of the optical head device shown in FIG.

5 is a schematic diagram illustrating a lens holder of the actuator shown in FIG. 3 and its vicinity.

FIG. 6 is a schematic diagram showing a configuration of a recording medium (optical disc) applicable to the optical disc apparatus shown in FIGS. 1 to 5 and capable of increasing the recording density.

FIG. 7 is a graph showing the relationship between the numerical aperture NA of an objective lens having a small degree of spherical aberration affecting the beam diameter of a laser beam converged on a recording layer of an optical disc and the thickness d of a protective layer of the optical disc D;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical disk device, 3 ... Optical head device, 36 ... Objective lens, 101 ... Transparent substrate (protection layer), 102 ... Recording mark (guide groove), 103 ... Recording layer, 104 ... Resin layer, 111 ... Transparent substrate (Protection) Layer 112, recording marks (guide grooves) 113 recording layer 114 single-sided recording disk 115 adhesive layer D optical disk.

Claims (5)

[Claims] 1. A recording material capable of reproducing and recording information by irradiating a recording material on one surface of a substrate and irradiating the recording material with coherent light supplied through a lens device. In the medium, when the distance between the light incident surface of the transparent protective layer for protecting the recording material and the recording material is d, and the numerical aperture of the lens device is NA, NA ⁴ × A recording medium characterized by satisfying d ≦ 0.0744. A recording material is provided on one surface of a substrate, and the recording material is irradiated with coherent light supplied via a lens device to thereby enable recording and reproducing of information. In the medium, the distance d between the surface of the transparent protective layer that protects the recording material and on which the light is incident and the recording material, and the refractive index n of the material used for the protective layer are as follows:
When the numerical aperture of the lens device is NA, (n ² −1) / 8 n ³ × NA ⁴ × d ≦ 3.52 × 1
Recording medium and satisfies the 0 ^-3. 3. A recording material capable of reproducing and recording information by irradiating a coherent light supplied via a lens device to the recording material provided on one surface of the substrate. In the medium, the numerical aperture of the lens device is NA
The distance between the recording material and the surface of the transparent protective layer that protects the recording material, on the side where the light is incident, is d, and the refractive index of the material used for the protective layer is n, (N ² −1) / 8n ³ × NA ⁴ × d ≦ 3.52 × 1
Recording medium and satisfies the 0 ^-3. 4. A recording material on which information is selectively recorded in accordance with a difference in light intensity of the light beam when the state is changed by irradiation of the light beam, and a support for supporting the recording material. On the other hand, in an optical head device for irradiating a recording medium having a transparent protective layer provided so as to cover a recording material with a light beam, a numerical aperture NA of an objective lens of the optical head device is equal to that of the protective layer of the recording medium. An optical head device, wherein the distance d between the surface on which the light is incident and the recording material is set to satisfy NA ⁴ × d ≦ 0.0744. 5. A recording material on which information is selectively recorded in accordance with a difference in light intensity of the light beam, the state being changed by irradiation with the light beam, and a support for supporting the recording material. On the other hand, in an optical head device for irradiating a recording medium having a transparent protective layer provided so as to cover a recording material with a light beam, a numerical aperture NA of an objective lens of the optical head device is such that a transparent material for protecting the recording material is provided. Assuming that the distance between the light-incident side of the protective layer and the recording material is d, and the refractive index of the material used for the protective layer is n, (n ² -1) / 8n ³ × NA ⁴ × d ≤ 3.52 × 1
0 ^-3 optical head apparatus characterized by being configured to satisfy.