JPH08241535A - Optical disk and reproducing method thereof - Google Patents

Abstract

PURPOSE: To obtain an optical disk capable of high density reproduction above the limitation of reproduction due to the wavelength of light from a light source and the lens NA. CONSTITUTION: This optical disk has a magneto-optical material layer 3 made of a rare earth element-transition metal ferromagnetic material on the transparent substrate 2 with pits 2A formed in accordance with information. At the time of reproduction, the magnetization directions of the magneto-optical material layer 3 are aligned in one direction and the layer 3 is irradiated with laser light for reproduction while applying an external magnetic field in a direction reverse to the one magnetization direction. A rear part in the irradiated spot attains to a high temp. and this high-temp. part is subjected to the inversion of magnetization by the external magnetic field. Information is reproduced by detecting the change of the quantity of light reflected from the part subjected to the inversion of magnetization or a part not subjected to the inversion of magnetization in accordance with the pits 2A.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disc for reproducing information by irradiating a laser beam, and more particularly to an optical disc suitable for high density recording.

[0002]

2. Description of the Related Art Optical discs such as compact discs and video discs are constructed by forming information as phase pits due to unevenness on a transparent substrate, and further forming an aluminum reflection film, a protective film, etc. thereon. In such an optical disc, the signal is read (reproduced) by irradiating the disc surface with the reading light and detecting the magnitude of the reflected light amount by utilizing the light diffraction in the phase pits. In this case, the reproduction resolution of the signal is almost determined by the wavelength λ of the light source of the reproduction optical system and the numerical aperture NA of the objective lens, and the reproduction limit is the spatial frequency 2NA / λ. Therefore, it is conceivable to shorten the wavelength λ of the light source in order to increase the recording density, or to reduce the diameter of the spot light of the reproducing device by using a high NA lens.

Therefore, in recent years, in order to increase the recording density of an optical disk, researches have been made to shorten the wavelength of laser light used for reproduction and to use a high NA lens to substantially reduce the diameter of spot light of a reproducing device. Has been done. For example,
In the technology of shortening the laser light wavelength, research on a semiconductor blue laser and research on setting the laser light wavelength to about 800 nm to 400 nm by using a second harmonic generation element (SHG) have been conducted. In terms of stability, performance, price, etc., these are not at the stage of being put to practical use yet, but if put to practical use in the future, it will be possible to record information at a higher density than the current optical disc system. .

[0004]

However, the wavelength of a semiconductor laser currently in practical use is only about 680 nm, and when a high NA lens is used, the depth of focus becomes shallow, and the distance between the lens and the disk is small. Precision is required, the manufacturing precision of the optical disc becomes strict, and the NA of the lens
Can not be so high, and the practical lens NA is 0.6 at most. As described above, there is a limit to shortening the wavelength of the light source and increasing the NA of the objective lens, and it is difficult to dramatically improve the recording density by these.

Therefore, the present invention has been made in view of the above points, and an object thereof is to provide an optical disk capable of high-density reproduction exceeding a reproduction limit by a light source wavelength or a lens NA. .

[0006]

As a means for achieving the above object, the present invention provides a laser having a light-transmissive substrate on which information is recorded by pits having an optically readable and recessed shape. In an optical disc in which the information is reproduced by detecting the concave and convex pits using light, Tb or Dy, Fe and Co are formed on the light transmissive substrate.
And reproducing the magneto-optical material layer composed of a ternary alloy with the composition, so as to partially reverse the magnetization of the magneto-optical material layer within the irradiation spot of the reproducing laser beam for detecting the uneven pits. The present invention is intended to provide an optical disk characterized in that the effective spot diameter of the laser light for use is reduced.

As a means for achieving the above object, the present invention has a "light-transmissive substrate on which information is recorded by pits having an optically readable and recessed shape,
In an optical disc in which the information is reproduced by detecting the concave-convex pits using a laser beam, a magneto-optical disc formed of a quaternary alloy of Gd or Dy, Tb, Fe, and Co on the light-transmissive substrate. An effective spot diameter of the reproducing laser light having a material layer and being configured to partially reverse the magnetization of the magneto-optical material layer within an irradiation spot of the reproducing laser light for detecting the uneven pits. It is intended to provide an "optical disk characterized by making the size smaller."

As a means for achieving the above object, the present invention provides a method for reproducing an optical disk according to any one of claims 1 to 4, wherein a laser beam for reproduction is irradiated to detect the concave and convex pits. Beforehand, the magnetization direction of the magneto-optical material layer is aligned in one direction, and the reproducing laser light is irradiated while applying an external magnetic field opposite to the magnetization direction aligned in this one direction, and the magneto-optical material. The layer is partially magnetized in the spot in the direction of the external magnetic field, and the change in the amount of reflected light from either the part where the magnetization is reversed or the part where the magnetization is not reversed is detected according to the uneven pits. In this way, a method for reproducing an optical disk, which is characterized in that the above-mentioned information is reproduced.

In recent years, a nonlinear optical material whose optical characteristics such as light transmittance, reflectance, and refractive index change according to temperature and light intensity is provided on an optical disc in a layered form, and the difference in temperature distribution and light intensity distribution in an irradiation spot is provided. By making use of the fact that it is possible to mask, a part of the irradiation spot is masked, and research is being conducted to reproduce minute pits that are below the reproduction limit determined by the light source wavelength and lens NA. Examples of the non-linear optical material include a pigment material whose light transmittance and refractive index reversibly change due to temperature change and light intensity, and a phase change material which utilizes the difference in light reflectance and refractive index between crystal and amorphous.

The optical disc of the present invention does not use the so-called non-linear optical material described above, but uses a rare earth (RE) -transition metal (TM) amorphous perpendicular magnetic material that has been conventionally used as a magneto-optical recording material. And
The magnetization of the magneto-optical material is partially reversed in the light spot by the external magnetic field. Since the polarization plane of the reflected light is different between the magnetization-reversed portion and the non-magnetization-reversed portion, the difference between the polarization planes is used to make the light of either the magnetization-reversed portion or the non-magnetization-reversed portion. By cutting, the minute pits below the reproduction limit determined by the light source wavelength and the lens NA can be reproduced.

[0011]

When the laser light spot for detecting the uneven pits is irradiated, the magnetization direction of the magneto-optical material layer is preliminarily aligned in one of the upper and lower directions by the initialization magnetic field. Then, at the time of irradiation of the laser light spot, a reading magnetic field opposite to the magnetization direction of the magneto-optical material layer is applied to the laser light spot. Since the coercive force of the magneto-optical material whose temperature has risen due to the laser light irradiation decreases, the high temperature portion in the spot is magnetized in the direction opposite to the magnetization direction in which the magneto-optical material layer is magnetized in advance. Here, the laser light spot is moving in the track direction on the optical disk, and a high temperature portion in the spot is generated rearward with respect to the spot traveling direction due to the time difference from the laser light irradiation to the temperature rise. The Kerr rotation angle (or Faraday rotation angle) of the reflected light from the high temperature portion behind the spot is different from the reflected light from the portion in the spot where the magnetization is not reversed. Therefore, an analyzer is provided in the optical pickup to cut off either the reflected light of the part where the magnetization is not reversed or the part where the magnetization is reversed in the light spot, and the amount of light transmitted through the analyzer is detected. By doing so, the information is reproduced.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram showing the structure of an embodiment of the optical disc of the present invention. The figure shows a part of the cross-sectional structure in the track direction. Optical disc 1 shown in FIG.
Is composed of a light transmissive substrate 2, a magneto-optical material layer 3, and a protective film layer 4 which are laminated in this order. The transparent substrate 2
As the material, a light-transmitting resin such as polycarbonate, polymethacrylic acid ester resin, or epoxy resin, glass, or the like which is used as a substrate in a normal optical disk can be used. Further, on the light transmissive substrate 2, pits 2A having an uneven shape corresponding to information are formed by a known method. The size, recording wavelength, and track pitch of the pit 2A are smaller than those of the current CD, and information is recorded at a higher density than that of the CD.

The magneto-optical material layer 3 uses a perpendicular magnetic film similar to that conventionally used as a recording material, and has a coercive force Hc of about 2 to 4 kOe at room temperature. ) -Transition metal (TM) ferrimagnetic material is used. Therefore, there is an advantage that the manufacturing equipment for manufacturing the optical disc 1 and the like can be changed to the minimum necessary amount, and the cost increase for manufacturing the optical disc due to the high density recording can be minimized.

Specifically, the above-mentioned magneto-optical material is a ternary alloy of Tb or Dy, Fe and Co, or Gd or Dy.
And a quaternary alloy of Tb, Fe and Co is used. The composition ratio of the magneto-optical material layer made of a ternary alloy is Tb _x (Fe _1-y C
o _y ) _1-x or Dy _x (Fe _1-y Co _y ) _1-x (0.15 ≦ x
≦ 0.35,0 <a y ≦ 0.50), the composition ratio of the magneto-optical material layer by quaternary alloy _{is, Gd x Tb y (Fe 1} -z Co z)
_1-xy or _{Dy x Tb y (Fe 1-} z Co z) 1-xy (0.
07 ≤ x ≤ 0.18, 0.07 ≤ y ≤ 0.18, 0 <z ≤ 0.50).
The magneto-optical material layer 3 needs to have a film thickness of 10 nm or more and a composition ratio within the above range in order to stabilize the magnetization and direct the magnetization in a direction perpendicular to the film surface. In addition,
When the composition ratio of the ternary alloy is 0.50 <y and the composition ratio of the quaternary alloy is 0.50 <z, the Curie point or the magnetic compensation temperature has a large composition dependency and is not practical. Furthermore, when the ranges of x in the composition ratio of the ternary alloy and x, y in the composition ratio of the quaternary alloy are set outside the above composition range, the easy axis of magnetization may be aligned in the direction perpendicular to the film surface. Since it becomes difficult and the rectangular hysteresis characteristic is deteriorated, a practical reproducing characteristic cannot be obtained.

The protective film layer 4 is provided as necessary for the purpose of protecting the medium. The protective film layer 4 can be easily formed by providing an ultraviolet curable resin by spin coating.

The optical disc 1 has a structure in which only the magneto-optical material layer 3 is provided on the substrate 2. However, as shown in FIG. 2, the magneto-optical material layer 3 is used to increase the difference in the amount of light of the linearly polarized light component. The dielectric interference layer 5 may be provided so as to be sandwiched, and the reflection layer 6 made of a substance having a high light reflectance such as aluminum or gold may be provided.

Next, a reproducing apparatus for the optical discs 1 and 11 will be described. FIG. 3 is a schematic configuration diagram of a main part of the optical disc reproducing apparatus in FIG. 1 or 2. It should be noted that FIG. 1 shows a schematic configuration centering on the optical pickup section of the reproducing apparatus, and other elements such as a focus actuator, a tracking actuator, and a demodulation circuit are omitted. The optical pickup 21 of the reproducing apparatus shown in the figure includes a laser light source 22, a collimating lens 23,
It is composed of a polarizer 24, a half mirror 25, an objective lens 26, an analyzer 27, a photodetector 28, and the like.
A generating initialization magnet 29 is provided, and a reading magnet 30 for generating a reading magnetic field 30A is provided at a position facing the objective lens 26 via the mounted optical disks 1 and 11.

The laser light L output from the laser light source 22 is collimated by the collimator lens 23, converted into light having a polarization plane in the direction perpendicular to the plane of the drawing by the polarizer 24, and then the half mirror 25 is moved. The objective lens 26 irradiates the pits 2A on the optical discs 1 and 11 as a focused spot via the objective lens 26. Irradiated laser light L
Is reflected by the magnetic material layer 3 (reflection layer 6) (laser light L
′), It returns to the half mirror 25 through the objective lens 26, and is moved to the analyzer 27 by the half mirror 25.
The light path is changed, and only light having a polarization plane in a specific direction is incident on the photodetector 28 by the analyzer 27.

The analyzer 27 transmits only the light having a specific plane of polarization among the reflected laser light L'and cuts (or attenuates) the other light.
It is provided inside. Therefore, by appropriately setting the polarization plane of the light passing through the analyzer 27, the irradiation laser beam L is reflected from a high temperature portion (a portion where magnetization is reversed) or a portion other than the high temperature portion (a portion where magnetization is not reversed) in the spot. Only light can be made incident on the photodetector 28.

The initialization magnet 29 and the reading magnet 30 are composed of permanent magnets, electromagnets or the like. The initializing magnetic field 29A from the initializing magnet 29 is a large magnetic field with respect to the coercive force Hc of the magneto-optical material layer 3 and is, for example, 5
It is set to about kOe. In addition, the read-out magnet 3
The read magnetic field 30A from 0 has a magnitude corresponding to the coercive force Hc of the magneto-optical material layer 3 when the temperature rises, for example, 100 to 5
It is set to 00 Oe. Although the initialization magnetic field 29A is directed upward and the read magnetic field 30A is directed downward in the figure, the initialization magnetic field 29A and the read magnetic field 3 are shown.
If the directions of the magnetic fields with 0A are opposite to each other, the magnetic field directions as shown in the figure are not limited.

Next, the principle of reducing the effective spot diameter of the irradiation spot by the optical discs 1 and 11 of this embodiment will be described with reference to FIG. FIG. 4 is a diagram for explaining the reproduction principle of the optical disc in FIG. 1 or 2. It should be noted that FIG. 9A is an enlarged plan view of the vicinity of the area where the laser beam spot is irradiated, and FIG. 9B is an enlarged side view thereof. Further, FIG. 6C is a diagram showing a light intensity distribution on the dashed-dotted line X in FIG.
The temperature distribution of the magneto-optical material layer 3 on the alternate long and short dash line X is shown.

As shown in FIG. 1B, the magnetization direction of the magneto-optical material layer 3 is changed to the initializing magnetic field 2 before the reproduction laser beam is irradiated.
9A aligns in one direction (the direction of arrow 31 in FIG. 3B). That is, the laser beam spot is traveling to the right in the figure, and the initialization magnet 29 is located forward of the traveling direction.
Is provided, the magnetization direction of the magneto-optical material layer 3 is aligned before the reproduction laser light irradiation by the initialization magnetic field 29A of the initialization magnet 29.

Further, the initialization magnetic field 29A is provided near the area where the reproducing laser light for detecting the uneven pits 2A is irradiated.
A read magnetic field 30A in the opposite direction to is applied. Here, it is assumed that the reproduction laser light has such a light intensity that the temperature of the rear portion in the spot rises near the Curie point due to the laser light irradiation. Further, when the laser light spot is irradiated, even if the light intensity distribution has a Gaussian distribution as shown in FIG. 6C, the temperature distribution is the same due to the time difference until the temperature rises because the optical disc is rotating. As shown in D), the temperature in the rear part of the laser beam spot traveling direction becomes high. Therefore, the shaded area A in FIG.
As shown by, the rear part in the laser beam spot becomes the high temperature part A.

When the high temperature portion A is generated in the magneto-optical material layer 3 as described above, the coercive force Hc of the magneto-optical material layer 3 becomes small in the high temperature portion A, so that the read magnetic field 30A opposite to the initialization magnetic field 29A. Direction (arrow 32 in the same figure (B))
The magnetization is reversed. Here, in general, when the magnetic material rises in temperature and the coercive force Hc becomes small, the magnetization reversal sufficiently occurs even with a weak external magnetic field. Therefore, considering the influence on the part where the temperature of the magneto-optical material layer 3 has not risen, the read magnetic field 30A is set to be smaller than the coercive force Hc of the magneto-optical material layer 3, for example, 100 to 500 Oe.

Further, since the high temperature portion A and the other portion B in the laser beam spot shown in FIG. 6A have different magnetization directions, the Kerr rotation angle (or Faraday rotation angle) of the reflected light is different. become. Therefore, with respect to such reflected light, the analyzer 27 as in the optical pickup 21 described above.
If the reflected light of the high temperature portion A is cut by using, it becomes the same as that the pits 2A-A in the high temperature portion A are masked and are not detected. Then, the amount of light transmitted through the analyzer 27 is equal to the pits 2A-B in the area B in the spot.
The information can be reproduced by detecting the difference in the light amount. If the light cut by the analyzer 27 is the reflected light from the area B, the same effect as if the pits 2A-B in the area B are masked and not detected will be obtained.

As described above, the optical disks 1, 1 of this embodiment
According to 1, even if there are a plurality of pits in the same spot, it is possible to read only a single recording pit, and it is possible to eliminate intersymbol interference and crosstalk. Therefore, it is possible to reproduce the minute pits which are below the read limit determined by the lens NA and the laser light source wavelength λ.

[0027]

As described above, according to the optical disc of the present invention, a ternary alloy of Tb or Dy, Fe and Co, or Gd or It has a magneto-optical material layer made of a quaternary alloy of Dy, Tb, Fe, and Co, and partially reverses the magnetization of the magneto-optical material layer within an irradiation spot of a reproducing laser beam for detecting uneven pits. Since the effective spot diameter of the reproducing laser light is made small by utilizing the magneto-optical effect, the reproduction is performed by the light source wavelength and the lens NA even though the layer structure is almost the same as the conventional magneto-optical disk. High density reproduction beyond the limit is possible. In addition, the manufacturing equipment and the like for manufacturing the optical disk can be changed to the minimum necessary amount, and the cost increase for manufacturing the optical disk due to the high density recording can be minimized.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of an embodiment of an optical disc of the present invention.

FIG. 2 is a diagram showing the structure of another embodiment of the optical disc of the present invention.

FIG. 3 is a schematic configuration diagram of a main part of the optical disc reproducing apparatus in FIG. 1 or FIG.

FIG. 4 is a diagram for explaining the reproduction principle of the optical disc in FIG. 1 or FIG.

[Explanation of symbols]

 1, 11 Optical disc 2 Light transmissive substrate 2A Pit 3 Magneto-optical material layer 21 Optical pickup 27 Analyzer 29 Initializing magnet 29A Initializing magnetic field 30 Readout magnet 30A Readout magnetic field

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G11B 11/10 586 9296-5D G11B 11/10 586G

Claims (5)

[Claims] 1. A light-transmitting substrate on which information is recorded by means of optically readable and recessed pits, and the information is reproduced by detecting the pits and projections using laser light. In the optical disc described above, a magneto-optical material layer made of a ternary alloy of Tb or Dy, Fe, and Co is provided on the light-transmissive substrate, and a reproducing laser beam irradiation spot for detecting the uneven pits is provided. The optical disk is characterized by partially reversing the magnetization of the magneto-optical material layer to reduce the effective spot diameter of the reproducing laser light. 2. The optical disc according to claim 1, wherein the magneto-optical material layer is Tb _x (Fe _1-y Co _y ) _1-x or Dy _x (Fe _1-y Co _y ) _1-x (0.15 ≦ x ≤ 0.35, 0 <
An optical disc having a composition ratio of y ≤ 0.50). 3. A light-transmitting substrate on which information is recorded by optically readable and recessed pits, and the information is reproduced by detecting the pits and projections using laser light. In the above optical disc, Gd or Dy, Tb, Fe and C are formed on the light transmissive substrate.
a magneto-optical material layer made of a quaternary alloy with o and being configured to partially reverse the magnetization of the magneto-optical material layer within an irradiation spot of a reproducing laser beam for detecting the uneven pits. An optical disc characterized in that an effective spot diameter of a reproducing laser beam is reduced. An optical disk according to claim 3, said magneto-optical material layer _{Gd x Tb y (Fe 1-} z Co z)
_1-xy or _{Dy x Tb y (Fe 1-} z Co z) 1-xy (0.0
An optical disk characterized in that it has a composition ratio of 7≤x≤0.18, 0.07≤y≤0.18, 0 <z≤0.50). 5. The method for reproducing an optical disc according to claim 1, wherein the magnetization direction of the magneto-optical material layer is one direction before irradiation with a reproduction laser beam for detecting the uneven pits. And irradiating the reproducing laser beam while applying an external magnetic field opposite to the magnetization direction aligned in this one direction, the magneto-optical material layer is partially directed in the spot in the direction of the external magnetic field. The information is reproduced by reversing the magnetization and detecting a change in the light amount of the reflected light from either the reversing portion or the non-reversing portion depending on the uneven pits. How to play an optical disc.