JPH01169753A - Optical recording medium and reproducing method therewith - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to an optical recording medium with improved volumetric recording density and a reproduction method using the same.

Technical background of the invention and its problems Transition metals such as iron and cobalt, terbium (Tb),
An amorphous thin film made of an alloy with a rare earth element such as gadolinium (G(1)) has an axis of easy magnetization perpendicular to the film surface, and light is applied to a part of the film that is entirely magnetized in one direction. It is known that by focusing and avoiding irradiation, it is possible to form a small reversed magnetic domain in the opposite direction to the overall magnetization direction.By setting the presence or absence of this reversed magnetic domain to "1" and corresponding to rOJ, the above It becomes possible to record digital signals on amorphous thin films such as magneto-optical recording.

Although such magneto-optical recording is superior to conventional magnetic recording in terms of areal recording density, it is necessarily inferior in terms of volumetric recording density. For example, [as reported by Mr. Yokoyama (Yokoyama: Research on high-density magnetic recording technology, NHK Broadcasting Technology Research Institute Foundation Commemoration Lecture Proceedings, p. 14, June 5, 1985)], normal magnetic recording is performed. While a household VTR, which is a typical magnetic recording medium, can record 8 hours of television programs with a volume of 1aax 104 x 25 #3 (including the case), a magneto-optical disk is a magneto-optical medium that performs magneto-optical recording. With a volume of 334 x 268 x 7 #3, only two hours of programs can be recorded. That is, while the volume recording density (recordable time/volume of recording medium) is 59 seconds/7 in the case of tape, it is 12 seconds/7, which is smaller in the case of disk. This is because tapes are thin and can be stacked in multiple layers and placed in a case, whereas disks used in magneto-optical recording systems use their surface Cp only two-dimensionally. This is the cause.

Incidentally, the good searchability derived from the disk shape is a major advantage of the disk method over the tape method. Therefore, in the field of magneto-optical recording, there is a need to take advantage of the advantages of such disks and to improve the volumetric recording density. In particular, when considering use as image memory, the amount of information to be recorded on high-definition televisions, etc. will continue to increase, and the demand for improving the volumetric recording density of disk-shaped recording media will continue to increase. There is.

Generally, in order to increase the recording capacity in a recording/reproducing system using light, it is sufficient to condense the light used for recording or reproduction to a smaller size and increase the recording density. However, the lower limit of the size that can be focused is determined by the wavelength λ of the light used and the numerical aperture (NA) of the condensing lens, and the wavelength λ is approximately 8001
In order to condense light even smaller than the current state of concentrating m light to about 1 μTrL, a lens with a larger numerical aperture must be used, which makes the lens heavier and larger, so there is no practical improvement. It cannot be a strategy. It is also possible to focus the light into a smaller size by using light with a shorter wavelength as a light source, but in order to double the recording density using this method alone, the wavelength must be halved. First, considering the current technological trend of shortening the excavation wavelength of semiconductor lasers, it is not easy to double the recording density.

Therefore, in order to increase the recording density in optical recording, photochemical hole firing (PHB) is used.
Attempts such as multiplex recording in the wavelength region using phenomena such as
U.S. Pat. No. 4,101,976 (197B)) has also been carried out, but there are disadvantages such as the phenomenon does not appear unless it is at an extremely low temperature, and the yield of ``child'' is not sufficient, so it still needs to be solved for practical use. Many problems remain.

In addition, in the field of magneto-optical recording, multi-layered media,
Attempts to greatly increase the recording density or data transfer speed (for example, Mr. Taki, Mr. Matsuda, Mr. Mizutani, et al., "Reproduction wavelength dependence of dual-layer magneto-optical recording media", 33rd Applied Physics Union Lecture Spring 1986 21)-H-7 Lecture Proceedings, p. 110), but this technology requires a complicated optical system because it uses light of different wavelengths to identify the layer to be recorded. Moreover, when a semiconductor laser is used as a light source, there is a disadvantage that the range of wavelengths that can be changed cannot be widened. Such inconveniences are not limited to magneto-optical recording. Attempts at multilayering in optical recording, in which recording layers consisting of many types of dyes with different absorption wavelengths are laminated on a substrate (for example, the Japa
The same is true for N Display, 3B (1983)).

Purpose of the Invention The present invention has been made in view of the above circumstances, and aims to provide an optical system that can be reproduced with a relatively simple and inexpensive reproduction device, even though the volume recording density can be increased several times. The purpose is to provide a recording medium and a reproduction method using the same.

In order to achieve this objective, the optical recording medium of the present invention is based on research to improve optical recording, which has already been developed to a stage close to practical use, and an artificial lattice film made of laminated ultra-thin films. It was invented through the research of , and an intermediate layer made of a transparent material having a film thickness that separates the recording layers so that the two or more recording layers are not located within the depth of focus of the light used for recording or reproduction.

Further, the reproduction method according to the present invention includes at least two recording layers made of an optical recording film capable of recording and reproducing a predetermined signal by converging and irradiating light, and laminated between these recording layers, An arbitrary depth in an optical recording medium including an intermediate layer made of a transparent material having a film thickness that separates the recording layers so that the two or more recording layers are not located within the focal depth of the light used for recording or reproduction. After moving the objective lens so that the reproduction light is focused near the recording layer, the focus servo mechanism is activated to adjust the focal position of the light focused through the objective lens to the recording layer at the desired depth. The information is then reproduced by detecting changes in the intensity of light transmitted through the recording medium.

According to such an optical recording medium according to the present invention, since at least two or more n layers of recording layers made of optical recording films are laminated, the volume is almost the same as that of a conventional optical recording medium. It is possible to record n times more information. therefore,
According to the optical recording medium according to the present invention, it is possible to record or reproduce high-density information such as television signals over a long period of time using fewer recording media.

A major advantage of the optical recording medium according to the present invention is that the recording capacity can be improved by using basically the same recording and reproducing device as that of the conventional system.

In particular, in order to reproduce information recorded on such an optical recording medium according to the present invention, if the above-described reproduction method according to the present invention is used, it is possible to easily reproduce the information by just slightly changing the control method of the conventionally used reproduction device. It is convenient because it can be done at any time.

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 1 is a cross-sectional view of an optical recording medium according to an embodiment of the present invention;
FIGS. 2(a) and (b) are schematic diagrams showing the method for reproducing an optical recording medium according to the present invention, and FIGS. 3(a) and (b) are diagrams showing the operation of the focus servo mechanism used in the reproducing method according to the present invention. FIG.

As shown in FIG. 1, the optical recording medium 10 according to the present invention includes:
It has a structure in which recording layers 14 and intermediate layers 16 are alternately laminated on a substrate 12, and a protective IWA 18 is formed on the surface. In the embodiment shown in FIG. 1, the heat retaining film 1B and the intermediate layer 16.
Although only three recording layers 14 are laminated between the recording layer 14 and the substrate 12, the present invention is not limited to this, and more layers may be laminated, or two layers may be laminated.

[Substrate] The substrate 12 is preferably transparent because it allows reproduction light to pass through it quickly, and specifically, in addition to inorganic materials such as glass and aluminum, polymethyl methacrylate, polycarbonate, and polycarbonate are used. and polystyrene, an amorphous polyolefin, poly 4-methyl-1-
Pentene, epoxy resin, polyether sulfofone,
11 materials such as polysulfone and polynyderimide can be used.

[Recording Layer] The recording layer 14 is made of an optical recording film that can record and reproduce a predetermined signal by irradiating it with focused light. The characteristics of the recording layer 14 do not need to be the same for each layer;
The thickness, Faraday rotation angle, composition, etc. can also be changed from layer to layer as necessary.

The optical recording film constituting the recording layer 14 may be, for example, a magneto-optical recording film or an optical recording film made of a photochromic material, but is not particularly limited thereto.

In the present invention, the optical recording film constituting the recording layer 14 includes:
A magneto-optical recording film is preferably used. In the following explanation,
An example in which a magneto-optical recording film is used as the recording layer 14 will be described.

The magneto-optical recording film is made of an alloy of transition metals such as iron and cobal-1 and rare earth elements such as terbium (Tb) and gadolinium (Gd), or other materials (for example, oxide magnetic materials such as garnet-1). It is an amorphous thin film made of an alloy containing etc., and has an axis of easy magnetization in a direction perpendicular to the film surface. In the present invention, the magneto-optical recording film constituting the recording layer 14 is
Although not particularly limited, it may consist of the following components, for example.

A magneto-optical recording film generally consists of at least one element selected from (i>3d transition metals), (i) at least one element selected from rare earths, and (iii>other elements).

(i) As the 3d transition metal, F e N Co -T
+, V, Cr, Mn, Ni5Cu, Zn, etc. are used, and among these, Fe, Co, or both are preferable.

This 3d transition metal is preferably included in the magneto-optical recording film.
It is present in an amount of 5 to 75 atom %, particularly preferably 5 to 70 atom %, more preferably 5 to 75 atom %.

In addition to the above (i>), the magneto-optical recording film contains (i) at least one rare earth element selected from the following group.

Gd, Tb, Dy, 1-1o, Er
, Tm, Yb, LtJ, La, Ce,
Pr, Nd, pHl, 3m.

u Among these, Gd, Tb, Dy, Ho, Nd,
Sm and pr are preferably used.

At least one rare earth element selected from the above group is contained in the magneto-optical recording film, preferably in an amount of 5 to 50 atomic %, more preferably 8 to 45 atomic %, particularly preferably 10 to 45 atomic %.
However, it is present in an amount of 40 atomic percent.

Magneto-optical recording films with the above composition have an axis of easy magnetization perpendicular to the film surface, and are often amorphous thin films capable of perpendicular magnetic and magneto-optical recording, exhibiting a rectangular loop with good Kerr hysteresis. This can be confirmed by wide-angle X-ray diffraction.

Note that a square loop with good Kerr hysteresis means the saturated Kerr rotation angle (1 for θ), which is the Kerr rotation angle at the maximum external m field, and the Kerr rotation angle at zero external magnetic field, and the Kerr rotation of the adult seedling. This means that the ratio of θ to 2/θ to 1 is, for example, 0.8 or more.

[Intermediate layer and protection r4] When the recording layer 14 is the above-mentioned magneto-optical recording film,
The intermediate layer 16 controls the interaction between each recording layer 14, and
This is to enable each recording layer 14 to independently form and maintain a magnetic domain pattern, and its material is transparent enough to allow recording light to pass through, and has a magnetic influence on each recording layer 14. No non-magnetic properties are required. Furthermore, the thickness of the intermediate layer 16 is determined based on conditions as described below.

The depth film 18 is for protecting the outermost recording layer 14, and its material is transparent enough to allow recording light to pass through, and non-magnetic so as not to have a magnetic effect on the recording layer 14. required.

Inorganic materials such as silicon dioxide (Si 02 ) are preferably used as materials for the intermediate layer 16 and the recording layer 18 that satisfy the above conditions (q), but other materials may also be used.

[Film Thickness] The thickness of the intermediate layer 16 according to the present invention is determined so as to satisfy the following two conditions.

(1) First of all, it is necessary that two or more recording layers 14 do not fall within the depth of focus of the recording light at the same time.

When two or more recording layers simultaneously fall within the depth of focus of the focused recording light for storing a predetermined signal in the recording layer,
This is because the same signal is stored in those recording layers by the recording light.

(2) Second, it is necessary to have a thickness sufficient to control the interaction between the recording layers 14 so that each layer has magnetic properties that can independently form and maintain a magnetic domain pattern. If the spacing between the recording layers 14 is too narrow, the magnetization corresponding to the signal stored in one recording layer 14 will affect the magnetization corresponding to the signal stored in the other recording layer 14, resulting in errors. This is because there is a risk that the signal will be memorized.

According to Mr. Imamura (Imamura: Journal of the Television Society).

365 (1985)), where λ is the wavelength of light used for recording or reproduction, and NA is the numerical aperture of the objective lens, the depth of focus is given by λ/(NA)2. λ=0.8 μn,
If NA=0.7, the depth of focus is 1.6 μm, so the minimum thickness of the intermediate layer 16 is this (1.6 μTrL).
) is fine. With such a film thickness, the above condition (2) is also satisfied. The thicker the intermediate layer 16 is, the easier it is to distinguish between the recording layers 14 during recording or reproduction, but the thickness of the intermediate layer 16 is determined by taking into consideration manufacturing efficiency.

Specifically, the thickness of the intermediate layer 16 is in the range of 1.6 to 50 μm, particularly preferably 1.6 to 10 μ7 rL.

Further, in the Q medium 10 according to the present invention, the recording layer 14
In order to determine the optimum film thickness, it is necessary to consider the following three conditions.

First, in the magneto-optical recording medium 10, in order to increase the recording density, the recording layer 14 needs to exhibit stable perpendicular magnetization, but in order to achieve this, the recording layer 14 must have a certain thickness or more. must not,.

The Tb-Fe film, which is a magneto-optical recording material, has a film thickness of 200 mm.
It is said that if the film is thinner than that of a human, it will not become a perpendicularly magnetized film. However, when the recording layer 14 is multilayered as in the present invention, even if the film thickness per layer is reduced to 20, each layer exhibits perpendicular magnetization, making magneto-optical recording possible. Are known.

Y, Tagami F, Ohtaki T, Hor
Ishika and K, Tsushima.

Proc of International Sym
Posium on Physics.

f ) lagnetic ) laterals (wo
rld 5 scientific, Sigap.

re, 1987) l), 275° ■Secondly, the information recorded on the recording medium 10 of the present invention is
Since it is reproduced by detecting the light transmitted through the medium, the recording 814 needs to be as thin as possible because the light must pass through the medium 10. Recording layer 14
When using an amorphous rare earth-transition metal alloy as
The maximum thickness of the entire recording layer through which light can pass is approximately 500 mm.

■Thirdly, in the case of the reproduction method using transmitted light, the recording layer 14
Since the strength of the signal from the recording layer 14 is proportional to the thickness of the recording layer 14, in order to clearly detect the signal, it is necessary to
The film thickness of No. 4 should be thicker.

The optimal film thickness of the recording layer 14 is determined in consideration of the number of layers, the wavelength of light used for recording, the light absorption rate of the recording layer, the sensitivity of the photodetector, etc., so as to satisfy these three conditions. Ru. Specifically, the thickness of the recording layer 14 is in the range of 20 to 500 people, particularly preferably 50 to 300 people.

The thickness of the protective film 18 is as follows:
The thickness may be sufficient as long as it is necessary and sufficient to protect 4 people, and is not particularly limited, but may be about 500 people, for example. Further, the thickness of the substrate 12 is not particularly limited, as long as it has a thickness of about 1 mm to provide appropriate rigidity to the recording medium 10.

[Recording Principle] Next, the principle of recording a predetermined signal on the above-mentioned magneto-optical recording medium 10 will be explained based on FIG. 1.

Each recording layer 14 is separated by an intermediate layer 16 such that two or more layers are not simultaneously present within the depth of focus of the recording beams A, B, and C, so that each recording beam A.

B and C can each focus on only one recording layer 14. Recording light A, B, C
is light such as a laser beam emitted from the same light source, and is controlled by a focus servo mechanism so that it is focused on each recording layer 14. What is the focus servo mechanism?
This is a mechanism for moving a lens forward and backward in the direction in which the light travels in order to control the focusing position of light such as a laser beam, and is also used in laser disc players and the like. The operation of the focus servo mechanism used in the present invention will be described in detail.

Recording layer 14 focused by focus servo mechanism
Information can be recorded using the same principle as normal magneto-optical recording. In the recording layer 14 which is out of focus, the temperature increase due to the laser beam is not sufficient, and therefore no change in the state of magnetization (recording, erasing) occurs.

[Reproducing Principle] In order to read (reproduce) the recorded signal or information from the optical recording medium 10 on which the signal or information (collection of signals) has been recorded in this way, it is necessary to read not only the surface recording layer 14 but also the internal recording layer 14. In order to also detect the information recorded in @14, the intensity of the light transmitted through the recording layer 14 is detected. When the recording layer 14 is the above-mentioned magneto-optical recording film, the rotation of the plane of polarization that the incident light receives upon reaching each recording layer 14 is detected as a change in the intensity of the light by an analyzer.

The depth of the recording layer to be focused on is controlled by the focus servo mechanism in the same manner as in the case of the recording principle described above. The reproduction light used to reproduce recorded information undergoes intensity changes depending on the state of the recording layer 14 that is in focus, and is not affected by layers that are out of focus. This is due to the following reasons.

That is, as shown in FIG. 2, taking as an example the case where the recording layer 14 is the above-mentioned magneto-optical recording film, the magnetization of each recording layer 14b and 14C other than the recording layer 14a from which the signal is to be reproduced is as follows.
Either the magnetization direction is uniform (before recording), as shown in Figure (a), or the different magnetization directions are mixed in approximately the same number (after recording), as shown in Figure (b). be. Therefore, the recording layer 14b where the reproducing light is not focused;
As shown in FIG. 2(a), the magnetization of 14C only changes the direct current (DC) component of the reproduced signal because before recording, the signal from the recording layer 14b, which is magnetized in one direction, is mixed. As shown in FIG. 2B, after recording, since the diffused reproduction light is irradiated thereon, the signals with different magnetization directions overcome each other and do not affect the reproduction signal. Therefore, by such a reproducing method, the magnetization state of each layer in the multilayered recording layer 14 can be detected separately.

[Focus Servo Machine 1] As a method of the focus servo mechanism for recording or reproducing the recording medium 10 according to the present invention, known techniques such as the astigmatism method and the critical angle method can be used. By making improvements to the known focus servo system, it is possible to focus recording light or reproducing light on a layer of arbitrary depth. When the recording layer is one layer as in the conventional case, the relationship (S curve) between the position of the objective lens that focuses recording light or reproduction light on the recording medium and the focus error signal voltage in the focus servo mechanism is shown in Figure 3. The result is as shown in (a). A negative feedback circuit causes a current to flow through the objective lens actuator so that the difference between this voltage and the reference voltage becomes O, thereby keeping the distance between the recording layer and the objective lens constant. When the recording layer is multilayer, the focus error signal voltage is the third
This is a superposition of the curves in Figure (a) shifted in seven directions.

FIG. 3(b) shows the focus error signal voltage from the recording medium having the three recording layers 14 at t4, taking into account that the signal from the layer far from the objective lens becomes weaker due to absorption. This was requested by the camp. Figure 3 (1
)) As shown by ■, ■, ■, an S curve is formed for each layer, so it is possible to select any one layer and apply focus servo to that layer. Note that negative feedback can be applied only to regions where the S curve has a negative slope.

The distance between the recording layer and the objective lens can be changed by changing the current flowing through the objective lens actuator, so by changing the DC component of the current flowing through the objective lens actuator, any one layer can be selected and that recording layer can be changed. After moving the objective lens so that the reproduction light is focused in the vicinity, the focus servo is activated for the selected recording layer. According to the recording/reproducing apparatus having the above, each layer of the multilayer magneto-optical recording medium 10 can be independently addressed to record/reproduce information.

As explained above, according to the present invention, the recording layers are stacked such that the recording layers are spaced apart from each other so as not to be located within the depth of focus of the light used for recording or reproduction. Therefore, even though it is possible to increase the volume recording density of a recording medium several times, it is possible to slightly change the control method of the conventional relatively simple and inexpensive recording and reproducing device without using a special recording and reproducing device. It has the excellent effect of making recording and reproducing possible easily.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of an optical recording medium according to an embodiment of the present invention;
FIGS. 2(a) and (b) are schematic diagrams showing the method for reproducing an optical recording medium according to the present invention, and FIGS. 3(a) and (b) are diagrams showing the operation of the focus servo mechanism used in the reproducing method according to the present invention. FIG. 10... Optical recording medium 12... Substrate 14.
...Recording layer 16...Intermediate layer 18...Protective layer Fig. 1 Recording light A (a) Reproducing light D Fig. 2 (b)

Claims (1)

[Scope of Claims] 1) At least two recording layers made of an optical recording film capable of recording and reproducing a predetermined signal by converging and irradiating light; and a recording layer laminated between these recording layers; An optical recording medium comprising: an intermediate layer made of a transparent material having a film thickness that separates the recording layers so that the two or more recording layers are not located within the depth of focus of light used for recording or reproduction. 2) At least two recording layers made of an optical recording film that can record and reproduce a predetermined signal by converging and irradiating light, and a recording layer laminated between these recording layers, and these two or more recording layers An intermediate layer made of a transparent material having a film thickness that separates each recording layer so that the recording layer is not located within the focal depth of the light used for recording or reproduction, and a recording layer for reproduction at an arbitrary depth in the optical recording medium. After moving the objective lens so that the light of A reproduction method characterized in that information is reproduced by detecting changes in the intensity of transmitted light. 3) The reproducing method according to claim 2, wherein the means for moving the objective lens is a current-controlled objective lens actuator.