JPH05334726A - Optical recording medium and optical recording and reproducing method - Google Patents

Abstract

PURPOSE:To considerably reduce the spot diameter of laser beams on a focal plane as compared with the conventional diameter without varying the numerical aperture of a condensing optical system or the wavelength of the laser beams. CONSTITUTION:In order to reduce the spot diameter of laser beams (a) for reproduction on a focal plane determined by the wavelength of the laser beams (a) and the characteristics of a condensing optical system (b) such as a condenser, a nonlinear light absorbing layer (c) is interposed between the recording layer (d) of an optical recording medium and the exit lens surface. At this time, one side of the light absorbing layer (c) is made to coincide with the focal plane.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording medium having a high recording density and an optical recording / reproducing method.

[0002]

2. Description of the Related Art An optical recording medium capable of recording and reproducing information by using a light beam is drawing attention as a high density information recording medium. Such an optical recording medium is roughly classified into a read-only type optical disc for reading a previously formed recording pit and a writable type optical disc.

As a read-only type optical disk, VD
(Video disc) and CD (compact disc). Examples of the writable type optical disc include a write-once type optical disc that forms irregularities called pits on a substrate, a magneto-optical disc, a photochromic type disc, and a PHB type disc.

When reading (reproducing) recording pits in any medium, the spatial convergence of the laser light is utilized. The spot diameter on the focal plane of the reproduction laser beam determines the size of the record pit that can be read. The spot diameter depends on the wavelength of the reproducing laser beam and the characteristics of the condensing optical system.
The upper limit of the recording density of the optical recording medium has been determined by the wavelength of the laser beam and the characteristics of the focusing optical system.

[0005]

As described above, in order to increase the recording density, the characteristics of the condensing optical system are changed (for example, the numerical aperture of the condensing optical system is increased) or the reproducing laser used for recording and reproducing. It is necessary to shorten the wavelength of light. When the numerical aperture of the optical system is increased, the spot diameter on the focal plane becomes smaller, but at the same time the depth of focus becomes shallower, and the demand for mechanical accuracy such as surface wobbling of the medium becomes higher.

On the other hand, it is expected that shortening the wavelength of laser light will be difficult in a short period of time. Therefore, an object of the present invention is to drastically reduce the spot diameter on the focal plane without changing the numerical aperture of the condensing optical system and the wavelength of the reproduction laser light.

[0007]

In the present invention, a nonlinear light absorption layer (c) is inserted between the recording layer and the exit lens surface of the medium in the arrangement as shown in FIG. At this time, one surface of the non-linear light absorption layer is configured to coincide with the focal plane. With such a configuration, the spot diameter of the reproduction laser light can be made smaller than that in the case where the non-linear light absorption layer is not inserted.

Therefore, according to the present invention, the reproduction laser beam is spatially narrowed down, the recording layer is irradiated, and the change in the reflected light amount from the recording layer, the change in the transmitted light amount, or the change in the emitted light amount is detected to reproduce the record. Provided is an optical recording medium having a non-linear light absorption layer exhibiting a non-linear light absorption characteristic in which an absorption coefficient decreases with an increase in reproduction laser light intensity (claim 1).

Further, the present invention is characterized in that the reproducing laser beam is irradiated onto the recording layer after passing through the non-linear light absorbing layer exhibiting a non-linear light absorbing characteristic whose absorption coefficient decreases as the reproducing laser beam intensity increases. An optical recording / reproducing method is provided (claim 2).

[0010]

Operation As one of the nonlinear optical phenomena, nonlinear optical absorption (or absorption saturation) is known. This is the process by which the absorption coefficient of the medium decreases as the incident light intensity increases. Here, it is assumed that the condensing optical system has no aberration and the pupil is sufficiently large. If the incident light is a plane wave having a Gaussian intensity distribution, the light intensity distribution at the focal plane will also be Gaussian. Here, further consider a case where a non-linear light absorber is inserted near the focal plane between the focal plane and the exit lens surface.
If the condition that absorption saturation occurs efficiently is satisfied, linear absorption becomes dominant in the low-intensity skirt of the focused spot near the focal plane, and conversely, the central part of the high-intensity spot is absorbed by absorption saturation. It shows higher transmittance than in the case of linear absorption. As a result, the focused spot diameter is reduced by inserting a non-linear light absorber. FIG. 2 shows the intensity distribution (I ₁ ) in a section perpendicular to the optical axis of the laser light immediately before entering the nonlinear optical absorber and the intensity distribution (I ₂ ) immediately after passing through the nonlinear optical absorber. Here, r ₀ and r are distances from the central axis of the laser beam. When the entrance pupil is finite, the peripheral portion of the focused spot generally expands while vibrating strongly, but the spot diameter is reduced by the above effect.

Such a simple prediction is allowed when the non-linear light absorber is as thin as the wavelength of the light beam and is arranged near the focal plane. If a nonlinear optical absorber with an arbitrary thickness, one of which matches the focal plane, is inserted between the exit lens surface and the focal plane, the Maxwell-Bloch equation and the theory of diffraction must be developed without contradiction. And it is very difficult to even make a qualitative conclusion.

As described above, non-linear light absorption is a phenomenon in which the absorption coefficient of the medium decreases in proportion to the intensity of incident light. Therefore, even when a non-linear light absorption layer with an arbitrary thickness is inserted, it is possible to optimize the numerical aperture of the focusing optical system and the incident laser intensity so that the saturation absorption effect remarkably occurs only near the focal point. is there. The reading of recording from a general optical recording medium uses a reflective optical system as shown in FIG. In this case, the reproduction light is reflected by the recording layer and again passes through the nonlinear light absorption layer. It is difficult to accurately analyze how the reproduction light changes in this process, as in the above process. However, considering that the reproduction light is generally continuous light, the reproduction light reflected by the recording layer can effectively reach the photodetector through the portion where the absorption coefficient is reduced by the reproduction light that has been delayed. You can say that you can.

The non-linear light absorption layer is effective when formed adjacent to the recording layer (claim 3). As the recording layer of the optical recording medium of the present invention, for example, a recording pit formed on one surface of a non-linear light absorbing layer is coated with a reflective layer that reflects a reproduction laser beam (claim 4), or a non-linear light A reflective layer that reflects a reproduction laser beam is formed on one surface of the absorbing layer, and the reflective layer is partially removed according to a recording signal (claim 5).

The present invention will be described below with reference to examples.

[0015]

Example 1 An apparatus system shown in FIG. 4 was constructed by using a He--Ne laser of 633 nm as a reproducing light source and using a focusing optical system having a numerical aperture of 0.5. As the medium, an optical recording medium having the structure shown in FIG. 5 was used. Here, a material generally called semiconductor-doped glass was used for the nonlinear light absorption layer. This material is prepared by dispersing II-VI compound crystals such as CdSe and CdS or ternary crystallites thereof in glass, and controlling the composition and particle size distribution of the crystallites to obtain the desired wavelength. It shows a large saturated absorption in the region. The material used here has a thickness of 1.2 mm and an optical density of 0.1 at 640 nm and 633 nm.
The values were 1.0 and 2.0 at 625 nm. The recording layer is a metal film (A
u, film thickness 1000Å) was formed on the semiconductor-doped glass by a sputtering method, and then the metal film was peeled off in a spot shape having a diameter of about 0.3 μm according to the presence or absence of a recording signal. The peeling of the metal film is performed according to ordinary photolithography, and an Ar ion laser is used as an exposure light source.
A 457.9 nm oscillation line was used. The numerical aperture of the exposure optical system is 0.93
Met. Also, for comparison, a comparative sample was prepared in which an Au thin film was sputtered on a normal transparent glass substrate to form an identical recording layer. The recording layer was sealed with an acrylic adhesive and an acrylic plate.

Using the apparatus shown in FIG. 4, the produced optical recording medium was periodically finely moved, and the signal appearing on the photodetector (g) was observed with an oscilloscope. In the optical recording medium of the present invention, a periodic signal corresponding to the reproduction signal could be observed, but it was found that the comparison sample in which the recording layer was formed on the glass substrate could not reproduce the recording (FIG. 6).

[0017]

Example 2 A dye laser having a wavelength of 620 nm was used as the light source, the same apparatus as that in FIG. 4 was used for the other components, and the optical recording medium shown in FIG. 7 was used. Here, the non-linear light absorption layer is formed of PMMA to which zinc tetraphenylporphine is added in an amount of 0.05 mol / l, and the recording layer is formed with recording pits on the above-mentioned non-linear light absorption layer by a template method, and then a reflective film (Al) is sputtered. Was formed into a film and manufactured. The recording layer was sealed with an acrylic adhesive and an acrylic plate. The diameter of the recording pit is 0.35μ
m, depth is about 150 nm. As in Example 1, as a comparative sample, a sample in which the non-linear light absorbing layer was replaced with a transparent PMMA substrate (without addition of zinc tetraphenylporphine) was also manufactured.

The reproduction signal was detected according to the method of the first embodiment. As a result, zinc tetraphenylporphine / P
The recorded signal could be reproduced with the sample using MMA as the non-linear light absorbing material, but PMMA containing nothing was added.
No reproduced signal was obtained in the comparative sample in which the recording layer was formed on the substrate.

[0019]

Example 3 The same apparatus as in Example 1 was used, and the optical recording medium shown in FIG. 8 was used. Here, the substrate (r) was made of transparent PMMA, and recording pits having a diameter of about 0.35 μm were formed by the casting method. The non-linear light absorption layer was formed by using the semiconductor-doped glass used in Example 1 on the surface of the PMMA substrate on which the pits were formed by a sputtering method.
The film thickness is about 3 to 10 μm. However, in order to obtain a sufficient optical density with a thin film, the concentration of CdSe or CdS is made extremely high. Optical density of 2.0 at 620 nm, respectively
As described above, the values were 1.0 at 633 nm and 0.1 at 640 nm. After that, a reflection film (Al) was laminated in 1000 liters and sealed with an acrylic adhesive and an acrylic plate. Further, for comparison, a comparative sample was prepared in which after the formation of the recording pits, the reflection film was formed and sealed without forming the semiconductor-doped glass as the nonlinear light absorption layer.

The recorded signal was reproduced according to the method of the first embodiment. At this time, as a light source for reproduction, 633 nm He-N
An e-laser was used. Recording was able to be reproduced in the sample in which the non-linear light absorption layer (1) was formed, but no reproduced signal was obtained in the sample for comparison in which the nonlinear light absorption layer (l) was not formed. The semiconductor-doped glass can be formed by a spin coating method such as a sol-gel method in addition to the sputtering method performed in this embodiment.

[0021]

As described above, according to the optical recording medium of the present invention, the spot light of the reproducing light can be reduced without changing the numerical aperture of the reproducing optical system and the wavelength of the reproducing light source. Higher density recording is possible. Further, according to the optical recording / reproducing method of the present invention, the resolution in photolithography can be dramatically improved.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of an optical recording medium of the present invention.

FIG. 2 is a diagram showing in-beam intensity distributions of laser light immediately before and immediately after entering a nonlinear light absorber.

FIG. 3 is a conceptual diagram showing a reflective optical system used for recording and reading on an optical recording medium.

FIG. 4 is a conceptual diagram of an apparatus system used for reproducing the optical recording medium of the present invention.

FIG. 5 is an explanatory diagram showing the structure of an embodiment of the optical recording medium of the present invention.

FIG. 6 is a graph showing changes in the amount of detected light with respect to the displacement of the medium.

FIG. 7 is an explanatory diagram showing the structure of an example of the optical recording medium of the present invention.

FIG. 8 is an explanatory diagram showing the structure of an example of the optical recording medium of the present invention.

[Explanation of symbols]

 a reproducing laser light b condensing lens c non-linear light absorption layer d recording layer e disk substrate f recording protection layer g photodetector h piezoelectric element i oscilloscope j piezoelectric element driving power source l semiconductor-doped glass m metal film n adhesive agent acrylic plate p Zinc tetraphenylporphine / PMMA q Al film r PMMA

Claims (5)

[Claims] 1. An optical recording medium for reproducing a recording by spatially narrowing a reproduction laser beam, irradiating a recording layer, and detecting a change in the amount of reflected light from the recording layer, a change in the amount of transmitted light, or a change in the amount of emitted light. An optical recording medium having a non-linear light absorption layer exhibiting a non-linear light absorption characteristic in which an absorption coefficient decreases with an increase in intensity of a reproduction laser beam. 2. An optical recording / reproducing method for reproducing a record by spatially narrowing a reproducing laser beam, irradiating a recording layer, and detecting a change in the amount of reflected light, a change in the amount of transmitted light, or a change in the amount of emitted light from the recording layer. In the optical recording / reproducing method, the reproducing laser light is transmitted through a non-linear light absorbing layer exhibiting a non-linear light absorbing property whose absorption coefficient decreases with an increase in the reproducing laser light intensity, and then irradiates the recording layer. .. 3. The optical recording medium according to claim 1,
An optical recording medium, wherein the non-linear light absorbing layer and a recording layer are formed adjacent to each other. 4. The optical recording medium according to claim 1,
An optical recording medium, wherein the recording layer is formed by coating a recording pit formed on one surface of the nonlinear light absorbing layer with a reflective layer that reflects a reproduction laser beam. 5. The optical recording medium according to claim 1,
The recording layer is a reflective layer formed on one surface of the non-linear light absorbing layer for reflecting a reproduction laser beam, and the reflective layer is partially removed according to a recording signal. Medium.