JPH065584B2 - Optical recording medium - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical recording medium.

(Prior art and problems) Various methods are proposed for recording information by converging a laser beam to a minute spot with a condenser lens and irradiating it on a recording medium surface to form a hole in the recording medium surface. Has been done.

However, in the recording method with or without holes, the density in the plane of the recording medium is determined by the beam spot diameter with which the recording medium is irradiated, so it is difficult to increase the recording density in the plane without changing the beam spot diameter. Therefore, a method of increasing the recording capacity by stacking several recording layers has been proposed (Proceedings of the 59th Annual Conference of IEICE General Conference S5-12).

FIG. 3 shows an example of the structure of a recording medium used in such a system. A blue color developing agent 41, a red color developing agent 45, a green color developing agent 46, and a color developing agent 43 are provided above and below the first, second, and third light absorbing layers 42, 44, 47.
When the first light absorption layer is irradiated with light, it becomes blue,
The second and third light absorption layers exhibit red and green colors, respectively. Recording and reproduction are performed on each light absorption layer by changing the focal position of the beam spot in the depth direction.

However, in order to distinguish each light absorbing layer by the difference in the depth of focus of the beam spot, it is necessary that the distance between each light absorbing layer is 10 μm or more. This is because if the distance between layers is closer than this, crosstalk occurs and it becomes difficult to reproduce each layer independently. Therefore, it is required to form each layer uniformly with a considerable thickness, but it is difficult to form a recording medium having a large area. Beam spot is 10μ in the depth direction
It is necessary to move m or more, which complicates the control of the spot position. Further, when the third light absorbing layer is irradiated with light, the first and second light absorbing layers also absorb light, resulting in a large loss of irradiation power.

(Object of the Invention) The object of the present invention is to eliminate the above-mentioned drawbacks, and
The present invention proposes a recording medium whose capacity can be increased.

(Means for Solving the Problems) The optical recording medium of the present invention is a substrate in which a recording layer having a refractive index n and a thickness d in which a hole is formed by light irradiation has a refractive index smaller than the refractive index n. Further, the thickness d is provided above, and the thickness d is determined so as to satisfy d ≦ λ / 4n (where λ is the wavelength of light used for reproducing recorded information), and the depth of the hole formed by light irradiation is set. It is characterized in that there are two or more types of height _tm (m = 1, 2 ...) And 0 <t ₁ <t ₂ ... ≤d.

FIG. 1 is a diagram showing a configuration example of a recording medium according to the present invention. The recording layer 1 is formed on the substrate 2. The depth of the hole can be controlled by changing the intensity of irradiation light or irradiation time.

In the configuration of FIG. 1, the reflectance of the recording layer surface varies depending on the thickness of the recording layer. For example, as a recording layer using a glass substrate, using vanadium oxide phthalocyanine (VOPc),
When 8300Å is used as the recording / reproducing wavelength, the complex refractive index of the recording layer is (2.9-j0.8). FIG. 2 shows the change in reflectance with the thickness of the recording layer. Thickness d = 8300 / (4 × 2.9)
The highest reflectance is obtained near 700Å. This is because the reflection from the recording layer surface and the reflection from the boundary between the recording layer and the substrate overlap in phase. It can be seen that if the thickness is thinner than this layer, the reflectance monotonously decreases. When the holes are formed by the light irradiation, the thickness of the recording layer is reduced and the reflectance is lowered inside the holes. It is clear from FIG. 2 that the reflectance varies depending on the remaining film thickness inside the hole. That is,
If the layer thickness d is set to d ≦ λ / 4n, the reflectance is uniquely determined when the depth of the hole is determined. As a result, multi-value recording is possible by changing the depth of the hole and the reflectance. For example, in the recording medium of Fig. 2, the layer thickness is set to 700 Å and the depth of the hole is changed to 300 Å, 500 Å, 700 Å to change the reflectance to 32%, 1
It was possible to change in 3 steps of 5% and 4%, and it was possible to record 4-level information.

(Effect of the Invention) According to the present invention, it is possible to record by changing the depth of the hole. That is, since multi-value recording is possible, it is possible to easily realize optical recording having a larger capacity than conventional optical recording. In addition, since the reflectance changes greatly when the hole depth changes by only a few angstroms, the beam spot does not move much even in multi-value recording, and corner control is simple. The light absorption by the layer is small and the irradiation power loss is small (because the change in depth is small).

[Brief description of drawings]

FIG. 1 is a diagram showing an example of the structure of a recording medium according to the present invention, FIG. 2 is a diagram showing the reflectance change with the recording layer thickness, and FIG. 3 is a diagram showing a conventional multi-layer recording medium. In the figure, 1 ... Recording layer, 2 ... Substrate, 41 ... Blue color former, 45 ...
Red coloring agent, 46 ... Green coloring agent, 42, 44, 47 ... Light absorbing layer, 43
… It is a color developer.

Claims (1)

[Claims] 1. A recording layer having a refractive index n and a thickness d in which a hole is formed by light irradiation is provided on a substrate having a refractive index smaller than the refractive index n, and the thickness d is d. ≦ λ / 4n (where λ is the wavelength of the light used for reproducing the recorded information), and the depth t _m (m = 1, 2 ...) Of the hole formed by the light irradiation. The optical recording medium is characterized in that two or more kinds are present and 0 <t ₁ <t ₂ ... ≦ d.