JPH0673184B2 - Optical information reproducing method and device - Google Patents

Description

The present invention relates to an information reproducing method and apparatus for an optical information recording medium.

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

However, in the recording method with or without holes, the density in the plane of the medium is determined by the diameter of the beam spot irradiated on the medium, so it is difficult to increase the recording density in the plane without changing the beam spot diameter. As a solution to such a problem, a method has been proposed in which the recording capacity is increased by stacking recording layers in the thickness direction of the medium (Proceedings of the Annual Conference of the Institute of Electronics and Communication Engineers, 1984, S5-12).

This has a structure having layers of different color developing agents (for example, blue, red and green color developing agents) and a developer above and below the first, second and third light absorbing layers, and the first light absorbing layer. When the absorption layer is irradiated with light, blue is emitted, and when the second and third light absorption layers are irradiated with light, red and green are emitted, respectively. The light absorption layer can be selected by changing the focal position of the irradiation beam in the depth direction.
Recording and reproduction are performed on each absorption layer. However, if the distance between the layers is too close, crosstalk occurs and it becomes difficult to record or reproduce each layer independently, and the distance between the absorption layers is 10 μm due to the depth of focus of the beam spot.
The above is necessary.

Further, it is required to form each layer uniformly with a considerable thickness, and it is difficult to form a recording medium having a large area.

Therefore, in a recording medium having one or three recording layers in which holes are formed by light irradiation, information is recorded by forming holes having different depths from the surface of the recording medium to increase the recording capacity. Can be considered.

FIG. 5 is a view in which holes having different depths are formed in one recording layer 31 formed on the substrate 30. Such holes can be formed by changing the irradiation time and irradiation power of light. Since the depth is 1 μm or less, it is not necessary to change the position of the focal point of the beam spot in the depth direction.

FIG. 6 shows a recording medium in which four different recording layers 32 to 35 are laminated on the substrate 30. By changing the number of recording layers forming the holes, holes having different depths can be formed. Since there are four recording layers in FIG. 6, four kinds of holes having different depths can be formed, and each of them has information of "1", "2", "3", "4" according to the depth. be able to. It can be seen that the recording capacity is doubled because the information of “0” and “1” is conventionally used with and without holes. In principle, if the number of layers is increased, the capacity is further increased.

It can be seen that in the multilayer recording medium, the minimum light irradiation power required for hole formation is set to be lower in the upper layer so that the irradiation power can be changed and holes having different depths can be formed. When the irradiation power is low, holes are formed only in the upper layer and as the power is increased to the lower layer.

The depth of the hole can be detected by changing the amount of reflected light.
Due to the interference effect of the reflected light from the inside and the outside of the hole, a change in the amount of light occurs with respect to the depth of the hole as shown in FIG. The minimum amount of light is obtained at a depth of 1/4 of the wavelength. Therefore, if the depth of the hole is shallower than this, the depth can be detected by the difference in the amount of reflected light.

However, since the above-mentioned reproducing method determines the depth based on the intensity of light, it is susceptible to the nonuniformity of the reflectance of the medium. For example, FIG. 7B shows the amount of reflected light at the portion where the reflectance of the medium is reduced. If the reflectance of the medium is reduced by 50%, the amount of reflected light is reduced to 50%. In order to determine the presence or absence of a hole even if the reflectance fluctuates in this way, it is necessary to set the threshold value for separating the peaks and valleys of a and b to the value of T in FIG. The limit is to obtain binary information of. As described above, conventionally, it is difficult to obtain multi-valued depth information of a hole from the amount of reflected light. When the depth of the hole is λ / 4 or more, there is a hole depth of λ / 4 or less that can obtain the same amount of reflected light, so that the recorded information is not increased. As described above, conventionally, the depth information of the hole cannot be effectively used.

(Object of the Invention) An object of the present invention is an optical information reproducing method and apparatus which are not affected by the non-uniformity of the reflectance of the medium which eliminates the above-mentioned drawbacks and which can double the depth of the recording hole. To provide.

(Structure of the Invention) The optical information reproducing method of the present invention is an information reproducing method for an information medium in which information is recorded with height on the surface, and at least two or more monochromatic lights having different wavelengths are applied to the same portion of the information recording medium. It is characterized in that it has a step of irradiating, measuring the amount of light opposite or transmitted from the information medium, and obtaining the ratio of the amounts of light. Further, the apparatus of the present invention includes at least two monochromatic light sources having different wavelengths, a means for condensing monochromatic light from the light source, a focus position detecting means, and a reflected light amount or transmission of the monochromatic light from the object to be measured. Means for measuring the amount of light, and a circuit for obtaining the ratio of the amount of reflected light or the amount of transmitted light of each monochromatic light.

(Operation and Principle of the Invention) FIGS. 2 and 3 are views showing the principle of the optical information reproducing method according to the present invention. When a hole of depth α is recorded from the surface S _{1 of the} recording medium, the amount of reflected light obtained during signal reproduction is
It is obtained by the sum of the reflected light from S ₁ and the reflected light from the bottom S _{2 of the} hole. Now, the amplitude reflectance of the surface S ₁ is γ ₁ , the bottom of the hole
In order to simplify the calculation by setting the amplitude reflectance of S ₂ to γ ₂ , the cross-sectional area where the light beam (light amount A) irradiated on the recording medium is reflected on the surface S ₁ and the bottom S ₂ of the hole are defined. If they are equal, the reflected light amount R is obtained by the following equation.

In equation (1), the first term is the amplitude of the reflected light from the medium surface S ₁ , and the second term is the amplitude reflectance of the bottom S ₂ of the hole multiplied by the optical phase difference due to the depth of the hole. It represents the reflected light amplitude from the bottom S _{2 of the} hole. In the formula (1), the amount of reflected light is the minimum when the third term is -1, and the depth α of the hole satisfies the following formula.

α = (2n + 1) λ / 4 where n = 0,1,2, ... (2) Therefore, the amount of reflected light differs depending on the depth of the hole,
It also depends on the wavelength of the incident light. FIG. 3 is a diagram showing how the amount of reflected light changes with the depth of the hole for light of two wavelengths (λ ₁ , λ ₂ : λ ₁ <λ ₂ ). reflected light amount with respect to lambda ₁ wavelengths of light are reflected light amount with respect to a change to lambda ₂ wavelengths of light to minimize the time a depth α is lambda _1/4 of the holes of the hole depth α of lambda _2/4 When it changes to the minimum.
Therefore, when the depth of the hole is D, the amount of reflected light is different from the value of R ₁ for the light of wavelength λ ₁ and the value of R ₂ for the light of wavelength λ ₂ . The ratio K of the two reflected light amounts is given by the following equation.

However, FIG. 4 shows the relationship between m = 2 ₁ γ ₁ γ ₂ / (▲ γ ² ₁ ▼ + ▲ γ ² ₂ ▼) reflected light amount ratio K and the depth α of the hole. Reflection light intensity ratio, when n is an integer, α = (2n-1) λ 1/4
In minimum value, alpha = become maximum at _{(2n-1) λ 2/} 4. In the range of this figure the depth of the hole in the 0~λ _1/4 or λ ₂ / 4~3λ _1/4, the depth values of the hole from the reflected light amount ratio K is determined uniquely. Further, in the R of the equation (1) and the K of the equation (3), the variation due to the change of γ ₁ and γ ₂ is small. For example, the reflected light amount ratio K does not change even when the reflection amplitudes γ ₁ and γ ₂ are similarly reduced by 50%.
Therefore, it is possible to set a threshold value like a, b, c, d, and e in FIG. 4 and read it as a multivalued value (1, 2, 3, 4, 5, 6) from the reflected light amount. . Therefore, it is possible to obtain the depth of the hole of the recording medium by making laser light having different wavelengths incident on the recording medium, detecting the reflected lights separately, and obtaining the ratio of the detected light amounts of both, and to obtain the multivalued recording information. Can be read as Therefore, in the present invention, recording / reproducing more than twice as much as the conventional one can be performed.

(Embodiment) FIG. 1 is a diagram showing an embodiment of an optical information reproducing method and apparatus according to the present invention. Wavelength λ ₁ emitted from semiconductor laser 1a
The light 2a is converted into a parallel light flux by the lens 3 and passes through the dielectric mirror 4. Light 2b of wavelength λ ₂ emitted from semiconductor laser 1b
Is converted into a parallel light flux by the lens 3 and reflected by the dielectric mirror 4. After the light of wavelengths λ ₁ and λ ₂ passes through the polarizing prism 6,
The light is converted into circularly polarized light by the quarter-wave plate 7, reflected by the reflecting mirror 8, and then condensed by the lens 9 into the hole recorded on the recording track 21 on the recording medium 20. The reflected light from the recording medium 20 passes through the lens 9 and the reflecting mirror 8, then returns to linearly polarized light again by the 1/4 wavelength plate 7, and is reflected by the polarization beam splitter 6. Furthermore the reflected light is divided into the light 13 of the light 12 and wavelength lambda ₂ wavelength lambda ₁ by the dielectric mirror 11, the light 12 of the wavelength lambda ₁ is the amount of light is detected by the photodetector 14. The light 13 having the wavelength λ ₂ is split into two by the half mirror 15, one of which is detected by the focusing detector 16 and the other of which is detected by the tracking detector 17. The comparator 18 calculates the ratio between the sum signal of the focusing detector 16 and the tracking detector 17 and the detection signal of the photodetector 14, and this output is obtained as a detection signal for the depth of the hole.

When the wavelengths λ ₁ and λ ₂ of the semiconductor lasers 1a and 1b are 780 nm and 830 nm, and the recording medium has the reflectances γ ₁ ² and γ ₂ ² of 0.5, the detection signal obtained from the equation (3) is used. Fourth as the value of K
Figure similar is obtained about the depth of the hole of the λ ₂ / 4~3λ _1/4
It was possible to decide over 1/2 wavelength.

(Effects of the Invention) According to the present invention, it is possible to detect twice as much recording information as in the conventional reproducing method, and it is possible to realize optical recording with a large capacity. Further, the depth of the hole can be detected without being affected by the unevenness of the reflectance of the surface of the recording medium.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of an optical information reproducing method and apparatus according to the present invention, FIGS. 2 and 3 are diagrams showing the principle of the optical information reproducing method according to the present invention, and FIG. 4 is an optical diagram according to the present invention. FIG. 5 is a diagram showing an effect of the information reproducing method, FIG. 5 is a diagram showing an example of a single layer recording medium used in the present invention, FIG. 6 is a diagram showing an example of a multilayer recording medium used in the present invention, and FIG. 7 is a conventional reflection. It is a figure which shows a light amount detection signal. In the figure, 1a, 1b ... laser, 3, 9 ... lens, 4, 11 ...
… Council dielectric mirror, 6 …… Polarizing beam splitter, 7
...... 1/4 wave plate, 14 ...... Receiver, 15 ...... Half mirror, 1
6 ... Focusing detector, 17 ... Tracking detector, 18 ... Comparator, 20 ... Recording medium.

Claims (2)

[Claims] 1. An optical information reproducing method for an optical recording medium having holes having different depths from the surface of the recording medium, wherein the optical recording medium has two wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ). 0
lambda _1/4, or n a when integers of 1 or above (2n-1) λ ₂
/ 4~ (2n + 1) hole at a depth of lambda _1/4 is being formed, 2
And irradiating the same location of the optical recording medium with monochromatic light of three different wavelengths, measuring the amount of light reflected or transmitted from the optical recording medium for each wavelength, and determining the ratio of the amounts of light. Optical information reproduction method. 2. A hole having different depths from the surface of the recording medium is provided, and 0 to 2 wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ) are provided.
lambda _1/4, or n a when integers of 1 or above (2n-1) λ ₂
/ 4~ (2n + 1) and the optical recording medium having a hole at a depth of lambda _1/4 is formed, two different monochromatic light of wavelength multiplexes condensing a means of monochromatic light from the respective light source And a focus position and track position detecting means, and means for measuring the reflected light amount or transmitted light amount of the monochromatic light from the object to be measured, and further obtaining the ratio of the reflected light amount or the transmitted light amount of each of the monochromatic light. An optical information reproducing apparatus comprising a circuit.