JPH02158932A - Optical information recording medium and optical information reproducing device - 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 [Object of the Invention] (Industrial Application Field) The present invention relates to an optical information recording medium and an optical information reproducing device.

(Prior art) A device that records information along spiral or concentric tracks on the surface of a disk-shaped recording medium and optically reproduces the information by irradiating it with a light beam (mainly a laser beam) is in practical use. has been made into

Typical examples include compact disc type digital audio discs and laser vision type video discs. In addition, products have been put into practical use such as document file systems using optical disks that allow users to record information as they wish, and products that are used as peripheral storage devices for computers that require a higher degree of reliability.

Furthermore, in recent years, there has been active research and development toward the commercialization of rewritable optical disk devices that can erase and rewrite recorded information, as well as optical card memory devices that apply similar technology to card-shaped recording media. is in progress.

When reproducing information recorded on these optical information recording media, a light beam is irradiated onto the recording film from the transparent substrate side to prevent the reproduced signal from deteriorating due to dust, scratches, etc. , a method of detecting its reflected or transmitted light is generally used. In this method, reflection of light on the light beam incident side of the substrate is unavoidable, which causes problems such as an increase in reproduced signal noise, a decrease in the reproduced signal level due to a decrease in light utilization efficiency, and instability of the focusing servo. There is.

In order to solve this problem, a method has been proposed in which an antireflection film is formed on the surface of the substrate on the light incident side to reduce the amount of reflected light (Japanese Patent Application No. 61-285749). however,
This method has a problem in that when a light beam is focused using a condensing lens with a large numerical aperture to increase the recording capacity, the reproduced signal intensity decreases.

Generally, the optical thickness n-d (n: refractive index, d: film thickness) of an anti-reflection film is set to λ/4 for a specific wavelength λ, and the amount of reflected light for light rays incident from the front is set to λ/4. It is configured to minimize the Therefore, as the angle of incidence of incident light increases, the amount of reflected light increases, and the performance of a condensing lens with a large numerical aperture cannot be fully utilized. That is, it is expected that the reproduction resolution by optical detection will increase in proportion to the numerical aperture of the condenser lens, but conventional antireflection films have not been able to provide resolution commensurate with the increase in numerical aperture.

(Problems to be Solved by the Invention) As described above, in optical information recording media provided with conventional antireflection films, when a condensing lens with a large numerical aperture is used in response to improved recording density, the amount of reflected light is reduced. As a result, the performance of the condensing lens is not fully utilized, resulting in a problem in that sufficient reproduction resolution cannot be obtained.

The present invention was made to solve these conventional problems, and an object of the present invention is to provide an optical information recording medium and an optical information reproducing device that can reproduce information recorded in high density with higher resolution. do.

[Structure of the Invention] (Means for Solving the Problems) The present invention provides readable information that is recorded on a substrate surface or a recording film formed on the substrate by irradiation with a light beam, and further provides information on the substrate surface on which the information is recorded. Alternatively, in an optical information recording medium in which an anti-reflection film is provided on the incident side of the light beam from the recording film, the anti-reflection film is able to absorb the light when the center wavelength of the light beam is λ and the maximum angle of incidence of the light beam is θc. Of the beams, 0<
For light incident at a predetermined angle θ within the range θ<θc, the optical path length difference from the front and back surfaces is (m+1/2)λ (where m is an integer).

In addition, the present invention records information that can be read by irradiating a light beam on a substrate surface or a recording film formed on the substrate, and further provides anti-reflection protection on the light beam incident side from the substrate surface or recording film on which the information is recorded. In an optical information reproducing device that reproduces information by irradiating a light beam onto an optical information recording medium provided with a film through a condensing lens and detecting the reflected light or transmitted light through the condensing lens, When the center wavelength of the light beam is λ and the maximum incident angle of the light beam determined by the numerical aperture of the condensing lens is θc, the prevention film is designed to prevent the light beam from being incident at a predetermined angle θ within the range of 0<θ<θc. For the light reflected from the front and back surfaces, the difference in optical path length is (
m+l/2)λ.

Note that the difference in optical path length of the reflected light from the front and back surfaces of the anti-reflection film is (
The lower limit of the angle of incidence θ, which corresponds to m+1/2)λ, may be any value exceeding 0, but it is particularly desirable to be θc/2 or more. If θ is less than θc/2, a sufficient antireflection effect for incident light near the edge of the light beam cannot be obtained.

(Function) In this way, for light incident at a predetermined angle θ within the range 0<θ<θc, the optical path length difference between the reflected light from the front and back surfaces is (
By providing an antireflection film having an angle of .lambda.m+1/2), the reflectance from the medium surface for incident light at an angle .theta. is minimized.

When reproducing information by irradiating an optical recording medium with a light beam and detecting reflected or transmitted light, the light that is incident at a large angle of incidence contributes more to improving the reproducing resolution.
Minimizing the reflectance for light at an angle θ (0 x θ x θc) as in the present invention means that the reproduction signal strength for information recorded at high density is improved.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a cross-sectional view schematically showing the structure of an optical information recording medium according to an embodiment of the present invention, in which a recording film 12, a reflective film 13, and a protective film 14 are provided on one surface of a transparent substrate 11. are stacked in sequence. As the transparent substrate 11, for example, a resin substrate such as polycarbonate or polymethyl methacrylate, or a glass substrate is used. The recording film 12 may be a write-once type recording film, such as a Te-0x film, which can basically be written only once, or a rewritable type recording film, such as a magneto-optical recording film typified by the RE-7M film. It will be done. For the reflective film 13, a light reflective film such as an aluminum film is used.

Further, as the protective film 14, 5i02 or other film is used.

An antireflection film made of a dielectric thin film such as a Si3N4 film is provided on the other surface of the substrate 11, that is, on the surface on which the light beam 17 focused by the condensing lens 16 is incident when viewed from the recording film 12. 15 is formed. In FIG. 1, θc is the maximum incident angle of the light beam 17, that is, the incident angle of light from the aperture edge of the condenser lens 16, and increases as the numerical aperture (NA) of the condenser lens increases ( NA=sinθc).

As is well known, the antireflection film utilizes the interference effect of a thin film, and is configured so that the reflected light from the front and back surfaces of the thin film cancel each other out due to the difference in optical path length. In a normal anti-reflection film, if the center wavelength of the light beam is λ, then the optical path length difference between the light reflected from the front and back surfaces relative to the incident light from the direction perpendicular to the film surface is 2·n-d (n: the difference in anti-reflection effect). refractive index, d
; film thickness) is (m+l/2)λ, for example, λ/2 (this is m
−0)).

nd is also referred to as the optical thickness of the antireflection film. As a result, for incident light from the front direction (θ-0), the reflected light from the front and back surfaces of the antireflection film have opposite phases and cancel each other out, and the reflectance becomes minimal.

However, in such conventional anti-reflection coatings, the incident angle is θ
For incident light of (0 x θ x θc), the optical path length difference between the reflected light from the front and back surfaces decreases, and ΔL-2n-d 1-(si
Since nθ/n)2, the reflected light from the front and back surfaces do not cancel each other out sufficiently, resulting in loss due to reflection.

In order to solve such problems, in the present invention, the reflective film +1
The film thickness d and material (refractive index n) of the film 15 are set to satisfy the following formula.

ΔL-2・n-dv'T(slrl/n)] --
(m+1/2)λ In other words, the anti-reflection film 15
Optical path length difference ΔL-2・n-d 1 of reflected light from the front and back surfaces of
-(sinθ/n)2 is (m+1/2)λ, for example, λ/
2 and the reflectance is minimized.
5 is configured. Note that the optical path length difference ΔL is obtained from FIG. 2 as follows.

ΔL-(n ・AC+n-CD)-AB2・d Mausoleum n @co8B / 2 ” dL anθ'・
sinθsinθ' 9sinθ -2・d(-..86.) COSθ' Here, from n-5inθ/s1nθ', ΔL-
2・n-d (..8.l?/ (1-sin2θ')
)-21InlIdlIcos θ' -2・n-dl¥-πT17 -2on-d〆";πe/n)".

In the optical information recording medium shown in FIG. 1, as the recording density increases, the optical changes (irregularities, bits, magnetization reversal, etc.) in the recording film 12 become more minute. When reading such optical changes using a light beam, the components near the edges of the light beam, that is, the reflected light of light that is incident on the medium at a large angle, reflect minute optical changes, and the reproduction resolution is affected. This greatly contributes to improvement. Therefore, if the anti-reflection film 15 is constructed so that the reflectance for light with a large incident angle θ (0 x θ<θc1, especially θc/2 x θc) is minimized as described above, a high Information recorded at high density can be reproduced with good resolution, and a reproduced signal with high signal strength can be obtained.

Note that the lower limit of the incident angle θ at which the optical path length difference ΔL of the reflected light from the front and back surfaces of the antireflection film 15 is λ/2 is preferably θc/2. Reflected light from light with θ less than θc/2 is difficult to reflect minute optical changes in signals recorded in high density, so in order to increase effective reproduction resolution, it is necessary to reflect light from light with an incident angle greater than this. This is because it is necessary to enhance the prevention effect.

FIG. 3 shows a schematic configuration of an optical information reproducing apparatus according to an embodiment of the present invention. The light source 1 is, for example, a semiconductor laser, and the light beam emitted from the light source 1 is made into parallel light by a collimator lens 2, and then connected to a beam splitter 3 and a condenser lens 4 (corresponding to the condenser lens 16 in FIG. 1). ) through
The optical information recording medium 5 shown in FIG. 1 is irradiated with light. The light beam irradiated onto the optical information recording medium 5 passes through the anti-reflection film 15 and the transparent substrate 11, irradiates the recording film 12, and is reflected by the reflective film 13 below the B film 12. The reflected light passes through the recording film 12, the transparent substrate 11, and the antireflection film 1.
5, passes through a condensing lens 4, is reflected by a beam splitter 3, and then passes through a lens 6 to a photodetector 7.
incident on .

As a result, a reproduced signal output is taken out from the photodetector 7.

FIG. 4 is a sectional view schematically showing the structure of an optical information recording medium according to another embodiment of the present invention, in which a reflective film 2 is provided on a substrate 21.
2. Recording film 23, transparent protective film 24, and antireflection film 25
are stacked in sequence.

In this embodiment, the substrate 21 does not need to be particularly transparent. The reflective film 22 and the recording film 23 are the same as those shown in FIG. The transparent protective film 24 also serves as an intermediate layer for protecting the recording film 23 and improving recording and reproducing characteristics. Further, the anti-reflection film 25 is 0<θ<θ as in the embodiment shown in FIG.
The structure is such that the optical path length difference between reflected light from the front and back surfaces of incident light at an angle θ c is (m+1/2)λ, for example, λ/2.

In this embodiment, the light beam 27 focused by the condenser lens 26 is irradiated from the recording film 22 side without passing through the substrate 21. In this case, since the recording film 22 is located near the surface of the substrate 23, the diameter of the light beam 27 on the recording film 23 can be sufficiently narrowed down even if the aperture radius of the condenser lens 26 is small. In other words, if a lens with the same aperture radius as the condenser lens 16 in the embodiment shown in FIG. 1 is used as the condenser lens 26, the diameter of the light beam 27 on the recording film 23 can be further narrowed down. . Therefore, in combination with the fact that the antireflection film 25 is configured so that the reflectance is minimized for light having a large incident angle θ as described above, it is possible to further increase the reproduction resolution.

The present invention is not limited to the embodiments described above; for example, in the embodiments, write-once or rewritable optical information recording media have been described; As in, information is transferred to the substrate surface through optical changes (
The present invention can also be applied to read-only discs recorded as pit strings, etc.). Further, the shape of the medium is not limited to a disk shape, and may be, for example, a card shape.

Further, in the embodiment, information is reproduced by detecting reflected light, but the present invention is also effective when detecting transmitted light and performing reproduction.

Further, in the examples, the case where the anti-reflective film is a single layer was explained as an example, but the present invention is also effective in the case of a multi-layer film, and in that case, the anti-reflective film of each layer should be do it. In addition, the present invention can be implemented with various modifications without departing from the scope of the invention.

[Effects of the Invention] According to the present invention, the effect of the anti-reflection film is increased particularly for light that enters the optical information recording medium at a large angle of incidence, that is, light near the edge of the beam. It becomes possible to improve the reproduction resolution,
Information recorded at high density can be reproduced with high signal strength.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view schematically showing the structure of an optical information recording medium according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view showing the structure of the antireflection film from the front and back surfaces of the antireflection film in the same embodiment with respect to light incident at an incident angle θ. FIG. 3 is a diagram showing the configuration of an optical information reproducing device according to an embodiment of the present invention, and FIG. 4 is a diagram showing the structure of an optical information reproducing device according to another embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing the configuration of a medium. DESCRIPTION OF SYMBOLS 1... Light source, 2... Collimer lens, 3... Beam splitter, 4... Condensing lens, 5... Optical information recording medium, 6... Lens, 7... Photodetector, 11...
- Transparent substrate, 12... Recording film, 13... Reflective film,
14... Protective film, 15... Antireflection film, 21...
Substrate, 22... Reflective film, 23... Recording film, 24...
- Transparent protective film, 25... antireflection film. Applicant's representative Patent attorney Takehiko Suzue No. 1rM Figure 4 Figure 2

Claims (2)

[Claims] (1) Readable information is recorded on the substrate surface or a recording film formed on the substrate by irradiation with a light beam, and an anti-reflection film is placed on the light beam incident side of the substrate surface or recording film on which the information is recorded. In the provided optical information recording medium, the anti-reflection film is arranged so that when the center wavelength of the light beam is λ and the maximum incident angle of the light beam is θc, 0<θ of the light beam is
For light incident at a predetermined angle θ within the range <θc, the optical path length difference between the reflected light from the front and back surfaces is (m+1/2)λ(
(where m is an integer). (2) Readable information is recorded on the substrate surface or a recording film formed on the substrate by irradiation with a light beam, and an anti-reflection film is further placed on the light beam incident side of the substrate surface or recording film on which the information is recorded. In an optical information reproducing device that reproduces information by irradiating a light beam through a condensing lens onto an optical information recording medium provided and detecting reflected light or transmitted light thereof through the condensing lens, the reflection The prevention film has a central wavelength of the light beam λ and a maximum incident angle of the light beam determined by the numerical aperture of the condensing lens θc.
When, among the light beams, for the light incident at a predetermined angle θ within the range of 0<θ<θc, the optical path length difference between the reflected light from the front and back surfaces is (m+1/2)λ (however, (m is an integer).