JPH0685218B2 - Magneto-optical reproducing device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a magneto-optical reproducing device for reading information written on a magneto-optical material.

2. Description of the Related Art A magneto-optical reproducing apparatus is known that reads information written by locally magnetizing a magneto-optical material having spontaneous magnetization and a large light transmittance in a vertical direction by utilizing a magneto-optical effect. Has been. Generally, in such a device, the Kerr effect or the Faraday effect is used, in which when the linearly polarized light is incident parallel to the magnetization direction of the magneto-optical material, the vibrating surface of the reflected light or the transmitted light rotates in relation to the magnetization direction. As a result, the information written on the magneto-optical material is read. However, according to the conventional magneto-optical reproducing apparatus, an analyzer (light detecting device) for detecting linearly polarized light whose vibrating surface is rotated by the Kerr effect or the Faraday effect is usually required, and the Kerr effect or the Faraday effect is required. Due to the extremely small rotation angle of the linearly polarized light vibrating surface (about 0.3 to 0.5 degree), it is necessary to use a differential detection device etc. to reliably detect weak changes in the linearly polarized light transmitted through the analyzer. There is an inconvenience that the device is complicated and expensive.

On the other hand, as described in, for example, Japanese Unexamined Patent Publication No. 57-50329, two types of magnetization included in transmitted light or reflected light from the magneto-optical material by irradiating the magneto-optical material with circularly polarized light. An apparatus for reading information by interference due to a phase difference between two types of light from a region is disclosed. According to such a technique, an expensive analyzer for detecting the Kerr rotation angle of linearly polarized light and a differential detection device for detecting a weak change of a linear change that has passed through the analyzer are unnecessary, and the magneto-optical There is an advantage that the reproducing apparatus can be configured easily and inexpensively.

However, in the above conventional device, information is read out based on the phase difference Δ between two types of light from two types of magnetized regions included in the transmitted light or the reflected light from the magneto-optical material. As a result, it is necessary to make two regions of the + M magnetization region and the −M magnetization region exist in the circularly polarized irradiation spot, and therefore the diameter of the irradiation spot cannot be made relatively small with respect to the magnetization region. There is a drawback that the track spacing becomes wide and the recording density in the magneto-optical material cannot be increased.

The present invention has been made in the background of the above circumstances,
It is an object of the present invention to provide a magneto-optical reproducing device capable of further increasing the recording density of the magneto-optical material.

Means for Solving the Problem The gist of the present invention for achieving the above-mentioned object is to form information in a layered form and locally magnetize in a direction perpendicular to the layer surface. Circularly polarized light is irradiated to the magneto-optical material parallel to the direction of the magnetization, and when the irradiation position of the circularly polarized light on the magneto-optical material and the position where information is written by the magnetization match, By detecting a change in the amount of reflected light that has undergone multiple reflection depending on the thickness of the layer within the layer of the magneto-optical material, or the amount of transmitted light from within the magneto-optical material that occurs depending on the direction of magnetization at the position The information written on the magneto-optical material is read out.

In this way, the magneto-optical material which is formed in a layered form and in which information is written by locally magnetizing in a direction perpendicular to the layer surface is parallel to the magnetization direction. When circularly polarized light is radiated on, and the irradiation position of the circularly polarized light on the magneto-optical material and the position where the information is written by the magnetization match, it occurs depending on the direction of the magnetization at that position,
A change in the amount of reflected light that has undergone multiple reflection or the amount of transmitted light from within the magneto-optical material is detected depending on the thickness of the layer of the magneto-optical material. The written information is read.

Therefore, since it is not necessary to provide two types of magnetized regions in the circularly polarized irradiation spot, the diameter of the irradiation spot can be made relatively small with respect to the magnetized region, and the track spacing can be narrowed. The recording density of the material can be increased.

Embodiment Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

In FIG. 1, a laser beam output from a laser light source 10 is linearly polarized by a polarizer 12, then passes through a polarization beam splitter 14, a 1/4 wavelength plate 16 and a disc-shaped magneto-optical disk.
It is vertically incident on 18. And the magneto-optical disk 18
The laser beam reflected from is again incident on the polarization beam splitter 14 via the 1/4 wavelength plate 16, is reflected by the polarization beam splitter 14, and reaches the photodiode 20 for detecting the amount of reflected light. . Polarizer 12
Is for linearly polarizing the laser beam output from the laser light source 10 (P-polarized with respect to the joint surface of the polarization beam splitter 14 described later). The polarization beam splitter 14 is a kind of polarization filter having a cemented surface 14a inclined at 45 ° with respect to the incident light, and allows P polarized light to pass through the cemented surface 14a, but allows S polarized light to travel in the traveling direction of the incident light. On the other hand, it efficiently reflects in the direction perpendicular to the surface. 1/4 wave plate 16
Is arranged such that its optical axis is inclined by 45 ° with respect to the vibration direction of the P-polarized light incident from the polarization beam splitter 14 in a plane perpendicular to the traveling direction of light, and the P-polarized light is converted into circularly polarized light. To make it transparent. Conversely, when circularly polarized light is incident, it is converted into linearly polarized light. Here, there are two types of circularly polarized light, one having a right-handed electric vector and the other having a left-handed electric vector with respect to the traveling direction.
In the above, the right-handed or left-handed circularly polarized light is converted into, for example, P-polarized light or S-polarized light when viewed from a fixed direction, while the P-polarized light or S-polarized light is converted into left-handed or right-handed circularly polarized light, respectively. ing. The magneto-optical disk 18 has a film thickness d, for example, as shown in FIG.
Is a glass substrate 24 and an aluminum reflective film
It is composed of a multilayer structure sandwiched by 26 and. As the magneto-optical material 22, amorphous GdTbFe, TbFe, GdCo,
Polycrystalline MnCuBi, single-crystal TbCFeO ₃ , garnet, etc. A substance that causes additional optical anisotropy in the presence of magnetization is used, and it is locally magnetized in a predetermined magnetization direction perpendicular to the plane. Information is thereby written. Glass substrate
24 supports and protects the magneto-optical material 22,
A resin having excellent translucency such as an acrylic resin may be used. Since the aluminum reflection film 26 is for increasing the amount of light reflected by the magneto-optical material 22, it may be another dielectric layer if a decrease in the amount of reflected light due to polymorphism is allowed. The photodiode 20 detects the amount of reflected light (light intensity) from the magneto-optical disk 18 that is incident from the polarization beam splitter 14, and outputs an electric signal according to the amount of incident light. .

Next, the function and effect of the magneto-optical reproducing device configured as described above will be described.

The laser beam that is P-polarized by the polarizer 12 passes through the polarization beam splitter 14 efficiently, and then is transmitted to the 1/4 wavelength plate 16
Circularly polarized light by the 1/4 wavelength plate 16
Is determined by the direction of the angle of inclination (45 °) between the optical axis of and the plane of vibration of the P-polarized light. For example, the light is circularly polarized light clockwise to the traveling direction. Then, when the right-handed circularly polarized light is made incident on the magneto-optical disk 18, the boundary surface between the glass substrate 24 and the magneto-optical material 22, and the magneto-optical material 22 and the aluminum reflection film 26.
It is reflected by the boundary surface of and and returned to the quarter-wave plate 16 again. At this time, from the magneto-optical disk 18 to the quarter-wave plate 1
The reflected light returned to 6 is circularly polarized light counterclockwise with respect to the traveling direction, which is opposite to the incident light to the magneto-optical disk 18, but when viewed from the 1/4 wavelength plate 16, clockwise circularly polarized light is incident. This means that the reflected light that has entered the quarter-wave plate 16 is converted into S-polarized light and entered into the polarization beam splitter 14. Since the polarization beam splitter 14 allows only the P-polarized light to pass therethrough as described above, the S-polarized light is reflected and converted to the 90-degree direction, and is incident on the photodiode 20. The amount of light incident on the photodiode 20 changes depending on the difference in the perpendicular magnetization direction of the magneto-optical material 22, so that the information written by locally magnetizing the magneto-optical material 22 in the direction perpendicular to the plane of The signal is read by the electric signal output from the photodiode 20 as the magneto-optical disk 18 moves.

That is, in general, when the traveling direction of the electromagnetic wave is parallel to the magnetization direction of the magneto-optical material 22, it corresponds to a so-called Faraday arrangement, so that the eigenmodes of light are positive and negative circular polarized light.
E ₊ , E ₋ (circularly polarized light whose electric vector is clockwise and counterclockwise circularly polarized), and the (complex) refractive index n ′ of the magneto-optical material 22 for those circularly polarized light is expressed by the following equation (1). It

▲ n ^′ _± ▼ ² = (n _± + ik _± ) ² = ε _⊥ ± iε ′ ……
(1) where n _± is the refractive index and k _± is the attenuation coefficient. ε _⊥ is the diagonal component (xx, yy component) of the dielectric constant tensor of the magneto-optical material, and ε'is the non-diagonal component (xy component). i is a complex number.

The sign of ± in the above formula (1) corresponds to the magnetization direction of the magneto-optical material, and the sign of ± is reversed if the magnetization direction in the magneto-optical material 22 is opposite even if the incident condition of circularly polarized light is the same. It should be done. Therefore, the amplitude reflectance r ^± from the layered magneto-optical material 22 is expressed by the following equation (2).

r ^± = {▲ r ^± ₁₂ ▼ + ▲ r ^± ₂₉ ▼ exp (-iδ ^± )} / {1 + ▲ r ^± ₁₂ ▼ ・ ▲ r ^± ₂₉ ▼ exp (-iδ ^± )}…
(2) The above δ ^± is a phase term due to multiple reflection, and given that the thickness of the magneto-optical material 22 is d and the wavelength of the light source is λ, it is expressed by δ ^± = 4π ▲ n ^′ _± ▼ d / λ It is a thing. Also, ▲ r ^± ₁₂ ▼, ▲ r ^± ₂₉ ▼
= Is called the reflection coefficient of _{^{Fresnel, ▲ r ± 12 ▼ (n}} 1 - ▲ n '± ▼) / (n 1 + n' ±) ▲ r ± 29 ▼ = (▲ n '± ▼ -n 3) It is represented by / (▲ n ^' _± ▼ + n ₃ ).

Therefore, it is clear from the equation (2) that the amplitude reflectance r ^± due to ▲ n ^′ ₊ ▼ and the amplitude reflectance r ⁻ due to ▲ n ^′ ₋ are different from each other, and the right-handed circularly polarized light is perpendicularly polarized. The amount of reflected light when entering the magneto-optical material 22 is different from the amount of reflected light when entering counterclockwise circularly polarized light in the same manner.
In other words, when circularly polarized light with clockwise or counterclockwise constant rotation is continuously incident, the amount of reflected light changes into two types depending on the difference in the direction of magnetization of the magneto-optical material. The signal (information) written by locally magnetizing the magneto-optical material 22 is detected based on the change of the. Note that FIG. 3 is a magnetization curve of the magneto-optical material 22 experimentally confirmed by the above method in order to show that the magnetization direction of the magneto-optical material 22 changes the reflected light amount.

As described above, according to this embodiment, since the photodiode 20 detects the change in the reflected light amount corresponding to the perpendicular magnetization direction of the magneto-optical material 22, the information written in the magneto-optical disk 18 is reproduced. , An expensive analyzer (detector) for detecting the rotation of the vibrating surface of the reflected linearly polarized light, or a weak change of the detected linearly polarized light as compared with the conventional magneto-optical reproducing device. The differential detection device and the like are unnecessary, and the device is simple and inexpensive. That is, the conventional device utilizes the Kerr effect or the Faraday effect in which when the linearly polarized light is incident on the magneto-optical material, the vibrating surface of the reflected light or the transmitted light rotates according to the magnetization direction of the magneto-optical material. For example, it is configured as shown in FIG. The laser beam linearly polarized by the polarizer 12 passes through the half mirror 28 and the magneto-optical disk 18
Incident on the half mirror
The light is reflected from 28 toward the half mirror 30, and is separated by the half mirror 30 into light in two directions toward the analyzers 32 and 34. The analyzers 32 and 34 are set so as to respectively pass two kinds of linearly polarized light whose vibrating surfaces are rotated corresponding to two kinds of perpendicular magnetization directions of the magneto-optical material 22 in the magneto-optical disk 18. The photodiodes 36 and 38 receive the light that has passed through the analyzers 32 and 34. The output signals of the photodiodes 36 and 38 are respectively supplied to the input terminals of the differential amplifier 40, and the differential amplifier 40 is adapted to generate an output signal based on the signal difference supplied to the photodiodes 36 and 38. Generally, the Kerr rotation angle or the Faraday rotation angle is an extremely small angle of about 0.3 to 0.5 degrees, and a precise and expensive detection for detecting the rotation direction of the vibrating surface of the linearly polarized light reflected from the magneto-optical material 22. Photons 32 and 34 are required and the output signal S /
A differential detection device such as the differential amplifier 40 for increasing the N ratio is required, and the device is complicated and expensive.

On the other hand, according to the present embodiment, the analyzers 32, 34, one of the photodiodes 36, 38, and the differential amplifier 40 are unnecessary, and the device is greatly simplified and the cost is reduced.

Further, according to this embodiment, since the half mirrors 28 and 30 are not used unlike the conventional device, there is an advantage that the loss of the light amount is eliminated. That is, in FIG. 4, the half mirror 28 allows half of the light amount from the laser light source 10 to pass therethrough,
The half mirror 30 passes half of the reflected light from the magneto-optical disk 18 through the analyzers 32 and 38 to the photodiode 36 and
Since the light is incident on each of the lasers 38, even if the loss of the other portions is assumed to be zero, 1/4 of the amount of light output from the laser light source 10 will be received by the photodiodes 36 and 38, respectively. On the other hand, according to the present embodiment, the light quantity of the laser light output from the laser light source 10 is effectively used, and the shortage of the light quantity is eliminated.

However, if the loss of the amount of light output from the laser light source 10 is tolerated, the present invention can be applied to a magneto-optical reproducing device using a half mirror. The magneto-optical reproducing apparatus shown in FIG.
It is configured by replacing the polarization beam splitter 14 of the illustrated apparatus with a half mirror 42. According to the present embodiment, the laser beam linearly polarized by the polarizer 12 passes through the half mirror 42 to have a light amount of approximately 50%, and the reflected light reflected from the magneto-optical material 18 is 1/4. After reaching the half mirror 42 through the wave plate 16, the photodiode 20
The amount of light is approximately 50% even when reflected toward, but the expensive analyzers 32, 34 and the like are not necessary, and the effect that the device is configured easily and inexpensively is the same as in the above-described embodiment. You can get it.

FIG. 6 shows an example of the optimum sectional structure of the magneto-optical disk 18 when a signal is read using the magneto-optical reproducing apparatus to which the present invention is applied. That is, the magneto-optical material 22 is sandwiched between the surface reflection preventing dielectric film 44 and the dielectric film 46, and these are fixed on the aluminum substrate 48 which is a reflection metal substrate. Dielectric films 44 and 46 are SiO ₂ and SiO _2.
And the film thickness is 1100Å and 1800Å respectively. Further, the magneto-optical material 22 is GdTbFe having a film thickness of 125Å and has a refractive index n'as expressed by the following equation (3).

n ′ = (2.3−3.0i) ± (0.025−0.01i) (3) According to an experiment conducted by the present inventor, the magneto-optical disk 18 having the above-described structure is used.
When using, the magneto-optical material 22 is about 1% depending on the difference in the magnetization direction.
Reflectance difference {(| r ⁺² | − | r ⁻² |) × 100}, in other words about 3% reflectance ratio {(| r ⁺² | − | r ⁻² |) / | r ^{+ 2} | × 100} was obtained. In order to obtain high efficiency when using the multiple reflection as described above, the refractive index of the dielectric layer sandwiching the magneto-optical material 22 is required to be a value that cannot be realized. In the structure of the magneto-optical disk 18, if the dielectric film 46 and the aluminum substrate 48 (metal reflective film) are used as the lower layer of the magneto-optical material 22 as the final film structure, the virtual refractive index optimum for multiple reflection can be obtained.

Although the detailed description has been given above with reference to the drawings showing the embodiment of the present invention, the present invention is also applied to other aspects.

For example, in the embodiment described above, the information written on the magneto-optical material 22 is reproduced based on the change in the amount of reflected light when circularly polarized light is incident on the magneto-optical material 22. Since the amount of incident light is the sum of the amount of reflected light and the amount of transmitted light, it can be reproduced by changing the amount of transmitted light emitted from the magneto-optical material 22. That is, the (1) t ^± amplitude transmittance of the light transmitted through the magneto-optical material 22 which holds the formula is represented by the following formula (4).

However, δ ^± = 4πn ′ _± d / λ ▲ r ^± ₁₂ ▼ = (n ₁ − ▲ n ^′ _± ▼) / (n ₁ + ▲ n ^′ _± ▼) ▲ r ^± ₂₉ ▼ = (▲ n ^′ _± ▼ -N ₃ ) / (▲ n ^' _± ▼ + n ₃ ) ▲ t ^± ₁₂ ▼ = 2n ₁ / (n ₁ + ▲ n ^' _± ▼) ▲ t ^± ₂₉ ▼ = 2 ▲ n ^' _± ▼ / (▲ n ^' _± ▼ + n ₃₎ Therefore, (4) refractive index ▲ n _^'± ▼ by amplitude transmittance t ⁺ a refractive index ▲ ^n' from the equation _- ▼ by amplitude transmittance t ^- and is clear that is different from each other The amount of transmitted light when right-handed circularly polarized light is vertically incident on the magneto-optical material is different from the amount of transmitted light when left-handed circularly polarized light is similarly incident. In other words, when circularly polarized light in the clockwise or counterclockwise constant rotation direction is incident, the amount of transmitted light changes into two types depending on the difference in the direction of the perpendicular magnetization direction of the magneto-optical material 22. The information written in the magneto-optical material 22 can be reproduced based on the change in the amount of light. Here, when using the reproduction method based on the change in the amount of transmitted light, it is necessary to make the thickness of the magneto-optical material 22 relatively thin, but a device for separating incident light and reflected light becomes unnecessary. , For example, the polarization beam splitter 14 of FIG.
The advantage is that the half mirror 42 in the figure is removed.

Further, the light source 10 may be not only a laser light source but also another monochromatic light source, and if the light source 10 originally outputs linearly polarized light such as an external resonator type laser light source, a semiconductor laser light source, etc., The polarizer 12 is unnecessary. Moreover, the first
When the polarization beam splitter 14 is used as in the illustrated embodiment, since only P-polarized light is transmitted, the polarizer 12 can be removed even if the light source 10 outputs circularly polarized light.

Further, in the apparatus shown in FIGS. 1 and 5, the optical elements such as lenses and mirrors are the laser light source 10 and the beam splitter.
14 or half mirror 42, magneto-optical disk 18 and 1 /
It is appropriately used between the four-wave plate 16 and between the photodiode 20 and the polarization beam splitter 14 or the half mirror 42 as necessary.

Although the disk-shaped magneto-optical disk 18 is used in the above-described embodiment, the shape may be drum-shaped or tape-shaped without any problem.

Also, instead of the 1/4 wave plate 16, a 1/4 wave such as a Fresnel rhombohedral is used.
An element that gives a phase difference of λ (π / 2) may be used.

Further, although the photodiode 20 is used to detect the amount of reflected light, it goes without saying that other light amount detection elements such as a photomultiplier tube may be used.

The above description is merely one embodiment of the present invention, and the present invention can be variously modified without departing from the spirit thereof.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of a magneto-optical reproducing apparatus to which the present invention is applied. FIG. 2 is a sectional view of the essential parts of the magneto-optical disk of FIG. FIG. 3 is a diagram showing a magnetization curve of the magneto-optical material experimentally detected using the apparatus of FIG. Fourth
FIG. 1 is a block diagram showing a conventional magneto-optical reproducing apparatus. FIG. 5 is a block diagram showing another example of an apparatus to which the present invention is applied. FIG. 6 is a partial cross-sectional view showing another configuration example of the magneto-optical disk. 18: Magneto-optical disk, 22: Magneto-optical material

Claims (1)

[Claims] 1. A circularly polarized light is irradiated to a magneto-optical material which is formed in layers and in which information is written by locally magnetizing in a direction perpendicular to the layer surface, in parallel with the direction of the magnetization. Then, in the layer of the magneto-optical material, when the irradiation position of the circularly polarized light on the magneto-optical material coincides with the position where the information is written by the magnetization, it occurs depending on the direction of the magnetization at the position. The information written on the magneto-optical material is read out by detecting a change in the amount of reflected light that has undergone multiple reflections or transmitted light from inside the magneto-optical material according to the thickness of the layer. Characteristic magneto-optical reproducing device.