JPH0412528B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present technology relates to a reflection type magneto-optical reproducing device that prevents a reduction in the intensity of a reproduced signal by providing a phase shifter.

(Background of the Invention) A magneto-optical recording medium is mainly composed of a perpendicularly magnetized film such as GdCo or GdTbFe.The direction of magnetization of this perpendicularly magnetized film is once aligned either upward or downward, and then recording is performed. By irradiating the desired area with a laser beam and heating the temperature of that area to, for example, above the Curie point of the magnetic material, the original magnetization direction is freely released, and at the same time, a weak magnetic field in the opposite direction is applied to that area. Then, the magnetization direction of that part is set to be opposite to that of the film, and then the laser beam irradiation is stopped to fix the magnetization in the opposite direction. This sets the magnetization direction of the temporary film to 0.
If the opposite direction is 1, then the laser beam irradiation will be recorded as 1 of the 0 and 1 digital signals.

The difference in the magnetization direction of the magnetized film recorded in this way, that is, upward and downward, can be determined by irradiating linearly polarized light.
The state of rotation of the polarization plane of the reflected light is read using a phenomenon (magnetic Kerr effect) that differs depending on the direction of magnetization. In other words, when the direction of magnetization is upward with respect to the incident light, the polarization plane of the reflected light is rotated by θrad relative to the polarization plane of the incident light, and when the direction of magnetization is downward, - Rotate θrad. Therefore, if the main axis of a polarizer (also called an analyzer) is placed at the end of the reflected light so that it is almost perpendicular to the -θ rad. plane, the light from the downward magnetized part will hardly pass through the analyzer, and the upward The amount of light transmitted from the magnetized portion of It is reproduced as a current strength signal based on the magnetization state.

A reproducing apparatus based on the above-described principle is called a reflection type magneto-optical reproducing apparatus, and this apparatus has a basic configuration as shown in FIG. 1 or 2, for example.

In other words, in FIG. 1, the polarized beam from the laser light source 1 is bent by 90 degrees in the direction of travel by the beam splitter 2, and then irradiated almost perpendicularly onto the recording medium 3, and the reflected light is passed through the boom splitter 2 again to the analyzer 4. The transmitted light is received by a detector 5 such as a photoelectric conversion element.

In contrast, in the apparatus shown in FIG. 2, a polarized beam from a laser light source 1 is transmitted through a beam splitter 2 to irradiate the recording medium 3 almost perpendicularly, and the reflected light is incident on the beam splitter 2. , the reflected light is guided to the analyzer 4, and the transmitted light is received by the detector 5.

A beam splitter 2 is required for both the apparatuses shown in FIG. 1 and FIG. 2, but depending on the beam splitter, the phase difference δ between the s-polarized light and the p-polarized light may not be zero. It has also been found that when the beam splitter 2 has a phase difference δ, the reproduced signal strength S decreases.

(Object of the Invention) Therefore, the object of the present invention is to prevent the reproduction signal strength S from decreasing when there is a phase difference in the beam splitter in a reflection type magneto-optical reproducing device using a beam splitter. The purpose is to prevent it.

(Summary of the invention) Now, (a) the Kerr rotation angle of the magnetic thin film is θ, the amplitude reflectance is r, (b) the phase difference of the beam splitter is δ, (c) the phase difference of the retarder is Δ, and the principal axis direction Assuming that the angle is φ (φ is essentially zero), (d) the azimuth of the analyzer is α, and (e) the azimuth of the polarized light emitted from the laser light source is zero, then the light passing through analyzer A is The light intensity I〓 is a complicated calculation, so if you omit the middle part, I〓∝｜r｜ ² ×θsin2αcosδ + 1/2 (1 + cos2α) + φsin2αsin ² Δ|
...(Formula 1) is derived.

Now, if the light intensity when the direction of magnetization of the magnetic thin film is the same as the direction of incidence of the incident light is I↑ and the light intensity when the direction of magnetization is opposite to the direction of incidence is Iα↓, then the light intensity difference, that is, the reproduction signal intensity S Since S=|Iα ↑|Iα ↓|, S∝|r| ² ×−θsin2αcosδ| (Formula 2).

In Equation 2, the optimal value of the azimuth angle α of the analyzer varies depending on the detector used, the laser as the light source, the analyzer, etc., but in any case, α is the extinction position (α = 1/
2πrad. i.e. 90°).

Therefore, it can be expressed as α=(1/2π+α′) rad. Since α′ is a sufficiently small angle, sin2α=sin(π+2α′)=−sin2α′ ≒−2α′ cos2α=cos(π+2α ′)≒cosπ=−1.

Therefore, assuming that the reflectance r is constant and θ is constant,
The light intensity difference, that is, the reproduced signal strength S is expressed as follows: S∝−θcosδ− (Formula 3).

Although we have so far explained the method of using the analyzer 4 to detect the rotation status (θ or -θ) of reflected light that has undergone Kerr rotation, there are other methods besides the direct method using such an analyzer. As shown in Fig. 3, a polarizing beam splitter 4' such as a Wallaston prism, a Thomson prism, a Rocillon prism, or a thin film type is used. A so-called differential method is also known in which the light intensity is divided into two and guided to each of the detectors 5a and 5b, and the difference in output from both detectors is obtained by a differential amplifier 6. The differential method may have an advantage over the direct method in terms of S/N ratio.

When the light intensity I is α=1/4πrad.I ⁴⁵ , α=3/4
Assuming I ¹³⁵ when πrad., in the case of the differential method, the reproduced signal strength S is expressed as S = - (I ⁴⁵ ↑ - I ¹³⁵ ↑) - (I ⁴⁵ ↓ - I ¹³⁵ ↓) - , by substituting Equation 1 into the right-hand side, the following is derived: S∝−θcosδ−……(Equation 4).

Therefore, even if it is a direct method using an analyzer,
Whether it is a differential method using a polarizing beam splitter,
It was found from equations 3 and 4 that the reproduced signal strength S is proportional to -θcosδ-.

In other words, it can be seen that when a beam splitter having a phase difference δ is used, the reproduced signal strength S decreases to a value multiplied by cos δ. For example, when δ is 1/3πrad., S is halved and the phase difference δ becomes a non-negligible entity.

The present inventor came up with the idea of correcting the phase difference δ using a newly provided retarder from the equation S∝−θcosδ−.

If the retarder 7 is provided between the medium 3 and the beam splitter 2 or between the beam splitter 2 and the analyzer 4 or the polarizing beam splitter 4' with an azimuth angle of approximately 0, as a result of similar calculation, S∝ -θcos(δ+Δ)-...(Formula 5) was found to be derived. (However, Δ is the phase difference of the retarder.) In this case, when the retarder 7 is disposed between the medium 3 and the beam splitter 2, the polarized beam passes through the retarder at two times: when it is incident and when it is reflected. However, since the retarder is provided so that the plane of polarization of the polarized beam coincides with the main axis of the retarder at the time of incidence, there is no effect of the retarder.

From Equation 5, by appropriately selecting Δ according to δ, −θcos(δ+Δ)− can be set to 1,
In this way, the reproduced signal strength S will not be reduced. Therefore, the present invention includes a light source 1 and a method for reflecting a polarized beam from the light source 1 toward a magneto-optical recording medium.
A reflective magnetic field comprising a beam splitter 2 that transmits the light reflected by the medium, an analyzer 4 that transmits the light that has passed through the beam splitter 2, and a detector 5 that receives the light that has passed through the analyzer 4. In the optical reproducing device, there is a phase difference Δ between the medium and the analyzer 4 that satisfies the following formula: -θcos(Δ+δ)-=1 (where δ is the phase difference given by the beam splitter to the transmitted light) A reflective magneto-optical reproducing device characterized in that the retarder is provided such that the azimuth angle of incident polarized light is zero and the principal axis azimuth φ of the retarder is substantially zero when based on the azimuth angle of the incident polarized light. This is what we provide.

The present invention also provides a reflective magneto-optical reproducing device that uses a polarizing beam splitter 4' instead of the analyzer 4, which splits the light transmitted/reflected by the beam splitter 2 into two orthogonal polarized lights. It is. In this case, a retarder is provided between the medium and the polarizing beam splitter.

In addition, in the magneto-optical recording medium 3, if the substrate or protective substrate of the medium has a phase difference δ' due to birefringence, when the incident polarization direction is made to match the axis of birefringence, the overall phase difference becomes δ + δ', cos(δ+
A similar effect can also be obtained by selecting Δ so that δ′+Δ)=1.

The retarders themselves used in the present invention are already widely known, and some of them are also available as commercial products. Generally, a retarder is made of a thin film of quartz, mica, or the like, and a retarder having an arbitrary phase difference Δ is easily available.

The present invention will be explained below with reference to Examples.

(Example 1) As shown in Fig. 4, a beam splitter 2 was used to convert the p-polarized beam of wavelength λ = 830 nm from the laser light source 3 into reflected light with a phase difference of -0.66 rad. (= -37.8°). After passing through, Kerr rotation angle θ=
GdTbFe of 0.007 rad. (=0.4°) is irradiated almost perpendicularly to the magneto-optical recording medium 3 having a magnetic thin film, and the reflected light is reflected by the beam splitter 2 and guided to the analyzer 4, where it is received by the detector 5. A magneto-optical reproducing device is used.

The beam splitter 2 used here is usually called a half mirror, and its structure is
As shown in the figure, on the slope of the BSC-7 prism 51 with a refractive index n=1.51, H layer: TiO ₂ (n=2.2)/optical film thickness 0.299λ F layer: SiO ₂ (n=1.453)/optical film 11 layers of two layers with a thickness of 0.384λ were deposited alternately in the order of H (LH) ⁵ .
After forming the prism 51 made of the same material,
It is made by closely adhering the slopes of This beam splitter 2 has a reflectance and transmittance of approximately 50% for p-polarized light, and provides a phase difference of -0.66 rad. (=-37.8°) between s-polarized light and p-polarized light upon reflection.

The phase difference δ of beam splitter 2 is -0.66 rad.
(=-37.8°), we obtain Δ=+0.66rad.(=+37.8°) as one of the solutions that satisfies the equation: -θcos(Δ+δ)-=1.

Therefore, we obtained a crystal retarder with a phase difference of +0.66 rad. (= +37.8°), which is shown as 7 in Fig. 4. It was installed so that it coincided with the plane of polarization.

As a result, the reproduced signal strength S was improved by 27% compared to the case where the phase shifter 7 was not provided.

Furthermore, the same results were obtained even when the retarder 7 was placed between the beam splitter 2 and the analyzer 4 as shown in FIG. Furthermore, the same results were obtained when a retarder having a phase difference Δ=+3.81 rad. (=217.8°) was used in place of the above retarder.

In the magneto-optical reproducing apparatus I as shown in FIG. 1, a similar improvement in the reproduced signal strength S is observed when a phase shifter is disposed at the position indicated by reference numeral 7 or 7'.

(Effects of the Invention) As described above, by providing a specific phase shifter according to the present invention, the decrease in the reproduced signal strength S when a beam splitter having a phase difference is used is eliminated.

[Brief explanation of drawings]

1 to 3 are explanatory diagrams showing the basic configuration of a conventional magneto-optical reproducing device. FIG. 4 is an explanatory diagram showing the basic configuration of a magneto-optical reproducing device showing an embodiment of the present technology. FIG. 5 is a sectional view of the beam splitter used in the example. FIG. 6 is an explanatory diagram showing the basic configuration of an apparatus I showing another embodiment of the present invention. Explanation of symbols of main parts, 1... Laser light source,
2... Beam splitter, 3... Magneto-optical recording medium, 4... Analyzer, 4'... Polarizing beam splitter, 5, 5a, 5b... Detector,
6... Differential amplifier, 7, 7'... Phase shifter.

Claims (1)

[Claims] 1. A light source 1, a beam splitter 2 that reflects a polarized beam from the light source 1 toward a magneto-optical recording medium, and transmits the light reflected by the medium, and the beam splitter. 2, and a detector 5 that receives the light that has passed through the analyzer 4. In the reflection type magneto-optical reproducing device, the following equation is established between the medium and the analyzer 4:
A retarder with a phase difference Δ that satisfies cos (Δ+δ) | = 1 (where δ is the phase difference given by the beam splitter to the transmitted light) is used as a reference, with the azimuth of the incident polarized light set to zero. 1. A reflection type magneto-optical reproducing device, characterized in that the retarder is provided so that its main axis azimuth angle φ is substantially zero. 2. A light source 1, a beam splitter 2 that reflects a polarized beam from the light source 1 toward a magneto-optical recording medium, and transmits the light reflected by the medium, and a beam splitter 2 that transmits the light that has passed through the beam splitter 2. Polarizing beam splitter 4' that splits the light into two orthogonal polarizations
and two detectors 5 that receive the light split by the polarizing beam splitter 4', between the medium and the polarizing beam splitter 4', the formula: |cos (Δ+δ) | = 1 (where δ is the phase difference given by the beam splitter to the transmitted light) When using a retarder with a phase difference Δ that satisfies the following, the azimuth of the incident polarized light is set to zero, and this is used as a reference. A reflection type magneto-optical reproducing device, characterized in that the main axis azimuth angle φ of the retarder is provided so as to be substantially zero. 3. A light source 1, a beam splitter 2 that transmits a polarized beam from the light source 1 toward a magneto-optical recording medium, and transmits the light reflected by the medium, and a beam splitter 2 that transmits the polarized beam from the light source 1 toward a magneto-optical recording medium, and transmits the light reflected by the beam splitter 2. In a reflective magneto-optical reproducing device comprising an analyzer 4 that transmits light, and a detector 5 that receives light that has passed through the analyzer 4, the following equation is established between the medium and the analyzer 4: |cos(Δ+δ)|=1( However, δ is the phase difference given by the beam splitter to the transmitted light). When using a retarder with a phase difference Δ that satisfies the following, the azimuth angle of the incident polarized light is set to zero, and the azimuth angle of the main axis of the retarder is calculated based on the azimuth angle of the incident polarized light. A reflection type magneto-optical reproducing device characterized in that it is provided so that φ is substantially zero. 4 a light source 1, a beam splitter 2 that transmits a polarized beam from the light source 1 toward a magneto-optical recording medium, and reflects the light reflected by the medium; a polarizing beam splitter 4' that splits the light into two orthogonal polarized lights;
In a reflective magneto-optical reproducing device consisting of two detectors 5 that receive light split by the polarizing beam splitter 4', the following equation exists between the medium and the polarizing beam splitter 4': |cos(Δ+δ ) | = 1 (where δ is the phase difference given by the beam splitter to the transmitted light) A retarder with a phase difference Δ that satisfies 1. A reflection type magneto-optical reproducing device, characterized in that the main axis azimuth angle φ of the child is substantially zero. 5. When the phase difference due to birefringence of the substrate or protective substrate of the medium is δ', the retarder has a phase difference Δ that satisfies the formula: |cos(δ+δ'+Δ)|=1 A reflective magneto-optical reproducing device according to any one of claims 1 to 4.