JPH0752530B2 - Phase-corrected reflection type magneto-optical reproducing device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present technology relates to a reflection-type magneto-optical reproducing device in which a reproduction signal intensity is prevented from decreasing by providing a phase shifter.

[0002]

2. Description of the Related Art Magneto-optical recording media are, for example, GdCo and GdTbFe.
Is mainly composed of a perpendicular magnetic film as described above, and the direction of magnetization of the perpendicular magnetic film is once aligned upward or downward, and a portion to be recorded is irradiated with a laser beam to change the temperature of the portion, for example. By heating 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 part, so that the magnetization direction of that part is opposite to that of the film. Then, the irradiation of the laser beam is stopped, and the magnetization in the opposite direction is fixed. As a result, if the magnetization direction of the film is set to 0 and the opposite direction is set to 1, the laser beam irradiation is recorded as 1 of 0 and 1 of the digital signal.

The difference in the magnetization direction of the thus recorded magnetic film, that is, the upward and downward irradiation of linearly polarized light, the phenomenon that the rotation state of the polarization plane of the reflected light differs depending on the magnetization direction (magnetic Kerr effect) Be read using. That is, when the polarization direction of the reflected light is rotated by θ rad. With respect to the polarization plane of the incident light when the magnetization direction is upward with respect to the incident light, and when the magnetization direction is downward with respect to the incident light, −
θ rad. rotate. Therefore, if the principal axis of the polarizer (also called analyzer) is placed almost perpendicular to the −θ rad. Plane at the tip of the reflected light, the light from the downward magnetized portion hardly passes through the analyzer, Since the light from the magnetized part is transmitted by the amount multiplied by sin2θ, if a detector (photoelectric conversion element) is installed in front of the analyzer, the recorded magnetization will increase when the recording medium is scanned at high speed. It is reproduced as a current intensity signal based on the state.

A reproducing apparatus based on the above-mentioned principle is called a reflection type magneto-optical reproducing apparatus, and this apparatus has a basic structure as shown in FIG. 3, for example. That is, in the device of FIG. 3, the polarized beam from the laser light source (1) is transmitted through the beam splitter (2) and is irradiated almost perpendicularly to the recording medium (3), and the reflected light is directed to the beam splitter (2). The incident light is guided to the analyzer (4), and the transmitted light is received by the detector (5).

In the apparatus of FIG. 3, a beam splitter (2) is required, but the phase difference δ between s-polarized light and p-polarized light may not be zero depending on the beam splitter. Then, it was found that the reproduction signal strength (S) was lowered when the beam splitter (2) had a phase difference δ.

[0006]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to reduce the reproduction signal strength (S) in a reflection type magneto-optical reproducing apparatus using a beam splitter when the beam splitter has a phase difference. It is to prevent doing.

[0007]

Means for Solving the Problems 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, and the phase difference of the phase shifter is Δ. , (A) the azimuth angle of the analyzer is (α), and (e) the azimuth angle of the polarized light emitted from the laser light source is zero, the analyzer (A)
Since the light intensity Iα after passing through is complicated calculation, omitting the way,

[0008]

[Number 4] ^{Iαα | r | 2 × | θsin2αcosб} + 1/2 (1 + cos2α) + φsin2αsin 2 △ |

.. (Equation 1) is derived. Now, let I be the light intensity when the direction of magnetization of the magnetic thin film is the same as the incident direction of the incident light.
α ↑, the light intensity when the direction of magnetization is opposite to the incident direction is I
If α ↓, the light intensity difference, that is, the reproduction signal intensity (S)
Is

[0010]

[Equation 5] S = | Iα ↑ −Iα ↓ |

Therefore,

[0012]

[Equation 6] S ∝ ｜ r ｜ ² × ｜ θsin2α cosб ｜ ………… (Equation 2)

[0013] The optimum value of the azimuth angle α of the analyzer in Equation 2 varies depending on the detector used, the laser as the light source, the analyzer, etc. In any case, α is the extinction position (α = 1/2 πrad. 90 °). Therefore, α = (1/2 π +
α ′) rad. and α ′ is a sufficiently small angle, so sin2α = sin (π + 2α ′) = − sin2α′≈−2α ′ cos2α = cos (π + 2α ′) ≈cosπ = −1 Become.

Therefore, when the reflectance r is constant and θ is constant, the light intensity difference, that is, the reproduction signal intensity (S) is

[0015]

[Equation 7] S ∝ ｜ θcosб ｜ ………… (Equation 3)

[0016] The method of using the analyzer (4) for detecting the rotation state (θ or −θ) of the reflected light that has undergone Kerr rotation has been described above, but a direct method using such an analyzer has been described. In addition, as shown in Fig. 4, a polarization beam splitter such as Wollaston prism, Thomson prism, Rochon prism, and thin film type.
(4 ') is used to divert light into polarized light components that are orthogonal to each other, and divides the light into two equal intensity components to guide each detector (5a), (5b), and both detectors are separated by a differential amplifier (6). There is also known a so-called differential method, which takes an output difference from. The differential method may be more advantageous than the direct method in terms of the S / N ratio.

The light intensity I is I ⁴⁵ when α = 1/4 π rad.
When I = ¹³⁵ when α = 3/4 π rad., the reproduced signal strength (S) in the case of the differential method is

[0018]

[Equation 8] S = ｜ (I ⁴⁵ ↑ -I ¹³⁵ ↑)-(I ⁴⁵ ↓ -I ¹³⁵ ↓) ｜

[Expression 1] is substituted into the right side,

[0020]

[Equation 9] S ∝ ｜ θcosб ｜ ………… (Equation 4)

Is derived. Therefore, whether the direct method using the analyzer or the differential method using the polarization beam splitter, the reproduction signal strength (S) is calculated from the equations 3 and 4 as follows:

[0022]

[Equation 10] ｜ θcosб ｜

It has been found to be proportional to That is, it can be seen that when the beam splitter having the phase difference δ is used, the reproduction signal strength (S) is reduced to a value obtained by multiplying cos δ. For example, when δ is 1/3 π rad., S is halved, and the phase difference δ cannot be ignored. The inventor

[0024]

[Equation 11] S ∝ ｜ θcosб ｜

From the equation (1), it was conceived that the phase difference δ should be corrected by a newly provided phase shifter. The phase shifter (7) is placed between the medium (3) and the beam splitter (2) or the beam splitter.
If the azimuth angle is set to almost 0 between (2) and the analyzer (4) or the polarization beam splitter (4 '), the same calculation result

[0026]

[Equation 12] S ∝ ｜ θcos (б + △) ｜ ………… (Equation 5)

It was found that (However, Δ is the phase difference of the phase shifter.) In this case, use the phase shifter (7) as the medium.
When it is placed between (3) and the beam splitter (2), the polarized beam passes through the phaser twice when it is incident and when it is reflected. Since the retarder is provided so as to coincide with the main axis, there is no influence of the retarder.

From equation 5, by properly selecting Δ according to δ,

[0029]

[Equation 13] | cos (б + △) |

Can be set to 1, so that the reproduction signal strength (S) is not reduced. Therefore, the present invention provides a light source (1), a beam splitter (2) which transmits a polarized beam from the light source (1) toward a magneto-optical recording medium, and transmits light reflected by the medium. , An analyzer (4) that transmits the light reflected by the beam splitter (2)
And a reflection type magneto-optical reproducing device comprising a detector (5) for receiving the light transmitted through the analyzer (4),
Between the medium and the analyzer (4), the formula:

[0031]

[Equation 14] | cos (△ ＋ б) ｜ = 1

(Where δ is the phase difference given to the reflected light by the beam splitter), a phase shifter having a phase difference Δ satisfying the reflected light is used as a reference when the azimuth angle of the incident polarized light is zero. The present invention provides a reflection type magneto-optical reproducing device characterized in that the main axis azimuth angle φ is provided to be substantially zero. The present invention also uses two beams of light reflected by the beam splitter (2) instead of the analyzer (4).
Polarization beam splitter splitting into two orthogonal polarizations
A reflective magneto-optical reproducing device using (4 ′) is provided. In this case, the phase shifter is provided between the medium and the polarization beam splitter.

In the magneto-optical recording medium (3), when the medium substrate or the protective substrate has a phase difference δ ′ due to birefringence, if the incident polarization azimuth is aligned with the birefringence axis, the overall phase difference is δ + δ. 'And therefore cos (δ + δ' + Δ)
The same effect can be obtained by selecting Δ so that = 1. The phase shifter itself used in the present invention is already widely known, and some of them can be obtained as commercial products. Generally, the retarder is made of a thin film such as quartz or mica, and a retarder having an arbitrary phase difference Δ is easily available.

The present invention will be described below with reference to examples.

[0035]

[Embodiment 1] As shown in FIG. 1, a p-polarized beam having a wavelength λ = 830 nm from a laser light source (3) is reflected by a phase difference −.
Beam splitter that gives 0.66 rad. (= −37.8 °)
After passing through (2), Kerr rotation angle θ = 0.007 rad. (=
0.4 °) GdTbFe Magneto-optical recording medium with magnetic thin film (3)
On the other hand, a magneto-optical regenerator (II) is used, which irradiates almost vertically, reflects the reflected light by a beam splitter (2), guides it to an analyzer (4), and receives it by a detector (5).

Beam splitter used here (2)
Is usually called a half mirror, and its structure is as shown in FIG. 4, and the BSC-7 prism (5
On the slope of 1), _two layers of H layer: TiO ₂ (n = 2.2) / optical film thickness 0.299λ L layer: SiO ₂ (n = 1.453) / optical film thickness 0.384λ are alternated in the order of H (LH) ^5. After forming 11 layers by vapor deposition (52), a prism (51) made of the same material is brought into close contact therewith. This beam splitter
In (2), both the reflectance and the transmittance are approximately 50% for p-polarized light, and the phase difference between s-polarized light and p-polarized light during reflection is -0.66 rad. (=-
37.8 °).

The phase difference δ of the beam splitter (2) is −
Since 0.66 rad. (= −37.8 °), the formula:

[0038]

[Equation 15] | cos (△ ＋ б) ｜ = 1

Δ = + 0.66 rad as one of the solutions satisfying
(= + 37.8 °) is obtained. So the phase difference + 0.66rad. (= +
We obtained a crystal phase shifter (37.8 °), and this is shown as (7) in Fig. 2, so that the principal axis is aligned with the p-polarized plane between the recording medium (3) and the beam splitter (2). installed. As a result, the reproduction signal strength (S) was improved by 27% as compared with the case where the phase shifter (7) was not provided.

The same result was obtained by installing the above-mentioned phase shifter (7) between the beam splitter (2) and the analyzer (4) in FIG. Further, even when a phase shifter having a phase difference Δ = + 3.81 rad. (= 217.8 °) was used in place of the above-mentioned phase shifter, similar improvement of the reproduction signal strength (S) was recognized.

[0041]

As described above, by providing the specific phase shifter according to the present invention, the reduction of the reproduction signal strength (S) when the beam splitter having the phase difference is used is eliminated.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a basic configuration of a magneto-optical reproducing device showing an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a beam splitter used in an example.

FIG. 3 is an explanatory diagram showing a basic configuration of a conventional magneto-optical reproducing device.

FIG. 4 is an explanatory diagram showing a basic configuration of a conventional magneto-optical reproducing device.

[Explanation of symbols]

 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 '... Reciprocator

Claims (3)

[Claims] 1. A light source (1), a beam splitter (2) for transmitting a polarized beam from the light source (1) toward a magneto-optical recording medium, and reflecting light reflected by the medium, In a reflection type magneto-optical reproducing device comprising an analyzer (4) which transmits the light reflected by the beam splitter (2) and a detector (5) which receives the light transmitted by the analyzer (4), the medium and the analyzer Between (4), a phase shifter having a phase difference Δ satisfying the equation: | cos (△ + б) | = 1 (where δ is the phase difference that the beam splitter gives to the reflected light), A reflection-type magneto-optical reproducing device characterized in that the azimuth angle of incident polarized light is set to zero, and the azimuth angle φ of the main axis of the retarder with reference to it is substantially zero. 2. A light source (1), a beam splitter (2) for transmitting a polarized beam from the light source (1) toward a magneto-optical recording medium and reflecting light reflected by the medium, and the beam. From the polarization beam splitter (4 ') that splits the light reflected by the splitter (2) into two orthogonal polarizations, and the two detectors (5) that receive the light split by the polarization beam splitter (4'). In the reflection type magneto-optical reproducing device, which is between the medium and the polarization beam splitter (4 ′),
Formula: [Equation 2] | cos (△ + б) | = 1 (where δ is the phase difference that the beam splitter gives to the reflected light). The reflection-type magneto-optical reproducing device is characterized in that the azimuth angle φ of the principal axis of the retarder is substantially zero with reference to it. 3. When the phase difference due to the birefringence of the substrate or protective substrate of the medium is δ ', the retarder satisfies the equation: | cos (б + б' + △) | = 1 The reflection-type magneto-optical reproducing device according to claim 1 or 2, which is a phase shifter having a phase difference Δ.