JPH0719402B2 - Magneto-optical disk playback device - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a magneto-optical disc reproducing apparatus suitable for obtaining a high signal-to-noise ratio.

[Background of the Invention]

The conventional device is shown in FIG. 1 as described in Sharp Technical Report, No. 22, March 1982.

The light from the semiconductor laser 1 is converted into parallel light by the lens 2, and converted into parallel light having a circular cross-sectional shape by the prism 3.
Focus on the magneto-optical disk 9 with the lens 5. Information is recorded on the magneto-optical disk 9 by the difference in the vertical direction of the vertical magnetization, and the difference in the direction of the vertical magnetization causes rotation of the polarization direction of the laser light (that is, Kerr effect) at the time of reflection. This rotation of the polarization direction is passed through the analyzer 7 to the photodetector 8
Detected by. A beam splitter 4 directs the reflected laser light from the disk toward the photodetector.

By the way, the beam splitter 4 and the prism 3 and the like used in this optical system are made of a dielectric multilayer film, and produce a phase difference when polarized light in the direction perpendicular to the paper surface of FIG. 1 and in the direction parallel to the paper surface is reflected or transmitted. . This phase difference causes a drop in the reproduced signal as described below.

[Object of the Invention]

It is an object of the present invention to provide a device for compensating for the phase difference introduced into an optical system and reproducing a signal with a high signal-to-noise ratio.

[Outline of Invention]

FIG. 2 is a diagram showing that the polarization state of the reflected light changes depending on the magnetization state of the area irradiated with the reproduction light. In the figure, I represents polarized light incident on the medium, R ⁺ represents polarized light reflected from a region magnetized below the film surface, and R ⁻ represents polarized light reflected from a region magnetized above the film surface. . θ
_K is called the force rotation angle. Rotate the analyzer by θ from the extinction position. As described above, it is assumed that the beam splitter, the prism, and the like generate the phase difference Δ in the x-axis and y-axis directions shown in FIG.

At this time, when the optical amplitude after the output of the analyzer from the region having one force rotation angle θ _K is calculated as A θ _K by the _Johnson vector method (see “Crystal Optics” Morikita Publishing), Here, T _θK : _optical rotation matrix, P _{0 °, R} ; reflectance in x direction
Rx, y matrix with optical elements having reflectance Ry, R _{Δ, 0 °} ; matrix with elements producing phase difference Δ, P _{0 °} T _−K ; analyzer matrix.

Amount of light entering the analyzer Because When the signal is defined as the difference between the amount of light from a region having a light intensity and mosquitoes one rotation angle - [theta] _K from the area with a force one rotation angle theta _K Is given. Therefore, the phase difference Δ introduced into the optical system
Reduces the signal by cos Δ.

Therefore, in the present invention, a phase compensator is provided in the optical system,
(EN) Provided is a magneto-optical disk reproducing device which eliminates a phase difference as a whole optical system and has a high signal to noise ratio.

Example of Invention

 An embodiment of the present invention is shown in FIG.

FIG. 3 shows the conventional apparatus of FIG. 1 with the phase compensator 10 inserted therein.

The phase compensator 10 may be, for example, a so-called Babinet Soleil compensator shown in FIG. That is,
The phase difference of other optical components is compensated for by adjusting the screw 14 to give the phase difference in the x direction and the y direction to the incident laser beam 13 arbitrarily.

That is, the phase difference in the x and y directions other than that of the phase compensator 10
_{When set to 0} , the effective thickness of the left-handed (or right-handed) optical element 11 of the phase compensator 10 and the right-handed (or left-handed) optical element 12
The effective thicknesses of d ₁ and d ₂ can be phase-compensated by setting as shown in the following equation.

ned ₁ −n ₀ d ₂ = mλ−δ ... Here, m is an integer, ne is an extraordinary ray refractive index, and n ₀ is an ordinary ray refractive index. However, By the way, the wavelength of the semiconductor laser 1 used in FIG. 3 is different even if it is created under the same conditions. Even with the same element, the wavelength changes when the drive current and the ambient temperature change. Therefore, it is desirable that the formula be satisfied even if the wavelength changes. That is, it is desirable to satisfy the expression obtained by differentiating the expression by the wavelength.

Select d ₁ and d ₂ that satisfy the formula and the formula at the same time. By solving the equation, the following equation is obtained.

Here, the units of d ₁ , d ₂ , λ, and δ are (m), m is an arbitrary integer, ne, n ₀ is no unit, and the unit of ψ ₀ is (rad).

If d ₁ and d ₂ are set as shown in Eq., The phase difference introduced into the optical system will always be compensated even if the wavelength of the laser light fluctuates.

If quartz is used as the material for the phase compensator shown in FIG. 4, ne = 1.548, n ₀ = 1.5 near the wavelength of the laser light of 800 nm.
39, again Is approximately equal to −1.65 × 10 ⁻⁵ (1 / nm).

The retardation introduced into the optical system depends on the multi-layer coating, but a retardation change of about 1 ° to 5 ° usually occurs for a wavelength change of 1 nm. That is, As a crystal size, a large one is expensive and a small one is difficult to manufacture. Therefore, the result of calculating the relationship between dδ / dλ and m by setting d ₂ from 1 mm to 20 mm in the formula Is shown in FIG.

The shaded area is desirable.

Next, one concrete example will be shown.

Laser wavelength (λ): 800 nm Phase difference (δ) of optical system: λ / 2 Wavelength derivative of phase difference (dδ / dλ): 3.5 Phase compensator material: Crystal d ₁ : 5.4929mm d ₂ : 5.5232mm m: 4 [Effects of the Invention] As described above, according to the present invention, it is possible to effectively eliminate the phase difference introduced into the optical system part of the magneto-optical disk reproducing device, and reproduce the signal with a high signal-to-noise ratio. can do.

[Brief description of drawings]

FIG. 1 is a diagram showing a conventional magneto-optical disc reproducing apparatus, and FIG.
FIG. 3 is a diagram for explaining the principle of signal reproduction, FIG. 3 is a diagram for showing a magneto-optical disc reproducing device of the present invention, and FIGS. 4 and 5 are diagrams for explaining a phase compensator used in the present invention. 1 ... semiconductor laser, 2 ... lens, 3 ... prism,
4 ... Beam splitter, 5,6 ... Lens, 7 ... Analyzer,
8 ... Optical detector, 9 ... Magneto-optical disk, 10 ... Phase compensator.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Takeshi Kato 1-280 Higashi Koigakubo, Kokubunji, Tokyo 1-280, Central Research Laboratory, Hitachi, Ltd. (72) Wen Saito 1-280 Higashi Koigakubo, Kokubunji, Tokyo Hitachi, Ltd. Central Research Laboratory (56) Reference JP-A-60-20342 (JP, A) JP-A-60-20341 (JP, A) JP-A-60-151855 (JP, A) JP-A-60-143461 (JP, A)

Claims (1)

[Claims] 1. A laser light source, an optical system for irradiating a magneto-optical disk with laser light emitted from the laser light source, and guiding light returning from the magneto-optical disk to a photodetector through an analyzer, and an optical system in the optical system. In the magneto-optical disk reproducing apparatus having the phase compensator arranged in the above, the phase compensator has adjusting means for varying the distance that the laser light passes, and the phase compensator satisfies the following condition. Magnetic disk player. conditions: However, d ₁ : Effective thickness of the left-handed (or right-handed) optical element of the phase compensator (m) d ₂ : Effective thickness of the right-handed (or left-handed) optical element of the phase compensator (m) m: Any Integer λ: wavelength of laser (m) ne: extraordinary ray refractive index of the phase compensator n ₀ : ordinary ray refractive index of the phase compensator ψ ₀ : by an optical system other than the phase compensator in XY Cartesian coordinates formed on a plane perpendicular to the optical axis of the laser beam X direction and Y
Phase difference in direction (rad)