JPS60113347A - Optical device of photomagnetic memory - Google Patents

Abstract

PURPOSE:To enable the improvement of S/N by controlling optical characteristics including the phase of a beam splitter, a mirror, an analyser etc. utilized for reproduction. CONSTITUTION:Reflected information light from a recording medium 1 is led to a detection side system by a polarization beam splitter 3 having function to improve the degree of polarization for reflected light and function to rotate polarization orientation and enlarge apparent magnetic optical rotation angle for transmitted light. Dielectric multilayer coating is made on the slanted face of a total reflection prism 6 to make phase deviation between S-wave and P- wave to be npi. Optical characteristics is given to the reflecting face of the beam splitter to give the relation of TP>TS between amplitude transmittivity TP for P-polarized light and amplitude transmittivity TS for S-polarized light and to make phase difference delta between transmitted P-wave and S-wave is npi-pi/4 <=deltanpi+pi/4 (n: integer).

Description

[Detailed Description of the Invention] Technical Field The present invention relates to a magneto-optical storage device that uses a magnetic film as a recording medium and records, reproduces, and erases information by irradiating the recording medium with a light beam such as a laser beam. particularly regarding its optical device.

<Prior Art> In recent years, optical storage devices have become memory devices f'? that are capable of higher density, larger capacity, and faster access. It has been widely studied. Among these, there is a device that forms a minute bit string on a storage medium and reproduces light by utilizing the diffraction phenomenon of a light beam in the pit portion, or a device that forms bit-like regions with different refractive indexes on the storage medium and changes the reflectance. Some devices for reproducing light using changes in transmittance have been put into practical use. However, the above-described devices only have the function of being read-only or capable of recording additional information, and an optical storage device that even has an erasing function, which is a major feature of a memory device, has not yet been put into practical use.

By the way, a magneto-optical storage device using a magnetic material as a storage medium can be an extremely useful optical storage device that allows easy recording and erasing. However, this magneto-optical storage device has problems such as its reproducing optical system being more complex than other optical storage devices and the quality of the reproducing signal being poor.

<Purpose> The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to improve the quality and reliability of optically reproduced signals.

<Embodiment> Hereinafter, an embodiment of an optical device of a magneto-optical storage device according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is an explanatory diagram of a schematic configuration of a reproducing apparatus for a magneto-optical storage device according to the present invention.

It-j is a magnetic recording medium having perpendicular magnetic anisotropy using an amorphous alloy thin film of a rare earth metal and a transition metal as a recording material. 2 semiconductor lasers that can emit laser light with a predetermined intensity; 3 collimating lenses that convert the emitted laser light into parallel light; It is a shaped prism that converts into a beam, and its entrance surface is coated with a non-reflection coating for S-polarized light. 5 is a polarizing beam splitter that has the function of improving the degree of polarization of reflected light and further rotating the polarization direction of transmitted light to increase the apparent magneto-optic rotation angle; The reflected information light from the recording medium l is detected by the polarizing beam splitter 5.
Guided by the side threads. More detailed effects of this polarizing beam splitter 5 will be described later.

It is a total reflection prism that bends the optical path 4908 times, and its slope is coated with a dielectric multilayer so that the phase shift between the S wave and the P wave is nπ, and the optical path is bent by 90 degrees while maintaining the state of incident polarization. Has the ability to change. Reference numeral 7 denotes an objective lens for forming a complete image of a minute light spot on the medium 1. 8
is a polarizing beam splitter which has the function of increasing the magneto-optical rotation angle of transmitted light, similar to the polarizing beam splitter 5 described above. 9 is a spot lens for projecting a light beam of a predetermined size and shape onto each photodetector (described later); 10 is a one-wavelength plate capable of rotating the polarization direction of the reflected information light in a predetermined direction; 11 is a polarizing beam splitter that separates S-polarized light, 12.13 is a polarizing beam splitter 1
A photodetector that receives information light detected by 1,
Usually 5iPIN photodiode or S1APD
(Avalanche Photo Diode) is used. 14 is a spot lens; 15 is an lindrical lens whose focal line is inclined by 45 degrees with respect to the tangential direction of the track on the recording medium I; This is a composite element type photodetector for detecting changes in the relative distance between the recording medium 1 and the objective lens 7, and in this embodiment, it also detects positional deviation information (tracking information) between the optical spot and the information track. It looks like it can be done.

Next, the action of the polarizing beam splitter 5.8 will be explained.

FIG. 2 is a vector diagram showing the optical characteristics of the polarizing beam splitters 5 and 8. The laser beam emitted from the laser 2 to the semiconductor f1 is set to become S-polarized light with respect to the polarization beam splitter 5, and reaches the recording medium 1 in the S-polarized state. The light reflected by the recording medium (+<l) and re-injected into the polarizing beam splitter 5 changes its polarization plane to the right or left by a small angle (Kerr rotation angle) depending on the magnetization state (upward or downward magnetization) of the recording medium l. α) Rotated polarized light, and if these are respectively M''-1M-, the polarized lights transmitted to the detection system side by the polarizing beam splitter 5 are M+'1M-'.As shown in the figure, the polarized beam splitter 5 The optical energy is reduced during the transmission process, but the rotation angle β of the polarization plane is 0, which means that the polarization beam rotation angle is increased.In addition, the rotation angle is further increased by the polarization beam splitter 8.However, the rotation angle β of the polarization plane is increased. The rotation angle increase ratio is when the phase shift between the S wave and the P wave reflected by the polarizing beam splitter is n7L, and when the above phase shift deviates from nπ, the increase ratio becomes smaller and the ellipticity increases. increases, the degree of signal modulation decreases, and the S/N fl tends to decrease.

The results of this calculation are shown in FIG. FIG. 3 shows the relationship between the phase shift δ between the transmitted P wave and S wave and the S/N when the approximate model shown in FIG. 4 is used. In FIG. 4, 21 is an analyzer and 22 photodetectors. As seen from FIG. 3, n feet - 2 ≦ δ ≦ n + 4 (n: integer) are required. The results shown in Figure 3 are relative values of S/N obtained when the analyzer 21 is set at the direction where the S/N is maximum. 45
The S/N ratio obtained when adjusting the angle (°) also shows a similar tendency.

Moreover, this S/N deterioration becomes even greater as the extinction ratio of the optical system deteriorates, and therefore, management of the phase shift δ of the polarizing beam splitter becomes extremely important. Further, this phase shift δ needs to be managed not only in the polarizing beam splitter 5.8 but also in other optical elements such as the total reflection prism 6.

Next, the optical characteristics that the polarizing beam splitters 5 and 8 should have will be explained. As mentioned above, if the polarizing beam splitters 5 and 8 are incorporated in the above configuration, the magneto-optic rotation angle can be increased to facilitate reproduction, and as a result of the increased rotation angle, the azimuth angle of the analyzer can be increased. Therefore, effects such as reducing the effect of the extinction ratio of the optical element on S/N deterioration can be expected, and it becomes easy to approach the S/N obtained when ideal reproduction is performed.

However, it is impossible for the polarizing beam splitters 5 and 8 to exceed the basic S/N 'i of the recording medium 1, and therefore, the function of the polarizing beam splitter 5.8 is to determine how to degrade the information signal of the recording medium 1. What is important is whether it can be guided to the detector without any interference. In general, a photodetector is equipped with a photoelectronic tank l] tube, an APD (avalanche
When using an element with amplification function such as Photo Diode), the biggest noise source is optical yacht noise, and its S/N ratio V,j,,S/Nci(p-o(p:
The amount of light to be detected can be expressed by the following equation: θ: magneto-optical 1i11 rotation angle). Therefore, the optical characteristic Tp is required to prevent the S/N' from deteriorating, and the magneto-optical rotation angle before the beam is input to the polarizing beam splitter T8 is θ. Then, approximately JT・θ.

It is sufficient if the value satisfies . That is, it is sufficient that the product of the light quantity and the magneto-optic rotation angle of the light exiting the polarizing beam splitter is similar to that before entering the polarizing beam splitter.

<Effects> According to the present invention described above, it is possible to improve S/N'i by managing the optical characteristics including the phase of the beam splitter, mirror, analyzer, etc. used for reproduction of the magneto-optical storage device. becomes.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a magneto-optical storage device according to the present invention, FIG. 2 is a vector diagram showing the polarization state of reproduced information light, and FIG.
The figure is a graph of the relationship between phase shift δ and S/N, and FIG. 4 shows a first explanation of the configuration of the reproduction model. In the figure, ■=Magnetic recording medium 2: Semiconductor laser 3: Collimating lens 4: Shaped prism 5° 8.11: Polarizing beam splitter 9.14 Varnished spot 15 Nijilinstrical lens 16: Complex element type photodetector Agent Patent attorney Yoshihiko Fukushi (and 2 others) Figure 1 S (S/N) ref lCdag) Figure 3

Claims (1)

[Claims] (1) A magnetic thin film having perpendicular magnetic anisotropy is used as a recording medium;
In an optical device of a magneto-optical storage device that reproduces information by irradiating the recording medium with a laser beam, a beam splitter is arranged in the optical path, and a P
The amplitude transmittance TP for polarized light and the amplitude transmittance T8 for S-polarized light have the relationship Tp)T8, and the phase difference δ between the transmitted P wave and S wave is ni−shi≦J≦nfi+7
1. An optical device for a magneto-optical storage device, characterized in that it is coated with a multilayer dielectric thin film that imparts optical properties such that (n: an integer). 2. Claim 1, characterized in that the magneto-optical rotation angle by the recording medium is set to θ, and the amplitude transmittance of P waves and S waves by the beam splitter is set to a value such that /θki1.
Optical device of magneto-optical storage bag [1q.