JPS62172550A - Pickup for optomagnetic recording medium - Google Patents

Abstract

PURPOSE:To prevent the deterioration in the C/N due to a light beam subjected to elliptically polarized light restored into a linearly polarized light by using a phase compensation plate to the light receiving means of a returned light from a recording layer. CONSTITUTION:When the light beam of linearly polarized light by an objective lens 6 is converged and focused onto the recording layer 9 of a recording medium 7 as a spot, projected and reflected then returned, the light beam is subjected to elliptic polarization by double refraction through a base 8, and the elliptic light is restored in the linearly polarized light with birefringence through a phase compensation plate 43 after being reflected in a half mirror 5. Then only the component corresponding to uni-magnetization direction is transmitted through an analyzer 11, and the recording information is reproduced by the photoelectric conversion signal output by a photodetector 13. Thus, the deterioration in the C/N due to the birefringence of the base 8 is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a pickup for a magneto-optical recording medium that uses phase compensation means to reduce the influence of birefringence of a substrate having birefringence.

[Background Art] In recent years, industries related to information such as computers have made remarkable progress, and the amount of information handled is rapidly expanding.

For this reason, information can be optically recorded at high density on a disc-shaped recording medium (hereinafter referred to as a disk) and reproduced at high speed using laser light instead of a conventional magnetic head. Optical recording and reproducing devices are attracting attention.

Among the above-mentioned optical recording and reproducing devices, those of the magneto-optical type that utilize the magneto-optical phenomenon (magnetic Kerr effect) are expected to be widely used in the future because they can rewrite and record or reproduce recorded information.

FIG. 6 shows the configuration of a pickup of a conventional magneto-optical recording/reproducing device.1 This pickup 1 converts a polarized light beam emitted from a laser diode 2 into a parallel beam using a collimator lens 3, After passing through the objective lens 6
Disk 7 as a disk-shaped optical recording medium by condensing light with
It should be irradiated to This disk 7 consists of a recording medium base 8 and a recording layer 9 made of ferromagnetic material formed on the base 8, and the condensed light passes through the base 8 and is focused and irradiated onto the recording layer 9. .

The polarization directions of the return light reflected by the recording layer 9 rotate in opposite directions depending on the direction of magnetization (of the irradiated portion), and the light that passes through the substrate 8 is focused by the objective lens 6. is,
A part of it is reflected by the half mirror 5, and the analyzer 11
Only the light component rotated in one direction passes through the condenser lens 12 and is received by a photodetector such as an avalanche photodiode (ΔPD) 13, and the recorded information is reproduced using the photoelectric conversion output of this photodetector.

Further, a part of the light transmitted through the half mirror 5 is reflected by the other half mirror 4, and the light reflected by the critical angle prism 14 is received by the 4-split photodetector 15, thereby performing the critical angle method. A focus error signal based on the push-pull method and a tracking error signal based on the push-pull method can be extracted, and the objective lens 6 can be controlled using these signals to perform focus servo and tracking servo.

By the way, in the above conventional example, when an injection molded PC board is used for the base 5!88, it exhibits optical birefringence, and for ordinary rays no = 1.5800. For extraordinary rays, the refractive index is ne =1.5806. In this case, the thickness of the substrate 8 whose optical axis is perpendicular is 1.211Il,
When a 110-No laser (wavelength 633nl) is incident at an incident angle of 30° (this 30° has a numerical aperture NA of 0.5).
For linearly polarized light (corresponding to the incident angle of the laser beam at the outermost circumference when using a lens of As shown in the figure, a phase difference occurs, resulting in ovalization. FIG. 8 shows how a phase difference occurs with respect to the incident angle when the angle e is 45 degrees. The reason why ovalization occurs due to the phase difference shown in these figures will be explained below with reference to FIG. 9.

FIG. 9 is an explanatory diagram showing how the objective lens 6 focuses the laser beam 16 onto a part of the PCL 18 and irradiates it in a spot shape. In the figure, only a portion of the PC group 5188 is shown.

Incidentally, the laser beam 16 is linearly polarized light (the direction of polarization is indicated by the reference numeral 19 in the figure) that is perpendicular to the radial direction of the base 8 (reference numeral 17 in the figure) and parallel to the base 8, and the laser beam that is incident on the surface of the base 8 Beam 16 has beam portions 31 and 32 incident perpendicularly and parallel to the direction of linear polarization.
and, for example, beam portions 33, 34 which are incident on beam portions 31, 32 at a 45 degree offset.

Since these beam portions 31, 32, 33, and 34 are focused toward the center of the beam, the beam portions 31, 32 having polarization directions perpendicular or parallel to this focusing direction retain their linearly polarized light on the surface of the base 8, respectively. death,(
(as S-polarized light or P-polarized light) and is refracted.

However, for example, the beam portions 33 and 34 which are incident at a deviation of 45 degrees become polarized light containing both P-polarized light and S-polarized light components by focusing. Therefore, when the light is refracted and transmitted, it is affected by birefringence by the substrate 8, and changes from linearly polarized light to elliptically polarized light while passing through the substrate 8. The reason for this is that the special application
-For details on No. 260615.

The ellipticalization to become elliptically polarized light is as shown in Figure 7.
The angle e between the radial direction and the plane of incidence becomes maximum at 45°.

[Problems to be Solved by the Invention] When the above-mentioned ovalization occurs, even if the analyzer 11 is set to pass only the reflected light beam in one direction of magnetization, the birefringence of the substrate 8 causes ovalization. Therefore, other light beams of signal components whose polarization direction has been rotated due to the magnetization of the recording layer 9 are also transmitted and mixed into the signal light beam, raising the noise level. This causes the C/N to deteriorate.

The present invention has been made in view of the above-mentioned points, and an object of the present invention is to provide a pickup for a magneto-optical recording medium that can prevent deterioration of C/N due to birefringence.

Means and operation for solving problem E] In the present invention, a phase compensating plate whose optical axis is perpendicular to the plate surface is provided in the light receiving part of the magneto-optical recording/reproducing optical system that receives light by the photodetector via the condensing lens. is arranged to reduce the influence of birefringence of the base.

[Example] Hereinafter, the present invention will be specifically described with reference to the drawings.

FIG. 1 shows a first embodiment of the invention.

A pickup 41 for a magneto-optical recording medium according to the first embodiment includes a light receiving section for reproducing information signals in which a light beam split by reflection by a half mirror 5 is received by a photodetector 13 in the pickup 1 shown in FIG. The structure is as follows.

The light beam reflected by the half mirror 5 passes through a 172-wave plate (also referred to as a λ/2 plate) 42 to
The phases of the S and P polarized lights in the light beam ellipticalized in step 5188 are made to have opposite phases (before entering this λ/2 plate 42,
For example, if the light beam is ovalized and rotates in the direction of a right-handed screw, after passing through the λ/2 plate 42, the phase of the two is shifted and the light beam rotates in the direction of a left-handed screw. ). After that, the objective lens 6 has the same diameter and the same numerical aperture NA.
After the light is focused by the condenser lens 12 and made into almost linearly polarized light through phase compensation through the phase compensation plate 43, only the light beam component rotated by one magnetization direction is analyzed by the analyzer 11. The light beam transmitted through the analyzer 11 is received by a photodetector 13.

The phase compensation plate 43 is made of the same material as the substrate 8, for example, the optical axis is perpendicular to the plate surface, and the thickness is twice that of the substrate 8. (This is because the base plate 8 passes back and forth twice, whereas the phase compensation plate 43 passes only once.

) The rest of the structure is the same as that shown in FIG. 6 above.

According to the first embodiment configured in this way, when the linearly polarized light beam is condensed by the objective lens 6 and irradiated in a spot-like manner onto the recording layer 9 of the recording medium 7, it is also reflected. When returning, the phase on the extraordinary ray side is delayed relative to the phase on the ordinary ray side, which has a small refractive index, due to the birefringence of the substrate 8, resulting in ovalization. λ/2 plate 42
After the phase relationship between the two is reversed by
In contrast to linearly polarized light being ovalized, elliptical light is converted back to linearly polarized light. Thus, only the light beam component corresponding to one magnetization direction is transmitted by the analyzer 11, and the recorded information can be reproduced by the photoelectric conversion signal output by the photodetector 13.

FIG. 2 shows the main parts of a second embodiment of the present invention.1 In the pickup 51 of this second embodiment, the objective lens 6 has a numerical aperture NA=0.5 as in the first embodiment. The above-mentioned NA
= smaller than 0.5 (that is, the focal length is long), for example, N
The one with A=0.2 is used. Therefore, between this condensing lens 52 and the analyzer 11 in front of the photodetector 13, there is a crystal plate (in this case, ne = 1.5473.n.

-1,5384) or the like is used. In this case, the light beam is transmitted through a phase compensation plate 5 such as a crystal plate.
Although the NA of the condenser lens 52 is considerably smaller than the NA of the objective lens 6, the influence of birefringence (per unit thickness) when passing through the lens 3 is due to the phase difference due to the difference in refractive index of the crystal plates Δn = ne - no, for example. The change is 1! ! ! ! Since it is considerably larger than the case of t8, a thinner substrate than this substrate 8 is used.

According to the second embodiment, by using a lens with a small NA as the condenser lens 52, a distance can be secured between the condenser lens 52 and the photodetector 13 to allow the phase compensation plate 53 to be actually inserted. At the same time, by using a phase compensation plate 53 that has a larger effect of birefringence than the substrate 8 (in other words, a plate with a larger phase change R per unit thickness), the thickness of the interposed phase compensation plate 53 can be reduced. A thin one is enough to perform its function.

Therefore, this second embodiment is effective when actually applied.

゛Next, the effectiveness of the present invention will be explained with the following experimental examples.

In this case, the disk 7 is injection molded with ne and no of about 1.5806 and 1.5800, respectively, and has a thickness of 1.
2mm (7) PCI board 8a, 900A as recording layer 9
A Gd-Tb-Fc perpendicularly magnetized film with a thickness of (this recording medium is designated as sample 7b) and two types were prepared.

Therefore, a comparison was made between the optical system according to the embodiment of the present invention shown in FIG. 2 and an IC case using a conventional optical system not shown in FIG.

Note that l used as the phase compensation plate in this example
@The thickness of the cut crystal plate is 0.56.0°64.0.7
Four types of 2.0.80 nua were prepared.

First, in the case of the conventional example, that is, the optical system shown in FIG.
The result of comparing the noise levels when using the PMMA substrate 8a and the PMMA substrate 8b is shown in FIG. 3(a), while the noise level for the PC substrate 18a when using the optical system of this example is shown in FIG. It is now shown in

It can be seen from FIG. 3(a) that the PC board 8a has a lower noise level. In addition, the range indicated by the reference numeral in FIG. 3(a) is the practical reproduction range.

On the other hand, as shown in FIG. 3(b), when the thickness is 0.72 ml, the noise level can be reduced to a level close to that of the PMMA substrate 8b in FIG. 3(a).

Note that the C/N in this measurement result was obtained by rotating the recording medium samples 7a and 7b by 900 rE) mr and using a 1 MHz carrier signal under 0 Snicol.

In the above experimental example, the optical system of this example is similar to the case where the PC board 8a is used with the PC board 8b.
N can be increased.

From this experimental example, it can be inferred that the C/N can be increased even for PCM discs under other conditions by adjusting the thickness of the phase compensation plate, etc.

FIG. 4 shows the main parts of a third embodiment of the present invention.

In the pickup 61 of this third embodiment, the λ/2 plate shown in FIG. 2 is not used, but a phase compensation plate 62 that exhibits birefringence characteristics opposite in polarity to that of the substrate 8 is used. In other words,
The phase compensation plate 62 used is one in which ne<no, whereas the base plate 7 has ne>no.

The rest is the same as the second embodiment.

FIG. 5 shows the main parts of a fourth embodiment of the present invention.

In the pickup 71 of the fourth embodiment, the second
In the embodiment, a half mirror 72 that transmits 50% of incident light and reflects 50% of the incident light is disposed between the λ/2 plate 42 and the condenser lens 12, for example.

As in the second embodiment, the phase compensation plate 52 and the analyzer 1
1 and received by the photodetector 13, and also for the reflected light beam, the phase compensation plate 52' having the same characteristics as described above,
The light is received by a photodetector 13' through an analyzer 11'.

The angle of the detector 11 is set so that the light beam for one magnetization direction is transmitted, whereas the other analyzer 11- is set at an angle so that the light beam for the opposite magnetization direction is transmitted. is set. In this way, the photoelectric conversion signals of the two photodetectors 13, 13, which have the same characteristics, are passed through the differential amplifier 73, and the recorded information is reproduced by the differential output thereof.

Since this fourth embodiment is designed to obtain differential output,
The C/N can be further increased.

Incidentally, a partial combination of each of the above embodiments is also subject to the present invention.

The present invention is not limited to PC boards, but can be widely applied to boards exhibiting birefringence.

In each of the above embodiments, the photodetector 13 for reproducing recorded information is
A phase compensating means is provided near the light receiving section for recording information reproduction, which is provided with a light beam, but a concave lens or the like is provided to widen the light beam before condensing it with the condenser lens 12 to adjust the diameter of the light beam. By increasing the size, even if a condensing lens with a large numerical aperture NA is used, the distance to the photodetector 13 (13') can be expanded, making it easier to insert a phase compensation plate or the like. In this case, a phase compensation plate may be interposed in the middle of the optical path that expands in diameter. Furthermore, multiple convex lenses are used to condense a parallel light beam, and the light beam that spreads out again after passing through the focal point is condensed into a parallel light beam, and then the condensing lens as described above is used to condense the light beam into a parallel light beam. The light may be made to enter the detector 13. In this case, the interposed position of the phase compensator may be either the condensing or expanding light beam portion, or the interposition may be divided into a plurality of parts.

[Efficacy of invention! ! ! ] As described above, according to the present invention, there is no birefringence and 157
Even if ovalization occurs in step 14, since a phase compensation means is provided to return this ovalized light beam to linearly polarized light, the C/N
can be increased.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing a first embodiment of the present invention, Fig. 2 is a block diagram showing the main parts of a second embodiment of the present invention, and Fig. 3 is a block diagram showing an experimental example of the present invention. Figure (a> is a characteristic diagram showing the noise level in a conventional example for comparison with the present invention,
Fig. 4(b) is a characteristic diagram showing the noise level when the thickness of the phase compensation plate is changed in the present invention, Fig. 4 is a configuration diagram showing the main part of the third embodiment of the present invention, Fig. 5 6 is a configuration diagram showing the main part of the fourth embodiment of the present invention, FIG. 6 is a configuration diagram showing the configuration of a conventional example, and FIG. A characteristic diagram showing the relationship in which a phase difference that causes ovalization occurs depending on the angle of incidence. Figure 8 shows the relationship in which a phase difference occurs depending on the angle of incidence when the angle between the radial direction and the plane of incidence is 45°. FIG. 9 is a perspective view showing how a linearly polarized light beam is focused and incident on a recording medium. 2...Laser diode 3...Collimator lens 4.5...Half mirror 6...Objective lens 7...Recording medium 8...Base 9...Recording layer 11...Analyzer
12... Condensing lens 13... Photodetector 41
...Pickup 42...λ/2 plate 43...
・Phase compensation plate 1st figure 2nd figure 3rd figure Light 3t-A /jj (deg)・π4
Figure 5 Figure 1 1 of 13 groups

Claims (1)

[Claims] A pickup for a magneto-optical recording medium that utilizes a magneto-optical phenomenon in which the polarization direction of light returned from the recording layer changes depending on the magnetization direction in the recording layer formed on a base having birefringence, A pickup for a magneto-optical recording medium, characterized in that a light receiving means for receiving the return light from the recording layer via the substrate with a photodetector is provided with a phase compensation plate.