JPH0312603A - Polarized light diffracting element and optical pickup device including the same - Google Patents

Abstract

PURPOSE:To compensate the phase difference between a P-polarized component and an S-polarized component generated in diffracted light of 0th order at a diffraction grating part by arranging the polarized light diffracting element in the optical path of reflected light from a magneto-optical recording medium to a photodetector. CONSTITUTION:A material with optical anisotropy is used for a substrate 22, so when diffracted light of 0th or 1st order is propagated in the substrate 22, the P-polarized and S-polarized components have a phase difference. This phase difference varies with the propagation distance in the substrate 22, so when the phase difference between the P-polarized and S-polarized light components of the diffracted light of 0th order needs to be eliminated by the polarized light diffracting element 21, the thickness of the substrate 22 is so set that the phase difference between the polarized light components of the diffracted light of 0th order due to the optical anisotropy of the substrate 22, and the phase difference between the polarized light components of the diffracted light of 0th order due to the polarization characteristics of the diffraction grating part 23 cancel each other. Consequently, the phase difference between the P-polarized and S-polarized light components of the diffracted light of 0th order can be compensated by the diffraction grating part 23.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a polarization diffraction element used in an optical pickup device for an optical memory element, and an optical pickup device equipped with this polarization diffraction element.

[Conventional technology]

A polarizing beam splitter is an important component in an optical pickup device for an optical memory device such as a magneto-optical disk. An example of a conventional optical pickup device for a magneto-optical disk is shown in FIG.

A laser beam emitted from a semiconductor laser 1 is converted into parallel light by a collimating lens 2, passes through a composite beam splitter 3, and is focused onto a magneto-optical disk 6 via a mirror 4 and an objective lens 5.

The reflected light that has been modulated according to the recorded information on the magneto-optical disk 6 is guided to the composite beam splitter 3 via the objective lens 5 and mirror 4, and is directed to the composite beam splitter 3 on the surface 3a of the composite beam splitter 3.
is reflected at right angles. Furthermore, a part is reflected by the surface 3b and passes through the spot lens 7 and the cylindrical lens 8,
It enters the four-divided photodetector 10, and the servo signal, that is,
A tracking error signal and a focus error signal are generated.

On the other hand, the light transmitted through the surface 3b of the composite beam splitter 3 is separated into two polarized components by the polarizing beam splitter 11, which enter the photodetectors 12 and 13, respectively, and the output signals of these photodetectors 12 and 13. Based on this, the signal recorded on the magneto-optical disk 6 is reproduced.

By the way, in the magneto-optical disk 6, the recording signal is detected using the Kerr effect on -a.

Now, in FIG. 4, it is assumed that the laser light irradiated onto the magneto-optical disk 6 is linearly polarized light having only a P-polarized component, as shown by . In that case, when the direction of magnetization on the magneto-optical disk 6 is upward, the polarization plane of the reflected light is +θ, as shown by ■.

Rotate a lot. Conversely, when the direction of magnetization on the magneto-optical disk 6 is downward, the plane of polarization of the reflected light rotates by one θ, as shown by ■. Therefore, by detecting the rotation of this plane of polarization, the recorded signal can be reproduced.

However, since the above-mentioned θ is generally an extremely small angle of 0.5 to 1.51, in order to obtain a high quality reproduction signal, it is necessary to take measures to make this angle apparently larger.

Therefore, in the optical pickup device shown in FIG. 3, the apparent θ is increased by giving polarization characteristics to the surface 3a or 3b of the composite beam splitter 3.

For example, the transmittance T of the P polarized light component on the surface 3b.

is 30%, the reflectance R2 is 70%, and the transmittance T of the S polarization component is
, is set to 100%, and the reflectance R8 is set to 0% (as shown in Fig. 5, the P polarized light component transmitted through the surface 3b is 30%).
%, but the S-polarized component does not decrease, so the apparent upper Kerr rotation angle θ8 (05 to 1.5") is θ'k (1,
7 to 5.0').

However, the optical pickup device as shown in FIG. 3 has drawbacks such as increased weight due to the increased number of parts and increased access time, which also results in increased cost.

Therefore, in recent years, diffraction elements with polarization characteristics have been used to
Attempts are being made to reduce the number of parts.

FIG. 6 shows an optical pickup device having such a polarization diffraction element. However, common components of the apparatus of FIG. 3 are designated with the same reference numerals.

In FIG. 6, laser light emitted from a semiconductor laser I is focused onto a magneto-optical disk 6 via a collimating lens 2, a beam splitter 14, a mirror 4 and an objective lens 5. Reflected light that has been modulated according to the recording signal on the magneto-optical disk 6 is guided to the beam splitter 14 via the objective lens 5 and mirror 4. Thereafter, the reflected light is reflected at right angles by the beam splitter 14, the polarized light is rotated by 90 degrees by a λ/2 plate (1/2 wavelength plate) 19, and then enters the polarization diffraction element 16 via the condenser lens 15. .

The polarization diffraction element 16 has a lattice interval set to approximately the wavelength of light, and thus has polarization characteristics. Further, as shown in FIG. 7, the surface of the polarization diffraction element 16 on which the grating is formed is divided into a plurality of regions in order to generate servo signals.

The 0th order diffracted light transmitted through the polarization diffraction element 16 is separated into two mutually orthogonal polarization components by the birefringent wedge-shaped optical element 17, and is incident on the two-split photodetector 18 to be recorded on the magneto-optical disk 6. A signal is detected.

On the other hand 1. The first-order diffracted light diffracted by the polarization diffraction element 6 is
The light enters the multi-divided photodetector 20, and a tracking error signal and a focus error signal are obtained by calculating the output signals of each of the divided photodetectors.

In addition, in the optical pickup device of FIG. 6, for example,
The 0th-order diffraction efficiency of the S-polarized light component is 30%, and the 1st-order diffraction efficiency is 7.
If the 0th-order diffraction efficiency of the P-polarized light component is set to 100%, and the 1st-order diffraction efficiency is set to 0%, the apparent Kerr rotation angle θ can be increased as described above.

In this configuration, the polarization diffraction element 16 has both the Kerr rotation angle increasing function of the composite beam splitter 3 in the apparatus shown in FIG. 3 and the servo signal generation function of the spot lens 7 and cylindrical lens 8. A reduction in points is achieved.

[Problem to be solved by the invention]

However, in the polarization diffraction element 16 shown in FIG. 6, which has a lattice spacing comparable to the wavelength of light, in the 0th or 1st order diffracted light, the polarization diffraction element 16
Since a phase difference occurs due to the polarization characteristics of the grating, the polarized light becomes circularly polarized light after passing through the polarization diffraction element 16.
A problem arises in that the quality of the reproduced signal deteriorates.

Since the above phase difference is caused by the polarization diffraction element 16 having polarization characteristics, it is impossible to compensate for the phase difference by optimizing the design of the polarization diffraction element 16.

Note that, for example, it is possible to compensate for the phase difference by inserting a phase compensation plate (not shown) between the polarization diffraction element 16 and the birefringent wedge-shaped optical element 17, but in that case, there is a problem that the number of parts increases. occurs.

[Means to solve the problem]

゛In order to solve the above problems, a polarization diffraction element according to the present invention is a polarization diffraction element in which a diffraction grating portion is provided on a flat substrate, and the substrate is made of a material having optical anisotropy. The phase difference between the P-polarized light component and the S-polarized light component of the diffracted light generated by the diffraction grating section and the phase difference between the P-polarized light component and the S-polarized light component caused by the propagation of the diffracted light in the substrate. This is characterized in that the thickness of the substrate is set such that the and the offset values cancel each other out.

Note that the spacing between the diffraction grating portions is preferably set to be approximately equal to the wavelength of the diffracted light.

Specifically, for example, the interval between the diffraction grating parts can be set in a range of 0.5 to 2 times the wavelength of the diffracted light.

Further, it is preferable that the substrate is formed of a material forming a uniaxial crystal.

In that case, for example, quartz can be used as the material forming the uniaxial crystal.

In that case, it is preferable that the diffraction grating section be formed parallel to the optical axis.

The diffraction grating section can be formed as a diffraction grating consisting of grooves provided in the substrate.

Further, the diffraction grating portion may be a gradient index diffraction grating formed by having a refractive index different from that of the remaining portion of the substrate.

The present invention also includes a light source, a light beam from the light source guided onto a magneto-optical recording medium, and a reflected light from the magneto-optical recording medium guided to a photodetector (based on an optical system and a Kerr rotation angle). In an optical pickup device comprising the above photodetector for detecting a recorded signal on a magneto-optical recording medium, the polarization diffraction element according to the present invention is provided in the optical path of reflected light from the magneto-optical recording medium to the photodetector. It is characterized by the fact that it is located.

[For production]

In the polarization diffraction element described above, since a material having optical anisotropy is used as the substrate material, 0-order or 1-order
When the next-order diffracted light propagates through the substrate, a phase difference will occur between the P-polarized light component and the S-polarized light component. This phase difference changes depending on the propagation distance in the substrate, so
In a polarization diffraction element, for example, P in the 0th order diffraction light
When it is necessary to eliminate the phase difference between each polarization component of polarized light and S-polarized light, it is necessary to eliminate the phase difference between each polarization component of the 0th order diffracted light due to the optical anisotropy of the substrate and the polarization characteristics of the diffraction grating. By setting the thickness of the substrate so that the phase differences that occur between each polarization component of the O-th order light cancel each other out,
It is possible to compensate for the phase difference between each polarization component of P-polarized light and S-polarized light that occurs in the 0th-order diffracted light in the diffraction grating section.

On the other hand, if it is necessary to eliminate the phase difference between each polarization component of P-polarized light and S-polarized light in the first-order diffracted light, similarly, the phase difference that occurs between each polarized light component of the first-order diffracted light due to the optical anisotropy of the substrate. The thickness of the substrate may be set so that the phase difference and the phase difference generated between the respective polarization components of the first-order diffracted light due to the polarization characteristics of the diffraction grating portion cancel each other out.

Note that in order to impart polarization characteristics to the polarization diffraction element, the spacing between the diffraction grating portions may be set to be approximately equal to the wavelength of the diffracted light.

Furthermore, if the substrate is formed of a material forming a uniaxial crystal and the diffraction grating section is provided parallel to the optical axis, even if the light incident on the polarization diffraction element is refracted or diffracted, the polarization direction with respect to the optical axis will be maintained. Since it does not change, it becomes easy to design the polarization diffraction element and the largest polarization anisotropy can be obtained. Thereby, the thickness of the substrate required to compensate for the phase difference between the polarization components of P-polarized light and S-polarized light generated in the diffraction grating portion can be reduced.

Further, since the optical pickup device for magneto-optical recording media according to the present invention uses the above-described polarization diffraction element according to the present invention, when detecting a recording signal based on the Kerr rotation angle, for example, If the Kerr rotation angle is to be detected based on the 0th-order diffraction light of the polarization diffraction element, the thickness of the polarization diffraction element substrate must be adjusted so that a phase difference does not occur between the P-polarized light and S-polarized light components in the 0th-order diffraction light. All you have to do is decide. As a result, the O-th order pointing light transmitted through the polarization diffraction element becomes linearly polarized light, so that recording signals can be detected accurately.

In addition, when detecting the Kerr rotation angle based on the first-order diffraction light of the polarization diffraction element, the thickness of the substrate of the polarization diffraction element is All you have to do is decide.

〔Example〕

An embodiment of the present invention will be described below based on FIGS. 1 and 2.

The optical pickup device according to this embodiment is basically the conventional optical pickup device shown in FIG. 6, except that the polarized diffraction element 21 shown in FIG. 1 is used in place of the polarized diffraction element 16 in the conventional example shown in FIG. It is configured similarly to the example. Therefore, detailed explanation regarding the optical pickup device itself will be omitted here.

Although the orientation of FIG. 1 showing the polarization diffraction element 21 of this embodiment does not correspond exactly to that of FIG. 6, the condenser lens 1
The light from 5 is guided to the polarization diffraction element 21 along the direction of arrow A, and then the 0th order directed light B of the polarization diffraction element 21 is guided to the photodetector 18 via the birefringent wedge-shaped optical element 17. Here, a recording signal on the magneto-optical disk 6 as a magneto-optical recording medium is detected. On the other hand, the first-order diffracted light C of the polarization diffraction element 2I is guided to the photodetector 20,
Here, a tracking error signal and a focus error signal can be obtained.

The substrate 22 in the polarization diffraction element 21 is made of a material forming a uniaxial crystal, such as quartz. As shown in FIG. 2, the surface of the substrate 22 on the magneto-optical disk 6 side is provided with diffraction gratings 23, 23, .・
is formed.

The diffraction gratings 23, 23, . . . are formed so that their grating lines are parallel to the optical axis of the substrate 22, which extends in a direction perpendicular to the plane of the paper. In this way, base! fIi22
If a material forming a uniaxial crystal is used as a material and the diffraction gratings 23, 23, etc. are formed parallel to the optical axis of the material, even if the light incident on the polarization diffraction element 21 is refracted or diffracted, the polarization with respect to the optical axis will be changed. Since the direction does not change, the polarization diffraction element 21
The design is easy, and the largest polarization anisotropy can be obtained.

The pitch of the diffraction grating 23, that is, the grating interval d, is approximately the same as the wavelength of the laser beam used for recording or reproduction, preferably 0 to 0 of the wavelength of the laser beam, in order to impart polarization characteristics.
．． It is set to about 5 to 2 times. For example, if the wavelength of the laser beam is 0.8 μm, the grating interval d is 0.5 μm, and the depth L of each groove forming the diffraction gratings 23, 23, etc. is 0.6 μm, then the S polarization component is 0. Next order diffraction efficiency η. , is 0.3.1
Next diffraction efficiency η1. is 0.7, the O-order diffraction efficiency η of the P-polarized light component. De is the 1st, 0.1st order diffraction efficiency η1. becomes O. As a result, the apparent Kerr rotation angle at the O-th instruction light B can be increased, as in the conventional example.

By the way, due to the polarization characteristics of the diffraction gratings 23, 23, . . . , a phase difference occurs between the polarization components of each P-polarized light and S-polarized light of the 0th-order and 1st-order index lights B-C. In this embodiment, since the recording signal on the magneto-optical disk 6 is reproduced by the O-order instruction light B, the phase difference between each polarization component of the 0-order instruction light B needs to be compensated for.

Therefore, the thickness T of the substrate 22 of the polarization diffraction element 21 is determined by the P
The phase difference between each polarization component of polarized light and S-polarized light, and the phase difference between both polarized light components of P-polarized light and S-polarized light of O-th order directed light B, which is caused by the 0th-order directed light B propagating through the substrate 22. The values are set so that they cancel each other out.

That is, since the substrate 22 has optical anisotropy, the P-polarized component of the 0th order light B becomes ordinary light and has a refractive index n. However, the S-polarized component of the 0th order light B becomes extraordinary light, and a refractive index na (≠no) is felt.

For example, if the substrate 22 is quartz, n6 = 1,
52,n,=1.48.

In that case, when the 0th-order pointing light B propagates through the substrate 22 by a length L, a phase difference Δψ between the polarization components in the P direction and the S direction is caused by the optical anisotropy of the substrate 22.

(rad) becomes λ. Therefore, by adjusting the thickness T of the substrate 22, the above Δ
ψ, and the phase difference Δψ between each polarization component of P-polarized light and S-polarized light caused by the diffraction gratings 23, 23, .

The sum of these should be 0, and they should cancel each other out.

As a result, the 0th order light B transmitted through the polarization diffraction element 21
Since there is no phase difference between the P-polarized light and S-polarized light components of can get.

In the above embodiment, the diffraction grating 23 consisting of grooves with a rectangular cross section was formed on the surface of the substrate 22 on the magneto-optical disk side.
By implanting impurities such as , Ag'', etc., it is also possible to create a gradient index diffraction grating whose refractive index is different from that of the rest of the substrate.In that case, the gradient index diffraction grating also makes the grating lines optically different from those of the rest of the substrate. It is preferable to set the lattice spacing to be parallel to the axis and to set the lattice spacing to be approximately equal to the wavelength of the laser beam.

〔Effect of the invention〕

As described above, the polarization diffraction element according to the present invention is a polarization diffraction element in which a diffraction grating portion is provided on a flat substrate.
The substrate is formed of a material having optical anisotropy, and the phase difference between the P-polarized light component and the S-polarized light component of the diffracted light generated by the diffraction grating portion and the P-polarized light caused by the propagation of the diffracted light through the substrate. The thickness of the substrate is set so that the phase difference between the polarized light component and the S-polarized light component cancels each other out.

As a result, since a substrate material having optical anisotropy is used, when the 0th-order or 1st-order diffracted light propagates through the substrate, there is a difference in position between the P-polarized light component and the S-polarized light component, respectively. This phase difference, which causes a phase difference, changes depending on the propagation distance in the substrate, so in a polarization diffraction element, for example, the phase difference between each polarization component of P polarization and S polarization in the 0th order diffracted light is If it is necessary to eliminate the phase difference between each polarization component of the 0th order diffracted light due to the optical anisotropy of the substrate, and the phase difference that occurs between each polarization component of the 0th order diffraction light due to the polarization characteristics of the diffraction grating part. By setting the thickness of the substrate so that the phase differences cancel each other out, it is possible to compensate for the phase difference between each polarization component of P-polarized light and S-polarized light that occurs in the 0th-order diffracted light at the diffraction grating section.

On the other hand, if it is necessary to eliminate the phase difference between each polarization component of P-polarized light and S-polarized light in the first-order diffracted light, similarly, the phase difference that occurs between each polarized light component of the first-order diffracted light due to the optical anisotropy of the substrate. The thickness of the substrate may be set so that the phase difference and the phase difference generated between the respective polarization components of the first-order diffracted light due to the polarization characteristics of the diffraction grating portion cancel each other out.

Note that in order to impart polarization characteristics to the polarization diffraction element, the spacing between the diffraction grating portions may be set to be approximately equal to the wavelength of the diffracted light.

Furthermore, if the substrate is formed of a material forming a uniaxial crystal and the diffraction grating section is provided parallel to the optical axis, even if the light incident on the polarization diffraction element is refracted or diffracted, the polarization direction with respect to the optical axis will be maintained. Since it does not change, it becomes easy to design the polarization diffraction element and the largest polarization anisotropy can be obtained. Thereby, the thickness of the substrate required to compensate for the phase difference between the polarization components of P-polarized light and S-polarized light generated in the diffraction grating portion can be reduced.

Further, the optical pickup device according to the present invention includes a light source, an optical system that guides a light flux from the light source onto a magneto-optical recording medium, and guides reflected light from the magneto-optical recording medium to a photodetector, and a Kerr rotation system. In an optical pickup device comprising the above photodetector that detects a recorded signal on a magneto-optical recording medium based on the angle, the present invention is provided in an optical path of reflected light from the magneto-optical recording medium to the photodetector. This is a configuration in which a polarization diffraction element according to the above is arranged.

As a result, when detecting a recording signal based on the Kerr rotation angle, for example, if the Kerr rotation angle is detected based on the 0th order diffracted light of the polarization diffraction element, P polarization and S polarization in the 0th order diffraction light can be detected. The thickness of the substrate of the polarization diffraction element may be determined so that no phase difference occurs between each polarization component. As a result, the O-th order light transmitted through the polarization diffraction element becomes linearly polarized light, so that recording signals can be detected accurately.

Note that when detecting the Kerr rotation angle based on the first-order diffraction light of the polarization diffraction element, the thickness of the substrate of the polarization diffraction element must be All you have to do is decide.

3 to 7 show conventional examples.

FIG. 3 is an explanatory diagram showing an example of an optical pickup device.

FIGS. 4 and 5 are explanatory diagrams each showing the principle of detecting a recording signal based on the Kerr rotation angle.

FIG. 6 is an explanatory diagram showing another optical pickup device.

FIG. 7 is a schematic plan view showing the grating pattern of the polarization diffraction element.

21 is a polarization diffraction element, 22 is a substrate, and 23 is a diffraction grating (diffraction grating portion).

Claims (1)

[Claims] 1. In a polarization diffraction element in which a diffraction grating portion is provided on a flat substrate, the substrate is formed of a material having optical anisotropy, and The thickness of the substrate is set so that the phase difference between the P-polarized light component and the S-polarized light component and the phase difference between the P-polarized light component and the S-polarized light component caused by the propagation of the diffracted light through the substrate cancel each other out. A polarization diffraction element characterized by: 2. The polarization diffraction element according to claim 1, wherein the spacing between the diffraction grating portions is set to be approximately equal to the wavelength of the diffracted light. 3. The interval between the diffraction grating parts is 0.5 to 2 of the wavelength of the diffracted light.
Claim 2, characterized in that the range is set to double
The polarized light diffraction element described in 2. 4. The polarization diffraction element according to any one of claims 1 to 3, wherein the substrate is made of a material forming a uniaxial crystal. 5. The polarization diffraction element according to claim 4, wherein quartz is used as the material forming the uniaxial crystal. 6. The polarization diffraction element according to claim 4 or 5, wherein the diffraction grating section is formed parallel to the optical axis. 7. The polarization diffraction element according to any one of claims 1 to 3, wherein the diffraction grating section is a diffraction grating made of grooves provided in a substrate. 8. According to any one of claims 1 to 3, wherein the diffraction grating portion is a gradient index diffraction grating formed by having a refractive index different from that of the remaining portion of the substrate. The polarized light diffraction element described above. 9. a light source, an optical system that guides the light beam from the light source onto the magneto-optical recording medium and guides the reflected light from the magneto-optical recording medium to a photodetector; In an optical pickup device comprising the above-mentioned photodetector for detecting a recorded signal, an optical pickup according to any one of claims 1 to 3 is provided in the optical path of reflected light from the magneto-optical recording medium to the photodetector. An optical pickup device characterized in that the polarization diffraction element according to the above is arranged.