JPH0823605B2 - Polarization diffraction element and optical pickup device including the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a polarization diffractive element used in an optical pickup device for an optical memory element and the like, and an optical pickup device including the polarization diffractive element.

[Conventional technology]

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

The laser light emitted from the semiconductor laser 1 is converted into parallel light by the collimator lens 2, passes through the composite beam splitter 3, and is focused on the magneto-optical disk 6 via the mirror 4 and the 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 the mirror 4, and the surface of the composite beam splitter 3 is guided.
It is reflected at a right angle at 3a. Further, a part of the surface 3b is reflected and passes through the spot lens 7 and the cylindrical lens 8,
The light enters the four-divided photodetector 10, and a 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 polarization components by the polarization beam splitter 11, enters the photodetectors 12 and 13, respectively, and is output to 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, a recording signal is generally detected by utilizing the Kerr effect.

Now, in FIG. 4, it is assumed that the laser light applied to the magneto-optical disk 6 is linearly polarized light having only a P-polarized component, as indicated by I. In that case, when the direction of magnetization on the magneto-optical disk 6 is upward, as shown by II, the polarization plane of the reflected light rotates by + θ _k . On the contrary, when the direction of magnetization on the magneto-optical disk 6 is downward, II
As indicated by I, the plane of polarization of the reflected light rotates by −θ _k . Therefore, by detecting the rotation of this polarization plane,
The recording signal can be reproduced.

However, since the above-mentioned θ _k is generally a very small angle of 0.5 ° to 1.5 °, in order to obtain a high quality reproduction signal, it is necessary to devise an apparently large angle.

Therefore, in the optical pickup device shown in FIG. 3, the surface 3a or 3b of the composite beam splitter 3 has a polarization characteristic to increase the apparent θ _k .

For example, the transmittance T _P of the P-polarized light component in the plane 3b 30%,
If the reflectance R _{P is set} to 70%, the transmittance T _S of the S-polarized component is set to 100%, and the reflectance R _S is set to 0%, as shown in FIG.
The P-polarized light component transmitted through 3b is reduced to 30%, but the S-polarized light component is not reduced, so the apparent Kerr rotation angle θ _k (0.5 °
To 1.5 °) is increased to _{θ 'k (1.7 ° ~5.0 °} ).

However, the optical pickup device as shown in FIG. 3 has drawbacks such as an increase in weight due to an increase in the number of parts and an increase in access time, resulting in a high cost.

Therefore, in recent years, it has been attempted to reduce the number of parts by using a diffraction element having a polarization characteristic.

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

In FIG. 6, the laser light emitted from the semiconductor laser 1 is focused on the magneto-optical disk 6 via the collimator lens 2, the beam splitter 14, the mirror 4 and the objective lens 5. The 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 the mirror 4. Thereafter, the reflected light is reflected at a right angle by the beam splitter 14, and the λ / 2 plate (1/2 wavelength plate) 19 rotates the 90 ° plane of polarization, and then the condenser lens 15
It is incident on the polarization diffraction element 16 via.

The polarization diffraction element 16 has a polarization characteristic because the lattice spacing is set to about the wavelength of light. 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 a servo signal.

The 0th-order diffracted light transmitted through the polarization diffraction element 16 is separated into two polarization components orthogonal to each other by the birefringent wedge-shaped optical element 17 and is incident on the photodetector 18 divided into two.
The recording signal on the magneto-optical disk 6 is detected.

On the other hand, the first-order diffraction diffracted by the polarization diffraction element 16 is incident on the multi-divided photodetector 20, and a tracking error signal and a focus error signal are obtained by calculating the output signals of the divided photodetectors. .

In the optical pickup device shown in FIG. 6, for example, the 0th-order diffraction efficiency of the S-polarized component is 30% and the 1st-order diffraction efficiency is
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 _k 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 and the servo signal generating function of the spot lens 7 and the cylindrical lens 8 in the device of FIG. A reduction in points is realized.

[Problems to be Solved by the Invention]

However, the sixth gap having a lattice spacing of about the wavelength of light
In the polarization diffractive element 16 of the figure, in the 0th-order or 1st-order diffracted light, between the polarization components of the P-polarized light and the S-polarized light,
Since a phase difference occurs due to the polarization characteristic in the grating, the polarized light becomes elliptically polarized light after passing through the polarization diffraction element 16, and the quality of the reproduced signal deteriorates.

Since the above-mentioned phase difference is caused by the polarization diffractive element 16 having the polarization characteristic, it is impossible to compensate the phase difference by optimizing the design of the polarization diffractive element 16 or the like. Note that, for example, the polarization diffraction element 16 and the birefringent wedge-shaped optical element
It is possible to compensate for the phase difference by inserting a phase compensating plate (not shown) between 17 and 17, but in that case, there is a problem that the number of parts increases.

[Means for solving the problem]

The polarization diffractive element according to the present invention is, in order to solve the above problems, in a polarization diffractive element in which a diffraction grating portion is provided on a flat plate-shaped substrate, the substrate is formed of a material having optical anisotropy, and , The phase difference between the P-polarized component and the S-polarized component of the diffracted light generated by the diffraction grating portion and the phase difference between the P-polarized component and the S-polarized component caused by the diffracted light propagating in the substrate cancel each other. The thickness of the substrate is set so as to meet each other.

The distance between the diffraction grating portions is preferably set to be substantially equal to the wavelength of the diffracted light.

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

Further, it is preferable that the substrate is made of a material that forms a uniaxial crystal.

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

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

The diffraction grating portion can be formed as a diffraction grating including a groove provided on the substrate.

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

The present invention also provides a light source, an optical system that guides a light beam from the light source onto the magneto-optical recording medium, and guides reflected light from the magneto-optical recording medium to a photodetector, and a magneto-optical system based on the Kerr rotation angle. In the optical pickup device including the photodetector for detecting a recording signal on a recording medium, the polarization diffraction element according to the present invention is arranged in the optical path of reflected light from the magneto-optical recording medium to the photodetector. It is characterized by being.

[Action]

In the above polarization diffractive element, a material having optical anisotropy is used as the material of the substrate, and therefore, the 0th or 1st order
When the next-order diffracted light propagates through the substrate, a phase difference occurs between the P-polarized component and the S-polarized component. Since this phase difference changes according to the propagation distance in the substrate, when it is necessary to eliminate the phase difference between the polarization components of P-polarized light and S-polarized light in the 0th-order diffracted light in the polarization diffraction element, The phase difference generated between the polarization components of the 0th order diffracted light due to the optical anisotropy of the substrate and the phase difference generated between the polarization components of the 0th order diffracted light due to the polarization characteristics of the diffraction grating portion cancel each other out. By setting the thickness of the above substrate, it is possible to compensate the phase difference between the polarization components of P-polarized light and S-polarized light generated in the 0th-order diffracted light in the diffraction grating portion without increasing the number of components.

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

In order to impart polarization characteristics to the polarization diffraction element,
The distance between the diffraction grating portions may be set to be substantially equal to the wavelength of the diffracted light.

Also, the substrate is formed of a material that forms a uniaxial crystal,
And, if the diffraction grating portion is provided in parallel with the optical axis, the polarization direction with respect to the optical axis does not change even if the light incident on the polarization diffraction element is refracted or diffracted, so that the design of the polarization diffraction element becomes easy, The largest polarization anisotropy can be obtained. As a result, the thickness of the substrate necessary for compensating 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, in the optical pickup device for the magneto-optical recording medium according to the present invention, since the above-mentioned polarization diffraction element according to the present invention is used, 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 diffractive element, the thickness of the substrate of the polarization diffractive element is adjusted so that there is no phase difference between the P-polarized light component and the S-polarized light component of the 0th-order diffracted light. Just decide. As a result, the 0th-order diffracted light that has passed through the polarization diffraction element becomes linearly polarized light, so that the recording signal can be accurately detected.

When the Kerr rotation angle is detected based on the first-order diffracted light of the polarization diffraction element, the P-polarized light and the S-polarized light in the first-order diffracted light are detected.
It suffices to determine the thickness of the substrate of the polarization diffraction element so that a phase difference does not occur between the polarization components of the polarized light.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

The optical pickup device according to the present embodiment is basically the same as the conventional optical pickup device shown in FIG. 6 except that the polarization diffractive element 21 shown in FIG. It is constructed similarly to the example. Therefore, a detailed description of the optical pickup device itself is omitted here.

FIG. 1 showing the polarization diffraction element 21 of the present embodiment does not exactly correspond to FIG. 6 in the orientation of the drawing, but the condenser lens 15
Is guided to the polarization diffraction element 21 along the direction of arrow A, and then the 0th-order diffracted light B of the polarization diffraction element 21 is guided to the photodetector 18 via the birefringent wedge-shaped optical element 17, Thus, the recording signal on the magneto-optical disk 6 as the magneto-optical recording medium is detected. On the other hand, polarization diffraction element
The 21st-order diffracted light C of 21 is guided to the photodetector 20, where a tracking error signal and a focus error signal are obtained.

The substrate 22 of the polarization diffraction element 21 is made of a material that forms a uniaxial crystal, for example, quartz. As shown also in FIG. 2, on the surface of the substrate 22 on the side of the magneto-optical disk 6, diffraction gratings 23, 23 ... Each having a rectangular cross section having a predetermined depth t and width are formed as diffraction grating portions. Has been formed.

The diffraction gratings 23, 23 ... Are formed so that their grating lines are parallel to the optical axis D of the substrate 22 extending in the direction orthogonal to the plane of the drawing. As described above, a material forming a uniaxial crystal is used as the substrate 22, and the diffraction grating 23 is parallel to the optical axis D thereof.
When 23 is formed, the polarization direction with respect to the optical axis D does not change even if the light incident on the polarization diffraction element 21 is refracted or diffracted, which facilitates the design of the polarization diffraction element 21.
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 light used for recording or reproduction in order to impart polarization characteristics, and is preferably 0.
It is set to about 5 to 2 times. For example, when the wavelength of the laser light is 0.8 μm, the grating interval d is 0.5 μm and the diffraction grating 23.
If the depth t of each groove forming 23 ... Is 0.6 μm, the 0th-order diffraction efficiency η _0S of the S-polarized component is 0.3, the 1st-order diffraction efficiency η _1S is 0.7,
The 0th-order diffraction efficiency η _0P of the P-polarized component is 1.0, and the 1st-order diffraction efficiency η
_1P becomes 0. This makes it possible to increase the apparent Kerr rotation angle in the 0th-order diffracted light B, 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 the P-polarized light and the S-polarized light of the 0th-order and 1st-order diffracted lights B and C. In this embodiment, since the recording signal on the magneto-optical disk 6 is reproduced by the 0th-order diffracted light B, it is necessary to compensate the phase difference of each polarization component of the 0th-order diffracted light B.

Therefore, the thickness T of the substrate 22 of the polarization diffraction element 21 is P polarization and S polarization in the 0th order diffracted light B generated by the diffraction gratings 23.
The phase difference between the polarized components of the polarized light and the 0th-order diffracted light B
It is set such that the phase difference between the P-polarized light component and the S-polarized light polarized component of the zero-order diffracted light B generated by propagating through the light beam 22 has a value that cancels each other.

That is, since the substrate 22 has an optical anisotropy, 0 P-polarized component of the diffracted light B becomes ordinary light, but feel the refractive index n _o, 0 S-polarized component of the diffracted light B becomes extraordinary light refractive index
Feel n _e (≠ n _o ). For example, if the substrate 22 is quartz,
n _o = 1.52 and n _e = 1.48.

In that case, when the 0th-order diffracted light B propagates through the substrate 22 by the length L, the phase difference ΔΨ _L (rad) between the polarization components in the P direction and the S direction due to the optical anisotropy of the substrate 22 is Becomes Therefore, by adjusting the thickness T of the substrate 22, the above ΔΨ _L
, And the sum of the phase difference ΔΨ _G between the P-polarized light components and the S-polarized light components generated by the diffraction gratings 23 ··· is nπ (n is an integer), and they may be offset each other. As a result, no phase difference occurs between the P-polarized light component and the S-polarized light component of the 0th-order diffracted light B that has passed through the polarization diffraction element 21, so that the 0th-order diffracted light B becomes linearly polarized light, and the recording signal based on the Kerr rotation angle The detection can be performed with high accuracy and a high quality recording signal can be obtained.

In the above-mentioned embodiment, the diffraction gratings 23, 23 ... Each having a rectangular cross-section are formed on the surface of the substrate 22 on the side of the magneto-optical disk. However, the diffraction grating portion is formed of, for example, Na ⁺ , K on the substrate. ⁺ , A
By implanting impurities such as g ⁺ , a refractive index distribution type diffraction grating having a refractive index different from that of the rest of the substrate may be used. Also in this case, it is preferable that the grating line of the gradient index diffraction grating is parallel to the optical axis D, and the grating interval is set substantially equal to the wavelength of the laser light.

〔The invention's effect〕

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 plate-shaped substrate, wherein the substrate is formed of a material having optical anisotropy, and The phase difference between the P-polarized component and the S-polarized component of the diffracted light generated by the portion and the phase difference between the P-polarized component and the S-polarized component caused by the diffracted light propagating in the substrate cancel each other out. The thickness of the substrate is set.

As a result, since the material having the optical anisotropy is used as the material of the substrate, when the 0th-order or 1st-order diffracted light propagates in the substrate, the difference between the P-polarized component and the S-polarized component is generated. There will be a phase difference. Since this phase difference changes according to the propagation distance in the substrate, when it is necessary to eliminate the phase difference between the polarization components of P-polarized light and S-polarized light in the 0th-order diffracted light in the polarization diffraction element, The phase difference generated between the polarization components of the 0th order diffracted light due to the optical anisotropy of the substrate and the phase difference generated between the polarization components of the 0th order diffracted light due to the polarization characteristics of the diffraction grating portion cancel each other out. By setting the thickness of the above substrate, it is possible to compensate the phase difference between the polarization components of P-polarized light and S-polarized light generated in the 0th-order diffracted light in the diffraction grating portion without increasing the number of components.

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

In order to impart polarization characteristics to the polarization diffraction element,
The distance between the diffraction grating portions may be set to be substantially equal to the wavelength of the diffracted light.

Also, the substrate is formed of a material that forms a uniaxial crystal,
And, if the diffraction grating portion is provided in parallel with the optical axis, the polarization direction with respect to the optical axis does not change even if the light incident on the polarization diffraction element is refracted or diffracted, so that the design of the polarization diffraction element becomes easy, The largest polarization anisotropy can be obtained. As a result, the thickness of the substrate necessary for compensating 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 guides a light source and a light beam from the light source onto the magneto-optical recording medium,
In an optical pickup device including an optical system that guides reflected light from the magneto-optical recording medium to a photodetector, and the photodetector that detects a recording signal on the magneto-optical recording medium based on a Kerr rotation angle, The polarization diffraction element according to the present invention is arranged in the optical path of the reflected light from the magneto-optical recording medium to the photodetector.

Accordingly, when the recording signal is detected 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, the P-polarized light and the S-polarized light in the 0th-order diffracted light are detected. It suffices to determine the thickness of the substrate of the polarization diffraction element so that a phase difference does not occur in each polarization component. As a result, the 0th-order diffracted light transmitted through the polarization diffractive element becomes linearly polarized light, so that the recording signal can be accurately detected.

When the Kerr rotation angle is detected based on the first-order diffracted light of the polarization diffraction element, the P direction and S in the first-order diffracted light are detected.
The thickness of the substrate of the polarization diffraction element may be determined so that a phase difference does not occur between the polarization components in the directions.

[Brief description of drawings]

1 and 2 show an embodiment of the present invention. FIG. 1 is a sectional view illustrating a polarization diffraction element. FIG. 2 is an enlarged vertical sectional view of the diffraction grating portion. 3 to 7 show a conventional example. FIG. 3 is an explanatory diagram showing an example of the optical pickup device. 4 and 5 are explanatory views showing the principle of detection of a recording signal based on the Kerr rotation angle. FIG. 6 is an explanatory view showing another optical pickup device. FIG. 7 is a schematic plan view showing a grating pattern of the polarization diffraction element. Reference numeral 21 is a polarization diffraction element, 22 is a substrate, and 23 is a diffraction grating (diffraction grating portion).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukio Kurata 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Within Sharp Corporation (56)

Claims (9)

[Claims] 1. A polarization diffractive element comprising a plate-shaped substrate provided with a diffraction grating portion, wherein the substrate is made of a material having optical anisotropy, and a P-polarized component of diffracted light generated by the diffraction grating portion. The thickness of the substrate is set so that the phase difference between the S-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 diffracted light propagating in the substrate cancel each other out. A polarization diffractive element characterized in that 2. The polarization diffraction element according to claim 1, wherein the distance between the diffraction grating portions is set to be substantially equal to the wavelength of the diffracted light. 3. The distance between the diffraction grating portions is 0.
The polarization diffractive element according to claim 2, wherein the polarization diffractive element is set in a range of 5 times to 2 times. 4. The polarization diffraction element according to claim 1, wherein the substrate is made of a material that forms 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, wherein the diffraction grating portion is formed parallel to the optical axis. 7. The polarization diffraction element according to claim 1, wherein the diffraction grating portion is a diffraction grating formed of a groove provided on a substrate. 8. The refractive index distribution diffraction grating formed by making the refractive index of the diffraction grating portion different from that of the remaining portion of the substrate, as claimed in any one of claims 1 to 3. The polarization diffractive element as described in 1. 9. A light source, an optical system for guiding a light beam from the light source onto a magneto-optical recording medium and guiding reflected light from the magneto-optical recording medium to a photodetector, and a magneto-optical system based on the Kerr rotation angle. An optical pickup device comprising the photodetector for detecting a recording signal on a recording medium, wherein the reflected light from the magnetooptical recording medium to the photodetector is in the optical path. An optical pickup device comprising the polarization diffraction element according to any one of the above.