JPH1090518A - Polarized light separating element and optical pickup device formed by using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarized light separating element which is good in mass productivity and is inexpensive and optical pickup device formed by using the same. SOLUTION: This polarized light separating element is formed by joining three sheets of parallel flat plates consisting of optical anisotropic crystalline bodies. The optical axes of the first, second and third crystalline bodies are nonparallel with the normal of the incident surface of the first crystalline body. A vector d2 and a vector d3 intersect orthogonally with each other and satisfy (vector d1)+(vector d3)=(vector d2) when the polarized light sepn. in the first crystalline body is defined as the vector d1, the polarized light sepn. in the second crystalline body as vector d2 and the polarized light sepn. in the third crystalline body as vector d3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup device for reproducing a signal recorded on a magneto-optical recording medium, and more particularly to a polarization plane of reflected light from the magneto-optical recording medium using an analyzer. The present invention relates to an optical pickup device for detecting a signal obtained as a rotation of the optical pickup.

[0002]

2. Description of the Related Art In recent years, magneto-optical recording media have been used in various fields as rewritable recording media having a large storage capacity.

[0003] Such a magneto-optical recording medium is provided with a recording layer (perpendicular magnetization film) made of a TbCo-based ferromagnetic alloy and a protective layer made of polycarbonate or the like in this order on a disk-shaped substrate. A guide groove for tracking is spirally provided on the surface of the recording layer. In this magneto-optical recording medium, the signal is transmitted in the magnetization direction (vertically upward or vertically downward).
In order to read the recorded information, it is necessary to detect the magnetization direction.

Generally, to detect the magnetization direction, a recording layer portion magnetized vertically upward or vertically downward is irradiated with a light beam, and the plane of polarization of the reflected light due to the magnetic Kerr effect (optical axis of the light beam) is detected. And rotation (a plane including the vibration direction) are detected (rotate clockwise when the incident direction is the same as the magnetization direction, and counterclockwise when the magnetization direction is the opposite). As an optical element used to detect the rotation of the polarization plane, Japanese Patent Publication No.
A Wollaston prism disclosed in JP-A-195522 or JP-B-6-77351 is known.

[Explanation Regarding Structure of Optical Pickup Device] An optical pickup device using the Wollaston prism will be described with reference to FIG. In the figure, a light beam generated from a semiconductor laser 51 is
Are separated into zero-order light for signal reproduction and focus servo, and ± first-order light for tracking servo, respectively, and a collimator lens 53, a polarizing beam splitter 54 (light beam is reflected in the direction of the objective lens 55) and an objective lens. 5
Irradiation is performed on the surface of the recording layer of the magneto-optical recording medium 56 via the optical recording medium 5. In this way, the reflected light of the light beam applied to the surface of the recording layer has its polarization plane rotated in accordance with the magnetization direction of the recording layer, and the objective lens 55 and the polarizing beam splitter 54 (the light beam is directed in the direction of the lens 57). Through the lens 57, the cylindrical lens 58, and the Wollaston prism 61, and is condensed on the light receiving surface 60a of the light receiving element 60 and converted into a voltage signal.

Here, the reflected light incident on the Wollaston prism 61 is split into three light beams (separated into two signal reproducing light beams and one focus servo light beam). The intensity is detected by the light receiving element 60. Since the separation ratio (light intensity ratio) of two signal reproduction light beams among the three light beams changes due to the rotation of the plane of polarization of the reflected light, the separation ratio (light intensity ratio) is detected. The magnetization direction, which is a recorded signal, can be detected.

The cylindrical lens 58 is an optical element that causes astigmatism, and when the light receiving surface 60a of the light receiving element 60 is at the focal position, FIG.
As shown in the figure, a circular spot 71 is formed on the light receiving surface 60a.
Are formed, and if the focus position is shifted, elliptical spots 72 and 73 are formed as shown in FIGS. Therefore, the focus can be adjusted by adjusting the spot shape of the focus servo light beam provided with the astigmatism by the cylindrical lens so as to be circular (generally called an astigmatism method). ).

[Description of Wollaston Prism] Next, FIG. 11 and FIG. 12 show the Wollaston prism.
This will be described in detail with reference to FIG.

FIG. 11A is a perspective view of a Wollaston prism, which is composed of a first crystal body 82 and a second crystal body 83. The first crystal body 82 and the second crystal body 82
In the crystal body 83, lithium niobate, quartz, rutile,
A uniaxial crystal such as calcite is used, and the optical axis 82a of the first crystal 82 and the second crystal 8 when viewed from the incident surface S1.
The angle formed by the third optical axis 83a is non-perpendicular (hereinafter, the angle formed by the optical axis 82a of the first crystal 82 and the optical axis 83a of the second crystal 83 is non-perpendicular) An existing Wollaston prism is called a three-beam Wollaston prism.)

FIG. 11B shows three light beams 8 on the exit surface S3 when a light beam 84 is incident on the three-beam Wollaston prism 81 (incident from the incident surface S1 side).
The process of separation into 6, 87 and 88 is shown (FIG. 11A is viewed from the direction A). Here, the light beam 84 incident from the incident surface S1 is an ordinary light component (light having a polarization plane orthogonal to the optical axis 82a of the first crystal body 82) and an extraordinary light component (first crystal) in the first crystal body 82. Light having a polarization plane parallel to the optical axis 82a of the body 82). The ordinary light component and the extraordinary light component travel along the same traveling path 85 in the first crystal 82, and in the second crystal 83, the respective polarization components are orthogonal to the optical axis 83 a of the second crystal 83. The light is separated again into a polarized light component having a plane of polarization and a parallel polarized light component.

That is, the three-beam Wollaston prism 8
The light beam passing through 1 is separated into the following four components (hereinafter, the refractive index felt by the ordinary light component is no and the refractive index felt by the extraordinary light component is ne).

(A) No is felt in the first crystal, and
(B) A light beam that felt no in the first crystal and a light beam that felt ne in the second crystal. (C) A light beam that felt no in the second crystal. Light beam that felt and felt no in the second crystal. (D) Light beam that felt ne in the first crystal and felt ne in the second crystal. (A), (D) The light beam of (a) travels along the same traveling path and becomes a light beam 87 for focus servo (becomes one light beam). One of the light beams (B) and (C) becomes a light beam 86 for signal reproduction, The other is a light beam 88 for signal reproduction.

FIG. 12 is an explanatory diagram for explaining the separation ratio of the three light beams. FIG. 11A shows a case where the three-beam Wollaston prism 81 of FIG. 11A is viewed from the incident surface S1 side (FIG. a) when viewed from the direction B).

[1] Regarding the angle between the oscillation direction of the light beam and the optical axis of the crystal (FIG. 12 (a)) 94a indicates the direction of linear polarization of the light beam incident on the three-beam Wollaston prism, and 92a Denotes the optical axis of the first crystal, and 93a denotes the optical axis of the second crystal. here,
The angle formed between the direction 94a of the linear polarization of the incident light beam and the optical axis 92a of the first crystal is α, and the angle formed between the optical axis 92a of the first crystal and the optical axis 93a of the second crystal. Is β. The optical axis 92a of the first crystal and the optical axis 93a of the second crystal are parallel to the entrance surface S1 and the exit surface S3.

[2] Separation into an ordinary light component and an extraordinary light component in the first crystal (FIG. 12B) The light beam entering the three-beam Wollaston prism is:
An extraordinary light component 95a having a plane of polarization parallel to the optical axis 92a of the first crystal and an ordinary light component 95a 'having a plane of polarization perpendicular to the optical axis 92a of the first crystal are separated. Here, assuming that the amplitude of the light beam incident on the three-beam Wollaston prism is 1, the amplitude Ae of the light of the extraordinary light component 95a and the amplitude Ao of the light of the ordinary light component 95a 'are obtained by the following equations.

Ao = sinα Ae = cosα Note that the extraordinary light component 95a and the ordinary light component 95a ′ travel on the same traveling path in the first crystal.

[3] Separation into an ordinary light component and an extraordinary light component in the second crystal (FIGS. 12 (c) and 12 (d)) The light beam of the extraordinary light component 95a separated by the first crystal is In the second crystal, an extraordinary light component 96a having a plane of polarization parallel to the optical axis 93a of the second crystal and an ordinary light component 96a 'having a plane of polarization perpendicular to the optical axis 93a of the second crystal (((c )), The light beam of the ordinary light component 95a '
The extraordinary light component 97a having a plane of polarization parallel to the optical axis 93a of the second crystal and the ordinary light component 97a 'having a plane of polarization perpendicular to the optical axis 3a of the second crystal ((d)) See again) and separate again. Accordingly, the amplitudes ((A) to (D)) of the four lights are obtained by the following equations.

Aoo = Aocosβ = sinα · cosβ Aoe = Aosinβ = sinα · sinβ Aeo = Aesinβ = cosα · sinβ Aee = Aecosβ = cosα · cosβ Aoo: No is felt in the first crystal, and in the first crystal, Aoe: the amplitude of the light beam that felt no in Aoe: the amplitude of the light beam that felt no in the first crystal, and felt ne in the first crystal Aeo: in the first crystal The amplitude of the light of the light beam that senses ne and feels no in the first crystal body Aee: the light amplitude of the light beam that senses ne in the first crystal body and senses ne in the first crystal body Amplitude Accordingly, the separation ratio (ratio of light intensity) of the three light beams is obtained by the following equation.

A: b: c = (sin ² α · sin ² β): (cos ² β): (cos ² α · sin ² β) (1) a, c: of the light beam for signal reproduction Light intensity b: Light intensity of light beam for focus servo In the above equation 1, the separation ratio of two light beams for signal reproduction (a:
c) The sin ² alpha: obtained by cos ² alpha, the segregation ratio (a: c)
Changes according to the rotation of the plane of polarization of the light beam (change in α). Therefore, the rotation of the plane of polarization can be detected by detecting a change in the separation ratio (a: c) of the two signal reproducing light beams.

When used in a pickup device, α is usually set to 45 ° in many cases.
The separation ratio (a: b: c) of the two light beams is obtained by the following equation.

A: b: c = (sin ² β) :( ² cos ² β) :( sin ² β) (2) As can be seen from the above equation 2, the signal reproducing light beam and the focus servo are used. The light beam separation ratio (a: b) is determined by β. Therefore, in order to change the separation ratio (a: b) between the signal reproducing light beam and the focus servo light beam, the angle between the optical axis 92a of the first crystal and the optical axis 93a of the second crystal is changed. I need to change.

As described above, even if a three-beam Wollaston prism is not used, a conventionally known two-beam Wollaston prism (the optical axis of the first crystal and the second crystal) can be used. The optical pickup device can also be configured using a Wollaston prism whose angle with the optical axis is a right angle. However, in this case, a beam splitter is provided in the optical path between the objective lens and the Wollaston prism to separate the light beams, and one of the light beams is used as a signal reproducing light beam and the other is used as a signal reproducing light beam. The optical pickup device must be used for a focus servo light beam, and the optical pickup device becomes large and complicated.

As another method, a light beam separated by a two-beam Wollaston prism is used as a light beam for signal reproduction, and at the same time, one or both light beams are used as light beams for focus servo. It can also be used. However, in this case, the accuracy of the focus servo signal decreases, and the quality of the reproduction signal deteriorates accordingly.

Therefore, the two-beam Wollaston prism described above tends not to be used in an optical pickup device for magneto-optical recording and reproduction in recent years.

As disclosed in Japanese Patent Application Laid-Open No. 63-247938, a method using a Savart plate or a birefringent plate is also different from the above two beam wollaston in that only two light beams can be obtained. This is similar to the case of the prism, and involves the enlargement of the optical pickup device and the deterioration of the signal quality, which is different from the optical pickup device according to the present invention.

[0026]

However, in the case of manufacturing a three-beam Wollaston prism, a complicated process of polishing and joining two crystals cut in different directions is required. It has been difficult to manufacture a three-beam Wollaston prism having excellent characteristics stably at low cost. Also, since the three-beam Wollaston prism joins crystals having inclined surfaces, one optical element is joined after joining a large-area crystal like an optical element joining parallel plate crystals. And the mass productivity was poor.

Since the three-beam Wollaston prism has a small separation angle, a desired separation distance can be secured by shortening the distance between the three-beam Wollaston prism and the light receiving element when used in an optical pickup device. Can not be done. Therefore, it has been difficult to reduce the size of the optical pickup device using the three-beam Wollaston prism.

Therefore, the present invention provides an inexpensive polarization separation element which can separate a light beam into three beams with a simple configuration, is mass-producible and is inexpensive, and an optical pickup device using the same. The purpose is to simplify the structure, reduce the cost, and reduce the number of manufacturing steps.

[0029]

According to a first aspect of the present invention, there is provided a polarization beam splitting element comprising three parallel flat plates made of an optically anisotropic crystal joined to each other. The optical axis of the crystal is not parallel to the normal to the incident surface of the first crystal, and the optical axis of the light beam incident on the first crystal from the surface on the incident surface is on the surface on the second crystal. The displacement from the arrival point of the ordinary ray to the arrival point of the extraordinary ray in the vector d1, the third crystal of the light beam incident on the second crystal from the surface on the first crystal side.
The displacement from the arrival point of the ordinary ray to the arrival point of the extraordinary ray on the surface of the crystal on the side of the vector d2 is defined by the vector d2, and the displacement on the exit surface side of the light beam incident on the third crystal from the surface on the second crystal side When the displacement from the arrival point of the ordinary ray to the arrival point of the extraordinary ray on the surface is vector d3, vector d2 and vector d3 are orthogonal, and (vector d1) + (vector d3) = (vector d
2) is satisfied.

According to a second aspect of the present invention, there is provided a polarization beam splitting element in which three parallel flat plates made of an optically anisotropic crystal are bonded, wherein the optical axes of the first, second, and third crystals are provided. But,
A light beam that is non-parallel to the normal to the incident surface of the first crystal and is incident on the first crystal from the incident surface side from the arrival point of the ordinary ray on the second crystal side surface of the light beam The displacement of the extraordinary ray to the arrival point is vector d1, and the extraordinary ray from the arrival point of the ordinary ray on the third crystal side of the light beam incident on the second crystal from the surface on the first crystal side. Is a vector d2, the displacement from the arrival point of the ordinary ray to the arrival point of the extraordinary ray on the surface on the emission surface side of the light beam incident on the third crystal from the surface on the second crystal side Is the vector d3
In this case, the vector d2 and the vector d3 are orthogonal, and (vector d1) + (vector d2) = (vector d
3) is satisfied.

According to the third aspect of the present invention, there is provided a polarization beam splitting element.
In the polarized light separating element described above, an angle β between the vector d1 and the vector d2 is 19.5 ° ≦ β ≦ 54.7 °.

According to a fourth aspect of the present invention, there is provided a polarization beam splitting element.
In the polarization separation element described above, an angle β between the vector d1 and the vector d2 is 35.3 ° ≦ β ≦ 70.5 °.

The optical pickup device according to claim 4 is
An optical pickup device using the polarization separation element according to any one of claims 1 to 4.

[0034]

The polarizing beam splitter of the present invention can split a light beam into three beams by the above-mentioned structure. Further, in the polarization beam splitter of the present invention, since the three separated light beams are parallel to each other, the adjustment of the optical system becomes easy when used in an optical pickup device.

[0035]

【Example】

(Embodiment 1) A method of separating a light beam into three beams using the polarization beam splitting element of the present invention will be described with reference to FIGS.

FIG. 1A is a perspective view of the polarization beam splitter of the present invention, and FIG. 1B is an explanatory diagram for explaining the direction of the optical axis of a crystal constituting the polarization beam splitter. The polarization splitting element shown in FIG. 1 is an optical element in which a first crystal 1, a second crystal 2, and a third crystal 3 are bonded. Is a parallel plate made of an optically anisotropic material exhibiting birefringence.

In the present embodiment, lithium niobate crystals are used as the first crystal 1, the second crystal 2, and the third crystal 3, but optically different materials such as calcite, rutile, and quartz are used. An isotropic material can also be used. A birefringent material having a large difference between the refractive index no perceived by ordinary rays and the refractive index ne perceived by extraordinary rays has a longer polarization separation distance, and is desirable. Is small, only the separation distance is reduced, and the effect of separating polarized light can be obtained.

FIG. 2 is an explanatory diagram for explaining a polarization separation distance in the optically anisotropic material, and shows one surface of a parallel plate (crystal 4) having a thickness t made of the optically anisotropic material. 3 shows a light beam 30 that is perpendicularly incident from the surface, and the light beam 30 is polarized and separated from the other surface into an ordinary ray 30a and an extraordinary ray 30b. Here, d in the figure is the crystal 4 of the optically anisotropic material.
The polarization separation distance in a plane including the optical axis 10 and the optical axis of the light beam 30 is shown.

This polarization separation distance can be obtained by the following equation, where α is the angle between the optical axis 10 of the crystal 4 made of an optically anisotropic material and the optical axis of the light beam 30 (where no is The refractive index felt by ordinary rays is shown, and ne represents the refractive index felt by extraordinary rays).

D = t × (ne−no) sin α × cos α / (ne cos α + no sin α) (3) where t represents the thickness of the crystal and the polarization separation of the present embodiment shown in FIG. In the element, the thickness of the first crystal 1 is set to 2.4.
The thickness of each of the second crystal body 2 and the third crystal body 3 was 1.74 mm. Also, as can be seen from Equation 3, since the polarization separation distance d changes depending on the value of α, d
Must be set so that d is maximized. When α = 45 ° with the lithium niobate used in this example, the separation distance between the ordinary ray and the extraordinary ray in the light beam having a wavelength of 780 nm is 1 m.
36 μm per m.

Next, the directions of the optical axes of the first crystal body 1, the second crystal body 2, and the third crystal body 3 constituting the polarization splitting device of the present embodiment shown in FIG. 1 will be described. . Here, as shown in FIG. 1B, the direction of the optical axis is X
Axis and the Y axis in a plane parallel to the incident surface 21, and the Z axis in the incident surface 2.
The angle α between the optical axis 10 and the Z-axis
(0 ° <α <90 °), and the angle β between the projection 10a of the optical axis on the XY plane and the X-axis (0 ° ≦
β ≦ 180 °).

The directions of the optical axis 11 of the first crystal 1, the optical axis 12 of the second crystal 2, and the optical axis 13 of the third crystal 3 shown in FIG. It is set as follows. In the following description, β of the optical axis 12 is set to β
1. Let β of the optical axis 13 be β2.

The direction (α, β) of the optical axis 11 of the first crystal 1: (45 °, 0 °) The direction (α, β) of the optical axis 12 of the second crystal 2: (45 °, 45 °) Direction of the optical axis 13 of the third crystal body 3 (α, β): (45 °, 135 °) Subsequently, three light beams incident on the polarization splitting element are described with reference to FIGS. The process of splitting into light beams is described.
Here, FIG. 3 is a perspective view showing a polarization separation process, and FIG. 4 is a view showing each surface ((a) incident surface 21 and (b)) of the polarization separation element.
Bonding surface 22 of first crystal 1 and second crystal 2, (c)
Bonding surface 23 between second crystal body 2 and third crystal body 3, (d)
It shows polarization separation at the exit surface 24). FIG.
As described in, the polarization separation distance depends on the value of α,
The direction of polarization separation is in the direction of the optical axis viewed from the normal direction of the incident surface.

Hereinafter, FIG. 4A shows an incident surface 21, FIG. 4B shows a bonding surface 22 between the first crystal 1 and the second crystal 2, and FIG. 4C shows a second crystal 3 and the third crystal. The polarization separation process will be described in the order of the bonding surface 23 of the body 3 and the (d) emission surface 24.

[1] Description of FIG. 4 (a) FIG. 4 (a) shows the direction 41 of the linear polarization of the light beam on the incident surface 21 in FIG.
Optical axis 11a of the first crystal, the optical axis 12a of the second crystal, and the optical axis 13 of the third crystal as viewed from the
a. In the figure, δ (in the counterclockwise direction, 0 ° ≦ δ ≦
180 °) is the angle between the direction 41 of the linear polarization of the light beam and the optical axis 11a of the first crystal, and β1 is the optical axis 11a of the first crystal and the optical axis of the second crystal. Β2 is the angle between the optical axis 11a of the first crystal and the third
Shows the angle formed by the optical axis 13a of the crystal of FIG. In the polarization splitting element of the present invention, β2 = β1 + 90 °.

[2] Description of FIG. 4B FIG. 4B shows the first crystal 1 and the second crystal 2 shown in FIG.
2 shows polarization separation at the bonding surface 22 of FIG. As shown in the figure, the light beam traveling in the first crystal and reaching the bonding surface 22 is an extraordinary ray whose polarization plane is parallel to the ordinary ray 42 perpendicular to the optical axis 11a of the first crystal. At 43, the light is separated along the optical axis 11a.

That is, in FIG. 3, the light beam incident from P11 is separated into an ordinary ray and an extraordinary ray in the first crystal.
The ordinary ray goes straight to P21 on the joint surface 22, and the extraordinary ray is refracted and reaches P22 on the joint surface 22. Therefore, joining surface 2
In 2, the light is split into two light beams.

The amplitude A of the light beam reaching P21
, P22 is given by the following equation.

A = SINδ A = COSδ [3] Description of FIG. 4 (c) FIG. 4 (c) shows the first crystal 2 and the second crystal 3 of FIG.
3 shows polarization separation at the bonding surface 23 of FIG. As shown in the figure, of the light beam traveling in the second crystal and reaching the bonding surface 23, the light beam traveling in the first crystal as the ordinary ray 42 has a polarization plane of the second ray. The light beam traveling as an extraordinary ray 43 in the first crystal to an extraordinary ray 45 parallel to the ordinary ray 44 perpendicular to the optical axis 12a of the crystal, has a polarization plane of the optical axis 12a of the second crystal. Along the optical axis 12a.

The displacement in this specification is a separation distance when an extraordinary ray traveling in the crystal is separated from the ordinary ray as in the case of the ordinary ray 46 and the extraordinary ray 47, for example. The direction of the displacement is a direction perpendicular to the traveling direction of the ordinary ray.

That is, in FIG. 3, the light beam reaching P21 is separated into an ordinary ray and an extraordinary ray in the second crystal, and the ordinary ray goes straight to P31 on the bonding surface 23, and the extraordinary ray is refracted. Then, it reaches P32 on the joining surface 23. The light beam that has reached P22 is separated into an ordinary ray and an extraordinary ray in the second crystal, and the ordinary ray goes straight to P33 on the joining surface 23, and the extraordinary ray is refracted and becomes P34 on the joining surface 23. Reach Therefore, the light is split into four light beams at the bonding surface 22.

The light beam reaching P31 (FIG. 4C)
, The amplitude A of the light beam that has reached P32 (the extraordinary ray 45 in FIG. 4C), and the amplitude A of the light beam that has reached P33 (the ordinary ray 46 in FIG. 4C).
, P34 (the extraordinary ray 4 in FIG. 4C).
The amplitude A of 7) is given by the following equation.

A = A SIN (90 ° −β1) = SINδ · COSβ1 A = A COS (90 ° −β1) = SINδ · SINβ1 A = A SINβ1 = COSδ · SINβ1 A = A COSβ1 = COSδ · COSβ1 [4] Description of FIG. 4 (d) FIG. 4 (d) shows polarization separation at the exit surface 24 of FIG. In the polarization separation element of the present invention, since β2 = β1 + 90 °, the light beam that has traveled in the second crystal as an ordinary ray travels in the third crystal as extraordinary light, and travels in the second crystal. The light beam that has traveled as an extraordinary ray travels through the third crystal as ordinary light. Therefore, the ordinary ray 44 of the joining surface 23 becomes an extraordinary ray 44 ′ at the exit surface 24, the extraordinary ray 45 of the joining surface 23 becomes an ordinary ray 45 ′ at the exit surface 24, and the ordinary ray 46 of the joining surface 23 becomes An extraordinary ray 46 ′ at the exit surface 24, and an extraordinary ray 47 at the joint surface 23
4 becomes an ordinary ray 47 '.

That is, in FIG. 3, the light beam (ordinary ray 44 in FIG. 4C) that has reached P31 is refracted at the joint surface 23,
The light beam (the extraordinary ray 45 in FIG. 4 (c)) that travels as an extraordinary ray in the third crystal and reaches P41 on the emission surface 24 and reaches P32 is regarded as an ordinary ray in the third crystal. The light beam (ordinary ray 46 in FIG. 4 (c)) that travels to P42 on the exit surface 24 and reaches P33 is refracted at the joint surface 23,
The light beam (the extraordinary ray 47 in FIG. 4 (c)) that travels through the third crystal as an extraordinary ray and reaches P at the emission surface 24 and reaches P34 is regarded as an ordinary ray inside the third crystal. The light advances to reach P43 on the emission surface 24. Here, the light beam that has reached P32 (the extraordinary ray 45 in FIG. 4C) and P33
Since the light beam (ordinary ray 46 in FIG. 4 (c)) that has reached both exits from P42 on the exit surface 24, the three light beams exiting from the polarization separation element exit from P41, P42 and P43. Light beam.

In order for the light beam reaching P32 and the light beam reaching P33 to both exit from P42 on the exit surface 24, the polarization separation distance d1, d1 in the first crystal body,
The polarization separation distance d2 in the second crystal and the polarization separation distance d3 in the third crystal must satisfy the following expression.

D1 = d2 + d3 (4) That is, when a triangle having three sides d1, d2, and d3 is considered, the angle between the side d1 and the side d2 is β1.
Then, the angle between the side d2 and the side d3 becomes a right-angled right triangle. Note that the light beams P41, P
42 and the separation distance between P42 and P43 are
It is equal to d1.

When the polarization separation in the first, second and third crystals is considered as a vector, a vector d1 (P21 to P22) indicating the polarization separation in the first crystal is considered.
, A vector d2 indicating the polarization separation in the second crystal (displacement from P31 to P322), and a vector d3 indicating the polarization separation in the third crystal (displacement from P33 to P32) Satisfies the following equation.

(Vector d1) + (vector d3) = (vector d2) (5) Further, the separation ratio (a: b: c) of the three light beams emitted from P41, P42 and P43 is: It is given by the following equation.

A: b: c = (SIN δ · COS β1) :( SIN β1) :( COS δ · COS β1) (6) In the above formula 6, separation of the light beams emitted from P41 and P43. The ratio (a: c) is obtained as SIN ² δ: COS ² δ, and the separation ratio (a: c) changes according to the rotation of the plane of polarization of the light beam (change in δ). Therefore, when used in an optical pickup device, rotation of the plane of polarization can be detected by detecting a change in the separation ratio (a: c) of the light beam.

Normally, when the polarization separation element is used in an optical pickup device, δ is set to 45 °. In this case, the separation ratio (a: b: c) of the three light beams is given by the following equation.

A: b: c = (COS β1) :( 2 SIN β1) :( COS β1) (7) As can be understood from the above equation (7), the light beam and the side ( The separation ratio (a: b) of the light beams of P41 and P43) is determined by β1. Therefore, by changing β1, the separation ratio between the light beam for signal reproduction (P41, P43) and the light beam for focus servo (P42) when used in an optical pickup device can be adjusted.

(Embodiment 2) The polarized light separating element of this embodiment is
As shown in FIG. 5, in the polarization separation element of the first embodiment,
The first crystal is rotated by 180 ° about the normal to the plane of incidence.

In this case, as shown in FIG. 6A, when a light beam having the same linear polarization direction 41 as that in the first embodiment is incident, the light beam becomes as shown in FIG. Then, on the joining surface 22, the light is separated into an ordinary ray 42 and an extraordinary ray 43 along the optical axis 11a. However, as shown in FIG. 5, the optical axis 11 of the first crystal body 1 constituting the polarization beam splitting element of the present embodiment.
Is set such that the optical axis of the first crystal in the polarization beam splitter of Example 1 is rotated by 180 ° about the normal to the plane of incidence, so that the direction in which the extraordinary ray 43 is separated is The opposite is the case with Example 1.

As shown in FIG. 6C, the ordinary ray 42 on the joining surface 22 is separated along the optical axis 11a into an ordinary ray 44 and an extraordinary ray 45 on the joining face 23, and The extraordinary ray 43 at the joining surface 23 separates into an ordinary ray 46 and an extraordinary ray 47 along the optical axis 11a. continue,
As shown in (d), the ordinary ray 44 on the joining surface 23 becomes an extraordinary ray 44 ′ on the exit surface 24, and the extraordinary ray 45 on the joining surface 23 becomes an ordinary ray 45 ′ on the exit surface 24. 23
Is an extraordinary ray 46 ′ at the exit surface 24,
The extraordinary ray 47 on the joint surface 23
7 '.

Here, in the case of the first embodiment, the extraordinary ray 4
5 ′ and the ordinary ray 46 ′ form one (center) light beam. In this embodiment, the ordinary ray 44 ′ and the extraordinary ray 4 ′ are used.
7 'becomes one (center) light beam. Accordingly, when the polarization separation in the first, second and third crystals is considered as a vector, the vector d1 indicating the polarization separation in the first crystal and the polarization separation in the second crystal are considered. And the vector d3 indicating polarization separation in the third crystal satisfy the following expression.

(Vector d 1) + (vector d 2) = (vector d 3) (8) The separation ratio (a: b: c) of the three light beams is given by the following equation.

A: b: c = (SIN δ · SIN β1) :( COS β1) :( COS δ · SIN β1) (9) Also, three lights when δ is set to 45 ° The beam separation ratio (a: b: c) is given by the following equation.

A: b: c = (SIN β1) :( 2 COS β1) :( SIN β1) (10) Here, the separation ratio which is the separation ratio between the light beam at the center and the light beam at the side. (A: b) In the case of Example 1, (COS
β1): (2 SIN β1) (Equation 7), and in this embodiment, (SIN β1): (2COS β1) (Equation 1)
0). Therefore, by rotating the first crystal by 180 ° about the normal to the plane of incidence, the separation ratio between the central light beam and the side light beam can be changed.

When the polarization separation element of the present invention is used in an optical pickup device, the separation ratio between the light beam at the center and the light beam at the side, that is, the separation between the light beam for signal reproduction and the light beam for focus servo. Separation ratio (a: b) which is the ratio
Is preferably from 1: 0.25 to 1: 4, and the value range of β1 in this case is 19.5 ≦ β in the first embodiment.
≦ 54.7, and in the second embodiment, 35.3 ≦ β ≦ 70.
5 A more preferable range of the separation ratio (a: b) is 1: 0.5 to 1: 2. In this case, the range of the value of β1 is 26.6 ≦ β ≦ 45.0 in the first embodiment. In the second embodiment, 45.0 ≦ β ≦ 63.4.

(Embodiment 3) An optical pickup device using the prism of the present invention will be described with reference to FIG. In the figure, a light beam generated from a semiconductor laser 51 is separated by a diffraction grating 52 into 0th-order light for signal reproduction and focus servo and ± 1st-order light for tracking servo, and each is collimator lens 53 and polarization beam splitter. The light is reflected on the surface of the recording layer of the magneto-optical recording medium 56 via the objective lens 55 (light beam is reflected in the direction of the objective lens 55) and the objective lens 55. In this manner, the reflected light of the light beam applied to the surface of the recording layer has its polarization plane rotated in accordance with the magnetization direction of the recording layer, and the objective lens 55 and the polarizing beam splitter 54
(The light beam is transmitted in the direction of the lens 57), the lens 5
7. The light is condensed on the light receiving surface 60a of the light receiving element 60 via the cylindrical lens 58 and the polarization splitting element 59 of the present invention, and is converted into a voltage signal.

Here, the light beam of the reflected light incident on the polarization beam splitter 59 of the present invention is split into three light beams (2
One light beam for signal reproduction and one light beam for focus servo).
Is detected. Since the separation ratio (light intensity ratio) of two signal reproducing light beams among the three light beams changes due to the rotation of the plane of polarization of the reflected light, this separation ratio (light intensity ratio) is used.
, The magnetization direction of the recording layer, which is a signal recorded, can be detected.

In the conventional three-beam Wollaston prism, since the separated three beams are not parallel to each other, the separation distance on the light receiving surface 60a of the light receiving element 60 depends on the position of the three-beam Wollaston prism. Although the change (the separation distance increases as the distance from the three-beam Wollaston prism to the light receiving element 60 increases), in the polarization separation element of this embodiment, the separated three beams are parallel to each other. Irrespective of the position where 59 is arranged, the separation distance on the light receiving surface 60a of the light receiving element 60 becomes constant. Therefore, when the polarization separation element of the present invention is used, the degree of freedom in designing the optical pickup device (the degree of freedom in the position where the polarization separation element 59 is arranged) increases, and the distance between the polarization separation element 59 and the light receiving element 60 is shortened. Therefore, the size of the optical pickup device can be easily reduced.

Further, as shown in FIG. 8, the light receiving element 60 and the polarization separating element 59 can be integrated, and in the case of being integrated, the light receiving element 60 and the polarization separating element 59 are separately installed. This eliminates the need for position adjustment, thereby reducing the number of assembly steps, the manufacturing cost, and the size of the apparatus.

[0074]

As described above, the polarization beam splitter of the present invention joins three parallel flat plates to form a light beam.
Can be separated into beams. Moreover, since the polarization beam splitting element of the present invention is composed of parallel plate crystals, it can be cut into one optical element after bonding a large area crystal. Therefore, an optical element that can separate a light beam into three beams can be stably manufactured at low cost.

Further, by using the polarization separation element of the present invention in an optical pickup device, it is possible to reduce the number of assembling steps of the optical pickup device, the manufacturing cost, and the size of the device.

[Brief description of the drawings]

FIG. 1 is a perspective view illustrating a configuration of a polarization beam splitter of Example 1.

FIG. 2 is an explanatory diagram for explaining a polarization separation distance in an optically anisotropic crystal.

FIG. 3 is a perspective view showing a polarization separation process in the polarization separation element of the first embodiment.

FIG. 4 is an explanatory diagram for explaining the operation principle of the polarization beam splitter of the first embodiment.

FIG. 5 is a perspective view illustrating a configuration of a polarization beam splitter of a second embodiment.

FIG. 6 is an explanatory diagram for explaining an operation principle of the polarization beam splitter of the second embodiment.

FIG. 7 is a schematic diagram showing a configuration of an optical pickup device according to the present invention.

FIG. 8 is a schematic diagram showing a configuration of an optical pickup device according to the present invention.

FIG. 9 is a schematic diagram showing a configuration of a conventional optical pickup device.

FIG. 10 is an explanatory diagram showing a spot shape on a light receiving element of the optical pickup device.

FIG. 11 is a perspective view for explaining the operation of a conventional three-beam Wollaston prism.

FIG. 12 is an explanatory diagram showing a polarization separation process of a conventional three-beam Wollaston prism.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 1st crystal body 2 2nd crystal body 3 3rd crystal body 11 Optical axis of 1st crystal body 12 Optical axis of 2nd crystal body 13 Optical axis of 3rd crystal body 11a 1st crystal Optical axis of the body (optical axis viewed from the incident direction of the light beam) 12a Optical axis of the second crystal (optical axis viewed from the incident direction of the light beam) 13a Optical axis of the third crystal ( (Optical axis viewed from incident direction) 21 Incident surface 22 Joining surface of first and second crystals 23 Joining surface of second and third crystals 24 Outgoing surface 31-34 Light beam 41 Direction of linear polarization of light beam 42-47, 44'-47 'Direction of linear polarization (ordinary ray, extraordinary ray)

Claims (5)

[Claims] In a polarization beam splitting element in which three parallel flat plates made of an optically anisotropic crystal are joined, the optical axes of the first, second and third crystals are set to the first crystal. From the arrival point of the ordinary ray to the arrival point of the extraordinary ray on the surface of the second crystal side of the light beam that is non-parallel to the normal to the incident surface and enters the first crystal from the surface on the incident surface side Is the vector d1, the second
The displacement of the light beam incident from the surface on the first crystal body side from the arrival point of the ordinary ray on the surface on the third crystal body side to the arrival point of the extraordinary ray on the crystal of the third crystal is represented by a vector d2. When the displacement from the arrival point of the ordinary ray to the arrival point of the extraordinary ray on the surface on the emission surface side of the light beam incident from the surface on the second crystal body side is defined as a vector d3, the vector d2 and the vector d3 is orthogonal, (vector d1) + (vector d3) = (vector d
2) A polarization separation / separation element characterized by satisfying the following. 2. In a polarization splitting element in which three parallel flat plates made of an optically anisotropic crystal are joined, the optical axes of the first, second, and third crystals are aligned with the first crystal. From the arrival point of the ordinary ray to the arrival point of the extraordinary ray on the surface of the second crystal side of the light beam that is non-parallel to the normal to the incident surface and enters the first crystal from the surface on the incident surface side Is the vector d1, the second
The displacement of the light beam incident from the surface on the first crystal body side from the arrival point of the ordinary ray on the surface on the third crystal body side to the arrival point of the extraordinary ray on the crystal of the third crystal is represented by a vector d2. When the displacement from the arrival point of the ordinary ray to the arrival point of the extraordinary ray on the surface on the emission surface side of the light beam incident from the surface on the second crystal body side is defined as a vector d3, the vector d2 and the vector d3 is orthogonal, (vector d1) + (vector d2) = (vector d
3) A polarization separation / separation element that satisfies the following. 3. The polarization separation element according to claim 1,
A polarization separation element, wherein an angle β between the vector d1 and the vector d2 is 19.5 ° ≦ β ≦ 54.7 °. 4. The polarization separation device according to claim 2, wherein
A polarization separation element, wherein an angle β between the vector d1 and the vector d2 is 35.3 ° ≦ β ≦ 70.5 °. 5. An optical pickup device using the polarization separation element according to claim 1.