JP3456610B2 - Polarization separation element, optical pickup and magneto-optical signal reproducing device - Google Patents

Description

Detailed Description of the Invention

[0001]

[Table of Contents] The present invention will be described in the following order. Industrial applications Conventional technology Problems to be Solved by the Invention Means for solving the problems Action Example (1) Polarization separation element (1-1) Biaxial crystal element (1-2) Uniaxial crystal element (2) Optical pickup (3) Magneto-optical signal reproducing device (4) Other embodiments The invention's effect

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization separation element.
For example, it is suitable for use in an optical pickup used for reproducing a magneto-optical signal.

[0003]

2. Description of the Related Art FIG. 11 shows the structure of an optical pickup used in a magneto-optical disk device. The optical pickup 1 is composed of a plurality of optical components 2 to 10.
The optical pickup 1 focuses the laser light emitted from the laser diode 2 onto the magneto-optical disk 11 via the grating element 3, the beam splitter 4, the collimator lens 5 and the objective lens 6, and reflects the reflected light. An image is formed on the photo detector 9 through the Ruston prism 7 and the multi-lens 8. Here, the Wollaston prism 7 is a polarization splitting element that utilizes the birefringence of crystals, and is used to split two polarized lights.

[0004]

However, the optical pick-up currently used has a problem that it is difficult to miniaturize and it is difficult to improve reliability because it is formed by positioning a plurality of optical components individually. It was

Therefore, it is desired to reduce the size of the optical pickup by realizing a composite optical element in which a plurality of optical components are integrated on a semiconductor substrate on which a light receiving element group is formed. All of the polarization separation elements currently used, including the prism 7, are of a type that separates polarized light by transmitting light rays, so that it is unsuitable for this type of composite optical element.

Therefore, as a polarization separation element suitable for this type of composite optical element, it is desired to realize a polarization separation element involving reflection of a light ray at least once in a crystal. Since there are differences depending on the polarization direction and the direction of the ray wavefront normal, there is a problem in that it is unavoidable that the light beam is separated at each reflection, and that the light flux corresponding to each polarization is diffused.

The present invention has been made in consideration of the above points, and it is an object of the present invention to propose a polarization separation element which is easy to handle even when each polarized light is reflected in a crystal.

[0008]

In order to solve such a problem, in the present invention, the normal line of the interface that reflects an incident light ray is one of three refractive indices of an index ellipsoid that represents the optical characteristics of a crystal. A polarization separation element is used which is arranged so as to match the direction corresponding to the refractive index and whose optical characteristics are controlled so as not to cause a phase difference and a reflectance difference for the P-polarized component and the S-polarized component during reflection. .

[0009]

The reflected P-polarized light component and S-polarized light component reflected inside or near the interface of the polarization beam splitting element do not separate even after reflection and proceed in the polarization beam splitting element.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings.

(1) Polarization Separation Element (1-1) Biaxial Crystal Element The structure of the polarization separation element according to the present invention will be described using an index ellipsoid showing the optical characteristics of crystals. Here, a crystal having three different refractive indices nx, ny, and nz that give an index ellipsoid, that is, a biaxial crystal will be described as an example. First, the reflection characteristics of light rays are confirmed assuming that the azimuth corresponding to one of the three refractive indices nx, ny, and nz that gives the refractive index ellipsoid coincides with the normal line of the reflective interface.

In this embodiment, the case where the azimuth corresponding to the refractive index nz is made coincident with the normal line of the reflecting interface will be described.
Incidentally, the directions corresponding to the refractive indices nx, ny and nz are the X'axis, the Y'axis and the Z axis in FIG. Here, the optical relationship between the reflected light and the incident light is confirmed assuming that the light ray incident on the index ellipsoid shown in FIG. 1 along the XZ plane is reflected on the XY plane. Incidentally, the vector k _in represents the wavefront normal direction of the incident light, and the vector k _out represents the wavefront normal direction of the emitted light (reflected light).

[0013] Now specific polarization and specific refractive index corresponding to the vector k _in the Yotsute two axes and the length of SetsuTsuta sectional refractive index ellipsoid in a plane passing through the are and origin perpendicular to the vector k _in expressed. A conceptual diagram of this cross section is shown in FIG. FIG. 2 is a sectional view in which the index ellipsoid is cut from the A direction. Two characteristic polarization directions are vector D ₁ and vector D
₂ and the corresponding intrinsic refractive indices are N ₁ and N
Expressed as ₂ .

At this time, it is verified which direction the light ray having the polarization direction of the vector D ₁ travels after being reflected. Figure 3
Shows how S-polarized light and P-polarized light normally change before and after reflection. Here, the term "normal" means that the incident light is totally reflected or is reflected by the optical thin film. When a phase difference occurs between the S-polarized light and the P-polarized light, it is shown in this figure. Can not.

Now, with respect to the polarization direction before reflection of the light ray having the characteristic polarization direction of the vector D _1, the polarization S _in shown in FIG.
And P _in are overlapped and displayed as shown in FIG. Incidentally, FIG. 4 is a diagram viewed from the direction A (direction of the vector k _in ).
Here, assuming that both the phase difference and the reflectance difference before and after reflection for P-polarized light and S-polarized light are suppressed, the polarization direction immediately after reflection is as shown in FIG. Incidentally, FIG. 5 is a view seen from the B direction (direction opposite to the vector k _out ). As can be seen from FIG. 5, the relationship between the polarization direction vectors D ₁ and D ₁ 'and the S polarization direction and the P polarization direction is preserved before and after reflection.

The direction of the vector k _out is generally not determined unless the intrinsic refractive index after reflection is determined. Therefore, assuming that the intrinsic refractive index does not change before and after reflection,
The incident angle α _in and the output angle α _out at are the same value.
At this time, the intrinsic polarization direction and the intrinsic refractive index corresponding to the vector k _out are as shown in FIG. 6 from the geometrical symmetry of this system. FIG. 6 is geometrically the same as FIG. 2, and it can be seen that the vector D ₁ representing the eigen polarization direction upon incidence and the vector D ₁ ′ representing the eigen polarization direction after reflection match. Similarly, it can be seen that the intrinsic refractive indices N ₁ and N ₁ ′ are the same.

That is, the exit angle α _out of the vector k _out representing the wavefront normal direction of the reflected light reflected in the crystal is the same angle as the incident angle α _in of the vector k _in representing the wavefront normal direction of the incident light. Since the vector D ₁ before reflection and the vector D ₁₁ after reflection match each other, it can be seen that the polarization components are not separated during reflection. This is also the case for the reflection of light rays with the eigenpolarization of vector D ₂ .

As described above, the azimuth corresponding to one of the three refractive indices of the refractive index ellipsoid representing the optical characteristics of the crystal is arranged so that it coincides with the normal line of the reflective interface, and in addition, reflection is performed. By using a crystal whose polarization characteristics are controlled so that a P-polarized component and an S-polarized component do not cause a phase difference and a reflectance difference, a light beam is reflected inside the polarization separating element or near its interface. It is possible to realize a polarization separation element in which light rays are not separated even in such a usage.

(1-2) Uniaxial Crystal Element Here, a uniaxial crystal will be described as an example of the special case of the biaxial crystal described in the preceding section. Uniaxial crystals are 3
It is a crystal in which two of two refractive indexes are equal. By the way, it will be explained using mathematical formulas that, in the case of a uniaxial crystal, the optical characteristics can be expressed relatively easily by a mathematical formula, and the same effect as that of a biaxial crystal can be obtained.

It is assumed that the polarized light is surface-reflected in the uniaxial crystal, and the reflection direction of the outgoing polarized light is obtained using FIG. Here, the vector e _sin represents a unit vector in the S polarization direction, and the vector e _pin represents a unit vector in the P polarization direction. At this time, if these unit vectors are used for the vector a _in representing the incident direction of arbitrary polarized light,

[Equation 1] And the vector a representing the emission direction of the plane-reflected polarized light
When _out similarly using these unit vectors, the following equation

[Equation 2] It can be expressed as.

Here, consider a case where the optical axis of the uniaxial crystal is arranged in a plane parallel to the reflection interface. This can be considered as a case where the rotation angle θ around the X axis of the optical axis is 0 in FIG. 8 and the rotation angle φ around the Z axis is a predetermined value. Incidentally, it is assumed that the incident light is reflected by the reflecting surface 11A. Now, in FIG. 7, assuming that the refractive indices of the incident light and the outgoing light (reflected light) are N _in and N _out , respectively, and the surface normal of the reflecting surface is R, the following formula is obtained from Snell's law.

[Equation 3] Holds.

Now, the rays propagating in the uniaxial crystal include ordinary rays (rays corresponding to the ordinary ray refractive index n _o of the uniaxial crystal and complying with Snell's law) and extraordinary rays (following Snell's law). When the light is reflected, it is usually separated into ordinary and extraordinary rays. That is, normally, when reflected, the ordinary light is separated into ordinary light and extraordinary light, and the extraordinary light is separated into ordinary light and extraordinary light. However, in the case of the crystal element described in the present embodiment, the ordinary light and the extraordinary light are kept as they are, depending on the setting conditions of the crystal axis. That is, the ordinary light remains the ordinary light even when reflected, and the abnormal light remains the abnormal light even when reflected.

In order to prove this, it is assumed here that ray separation does not occur, and it is shown that the conclusion is consistent. First, consider the case of ordinary light. By satisfying the condition of intrinsic refractive index N _in = N _out , the crystal has a wavefront normal direction vector k _{in of} incident light, a wavefront normal direction vector k _{out of} emitted light, unit vectors e _sin , e _sout and e. _pin , e
_Each of _pout is

[Equation 4] Becomes

Therefore, substituting the equation (4) into the equations (1) and (2) giving the incident polarization direction vector a _in and the exit polarization direction vector a _out , the following equation is obtained.

[Equation 5]

[Equation 6] Becomes

Here, the incident angle of the ordinary light incident on the reflecting surface is defined as −
The emission angle of the reflected light is treated as α, and the axis corresponding to the ordinary ray refractive index n _o of the uniaxial crystal is called the principal axis.
The axis corresponding to the other refractive index will be called the minor axis. Now, based on this definition, the principal axis direction vector e _c (α) of the outgoing light reflected on the reflecting surface is obtained. Since the rotation angle θ around the X axis is 0, the principal axis direction vector e _c (α) of the emitted light is

[Equation 7] Becomes On the other hand, the principal axis direction vector e _c (-α) of the incident light
Is the expression

[Equation 8] It can be expressed as.

Since the incident light is ordinary light here, if the incident polarization direction vector a _in = e _c (-α) is set, (5)
The coefficients A and B that give the equation and the equation (6) are respectively expressed by the following equations.

[Equation 9] Becomes

Therefore, the outgoing polarization direction vector a _out is

[Equation 10] Becomes This means that the ordinary light remains ordinary after reflection.

A similar relationship is obtained for extraordinary light. Here, assuming that the incident polarization direction vector a _in = e _L (−α), the output polarization direction vector a _out is also a _out = −
It can be shown that e _L (α) holds, and it can be seen that the extraordinary ray remains the extraordinary ray corresponding to the same refractive index. Incidentally, in the case of extraordinary light, unlike the case of ordinary light, the refractive index generally differs depending on the wavefront normal direction. From the above, it can be seen that when the light ray incident on the uniaxial crystal is reflected by the reflecting surface, the ordinary ray and the extraordinary ray are reflected as they are.

As described above, the azimuth corresponding to one of the three refractive indices of the refractive index ellipsoid showing the optical characteristics of the uniaxial crystal is made to coincide with the normal line of the reflective interface, and P at the time of reflection is used. By using a crystal in which the phase difference and reflectance difference of the polarization component and the S polarization component are suppressed as a polarization separation element, the light beam is separated even if it is used to reflect the light beam inside the crystal or near the crystal interface. It is possible to obtain a polarization beam splitting element that can reflect light without doing so. If this polarization separation element is integrated with a semiconductor element, the size of the optical pickup used for detecting a magneto-optical signal can be reduced.

(2) Optical Pickup This section describes an optical pickup for detecting a magneto-optical signal, which is constructed by using the polarization separation element described in the previous section.
The optical pickup 21 shown in FIG. 9 includes a laser diode LD, a polarization separation element 23, and a photodetector P in a housing 22.
The semiconductor substrate 24 and the like in which D1 to PD3 are integrated are integrally combined. Incidentally, the opening of the concave housing 22 is closed by the transparent member 25.

The polarization separation element 23 is the crystal element described in the previous section, and reflects the light beam twice inside the crystal. Here, the polarization separation element 23 is processed to have a substantially trapezoidal cross section, and the laser light is reflected by the slope facing the laser diode LD so as to irradiate the recording surface of the magneto-optical disk (not shown). There is. The polarization separation element 23 guides the laser light reflected on the recording surface of the magneto-optical disk into the crystal through the beam splitter film 26 attached to the slope of the polarization separation element 23, and a part of the polarization separation element 23. It is designed to output from the bottom surface of the to the semi-transmissive film 27 on the outside.

At this time, the light beam transmitted through the semi-transmissive film 27 is received by the photodetector PD1. On the other hand, the laser light reflected on the bottom surface of the polarization separation element 23 is reflected again by the total reflection film 28 provided on the top surface of the polarization separation element 23, and then emitted from the bottom surface of the polarization separation element 23. At this time, the emission position of the laser light emitted from the polarization separation element 23 differs depending on whether the laser light is the S-polarized wave component or the P-polarized wave component, and is collected as spot groups on the photodetectors PD2 and PD3, respectively. Glow.

The reason why the spot group is properly separated into two in this way is that the light rays are not separated when they are reflected inside the polarization separation element 23, and the output signals obtained from the photodetectors PD2 and PD3. A magneto-optical signal can be obtained by obtaining the difference between Incidentally, if the sum of the output signals of the photo detectors PD1 to PD3 is obtained, the pit signal can be obtained. The output signal of the photo detector PD1 and the photo detectors PD2 and PD
A focus error signal can be obtained by using at least one of the three output signals. Furthermore, the tracking error can be obtained from at least one of the output signals of the photo detectors PD1 to PD3.

According to the above construction, the polarization splitting element 23 and the light emitting element satisfying the above-mentioned conditions are arranged on the semiconductor substrate 24 on which the light receiving element is formed to form the optical pickup, thereby detecting the magneto-optical signal. It is possible to reduce the size and weight of the optical pickup. By thus configuring the optical pickup without combining individual components, it is possible to reduce the adjustment man-hours and improve the reliability.

(3) Magneto-optical signal reproducing device Finally, an example of an applied device using the optical pickup 21 will be described. FIG. 10 shows a magneto-optical disk reproducing device 31. The magneto-optical disk reproducing device 31 illuminates a predetermined position of the magneto-optical disk 32 with the laser light emitted from the optical pickup 21, and the laser light reflected at the predetermined position is received by the photo detectors PD1 to PD3. The optical pickup 21 converts the reflected light into an electric signal by the photo detectors PD1 to PD3, and the I / V amplifier 33 converts the reflected light into an electric signal.
Give to.

The I / V amplifier 33 is a photo detector PD.
The electrical signals output from 1 to PD3 are current-voltage converted and amplified, and the amplification result is output to the matrix amplifier 34.
Here, the matrix amplifier 34 calculates the input signal, and the tracking error signal S1, the focus error signal S2,
The magneto-optical signal S3 and the pit signal S4 are generated. The demodulation / decoding circuit 35 decodes the magneto-optical signal S3 and the pit signal S4 and outputs the decoded signal to the processing circuit in the subsequent stage.

According to the above construction, by using the small and lightweight optical pickup 21 as the optical pickup of the magnetooptical disc reproducing apparatus 31, a much smaller magnetooptical disc reproducing apparatus than the conventional one is realized. be able to.

(4) Other Embodiments In the above-described embodiments, the optical pickup using the crystal element according to the present invention has the structure shown in FIG. The arrangement and the like are not limited to this, and other modes can be adopted. For example, it is also applicable to those using a hologram optical element or the like as a constituent element.

Further, in the above-mentioned embodiment, the one for the magneto-optical disk has been described as the optical pickup.
The present invention is not limited to this, and can be widely applied to an optical pickup used for reading information from an optical recording medium such as a rewritable compact disk (MO-CD) or an optical tape.
Similarly, in the above-mentioned embodiment, the magneto-optical disk reproducing device has been described as the application device of the optical pickup.
The present invention is not limited to this, and can be widely applied to an optical reproducing device such as an optical tape recording device.

[0040]

As described above, according to the present invention, the normal line of the interface that reflects an incident light ray corresponds to one of the three refractive indices of the index ellipsoid representing the optical characteristics of the crystal. The light beams of the respective polarization components are handled by arranging them so as to match the azimuth and using as the polarization separation element those in which the phase difference and the reflectance difference of the P polarization component and the S polarization component at the time of reflection are suppressed. It is possible to realize a small-sized and lightweight polarization separation element that is easy to operate. Further, by using this polarization separation element, the optical pickup and the magneto-optical signal reproducing device can be further downsized.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an index ellipsoid showing the optical characteristics of a crystal.

FIG. 2 is a schematic diagram showing a cross section of an index ellipsoid viewed from a direction in which a light ray is incident.

FIG. 3 is a schematic diagram showing reflection of a polarized component.

FIG. 4 is a schematic diagram showing an intrinsic polarization direction viewed from a direction in which a light ray is incident.

FIG. 5 is a schematic diagram showing an intrinsic polarization direction viewed from a direction in which a reflected light beam is output.

FIG. 6 is a schematic diagram showing a cross section of an index ellipsoid viewed from a direction in which a reflected light beam is output.

FIG. 7 is a schematic diagram showing unit vectors of incident-side and reflection-side polarization components.

FIG. 8 is a schematic diagram showing an optical axis arrangement of crystals.

FIG. 9 is a schematic side sectional view showing a configuration of an optical pickup.

FIG. 10 is a block diagram showing a magneto-optical signal reproducing device.

FIG. 11 is a schematic diagram showing a configuration of a conventionally used optical pickup.

[Explanation of symbols]

21 ... Optical pickup, 22 ... Housing, 23 ... Polarization separation element, 24 ... Semiconductor substrate, 25 ... Transparent member, 2
6 ... Beam splitter film, 27 ... Semi-transmissive film, 28 ...
… Total reflection film, 31 …… Magneto-optical disk reproducing device, 32…
... Magneto-optical disk, 33 ... I / V amplifier, 34 ... Matrix amplifier, 35 ... Demodulation / decoding circuit, LD ... Laser diode, PD1 to PD3 ... Photo detector.

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02B 5/30 G02B 27/28

Claims (5)

(57) [Claims] 1. A normal line of an interface that reflects an incident light beam is arranged so as to coincide with an azimuth corresponding to one of three refractive indices of an index ellipsoid that represents optical characteristics of a crystal. A polarization separation element, wherein optical characteristics are controlled so that a phase difference and a reflectance difference do not occur between the P-polarized component and the S-polarized component when the incident light ray is reflected. 2. The polarization beam splitting element according to claim 1, wherein the crystal is a uniaxial crystal. 3. The polarization beam splitting element according to claim 1, wherein the crystal is a biaxial crystal. 4. A normal line of an interface for reflecting an incident light ray is arranged so as to coincide with an azimuth corresponding to one of the three refractive indices of the index ellipsoid representing the optical characteristics of the crystal. And a polarization splitting element whose optical characteristics are controlled so that a P-polarized component and an S-polarized component do not have a phase difference and a reflectance difference when the incident light is reflected, and the polarization splitting element. An optical pickup comprising a semiconductor substrate on which a light receiving element group for receiving light rays is formed. 5. A normal line of an interface that reflects an incident light beam is arranged so as to coincide with an azimuth corresponding to one of the three refractive indices of the index ellipsoid representing the optical characteristics of the crystal. And a polarized light separating element whose optical characteristics are controlled so that a P-polarized component and an S-polarized component do not have a phase difference and a reflectance difference when the incident light is reflected, and the polarized light separating element. An optical pickup having a light receiving element group for receiving a light beam, and a signal processing circuit for reproducing a magneto-optical signal based on the intensity difference between the P-polarized component and the S-polarized component received in the optical pickup. Magneto-optical signal reproducing device.