JPH05314563A - Optical head and signal detecting means - Google Patents

Abstract

PURPOSE:To provide an inexpensive optical head which can extremely reduce the number of optical parts by employing an optical anisotropic element. CONSTITUTION:The error signal and the magneto-optical signal are detected by a signal detecting means consisting of a parallel flat plate adhered with an optical anisotropic element 105, a non-phase beam splitter film 104 provided on the surface, a 4-split photodiode 108, and 2-split photodiode 109. Thus it is possible to obtain a super-compact and light-weight optical head consisting of only a semiconductor laser, an objective lens, an optical anisotropic element, and the photodiodes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head of an optical memory device.

[0002]

2. Description of the Related Art The structure of a conventional optical head is shown below. Laser light emitted from a semiconductor laser is collimated by a collimator lens, reflected by a beam splitter, and focused on a recording medium by an objective lens. The return light reflected by the recording medium passes through the beam splitter, and is then separated by the polarization beam splitter into a focus error signal detection system and a tracking error signal detection system. The focus error signal is obtained by the astigmatism method, and the tracking error signal is obtained by the push-pull signal. Therefore, the focus error signal is
The difference signal of the diagonal sum of the four-division photodiode is obtained, and the tracking error signal is obtained by the difference signal of the two-division photodiode. The magneto-optical signal is 4
It is obtained by the difference signal between the sum of the divided photodiodes and the sum of the divided photodiodes.

[0003]

However, since the beam is divided by the polarization beam splitter, there is a problem that the optical head becomes large and the cost of the optical head becomes high because of the large number of optical components. ..

Therefore, the present invention solves such a problem, and an object thereof is to provide an inexpensive optical head which is small in size and has a small number of optical components.

[0005]

The optical head according to the present invention comprises: (1) In an optical head for converging light on a recording medium and detecting a magneto-optical signal, the condensing light flux of return light from the recording medium It is characterized by having a plane-parallel plate having optical anisotropy.

(2) An optical thin film having a function of a beam splitter for separating laser light and having a phase difference between the p component and the s component suppressed to about 0 degree is provided on the surface of the plane-parallel plate having the optical anisotropy. Characterize things.

(3) The optical axis of the plane-parallel plate having optical anisotropy according to claim 1 is oriented at an angle of 45 degrees with respect to the polarization direction of the laser beam incident on the recording medium. ..

(4) Two optical anisotropic plates having parallel optical planes with optical anisotropy, each having an optical axis having an angle of 45 degrees with respect to the polarization direction of the laser light incident on the recording medium. It is characterized in that it is configured by laminating triangular prisms having properties.

The signal detecting means according to the present invention comprises: (5) At least one photoelectric conversion element at a position where the light separated into the ordinary ray and the extraordinary ray is condensed with respect to the optical axis, and at least one of the photoelectric conversion elements is divided into four. It is characterized in that a focus error signal for detecting focus is obtained from a difference signal of diagonal sums of the conversion elements.

(6) In the photoelectric conversion element, the photoelectric conversion element other than the 4-division photoelectric conversion element is a 2-division photoelectric conversion element, and a tracking error signal is obtained by a difference signal of the 2-division photoelectric conversion element.

(7) In each of the divided photoelectric conversion elements which receive the light separated into the ordinary ray and the extraordinary ray with respect to the optical axis, the magneto-optical signal is generated by the difference signal between the sums of the two sets of divided photoelectric conversion elements. 6. A signal is detected.
The signal detection means described.

(8) The return light from the recording medium of the sub beam for obtaining the tracking error signal is received on both sides of the photoelectric conversion element which receives the light separated into the ordinary ray and the extraordinary ray with respect to the optical axis. A photoelectric conversion element is provided, and a tracking error signal is obtained from each difference signal.

[0013]

【Example】

(Embodiment 1) FIG. 1 is a schematic view of an optical head according to the first embodiment of the present invention. The figure uses a finite objective lens. The laser light emitted from the semiconductor laser 101 is partially reflected by the beam splitter 104, and is focused by the finite objective lens 102 on the recording medium 103 where information is recorded and reproduced. Recording medium 10
The return light reflected by 3 returns to the objective lens 102. The return light is condensed by the objective lens 102 and partially transmitted by the beam splitter 104. Beam splitter 1
In No. 04, an optical thin film is configured so as to transmit 100% of S waves and 50% of P waves. Here, the magneto-optical signal component detects the rotation of the P wave by the Kerr rotation angle. Further, the reflectance of P waves is determined in consideration of the light utilization efficiency of the optical head, and it is not necessary to stick to 50%, and various values can be taken depending on the type of the optical head. Here, the beam splitter film is composed of a dielectric multilayer film in which the phase difference between the P component and the S component is suppressed to 10 degrees or less. The light transmitted by the beam splitter 104 is converted into two light beams 106 by the optically anisotropic substance 105.
It is separated into 107 and enters the photodiodes 108 and 109. FIG. 2 shows a schematic view of the optically anisotropic substance 105 viewed from the light incident side. Optical axis 2 of optically anisotropic substance 105
01 is set to an angle of 45 degrees with respect to the direction of the P wave 202. By this, the return light from the recording medium,
The component in the optical axis direction 201 becomes a light beam 106 and enters the photodiode 108. On the other hand, the component of the return light from the recording medium, which is perpendicular to the optical axis direction 201, becomes a light beam 107 and is incident on the photodiode 109. Therefore, a magneto-optical signal can be obtained by detecting the difference signal between the photodiode 108 and the photodiode 109. Further, the distance between the light beams 108 and 107 is determined by the optical anisotropic substance 10
Depending on the difference between the refractive indices of the ordinary ray and the extraordinary ray of 5 and the thickness, the larger the difference in the refractive index, the wider the spacing, and the thicker the wider.

(Embodiment 2) FIG. 3 shows the shape of a photodiode according to the second embodiment of the present invention. The photodiode on which the light beam 106 separated by the optically anisotropic substance 105 enters is divided into four, and 301, 302, 30
3, 304, the photodiode S1,
Photodiode S2, photodiode S3, and photodiode S4. Further, the photodiode on which the light flux 107 is incident is divided into two, and denoted by 308 and 309, which are respectively a photodiode S5 and a photodiode S6. The optically anisotropic substance placed in the focused light serves as a plane-parallel plate that produces astigmatism. Therefore,
When the recording medium and the objective lens are in focus, the spot on the 4-division photodiode has the shape shown in 305, and when the recording medium and the objective lens are farther from the focus, the spot on the 4-division photodiode is 306. When the recording medium and the objective lens are closer to the in-focus state, the spot on the 4-division photodiode is 307.
It becomes a horizontally long oval shape shown in. Here, the focus error signal is obtained by calculating the difference between the diagonal sums of the four-divided photodiodes. The calculation of the photodiode is shown in the following equation.

(S1 + S3)-(S2 + S4) Further, the light beam 107 separated by the optically anisotropic substance 105 is condensed on the two-division photodiode placed next to the four-division photodiode. When the recording medium and the objective lens are in focus, the spot on the two-divided photodiode has a shape shown by 310, and a tracking error signal is obtained by taking the difference between the photodiode 308 and the photodiode 309. The calculation formula is S5-S6.

The magneto-optical signal is transmitted to the photodiode 10
8 and the photodiode 109, the photodiode shown in FIG. 3 corresponds to the difference between the total sum of the four-divided photodiodes and the total sum of the two-divided photodiodes. Therefore, the arithmetic expression is (S1 + S2 + S3 + S4)-(S5 + S6).

(Embodiment 3) FIG. 4 shows an optically anisotropic substance based on the third embodiment of the present invention. A sectional view is shown in (a), and a shape seen from the incident side of the light beam is shown in (b). This is what constitutes a plane parallel plate by laminating triangular prisms made of a birefringent material. 40
1 is set so that the optical axis 404 has an angle of 45 degrees with respect to the polarization direction of the incident beam of the laser, and 402 shows the optical axis 405 with respect to the polarization direction of the incident beam of the laser and an angle of 45 degrees symmetrical to 404. Is set to have. Therefore, by the bonding surface 403 of this birefringent material,
Incident light in polarization direction 406 is split into two directions at an angle.

FIG. 5 shows a schematic view of an optical head using an optically anisotropic substance according to the third embodiment of the present invention.
This uses a finite objective lens like the optical head shown in the first embodiment. The laser light emitted from the semiconductor laser 101 is partially reflected by the beam splitter 104, and is focused by the finite objective lens 102 on the recording medium 103 where information is recorded and reproduced. The return light reflected by the recording medium 103 returns to the objective lens 102. The return light is condensed by the objective lens 102 and partially transmitted by the beam splitter 104. The beam splitter 104 is formed of an optical thin film so as to transmit 100% of S waves and 50% of P waves. The light transmitted by the beam splitter 104 is split into two light beams 502 and 503 having an angle by the optically anisotropic substance 501 and is incident on the photodiodes 504 and 505. As shown in FIG. 4, the optical axis direction 404 of the return light from the recording medium is changed.
Component becomes a light beam 502 and is incident on the photodiode 504. On the other hand, the component of the return light from the recording medium in the optical axis direction 405 becomes a light flux 503.
It is incident on 05. Therefore, a magneto-optical signal can be obtained by detecting the difference signal between the photodiode 504 and the photodiode 505. In addition, the photodiode 50
It is desirable that the distance between 4 and 505 be 200 microns or more, and if quartz is used as the birefringent material, a distance of 200 microns can be realized if the thickness is 1.5 mm and the effective diameter is 3 mm. For the focus error signal detection, tracking error signal detection, and magneto-optical signal detection, the same photodiode shape as that of the second embodiment shown in FIG. 3 is used, and the calculation is the same as that of the second embodiment.

(Embodiment 4) A schematic view of an optical head according to a fourth embodiment of the present invention is shown in FIG. The figure uses a finite objective lens. Semiconductor laser 10
The laser beam emitted from the beam splitter 1 is split into a sub beam for obtaining a tracking error signal and a main beam by a diffraction element 603, and then partially reflected by a beam splitter 104, and information is recorded by a finite objective lens 102. It is condensed on the recording medium 103 to be reproduced. The return light reflected by the recording medium 103 is the objective lens 102.
Return to. The return light is condensed by the objective lens 102 and partially transmitted by the beam splitter 104. The beam splitter 104 is constructed of an optical thin film so as to transmit 100% of S waves and 50% of P waves, and further, the phase difference between the p component and the s component is suppressed to 10 degrees or less.
Here, the magneto-optical signal is obtained by detecting the rotation of the P wave by the Kerr rotation angle. Beam splitter 104
The light transmitted by the light is separated into two light fluxes 502 and 503 by the optically anisotropic substance 501, and the photodiode 60
It is incident on 1,602. A photodiode based on the fourth embodiment is shown in FIG. Here, the four-divided photodiodes 701, 702, 703 and 704 are respectively referred to as a photodiode S1, a photodiode S2, a photodiode S3 and a photodiode S4, and the two-divided photodiodes 706 and 707 are respectively ,
Photodiode S5 and photodiode S6,
709 is a photodiode S7, and 710 is a photodiode S8. The return light from the recording medium of the main beam that is not diffracted by the diffractive element 603 is separated by the optically anisotropic substance 501, and is made into spots 705 on the four-divided photodiodes 701, 702, 703, 704, and the two-divided photodiode 706. , 707 as spots 708, respectively. On the other hand, the diffraction element 60
Similarly to the main beam, the sub-beam diffracted by the light beam 3 is spotted by the optically anisotropic substance 501.
14 and 711 and 713, and spots 711 and 712 are formed on the same photodiode 709 and spot 71.
3 and 714 are incident on the same photodiode 710.

The focus error signal can be obtained by detecting the difference signal of the diagonal sum of the four-divided photodiodes as in the second embodiment. The arithmetic expression is (S1 + S3)-(S2 + S4).

Since the tracking error signal is obtained by the sub-beam in which the information of the track groove is separated by the diffraction element 603, the photodiodes 709 and 710 are provided.
It is obtained by detecting the difference signal of. The calculation formula is S7-S8.

The detection of the track groove having wobble is performed by detecting the difference between the two-divided photodiodes 706 and 707. The calculation formula is S5-S6. The summation signal may be used for wobble detection of the track groove, and the arithmetic expression in that case is obtained by S1 + S2 + S3 + S4 + S5 + S6 or S5 + S6.

For the detection of the magneto-optical signal, it is necessary to use a calculation method excluding the sub-beam. Therefore, the magneto-optical signal is obtained by the difference between the sum of the four-division photodiodes and the sum of the two-division photodiodes. The calculation formula is (S1 + S2 + S3 + S4)-(S5 + S6).

The photodiodes S1, S2, S3,
S4, S5, S6, S7, and S8 are formed on a substrate that forms the same photodiode, and are housed in the same package.

[0025]

As described above, according to the present invention, the number of optical components is significantly reduced by using the plane-parallel plate made of the optically anisotropic substance in the optical head in the optical memory device. A compact, lightweight, and inexpensive magneto-optical signal detecting optical head has been realized.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an optical head according to a first embodiment of the present invention.

FIG. 2 is a schematic view showing an optically anisotropic element according to the first embodiment of the present invention.

FIG. 3 is a schematic diagram showing a photodiode shape and a signal detection method according to a second embodiment of the present invention.

FIG. 4 is a schematic diagram showing an optically anisotropic element according to a third embodiment of the present invention.

FIG. 5 is a schematic diagram showing an optical head according to a third embodiment of the present invention.

FIG. 6 is a schematic view showing an optical head according to a fourth embodiment of the present invention.

FIG. 7 is a schematic diagram showing a photodiode according to a fourth embodiment of the present invention.

[Explanation of symbols]

 101 Semiconductor Laser 102 Objective Lens 103 Recording Medium 104 Beam Splitter 105 Optical Anisotropy Element 108, 109 Photodiode

Claims (8)

[Claims] 1. An optical head for condensing light on a recording medium to detect a magneto-optical signal, wherein a converging light flux of return light from the recording medium is provided with a plane parallel plate having optical anisotropy. And an optical head. 2. An optical thin film having a function of a beam splitter for separating laser light and having a phase difference between the p component and the s component suppressed to about 0 degree is provided on the surface of the plane-parallel plate having the optical anisotropy. The optical head according to claim 1, wherein: 3. The optical axis of the plane-parallel plate having optical anisotropy according to claim 1, wherein the direction of the optical axis is 45 degrees with respect to the polarization direction of the laser light incident on the recording medium. Item 1. The optical head according to Item 1. 4. A plane-parallel plate having optical anisotropy according to claim 1, comprising two sheets each having an optical axis having an angle of 45 degrees with respect to a polarization direction of a laser beam incident on a recording medium. The optical head according to claim 1, wherein the optical head is configured by laminating a triangular prism having optical anisotropy. 5. The photoelectric conversion element at a position where the light separated into the ordinary ray and the extraordinary ray is condensed with respect to the optical axis, and at least one of the photoelectric conversion elements is divided into four, and the diagonal sum of each of the four divided photoelectric conversion elements is calculated. A signal detecting means for obtaining a focus error signal for detecting focusing based on a difference signal. 6. The photoelectric conversion element according to claim 5,
6. The signal detecting means according to claim 5, wherein a photoelectric conversion element other than the 4-division photoelectric conversion element is a 2-division photoelectric conversion element, and a tracking error signal is obtained from a difference signal of the 2-division photoelectric conversion element. 7. A magneto-optical signal is obtained by a difference signal between the sums of two sets of divided photoelectric conversion elements in each of the divided photoelectric conversion elements that receive light separated into an ordinary ray and an extraordinary ray with respect to the optical axis. 6. The signal detecting means according to claim 5, wherein 8. A photoelectric converter for receiving return light from a recording medium of a sub-beam for obtaining a tracking error signal on both sides of a photoelectric conversion element for receiving light separated into an ordinary ray and an extraordinary ray with respect to an optical axis. 6. The signal detecting means according to claim 5, further comprising a conversion element, wherein a tracking error signal is obtained from each difference signal.