JPH0214439A - Focus controller - Google Patents

Abstract

PURPOSE:To maintain the interval of a light beam approximately fixed without being influenced with the dislocation of the optical axis of each optical part and to facilitate an assembly and an adjustment by branching the light beam from a recording medium into first and second light beams to be approximately parallel to each other, answering the light beams, and adjusting the position of a focusing means to the recording medium. CONSTITUTION:A light beam L reflected at the recording film of an optical disk 1 is returned to a 1/4 wavelength plate 18, converted to a linearly polarized beam having a polarizing surface in an X direction, and made incident on first and second double refracting bodies 28 and 29 arranged on a light path. Here, the light beam is branched into first and second light beams in which the optical axes are mutually parallel, light beams L1 and L2 are made to radiate photodetectors 20X and 20Y, they are converted to electric signals, an objective 16 is driven in the optical axis direction, and the focus of the light beam is controlled. Thus, the interval D of the light beams incident from the first and second double refracting bodies can be maintained approximately fixed without being influenced by the dislocation of the optical axis of each optical part, and the assembly and adjustment are facilitated.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a device for detecting the focus state of an optical device, and more particularly relates to a device for detecting a focus state of an optical device, and more specifically, for detecting a focus state of an optical device. The present invention relates to a device for focusing a light beam onto an information recording medium for reading recorded information from the information recording medium.

(Prior Art) In recent years, image information recording and reproducing devices such as optical disk devices have been developed that can record image information such as documents, retrieve the image information as necessary, and reproduce it as a hard copy or soft copy. ing.

In an optical disk device, information is recorded or reproduced by irradiating a focused light beam toward a disk-shaped recording medium, that is, an optical disk. That is, during recording, a state change is caused on the recording surface by being irradiated with a light beam, and as a result, information is recorded on the optical disk as, for example, pits.

Further, during reproduction, a steady light beam is irradiated onto the information recording medium, and the intensity of the light beam is modulated with bits according to the recorded information. Information is reproduced by processing the intensity modulation of the light beam. During recording and reproduction, the optical disk is rotated at a constant linear velocity, and an optical head for directing a light beam toward the optical disk is linearly moved in the radial direction above the optical disk.

The optical head includes an objective lens that focuses a laser beam onto an optical disk, and the objective lens is supported so as to be movable in the direction of its optical axis in order to accurately irradiate a predetermined area with a focused beam. The movable mechanism of the focus control system moves the objective lens in the optical axis direction to maintain the objective lens in a focused state, and the light beam from the objective lens is focused on the optical disk. In order to keep the objective lens in focus, a defocus state on the recording medium is detected and the detection result is fed back to the objective lens movement mechanism.

A wedge prism method is known as a method for detecting this defocus.

In the wedge prism method, a light beam emitted from an information recording medium in accordance with the force focusing state passes through a condenser lens and is split into two beams by a wedge prism. In this case, the two light beams are emitted in different directions at specific angles with respect to the optical axis of the incident beam. The branched light beams are respectively irradiated onto two detectors placed at predetermined positions and converted into electrical signals. The electrical signal is processed in a predetermined manner, and a current corresponding to the forcing error is supplied to a voice coil for driving the objective lens. By supplying current to the voice coil, the objective lens is driven and the light beam is focused onto the information recording medium.

(Problems to be Solved by the Invention) In focus control using the wedge prism method, a light beam is split by a wedge prism into two beams that are emitted in different directions at a specific angle with respect to the incident optical axis. Therefore, the distance between the two detectors that detect the two beams largely depends on the distance between the wedge prism and the detectors in the optical axis direction. To achieve accurate forcing control, the wedge prism installation requires that the detector spacing be adjusted depending on the position.

An object of the present invention is to provide a new focus control device for detecting a focus state on an information recording medium.

Another object of the present invention is to solve the problem of position adjustment of the detector interval caused by positional deviation of optical components in the optical axis direction, thereby providing a focus control device that is easy to assemble and adjust.

[Structure of the Invention] (Means for Solving Issues) According to the present invention, a light source that generates a light beam, a focusing means for focusing the light beam from the light source onto a recording medium, and the light source first and second light beams disposed on the same optical path of the light beam from the recording medium and the light beam from the recording medium and splitting the light beam from the recording medium into first and second light beams substantially parallel to each other; A focus control device is provided comprising a refractor and means responsive to the first and second light beams for adjusting the position of the focusing means relative to the recording medium.

(Function) In this invention, since the interval between the light beams incident from the first and second birefringent bodies is kept substantially constant without being affected by the deviation of the optical axes of each optical component, assembly and adjustment are possible. becomes easier.

(Example) An example of the present invention will be described below with reference to the drawings.

FIG. 1 shows a side view of an optical disk device incorporating a focus control device of the present invention. In FIG. 1, an optical disk (recording medium) 1 has an information recording film made of tellurium or
It is formed by coating with a metal wave film such as bismuth. Groups 5 defining recording areas are formed concentrically on the disk-shaped substrate. An optical head 3 is provided facing the optical disc 1, and the optical disc 1 is rotated relative to the optical head 3 at a constant linear velocity during recording, reproduction, and searching. In the optical disc shown in FIG. 1, a group 5 defining a recording area extends in the Y direction.

The optical head 3 is equipped with a semiconductor laser 11 as a light source. In this invention, the semiconductor laser 11 is a photodetector 20X, which will be described later.

20Y, preferably a photodetector 20X
and the photodetector 20Y, and the light receiving surface of each photodetector and the light beam emitting surface of the light source 11 are arranged on substantially the same plane. A diverging laser beam L is generated from the semiconductor laser 11 . The light beam L is generated with its light intensity modulated according to the information to be written during recording to write information onto the recording film of the optical disc 1, and is generated with a constant intensity during playback when information is read from the recording film of the optical disc 1. be done. The semiconductor laser 11 generates a diverging light beam L that is linearly polarized in a specific direction, for example, in the Y direction in FIG. A linearly polarized light beam L is placed on its optical path.
and the second birefringent body 28, 29. In FIG. 1, the birefringent bodies 28 and 29 are joined to each other at their side surfaces and extend in a direction perpendicular to the paper (Y direction). Further, the optical axes A and B of the birefringent body are arranged so as to be inclined in different directions with respect to the optical axis. The light beam L incident on the birefringent bodies 28 and 29 is transmitted and incident on the lens 17. The incident light beam L is converted into a parallel light beam by a lens 17, and the 1/4 wavelength plate 1 is placed on the optical path.
8. The linearly polarized light beam L having a polarization plane in the Y direction, which is incident on the quarter-wave plate 18, has its phase shifted and is converted into a circularly polarized light beam L.

The circularly polarized light beam L is incident on an objective lens 16 for focusing the light beam onto the optical disk, whereby a convergent light beam L is focused onto the recording film of the optical disk 1.

The objective lens 16 is supported so as to be movable in its forward axis direction and in a plane perpendicular to the optical axis. When the objective lens 16 is placed at the optimal position on the optical axis, that is, at the focused position, the beam waist of the focused laser beam L emitted from the objective lens 16 is projected onto the surface of the recording film of the optical disc 1. , whereby a minimum beam spot is formed on the surface of the recording film of the optical disc 1. On the other hand, when the objective lens 16 is placed at the optimum position in a plane perpendicular to the optical axis (in a plane parallel to the recording film surface), that is, at the alignment track position, the beam spot formed on the optical disk 1 becomes the recording area. It is formed precisely on a defined track, so that the track is followed by a laser beam. By maintaining these two states (in-focus state and in-track state), writing and reading of information becomes possible. That is, during recording, a change in the state of bits, etc. is caused on the recording film by an intensity-modulated laser beam, and during reproduction, a laser beam with a constant intensity is intensity-modulated and reflected from a recording area formed by bits, etc. in a track. .

Divergent light beam L reflected by the recording film of the optical disc 1
is converted into a parallel light beam by the objective lens 16 at the time of focusing, and is returned to the quarter-wave plate 18 again. The light beam L returned to the quarter-wave plate 18 is again phase-shifted and converted from a circularly polarized beam to a linearly polarized beam having a plane of polarization in the X direction. The light beam L linearly polarized in the X direction is transmitted through the focusing lens 17 and is again incident on the first and second birefringent bodies 28 and 29 arranged on the optical path. At this time, the shapes of the first and second birefringent bodies 28 and 29 and the inclinations of the optical axes are set so that the incident light beam has desired characteristics. That is, the first and second birefringent bodies 28 and 29
A light beam L linearly polarized in the direction is arranged so as to be branched into first and second light beams whose optical axes are parallel to each other. The light beams L1 and L2 branched by the birefringent bodies 28 and 29 arranged in this way are detected by photodetectors 20X and 20 arranged close to both sides of the exit surface of the light Mll, respectively.
Y is irradiated.

The light beams L1 and L2 irradiated onto the photodetectors 320X and 2OY are converted into electrical signals in each photodetection area of the photodetector and processed in a predetermined manner. By processing the signal, the comparison circuit 41 generates a signal corresponding to the defocusing of the light beam on the optical disk 1, that is, a forcing error signal (F, E). In response to the generated signal, a current is supplied to the drive circuit 44, and in response, a current is supplied to the voice coil 8 to drive the objective lens 16 in the direction of its optical axis, thereby controlling the focus of the light beam. Further, tracking error signals (T, E
) is generated. In accordance with the tracking signal, a current is supplied to the drive circuit 45 for tracking control, and the optical head 3 is driven linearly in a plane perpendicular to the optical axis of the objective lens 16, whereby a predetermined track is aligned with the light beam. tracked by Further, all of the signals detected in the photodetection areas of the photodetectors 20X and 20Y are processed by a signal processing circuit 43 and used as information reproduction signals.

The arrangement of the birefringent bodies 28, 29 and the detector placed behind them is shown in FIG. In FIG. 2, the birefringent body 28
．． 29 is composed of a uniaxial crystal such as calcite and has a parallel plate shape. Each birefringent body has side surfaces parallel to the x-Z plane, and the side surfaces are bonded to each other so as to be in contact with the optical axis of the incident beam. Further, the incident surface and the exit surface of the first birefringent body 28 and the second birefringent body 29 are arranged perpendicular to the optical axis of the light beam L emitted from the light source 11 and the optical axis of the light beam L returned from the optical disk. be done. Furthermore, the first optical axis A of the first birefringent body 28 and the second optical axis B of the second birefringent body 29 are arranged so as to have a predetermined angle with respect to the optical axis of the light beam L. be done. Preferably, the optical axes A and B of the birefringent body are oriented in different directions and are arranged at equal angles to the optical axis of the incident light beam L. By setting the shape of the birefringent body and the angle of the optical axis in this way, all of the light beam L linearly polarized in the Y direction emitted from the semiconductor laser 11 is transmitted without being birefringent, and at the same time, the light beam L is transmitted from the optical disk. The light beams that are linearly polarized in the X direction and returned are birefringent in different directions, and are split into first and second light beams that are parallel to each other with the optical axis as a boundary, and are incident on photodetectors 20X and 20Y. Ru. In other words,
The birefringent body is provided so that the destination beam L emitted from the semiconductor laser is transmitted without being split, and the light beam L returned from the optical disk 1 is split into first and second light beams parallel to each other. ing.

Note that the light beam L is separated along the mutually contacting side surfaces of the birefringent material, that is, the X-2 plane, and the separating X-2 plane is perpendicular to the direction in which the groups on the optical disk extend (Y direction). It is set up to do so. Photodetector 20X, 20
The light beam irradiated onto Y is converted into an electrical signal in the photodetection region and processed in a predetermined manner. This allows forcing control, tracking control, and information retrieval.

The birefringence phenomenon of a light beam incident on a birefringent body is gj43
It will be explained in more detail with reference to the figures. Birefringent body 29 (
The entrance and exit surfaces of the birefringent body 29 (28) are arranged perpendicular to the optical axis of the incident light beam, and the optical axis A of the birefringent body 29 (28) is arranged from a plane perpendicular to the optical axis of the incident light beam L. When tilted at a certain angle in the Z direction, the birefringent body 29
(28) Among the light beams L incident on the optical axis, the light beam Lb has a vibration plane perpendicular to the optical axis and the plane (main cross section) including the optical axis.
(Ordinary rays) follow the normal laws of refraction and are transmitted straight through. Also, among the light beams incident on the birefringent body 29 (28), the light beam La having a vibration plane parallel to the main cross section
(extraordinary rays) are birefringent without following the normal laws of refraction. As a result, the light beam La (extraordinary ray) is displaced slightly parallel to the light beam Lb (ordinary ray), as shown in FIG. 3, for example.

With reference to FIG. 4, the case where a pair of birefringent bodies 28 and 29 are arranged will be considered. A pair of birefringent bodies 28.2
9 have the same shape and characteristics as shown in FIG. 2, and their optical axes ASB are predetermined at equal angles in different directions with respect to the optical axis of the light beam. Therefore, a light beam having a plane of vibration perpendicular to the main cross section of the birefringent body 28, 29, that is, a light beam linearly polarized in the Y direction, is simply transmitted, whereas a light beam having a plane of vibration perpendicular to the main cross section is transmitted through the birefringent body 28,29. The beam, that is, the light beam linearly polarized in the X direction, is birefringent depending on the inclination of the optical axis ASB of the birefringent body. Focus control according to the present invention is made possible by determining the polarization state of the light beam incident on the birefringent body as follows. That is, a light beam L linearly polarized in the Y direction as shown by the solid arrow is incident from the semiconductor laser 11 onto the birefringent bodies 28 and 29. The light beams linearly polarized in the Y direction that are incident on the birefringent body 28 and the birefringent body 29 are not birefringed and are therefore transmitted together. The transmitted light beam is converted into circularly polarized light by the quarter-wave plate 18 and irradiated onto the optical disk 1. The irradiated light beam is reflected by the recording surface and becomes 1/4
returned to the wave plate. The light beam transmitted through the quarter-wave plate is again converted into linearly polarized light and is incident on the birefringent bodies 28 and 29. At this time, the returned light beam L is linearly polarized in the X direction as shown by the dotted arrow. Therefore, the light beams incident on the birefringent body 28 and the light beams incident on the birefringent body 29 are birefringent depending on the inclination of their optical axes, respectively, and are turned into first and second light beams L1 parallel to each other. ,
Branched to L2. The axes of the split light beams L1 and L2 are connected to the photodetector 2 while maintaining a constant width interval.
Irradiated at 0X and 20Y.

For these reasons, the first and second birefringent bodies 28.
The interval between the light beams L1 and L2 emitted from the optical system 29 is kept substantially constant without being affected by the deviation of each optical component in the optical axis direction. Therefore, a focus control device is provided that is easy to assemble and adjust.

Further, in the present invention, the birefringent body is formed in the shape of a parallel plate, and its entrance surface and exit surface are arranged perpendicular to the optical axis. Therefore, it is easier to install than prisms and tilted glass plates, and the manufacturability is correspondingly increased and costs are reduced.

A photodetector 20120Y on which the first light beam L1 and the second light beam L2 are incident is shown in FIG. 7th
As shown in the figure, one photodetector 20X is composed of photodetection areas 2OA, 20B, 20H, and 20I that convert the irradiated light passing through the first birefringent body 28 into electrical signals; The photodetector 20Y is a photodetection area 20 that converts the irradiated light that passes through the second birefringent body 29 into an electrical signal.
It consists of detection areas C, 20D, 20F, and 20G. The photodetectors 20X and 20 are substantially coplanar with each other (X
- Y plane), and at the same time, they are arranged with a slight interval in the direction in which the light beam is separated, that is, in the X direction. A light source 11 such as a semiconductor laser is arranged in the gap between the photodetector 20X and the photodetector 20Y, and preferably the light receiving surface of each photodetector and the light beam emitting surface of the light source 11 are approximately the same.
placed on a plane. One of the photodetectors 20X is vertically divided into two parts by a dividing line 21 in the X direction, which serves as a non-detection area, and horizontally divided into two parts by a dividing line 23 in the Y direction. The other photodetector 20Y is vertically divided into two by a dividing line 22 in the X direction serving as a first non-detection area, and horizontally divided into two by a division vA24 in the Y direction. The distance between the dividing line 23 of the photodetector 20X and the dividing line 24 of the photodetector 20Y is equal to the distance between the light beams branched substantially in parallel by the birefringent bodies 28 and 29. The outputs of the separated detection areas 20A to 201 are used for focus correction, tracking correction, and information reproduction.

When the objective lens 16 is in a focused position with respect to the optical disc, an image as shown in No. 5B is projected onto the photodetector.

The light beam that has passed through the first birefringent body 28 reaches the photodetection areas 2OA, 20B, and 20H.

201 as a semicircular image. On the other hand, the light beam passing through the second birefringent body 29 reaches the photodetection areas 20C and 20D.
, 20F, and 20G as semicircular images in opposite directions. Parting lines 21 and 22 as first non-detection areas are predetermined so that each semicircular image is projected onto the left and right photodetection areas with equal intensity. In other words, as shown in FIG. 8A, the outputs of the photodetection areas 20A to 20I are
~ ■, the positions of the dividing lines 21 and 22 are such that the forcing error signals (F, E signals) are O during focusing.
The position is set to be equal to F, E-((■+■1■10) (■+■1■10■))-0.

As a result, when the objective lens 16 moves away from the in-focus position relative to the optical disc, the two projected semicircular images are formed smaller than the in-focus image, as shown in FIG. 5A, and the output is F, E>0.

Conversely, when the objective lens 16 approaches the in-focus position with respect to the optical disc, the two semicircular images projected onto the first group and the second group are smaller than the in-focus image, as shown in FIG. 5C. is also formed large, and the output becomes F and Ego.

On the other hand, dark areas caused by diffraction of the guide groove on the optical disk 1 appear parallel to the dividing lines 23 and 24 of the photodetection area, as shown in FIGS. 6A to 6C. 8th
As shown in Figure B, the deviation of the track position is caused by the tracking error signal (T, E) - + (■ + ■ + ■ 10 ■) (■ It is detected by comparing +■+■10■)). The position of the optical head is adjusted with respect to the optical disk according to the difference signal of the compared signals, that is, the track position deviation signal.

Next, a circuit for processing electrical signals detected by the photodetector will be described with reference to FIG. Photodetection area 2OA and 20
The output of G is supplied to an amplifier circuit 31A. Photo detection area 2
The outputs of 0D and 20H are supplied to the amplifier circuit 31B.

The outputs of the photodetection regions 20C and 201 are supplied to an amplifier circuit 31C.

The outputs of the photodetection regions 20B and 20F are supplied to an amplifier circuit 31D. Each signal supplied to each amplifier 31A to 31D is amplified and supplied to an adder circuit 36. The signal added by the adder circuit 36 is supplied to a control circuit (not shown) as an information reproduction signal. The signal from the amplifier circuit 31A is supplied to adder circuits 32 and 34. The signal from the amplifier circuit 31B is supplied to adder circuits 33 and 34. The signal from the amplifier circuit 31C is supplied to adder circuits 33 and 35. The signal from the amplifier circuit 31D is sent to the adder circuit 32.
and 35.

The output of the adder circuit 32 and the output of the adder circuit 33 are used to supply a forcing control signal to the voice coil 8. In other words, the output of the adder circuit 32 is the output of the comparator circuit 41.
The output of the adder circuit 33 is supplied to the non-inverting input terminal of the comparator circuit 41. In the comparison circuit 41, the addition result of the detection signals of the photodetection areas 2OA, 20G, 20B, and 20F and the photodetection areas 20C, 201, and 20D.

It is compared with the addition result of the detection signal of 20H, and an output corresponding to the difference signal, that is, a focus control signal is fed back to the driving circuit 44 for forcing. A current is supplied to a voice coil (not shown) that drives the objective lens 16 in the forward axis direction in response to a defocus detection signal fed back to the drive circuit 44 . The objective lens 16 is driven in accordance with the supplied current to correct defocus.

The output of the adder circuit 34 and the output of the adder circuit 35 are used to supply a tracking control signal to the voice coil 8. That is, the output of the adder circuit 34 is supplied to the inverting input terminal of the comparator circuit 42, and the output of the adder circuit 35 is supplied to the non-inverting input terminal of the comparator circuit 42.

In the comparison circuit 42, the photodetection areas 2OA and 20G
, 20C120I and the addition results of the detection outputs of the photodetection areas 20B, 20F, 20D, and 20H are compared, and an output corresponding to the difference, that is, a tracking deviation detection signal, is fed back to the drive circuit 45. .

A current is supplied to a voice coil (not shown) that drives the objective lens 16 in a plane perpendicular to its optical axis in accordance with a tracking control signal fed back to the drive circuit 45. This drives the objective lens 16 and corrects tracking deviation.

The output of the adder circuit 36 is supplied to a signal processing circuit 43.

The signal processed by this signal processing circuit 43 is used as a search signal for information reproduction.

In the focus control device described above, the diverging laser beam generated from the semiconductor laser 11 is
The light is incident on the birefringent bodies 28 and 29.

The incident light beam is transmitted without being birefringent and enters the lens 17. The light beam incident on the lens 17 is transmitted through the quarter-wave plate 18 and then incident on the objective lens 16. The incident light beam is focused toward the recording film of the optical disc 1 by the objective lens 16 .

In recording information, pits are formed in tracks on the optical disc 1 by irradiating the optical disc 1 with a laser beam (recording beam) modulated with a strong optical bottom.

In reproducing information, a steady laser beam (reproducing beam light) of weak light dust is irradiated onto the optical disc 1, and the intensity is modulated by the pits and reflected. The reflected light from the optical disc 1 is returned to the quarter wavelength plate 18 via the objective lens 16.

The light beam L returned to the quarter-wave plate 18 is transmitted through the lens 17 and enters the first and second birefringent bodies 28 and 29 again. The light beams incident on the first and second birefringent bodies 28 and 29 are split into a first light beam L1 and a second light beam L2 that are substantially parallel to each other with their optical axes as a boundary, and are sent to the photodetector 20X. photo detection areas 2OA, 20B, 201, 20
H and photodetection areas 20C and 20D of the photodetector 20Y.

Irradiated to 20G, 20F-+-. As a result, a signal corresponding to the intensity of the irradiated light beam is transmitted to the photodetection area 20A~
201, and those signals are outputted from amplifier circuits 31A to 31A.
31D.

A focusing operation in such a state will be explained. That is, the amplification circuit 31A.

The signal from 31D is supplied to adder circuit 32.

The signals from the amplifier circuits 31B and 31C are sent to the adder circuit 3.
3. Photo detection area 20A120G, 20B,
The detection signals from 20F are added by an adder circuit 32 and supplied to a comparator circuit 41.

Detection signals from the photodetection areas 20C, 20I, 20D, and 20H are added by an adder circuit 33 and supplied to a comparator circuit 41. As a result, in the comparison circuit 41, the photodetection areas 2OA, 20G, and 20B.

The addition result of the detection signal of 20F and the photodetection area 20C,
The addition results of the detection signals 201, 20D, and 20H are compared, and the difference signal, that is, the focus control signal, is fed back to the drive circuit 44. Current is supplied to a coil (not shown) in response to a focus control signal fed back to the drive circuit 44. Objective lens 1 depending on the supplied current
6 is driven in the optical axis direction to control the focus state of the light beam.

Also, the tracking operation will be explained. That is, the signals from the amplifier circuits 31A and 31C are supplied to the adder circuit 34. The above amplifier circuit 31B.

The signal from 31D is supplied to adder circuit 35.

Signals from the photodetection areas 2OA, 20G, 20C, and 201 are added by an adder circuit 34 and supplied to a comparator circuit 42. Photodetection areas 20B and 20F.

The detection signals from 20D and 20H are added by an adder circuit 35 and supplied to a comparator circuit 42. As a result, the photodetection areas 2OA, 20G, and 20C.

The addition result of the detection signals of 201 and the detection areas 20B, 2
The addition result of the detection signals of 0F, 20D, and 20H is compared in a comparator circuit 42, and an output corresponding to the difference, that is, a tracking control signal, is fed back to the drive circuit 45.

Current is supplied from the drive circuit 45 to a coil (not shown) in accordance with the tracking control signal. In accordance with the supplied signal, the objective lens 16 is driven on a plane perpendicular to its front axis to control the tracking position of the light beam.

Next, information reproduction will be explained. When reproducing information, a laser beam L with a constant low luminous intensity is emitted from the semiconductor laser 11.
is generated. The entire reproduction beam is intensity-modulated according to the recorded information and is incident on a photodetector. The outputs of the photodetector regions 20A to 201 of the photodetector are output from amplifier circuits 31A and 3.
1B, 31C.

31D. Amplification circuits 31A and 31B.

The recorded data is reproduced by processing the total output of the 31C131D in the signal processing circuit 43.

One embodiment of the invention described above describes a photodetector that is divided into eight regions where focus control, track position control and signal detection can be performed using the same detector. . However, this detector is not limited to an eight-divided detector, and other four-divided photodetectors or other photodetectors may be used.

Furthermore, in the embodiment described, the birefringent bodies 28, 29 are formed in the shape of parallel plates. However, the shape of the birefringent body may be changed as long as the same effect is achieved. Furthermore, the arrangement of the detector can be changed depending on the arrangement position of the birefringent body. For example, the two birefringent bodies in the embodiment are arranged such that their optical axes A and B are oriented in different directions and make equal angles to the optical axis of the incident light beam. However, even if the optical axes A and B of the two birefringent bodies are not arranged at equal angles to the optical axis of the light beam, similar detection is possible by predetermining the arrangement of the detectors.

(Effects of the Invention) In the present invention, the interval between the light beams incident from the first and second birefringent bodies is kept substantially constant without being affected by the deviation of the optical axes of each optical component.

Therefore, a focus control device is provided that is easy to assemble and adjust. Further, the birefringent body is formed in the shape of a parallel plate, and its entrance surface and exit surface are arranged perpendicular to the optical axis. Therefore, it is easier to install than a prism or a parallel flat plate such as glass arranged at an angle, and the cost is correspondingly reduced.

[Brief explanation of the drawing]

FIG. 1 is a side view showing a schematic configuration of an optical head equipped with a focus control device of the present invention, FIG. 2 is a perspective view showing essential parts of the focus control device of the present invention, and FIG. 3 is a birefringent body of the present invention. FIG. 4 is an optical path diagram of a light beam incident on two birefringent bodies having different optical axes, and FIG. 6A to 6C are front views of the photodetector showing focused and unfocused states of the light beam irradiated onto the photodetector shown in FIG. 7 is a front view of the photodetector shown in FIG. 2, and FIGS. 8A and 8B are shown in FIG. 7. FIG. 9 is an electric circuit diagram showing a method of processing signals detected by the photodetector shown in FIG. 7. FIG. 9 is an electric circuit diagram showing a method of processing all signals detected by the photodetector shown in FIG. DESCRIPTION OF SYMBOLS 1... Optical disk, 3... Optical head, 5... Group, 8... Voice coil, 11... Semiconductor laser (light source) 16... Objective lens, 17... Lens, 18... ...1/4 wavelength plate, 20X, 20Y...Photodetector, 20A-201...Photodetection area, 21-24
... parting line, 28 ... first birefringent body, 29 ...
Second birefringent body, 31A to 31D... amplifier circuit, 32
~36... Addition circuit, 41.42... Comparison circuit, 4
3...Signal processing circuit, 44.45...Drive circuit Patent attorney Takehiko Suzue

Claims (1)

[Scope of Claims] A light source for generating a light beam; a focusing means for focusing the light beam from the light source onto a recording medium; and a focusing means for focusing the light beam from the light source and the light beam from the recording medium. adjusting the position of first and second birefringent bodies disposed on an optical path and splitting a light beam from the recording medium into first and second light beams substantially parallel to each other, and the focusing means with respect to the recording medium; A focus control device comprising means responsive to said first and second light beams.