JP2653478B2 - Focus control device - Google Patents

Description

The present invention relates to a device for detecting a focus state of an optical device, and more particularly, to a device for recording information on an information recording medium. The present invention relates to an apparatus for focusing a light beam for reading recorded information from an information recording medium on the information recording medium.

(Prior Art) In recent years, an image information recording / reproducing device such as an optical disk device capable of recording image information such as a document, retrieving the image information as needed, and reproducing as a hard copy or a soft copy has been developed. I have. In an optical disk device, a converging light beam is irradiated onto a disk-shaped recording medium, that is, an optical disk, to record or reproduce information. That is, at the time of recording, a state change is caused on the recording surface by irradiating a light beam, and as a result, information is recorded on the optical disk as, for example, pits. At the time of reproduction, a steady light beam is irradiated on the information recording medium, and the light beam is intensity-modulated by pits according to the recorded information. Information is reproduced by detecting a change in the intensity of the modulated 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 to the optical disk is linearly moved in a radial direction on the optical disk.

The optical head includes an objective lens for focusing a laser beam on an optical disk, and the objective lens is supported so as to be movable in an optical axis direction in order to accurately irradiate a predetermined region with a converged beam. The objective lens is moved in the optical axis direction by the movable mechanism of the focus control system so that the objective lens is kept in focus, 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 moving mechanism. The wedge prism method is known as a method of detecting this defocus.

In the wedge prism method, a light beam emitted from an information recording medium according to a focusing state is split into two beams by a wedge prism via a condenser lens. At this time, the two light beams are emitted in different directions at a specific angle with respect to the optical axis of the incident beam. The split light beams are respectively irradiated on two detectors arranged at predetermined positions and converted into electric signals. The electric signal is processed by a predetermined method, and a current corresponding to a focusing error is supplied to a voice coil for driving the objective lens. The current is supplied to the voice coil to drive the objective lens, and the light beam is focused on the information recording medium.

(Problem to be Solved by the Invention) In the focus control using the wedge prism method, a light beam is split by a wedge prism into two beams that are emitted at different angles from each other at a specific angle with respect to an incident optical axis. Therefore, the distance between the two detectors that detect the two beams greatly depends on the distance between the wedge prism and the detector in the optical axis direction. To achieve accurate focusing control, the detector spacing must be adjusted according to the mounting position of the wedge prism.

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 the position adjustment of the detector interval caused by the displacement of the optical components in the optical axis direction, thereby providing a focus control device that is easy to assemble and adjust.

According to the present invention, a converging means for converging a light beam on a recording medium, and a focusing means arranged on an optical path of the light beam from the recording medium and incident thereon. Branching means for splitting the light beam into first and second light beams, and the first light beam from the branching means is arranged on the optical path of the first light beam at an angle different from each other with respect to its optical axis. First and second parallel plates for splitting one light beam into third and fourth beams substantially parallel to each other, and the third and fourth parallel plates for adjusting the position of the converging means in the optical axis direction with respect to the recording medium. A focus control device comprising means for responding to the second light beam from the branching means for adjusting the position of the focusing means relative to the recording medium in a plane perpendicular to the optical axis. You.

(Operation) In the present invention, the interval D between the light beams emitted from the first and second light refractors is kept substantially constant without being affected by the shift of the optical axis of each optical component, so that the assembling and adjustment are performed. Becomes easier.

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

FIG. 1 is a plan view of an optical disk device in which a focus control device according to the present invention is incorporated. In FIG. 1, an optical disk (recording medium) 1 is formed by coating an information recording film on a disk-shaped substrate such as glass or plastic as a metal film such as tellurium or bismuth.
Grooves 5 that define a tracking guide area are formed concentrically on the disk-shaped substrate. Optical disk 1
An optical head 3 is provided so as to face the optical disk 1. During recording, reproduction, and retrieval, the optical disk 1 is driven to rotate at a constant linear velocity with respect to the optical head 3. In FIG. 1, a groove 5 defining a recording area extends in the Z direction.

The optical head 3 includes a semiconductor laser 11 as a light source,
A divergent laser beam L is generated from the semiconductor laser 11. The laser beam L is generated by modulating the light intensity according to the information to be written at the time of recording the information on the recording film of the optical disk 1 and generating the laser beam L at a constant intensity at the time of reproduction reading the information from the recording film of the optical disk 1. Is done. The divergent laser beam L generated from the semiconductor laser 11 is converted into a parallel light beam by the collimator lens 13 and guided to the half prism 14. The laser beam L guided to the half prism 14 passes through the half prism 14 and is incident on the objective lens 16, and the laser beam L is focused by the objective lens 16 toward the recording film of the optical disc 1.

The objective lens 16 is movably supported in its optical axis direction and in an in-plane direction orthogonal to the optical axis. When the objective lens 16 is arranged at the optimum position on the optical axis, that is, at the in-focus position, the beam waist of the converging laser beam L emitted from the objective lens 16 is projected on the surface of the recording film of the optical disc 1. ,
Thereby, the 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 arranged at an optimum position in a plane perpendicular to the optical axis (in a plane parallel to the recording film plane), that is, at a combined track position, a beam spot formed on the optical disc 1 is determined as a recording area. The track is precisely formed on the track, and the track is tracked by the laser beam. These two states (focused state
By maintaining the combined track state, writing and reading of information become possible. That is, during recording, a state change of pits or the like is caused in the recording film by the intensity-modulated laser beam, and at the time of reproduction, a laser beam of a constant intensity is intensity-modulated and reflected in a recording area formed by pits or the like in a track. .

The divergent laser beam L reflected by the recording film of the optical disk 1 is converted into a parallel light beam by the objective lens 16 at the time of focusing, and is returned to the half prism 14 again. The laser beam L reflected by the half prism 14 is transmitted through a focusing spherical convex lens 17 and is incident on a beam splitter 30. Part of the incident light beam is reflected by the joint surface of the beam splitter 30, and the remaining light beam is transmitted. The reflected light beam La is focused and irradiated on the photodetector 21. The light beam La applied to the light detector 21 is converted into an electric signal in each light detection area and processed by a predetermined method. The tracking error signal is generated from the comparison circuit 42 by being processed in a predetermined manner. A current is supplied to the voice coil 8 via the drive circuit 45 in response to the tracking error signal, and the optical head 3 is driven in a plane perpendicular to the optical axis of the objective lens 16, whereby a predetermined track is driven by the light beam. Tracked. The light beam Lb transmitted through the joint surface of the beam splitter 30 is incident on the first and second light refractors 18 and 19 disposed on the optical axis. First
The second light refractors 18 and 19 are arranged on the optical path of the light beam and split into two parallel first light beams L1 and second light beams L2 at the optical axis of the incident light beam. The split light beams L1 and L2 are irradiated on the photodetector 20, respectively. The light beams L1 and L2 applied to the photodetector 20 are converted into electric signals in respective photodetection areas and processed by a predetermined method. A focusing error signal is generated from the comparison circuit 41 by the signal processing. The generated electric signal is supplied to the driving circuit 44.
Is supplied to the voice coil 8 to drive the objective lens 16 in the optical axis direction, thereby controlling the focus of the light beam. Further, the signal detected in the light detection area is processed by the signal processing circuit 43 and used as a read signal.

The function of the first and second light refractors 18 and 19 will be described in detail with reference to FIG. The first and second light refractors 18 and 19 are made of, for example, glass plates, and are preferably formed in a parallel plate shape having the same thickness. Each parallel plate is X-
It has side surfaces parallel to the Z plane, and the side surfaces are arranged so as to be in contact with the optical axis of the incident beam. Further, the entrance plane and the exit plane of the first and second parallel plates 18 and 19 are arranged so as to have a predetermined angle with respect to a plane perpendicular to the optical axis,
Preferably, the two parallel plates 18, 18 are arranged at equal angles in different directions with respect to the optical axis. Thereby, the incident light beam is split along a line in the X direction perpendicular to the direction (Y direction) in which the shadow of the group on the optical disk is projected. The light beams transmitted through the parallel plates 18 and 19 are mutually incident light beams L by tilting the respective entrance and exit surfaces of the first and second parallel plates 18 and 19.
The first is substantially parallel to the optical axis and has a predetermined interval D.
Are split into a light beam L1 and a second light beam L2. The distance D between the first light beam L1 and the second light beam L2 is equal to the distance between the first parallel plate 18 and the second parallel plate.
It depends on the refractive index n of 19, the thickness, and the angle θ between the parallel plates 18 and 19. This relationship is shown in FIGS. 3A and 3B.

FIG. 3A shows the optical paths of the first light beam L1 and the second light beam L2 emitted from the first and second parallel plates 18, 19 substantially in parallel to the optical axis of the incident light beam L. I have. In this figure, the parallel plates 18, 18 are formed of the same material having the same thickness. The thickness t of the parallel plates 18 and 19, the parallel plate 1
Assuming that the angle between the refractive indices n of 8 and 19 and the angle between the two parallel plates 18 and 19 is θ, the interval D between the optical axes of the light beams L1 and L2 is (1) in a range where the angle θ is relatively small. −1 / n) θ
It is approximately expressed as t. Therefore, the shift D of the optical axis of the light beam is caused by the condenser lens 17, the parallel plates 18, 19, and the photodetector.
It does not depend on the mutual distance of 20.

Also, FIG. 3B shows that the incident light beam is incorrectly at an angle δ with respect to the optical axis.
Shows the displacement of the optical axes of the two split light beams L1 and L2 generated when the light beams are incident on the two glass plates 18 and 19. When incident at an angle δ with respect to the optical axis,
The shift amount due to the parallel plate 19 is increased by (1-1 / n) δt, and the shift amount due to the parallel plate 18 is reduced by (1-1 / n) δt. Therefore, the distance D between the optical axes of the two light beams
Does not depend on the angle δ in a range where the angle θ between the two glass plates is relatively small. Thus, the parallel plate 1 of the present invention
The interval D between the light beams emitted from 8 and 19 is kept substantially constant without being affected by the displacement of each optical component in the optical axis direction. Therefore, a focus control device that is easy to assemble and adjust is provided.

The first light beam L1 and the second light beam L2 transmitted through the parallel plates 18 and 19 as the light refractors and branched as described above.
The incident photodetector 20 is shown in FIG.

As shown in FIG. 4, the light detector 20 is irradiated through the light detecting regions 20K and 20M and the light refractor 19 for detecting the light beam irradiated through the first light detector 18. And four light detection areas 20L and 20N for light beam detection.

The light detection area is the vertical direction (X direction) as the light non-detection area
Is divided into two by the dividing line 23, and the horizontal direction (Y direction)
Is divided into two parts by the dividing line 25. Outputs of the divided light detection areas 20K, 20L, 20M, and 20N are used for focus control and information reproduction.

When the objective lens 16 is located at the in-focus position with respect to the recording surface of the optical disk, an image as shown in FIG. 4B is projected on the photodetector. That is, a beam spot having a small area due to the light beam L1 that has passed through the first light refractor 18 is formed around the division line 23 that divides the light detection regions 20K and 20M. At the same time, a beam spot having a small area due to the light beam L1 passed through the second light refractor 19 is formed in the light detection area.
It is formed around a dividing line 23 dividing 20L and 20N. In other words, assuming that the signal outputs corresponding to the light intensities incident on the photodetection regions 20K to 20N are respectively, the dividing line 23 has FE = ｛(+) − at the time of focusing.
It is set so as to satisfy the condition of (+)｝ = 0.

By setting the spot size of the light beam and the configuration of the detector at the time of focusing as described above, it is possible to detect a focus shift.

For example, when the objective lens 16 moves away from the in-focus position with respect to the optical disc, a slightly focused light beam from the optical disc is incident on the parallel flat plates 18 and 19 as compared with the focused state. Therefore, an image as shown in FIG. 4A is projected on the detector 20. That is, a substantially semicircular beam spot is formed on the light detection region 20K and the light detection region 20N of the detector 20. At this time, the focusing error signal is FE =
{(+)-(+)}> 0.

Conversely, when the objective lens 16 is closer to the optical disc than the in-focus position, the light beam slightly diverged from the optical disc than the focused state is incident on the parallel flat plates 18 and 19. Therefore, an image as shown in FIG. 4C is projected on the detector 20. That is, a substantially semicircular beam spot is formed on the light detection areas 20L and 20M of the detector 20. At this time, the focusing error signal is FE = ｛(+) − (
+)｝> 0.

Next, a processing circuit for the electric signal detected by the photodetector 20 will be described with reference to FIG.

The output signals of the light detection areas 20K to 20N are amplified by the amplifier circuits 31K to 31N.
Respectively. The signals amplified by the amplification circuits 31K and 31N are supplied to the addition circuit 32. Amplifier circuit
The signal amplified by the amplifier 31L and the amplifier 31M is supplied to the adder 33. The signal output added by the adding circuit 32 is supplied to the inverting input terminal of the comparing circuit 41, and the signal output added by one adding circuit 33 is supplied to the non-inverting input terminal of the comparing circuit 41. A comparison circuit 41 compares the addition result of the detection signals of the light detection areas 20K and 20N with the addition result of the detection signals of the light detection areas 20L and 20M. The compared difference output, that is, an electric signal corresponding to the shift of the focus state is supplied to a voice coil (not shown) via the drive circuit 44. Thereby, the objective lens 16 is driven along the optical axis direction to correct the defocus.

The outputs of the amplifier circuits 31K, 31L, 31M, and 31N are all added by the signal processing circuit 43. The information recorded on the optical disc is reproduced based on the result of the addition.

On the other hand, the photodetector 21 for detecting the light beam La reflected by the joint surface of the beam splitter 30 is shown in FIGS. 6A to 6C.

As shown in FIGS. 6A to 6C, the photodetector 21 is divided into two detection areas 21A and 21B by a horizontal (Z direction) division line 35 as a light non-detection area at the center thereof. I have. As a result of the light beam incident on the optical disk being diffracted by the groove 5 extending in the Z direction, a shadow of the groove, that is, a band-shaped dark portion appears in the beam spot parallel to the dividing line 35. These band-shaped dark portions are used for control for keeping a light beam on a predetermined groove (track), that is, for tracking control. In other words, the difference between the light intensities of the light beams irradiated on the separated light detection areas 21A and 21B is changed according to the position of the shadow of the groove, so that the light is irradiated on the light detection areas 21A and 21B. By using the difference in light intensity, it can be used for tracking control.

For example, when the objective lens 16 is located at the matching track position with respect to the optical disc, as shown in FIG.
A light beam having the same light intensity is irradiated on the light detection areas 21A and 21B on the light. Therefore, a signal corresponding to the difference in light intensity, that is, a zero signal is generated from the photodetector. Further, when the objective lens 16 is displaced from the combined track position with respect to the optical disc in any direction, as shown in FIG. 6A or FIG. 6C, the intensity of the light beam irradiated on the light detection area 21A and the intensity of the light beam 21B are increased.
Different from the intensity of the light beam illuminated above. Therefore, a tracking error signal corresponding to the difference in light intensity is generated from the photodetector. The tracking error signal generated by the detector 21 is supplied to a voice coil 8 for driving the objective lens 16 via a drive circuit 45, whereby the position of the objective lens 16 is adjusted on a plane parallel to the optical disk. .

Next, a processing circuit for the electric signal detected by the photodetector 21 will be described with reference to FIG.

The output signal of the light detection area 21A is supplied to the amplifier circuit 31A,
The output signal amplified by the amplification circuit 31A is supplied to the inverting input terminal of the comparison circuit. The output signal of the light detection area 21B is supplied to the amplifier circuit 31B, and the output signal amplified by the amplifier circuit 31B is supplied to the non-inverting input terminal of the comparison circuit. The comparison circuit 42 compares the detection signal of the light detection region 21A with the detection signal of the light detection region 21B. A difference signal between the compared signals, that is, an electric signal corresponding to a focus shift is supplied to a voice coil (not shown) via the drive circuit 45. Thereby, the objective lens 16 is driven in a plane perpendicular to the optical axis, and the tracking deviation is reinforced.

In the above-described apparatus, the laser beam L reflected by the half prism 14 passes through the spherical convex lens 17 and is incident on the beam splitter 30, and is split into light beams La and Lb at the joint surface of the beam splitter 30. One of the light beams La reflected by the bonding surface is projected on the photodetector 21 and converted into an electric signal, and the converted signal is processed by a predetermined method and used as a tracking control signal. The light beam Lb transmitted through the other joint surface is the first and second light refractors 18,
19, and is split into a first light beam L1 and a second light beam L2 substantially parallel to each other with the optical axis thereof as a boundary, and
Irradiated on top. The first and second light beams L1 and L2 are projected onto the photodetector 20 and converted into electric signals, and the converted signals are processed by a predetermined method and used for focus control and information reproduction. . However, the arrangement of the device of the invention is not limited to these arrangements. For example, the light beam La reflected by the beam splitter 30 may be used for focus control, and the transmitted light beam Lb may be used for tracking guide. The detection of the information reproduction signal is performed by the photodetector 21
May be done. Further, the photodetector is formed of glass plates 18, 19 having a parallel plate shape.
However, the material and shape of the light refractor may be changed as long as the same effect is exhibited. Furthermore, the position of the detector can be changed accompanying the change of the light refractor. For example, in the embodiment, two light refractors are symmetrically arranged on the optical axis of the incident light beam. However, even if the two light refractors are not symmetrically arranged on the optical axis of the incident light beam, the same detection can be performed by pre-determining the arrangement of the detector and the positions of the dividing lines 23 and 25 on the detector. Is possible.

[Effects of the Invention] In the present invention, the distance D between the light beams emitted from the first and second light refractors is kept substantially constant without being affected by the shift of the optical axis of each optical component. Therefore, a focus control device that is easy to assemble and adjust is provided. Further, the light beam incident on the beam splitter 30 is divided into two light beams La and Lb.
And a focus error signal is detected from one of the light beams Lb, and a track error signal is simultaneously detected from the other light beam La. Therefore, highly accurate focusing control and tracking control are achieved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of an optical head including a focus control device of the present invention, FIG. 2 is a perspective view showing a main part of the focus control device of the present invention, and FIGS. 3A and 3B are on an optical axis. 4A to 4C show the in-focus state and the out-of-focus state of the light beam, showing the relationship between the two parallel flat plates arranged in FIG. FIG. 5 is an electric circuit diagram showing a method of processing a signal detected by the photodetector 20. 6A to 6C are front views of a two-segment photodetector showing a combined track state and a non-matched track state of a light beam, and FIG. 7 is an electric circuit showing a method of processing a signal detected by the light detector 21. FIG. 1 ... optical disk, 3 ... optical head, 5 ... groove, 8 ... voice coil, 11 ... semiconductor laser, 13 ...
Collimator lens, 14 half prism, 16 objective lens, 17 spherical convex lens, 18 first light refractor,
19: second photorefractive body, 20: photodetector, 20K-20N
Photodetection area, 21 photodetector, 21A, 20B photodetection area, 23, 25 dividing line, 30 beam splitter, 31
A, 31B, 31K to 31N: an amplification circuit, 32, 33 ... an addition circuit,
35 ... dividing line, 41, 42 ... comparison circuit, 43 ... signal processing circuit, 44, 45 ... driving circuit.

Claims (1)

(57) [Claims] A focusing means for focusing a light beam on a recording medium; and a focusing means disposed on an optical path of the light beam from the recording medium, for dividing an incident light beam into first and second light beams. Branching means, and on the optical path of the first light beam from the branching means, the side surfaces thereof are arranged so as to be in contact with the optical axis, and the incident surface and the outgoing surface of each other are in a plane perpendicular to the optical axis. The first light beam is arranged at a predetermined angle with respect to the recording medium, and the first light beam is directed in a direction perpendicular to a direction in which the groove shadow on the recording medium is projected and a third light beam substantially parallel to the optical axis. First and second parallel flat plates branching into a fourth light beam, and means responding to the third and fourth light beams for adjusting the position of the focusing means in the optical axis direction with respect to the recording medium; A plane perpendicular to the optical axis of the focusing means for the recording medium A focus control device comprising means for responding to said second light beam from said branching means for adjusting the position within the beam.