JP2886230B2 - Optical head and focus error detecting device using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to an objective lens for a recording surface of an optical disc incorporated in a device for detecting a focus error, in particular, an optical disc device for optically reproducing information. 1. Field of the Invention The present invention relates to an optical head suitable for detecting a focus error of an optical head and a focus error detecting device using the same.

(Prior Art) In an optical information recording medium, for example, an optical disk apparatus that reproduces information from an optical disk and records information on the optical disk, an objective lens that focuses a light beam from a light source on a recording surface of the optical disk during recording and reproduction. It is required that the focused state be maintained and a minimum beam spot of the light beam be formed on the recording surface. In response to such demands, the optical disc apparatus incorporates a focusing servo device for detecting a focus error of the objective lens with respect to the recording surface and displacing the objective lens in the optical axis direction based on the error.

As a method of detecting a focus error in an optical disk device, many methods are known, for example, a double knife edge method.

FIG. 1 shows the configuration of an optical system of a focus error detecting device employing the double knife edge method. The light beam reflected from the optical disk (not shown in FIG. 1) passes through an objective lens (not shown) in the opposite direction to the light beam directed to the optical disk, is separated from the light beam toward the optical disk by the beam splitter 101, and is separated by a wedge prism. Ten
2, 103 (or a two-piece prism) splits the light beam into two light beams, and furthermore, by a condenser lens 104, detection areas 105 of two detectors 105, 106 provided at different positions.
The light is condensed on a, 105b, 106a, 106b.

The state of the change in the spot shape of the light beam on the detection areas 105a, 105b, 106a, 106b of the photodetectors 105, 106 is shown in FIG.
FIG. 2C shows the change of the focus error signal and the reproduction information signal with respect to the focus error in FIG. 3, respectively. Assuming that output signals respectively corresponding to the detection areas 105a, 105b, 106a, 106b of the photodetectors 105, 106 are A, B, C, D, a focus error signal representing a focus error of the objective lens by an arithmetic circuit is (A +
D)-(B + C), the reproduction information signal is obtained by the calculation of A + B + C + D. As can be seen from these figures,
The double knife edge method has the advantage that the level change of the focus error signal near the focal point is large and the focus error detection sensitivity is high.

(Problems to be Solved by the Invention) The focus error detection device described above has various advantages,
In this conventional focus error detecting device, when the objective lens is maintained in a focused state where the minimum beam spot of the light beam is formed on the optical disk, as shown in FIG. Above, a very small circular spot is formed. Therefore, most of the spot is incident on the non-detection area of the photodetectors 105 and 106, so that the photodetector 10
The signal level output from 5,106 becomes extremely small.
Further, the width of the light non-detection areas of the photodetectors 105 and 106 is relatively large, and most of the beam spots are
When formed in 5,106 light non-detection areas, the signal level detected by the photodetector fluctuates greatly, and the focus error signal (A + D)-(B + C) becomes very unstable. . Also, the reproduction information signal A + B + C + D
Of the playback information signal
There is a problem that sufficient S / N cannot be obtained.

Furthermore, since the change in the spot shape is very sensitive to the focus error, there is a problem that the detection range of the focus error is very narrow. Therefore, there is a problem that it is difficult to appropriately cope with the retracting of the objective lens at the beginning of the focusing servo and a large focus error during normal recording / reproducing.

As described above, in the conventional focus error detection device based on the double knife edge method, the detection of the light beam detected by the photodetector in the focused state depends on the width of the non-light detection area of the photodetector. Since the signal level is small, it is difficult to perform stable focus error detection, the S / N of the reproduced information signal cannot be sufficiently obtained, and the detection range of the focus error is narrow.

Accordingly, it is an object of the present invention to provide a detection method in which the detection level detected by the photodetector is sufficiently large even when the objective lens is maintained in focus, and the focus error detection range is suitable for focus error detection. An object of the present invention is to provide an optical head and a focus error detecting device using the same.

(Means for Solving the Problems) In order to solve the above problems, a first optical head according to the present invention uses a light beam reflected from a recording surface of an optical memory as a first optical head.
A first optical element for converging light in a direction, and a first and second light beam along a dividing line extending in a second direction orthogonal to the first direction, the light beam having passed through the first optical element And a second optical element that gives the first and second light beams aberrations in directions opposite to each other along the second direction, and a non-detection region extending parallel to the division line. A first photodetector having two divided detection areas and detecting the first light beam from the second optical element to generate first and second detection signals; It has two detection areas separated by a non-detection area extending parallel to the line, and detects the second light beam from the second optical element to generate third and fourth detection signals. A second photodetector that generates light.

Here, the first optical means is constituted by a cylindrical lens arranged such that the light beam reflected from the recording surface of the optical memory is incident on the convex surface side and is emitted from the flat surface side, The optical element has a first surface in which a light beam incident side surface is bonded to a flat surface side of the cylindrical lens, and a first light beam exit side surface is arranged to be inclined in a direction opposite to each other with respect to the second direction.
And a second triangular prism.

A second optical head according to the present invention includes a first optical element that divides a light beam reflected from a recording surface of an optical memory into first and second light beams along a dividing line extending in a first direction. A second optical element that focuses the first and second light beams split by the first optical element in a second direction orthogonal to the first direction, and the second optical element from the second optical element. A third optical element for giving aberrations in the first direction to the first and second light beams, and two detection areas divided through a non-detection area extending parallel to the division line. A first light detector for detecting the first light beam from the third optical element to generate first and second detection signals, and a non-detector extending parallel to the dividing line The second light from the third optical element, having two detection regions separated through regions Characterized by comprising a second light detector for generating a third and fourth detection signal by detecting the over arm.

Here, the first optical element is constituted by a wedge prism, and the second optical element is such that the first and second light beams from the first optical element are incident on the convex surface side,
The third optical element is arranged on a flat surface side of the cylindrical lens, and the third optical element is arranged on the flat surface side of the cylindrical lens.
A first and a second refractive index changing portion divided along a direction, wherein the first and second refractive index changing portions change the refractive index along the first direction and in directions opposite to each other; It may be constituted by a refractive index distribution type optical element.

A third optical head according to the present invention has a focusing function of focusing a light beam reflected from a recording surface of an optical memory in a first direction, and a division extending the light beam in a second direction orthogonal to the first direction. A holographic element having a splitting function of splitting the first and second light beams along a line, and an aberration providing function of giving the first and second light beams an aberration along the second direction; It has two detection areas divided through a non-detection area extending in parallel with the dividing line, and detects the first light beam from the holographic element to generate first and second detection signals. A first photodetector, and two detection regions separated through a non-detection region extending parallel to the dividing line, and detecting the second light beam from the holographic element. The third generating the third and fourth detection signals Characterized by comprising a photodetector.

A fourth optical head according to the present invention converges a light beam reflected from a recording surface of an optical memory in a first direction and gives an aberration along a second direction orthogonal to the first direction. An element, a second optical element for splitting the light beam from the first optical element into first and second light beams along a dividing line extending in the second direction, and A second detection area divided by a non-detection area extending to the first optical element and detecting the first light beam from the second optical element to generate first and second detection signals. One photodetector, and two detection regions separated through a non-detection region extending parallel to the division line, and detecting the second light beam from the second optical element. A second photodetector for generating third and fourth detection signals.

Here, the first optical element may be configured by a cylindrical lens arranged to be inclined along the second direction, and the second optical element may be configured by a Foucault prism.

Furthermore, in the optical head according to the present invention, a light source for generating a light beam, and the light beam generated from the light source are arranged so as to be focused on a recording surface of an optical memory. An objective lens formed on the recording surface and forming a beam spot larger than the minimum beam spot on the recording surface when the object is out of focus may be further provided.

The focus error detection device according to the present invention is a device that detects a focus error with respect to the recording surface of an objective lens that focuses a light beam generated from a light source on the recording surface of an optical memory, and according to the above-described present invention. The optical head further comprises processing means for processing the first to fourth detection signals to generate a focus error detection signal corresponding to the focus error.

Here, one of the first and second photodetectors is located forward and the other is located at a position on the optical axis where the amounts of incidence on the two detection areas are equal when the focus error is 0. They may be arranged rearward, respectively.

(Operation) In the present invention, directional aberration is given to the first and second light beams, and an image of the beam is formed on the detection area at the boundary of the non-detection area on the detector, and the light beam is slightly Only an image of the beam is formed on the detection area. Therefore, the signal level from the detector can be kept sufficiently high.

(Example) Hereinafter, an example of the present invention is described with reference to drawings.

FIG. 4 shows a schematic configuration of a focus error detecting device for detecting a focus error according to the first embodiment of the present invention. In the apparatus shown in FIG. 4, a laser beam from a light source 1, for example, a semiconductor laser, is collimated by a collimating lens 2, and this collimated light beam is shaped by a beam shaping prism 3 and sent to a beam splitter 4. Incident. The light beam transmitted through the beam splitter 4 is guided to an objective lens 6 by a mirror 5 and is focused and irradiated as a minute spot on the recording surface of the optical disk 7 by the objective lens 6.

The objective lens 6 is supported by a suspension (not shown) so as to be movable along the optical axis of the objective lens 6, and is moved in the optical axis direction by a drive current supplied to the focusing coil 16. When the objective lens 6 is maintained in focus, a minimum beam spot of the light beam traveling from the objective lens 6 to the optical disk 7 is formed on the recording surface of the optical disk 7, and the objective lens 6 is moved to the in-focus position. When the optical disk 7 approaches or separates from the optical disk 7, a spot larger than the minimum spot is formed on the optical disk 7.

The light beam reflected from the recording surface of the optical disk 7 passes through the objective lens 6, is reflected by the mirror 5, is reflected by the beam splitter 4 and is on the convex side of the cylindrical lens 8, which is the first one-way condensing means. And is collected only in one direction (first direction).

As shown in FIG. 4, the flat surface on the light beam emission side of the cylindrical lens 8 is divided by a dividing line extending in a direction (second direction) orthogonal to the condensing direction (first direction) of the cylindrical lens 8. First and second aberration providing means
Are joined. The light emitting surfaces of the first and second triangular prisms 9 and 10 are inclined so as to form a predetermined angle with respect to a plane orthogonal to the optical axis, and are arranged so as to be opposite to each other in the second direction. . The light beam condensed by the cylindrical lens 8 is incident on these triangular prisms 9 and 10. The light beam is split into two substantially semicircular light beams along a dividing line orthogonal to the light converging direction of the cylindrical lens 8 and directed in different directions by refraction. The triangular prisms 9 and 0 impart aberrations in opposite directions to the respective light beams.

Two light beams emitted from the prisms 9 and 10 are detected by the first and second photodetectors 11 and 12, respectively. Photodetectors 11, 12
Is a pair of detection areas 11a, 11b, and detection area 1
2a and 12b, each pair of detection areas is separated by a non-detection area extending in parallel to sides of the exit surfaces of the prisms 9 and 10 that are in contact with each other.

Output signals of the photodetectors 11 and 12 are amplified by the amplifier 13 and input to the arithmetic circuit 14. The arithmetic circuit 14 is composed of an addition circuit and a subtraction circuit. Output signals corresponding to the two detection areas 11a and 11b of the first photodetector 11 are A and B, and the output signals of the second photodetector 12 are two. When the output signals corresponding to the two detection areas 12a and 12b are C and D, a focus error signal is generated by an operation of (A + D)-(B + D), and A + B + C + D
A reproduction information signal is generated by the following operation. The focus error signal is supplied to the objective lens drive circuit 17, and in response to the focus error signal, a drive current is supplied from the drive circuit 17 to the objective lens 6.
Is applied to a focusing coil 16 that displaces the objective lens 6 in the optical axis direction, and the objective lens 6 is displaced in the optical axis direction. As a result, the focus error of the objective lens 6 is corrected and the
Are kept in focus. The information reproduction signal is supplied to a processing circuit (not shown).

FIGS. 5A and 5B show the optical detectors 11 and 1 with respect to the error of the focal position of the objective lens 6 with respect to the recording surface of the optical disc 7.
This shows how the focusing state of the light beam incident on the second detection areas 11a, 11b, 12a, 12b changes. Detection areas 11a, 11
The light beam images formed on b, 12a, and 12b have an asymmetric shape with respect to the non-detection area that divides the detection area, and are focused on the photodetector even in a state where the focus error is 0. , Spread over a relatively large area, and its light amount detection level is larger than the detection level at the time of focusing in the conventional double knife edge method. Therefore, according to the optical system as shown in FIG. 4, the level of the detection signal output from each detection area can be increased, and as a result, the level of the reproduction signal can be increased and a stable focus error signal can be obtained. Can be generated.

The principle by which images having the shapes shown in FIGS. 5A and 5B are formed on the photodetectors 11 and 12 will be described with reference to FIG. As shown in FIG. 6, when a circular incident beam 61 sequentially passes through a cylindrical lens 62 and a triangular prism 63, the incident beam firstly forms a linear image (focal line) on a surface having a detection area by the cylindrical lens 62. Form. When this beam subsequently enters the triangular prism 63, the optical path length of the light beam of the light beam in the triangular prism 63 differs depending on the position on the y-axis.
The light beam passing through the small band-shaped region extending in the direction is incident on the triangular prism 63 depending on the position on the y-axis.
Focus at different positions on the axis). Therefore, the cross-sectional shape of the beam in a plane perpendicular to the optical path of the light beam changes as indicated by reference numerals 64, 65, and 66 depending on the position on the optical axis. In other words, when the photodetector is placed at a certain position, the image of the light beam on the detection area of the photodetector changes with the focus error. In the cross section of the incident beam, the position on the optical axis where the small band-shaped light beam in the x direction including the optical axis converges is referred to as a reference focal point of this optical system in this specification.

The triangular prism 63 shown in FIG. 6 is divided into two along a direction orthogonal to the light condensing direction of the cylindrical lens 62, that is, a direction parallel to the generatrix of the cylindrical lens 62, and the light exit surface is optical axis. And the directions of the inclinations are opposite to each other, the arrangement of the optical system shown in FIG. 4 is obtained. Due to the principle of image change of the light beam shown in FIG. 6, in the optical system shown in FIG. 4, the spot shapes of the light beams incident on the photodetectors 11 and 12 are shown in FIGS. 5A and 5B. As described above, the detection signal having a sufficiently large level is obtained from the detectors 11 and 12 because the detection signal continuously changes in accordance with the focus error and has a relatively large spread even during focusing.

Also, the reference focusing point of the light beam when the first and second photodetectors 11 and 12 are in focus (when the focus error is 0), respectively.
That is, when the light beams detected in the detection regions 11a, 11b, 12a, 12b are arranged at positions on the optical axis at which the light amounts are substantially equal, the images of the light beams incident on the photodetectors 11, 12 are the 5A It is changed as shown in the figure. With respect to such an arrangement, when one of the photodetectors 11 and 12 is biased forward and the other is biased backward from the reference focal point, the photodetectors 11 and 1 are disposed.
The image of the light beam incident on 2 is changed as shown in FIG. 5B. In FIG. 5B, compared to FIG. 5A, the spot size particularly at the time of focusing becomes larger, the portion of the light beam spot formed in the non-detection area becomes smaller, and the light amount level detected by the photodetector becomes smaller. It will be appreciated that it increases.

FIG. 7 shows the characteristics of the signal output from the arithmetic circuit 14 shown in FIG. 4, and shows the focus error signal (A + D).
The range where − (B + C) shows a substantially linear change with respect to the focus error, that is, the focus error detection range is larger than the characteristic of the conventional double knife edge method shown in FIG.
Also, by increasing the detection signal levels from the photodetectors 11 and 12 at the time of focusing, the reproduction information signal A + B + C + D can be sufficiently increased.

Next, another embodiment of the present invention will be described with reference to FIGS. FIG. 8 shows an optical system according to a second embodiment of the present invention, and FIG. 8 shows only parts different from the first embodiment. The reflected light beam from the recording surface of the optical disk 7 guided by the beam splitter 4 is split into two light beams by a wedge prism 21 as a splitting means, and is incident from a convex side on a cylindrical lens 22 as a light collecting means. You. Cylindrical lens
On the flat surface side of 22, there is provided a refractive index distribution type optical element 23 which is an aberration imparting means.

As shown in FIG. 9, the refractive index distribution type optical element 23 is divided along a dividing line (shown by a broken line) perpendicular to the light collecting direction of the cylindrical lens 22, that is, parallel to the generating line of the cylindrical lens. It comprises portions 23a and 23b where the first and second refractive indexes are distributed. The refractive index of these refractive index distribution portions 23a23b is gradually changed in the direction along the dividing line of the wedge prism 21, and further, the refractive indexes are gradually changed in directions opposite to each other. The refractive index distribution portions 23a and 23b are divided by the wedge prism 21,
The two light beams condensed by the cylindrical lens 22 are respectively incident, and the refractive index distribution portions 23a and 23
b, but in the refractive index distribution portions 23a and 23b, since the refractive index is gradually changed, each of the light beams of the light beam passing therethrough passes through the optical path having a different optical path length. . That is, the prism elements 9 and 10 shown in FIG.
As in the case where light beams pass through, the light beams passing through the refractive index distribution portions 23a and 23b are given aberrations. As shown in FIG. 10, the light emitted from the gradient index optical element 23 is divided into first and second photodetectors 25 and 26 corresponding to the gradient index elements 23a and 23b by respective detection region dividing lines. Are detected by an integrated photodetector 24 arranged and integrated so as to be arranged in parallel. According to the optical system shown in FIGS. 8 to 10,
An image similar to that shown in FIG. 5A is formed on photodetector 24.

FIG. 11 shows an optical system of a focus error detecting device according to a third embodiment of the present invention. In this optical system, a second cylindrical lens for condensing a light beam following the first and second triangular prisms 9 and 10 in the first embodiment.
31 are arranged. This cylindrical lens 31
The generatrix is arranged so that the generatrix is orthogonal to the generatrix of the first cylindrical lens 8, and converges a light beam in a second direction orthogonal to the direction of condensing the first cylindrical lens 8.

When such a cylindrical lens 31 is added, the spread angle of the two light beams emitted from the lens 31 becomes smaller. Therefore, as a photodetector, as shown in FIG. 12, an integrated light obtained by integrating the first and second two-divided photodetectors 33 and 34 such that the respective detection area dividing lines are arranged side by side on the same line. Using a detector 32, the detection area of which is a triangular prism
It is possible to adopt a configuration in which the optical system is arranged so as to be perpendicular to the traveling direction of the light beams incident on the light sources 9 and 10, so that the optical system can be easily arranged and the installation space is reduced.

FIG. 13 shows an optical system of a focus error detecting device according to a fourth embodiment of the present invention. In this optical system,
A finite objective lens 6 is used. When a finite objective lens 6 is used, the reflected light beam from the recording surface of the optical disc 7 is converted into a focused light beam by the objective lens 6. As the photodetector 44, an integrated photodetector similar to that shown in FIG. 12 can be used.

FIG. 14 shows an optical system of a focus error detecting device according to a fifth embodiment of the present invention. In the optical system shown in FIG. 14, a single holographic element 51 realizes a first condensing means for converging light in one direction, a dividing means for dividing a light beam, and an aberration applying means for giving an aberration to the light beam. Is done. As shown in FIG. 15, the holographic element 51 converges the light beam in only one direction, and imparts an aberration 51a that tilts the linearly focused image in the optical axis direction.
51b, and regions 51c and 51d for dividing the light beam into two. Also in this optical system, the twelfth
An integrated photodetector similar to the one shown is used.
As shown in FIGS. 16A and 16B, the cross-sectional shape of the light beam emitted from the holographic element 51 of the optical system shown in FIG. 14 changes as it progresses.

According to the fifth embodiment, there is an advantage that the optical system for detecting the focus error can be further reduced in size and the installation space is further reduced.

FIG. 17 shows a focus error detecting device according to a sixth embodiment of the present invention. In the optical system shown in FIG. 17, the light beam reflected from the recording surface of the optical disc is transmitted through the objective lens 6 and the mirror 5 to the beam splitter 4.
And is incident on a cylindrical lens 8 which is a converging / aberration providing means inclined with respect to the optical axis. This cylindrical lens 8
The light beam is converged in one direction and the cylindrical lens 8 is arranged to be tilted, so that the light beam is given an aberration and the focal line of the light beam condensed in one direction is tilted. . The light beam transmitted through the cylindrical lens 8 is incident on the Foucault prism 109, which is a dividing means whose ridge line is arranged in a direction orthogonal to the light collecting direction of the cylindrical lens. Deflected in the direction.

The two light beams split by the Foucault prism 109 are incident on first and second split photodetectors 11 and 12. The photodetectors 11, 12 have respective detection surfaces 11a, 11b, 1
As shown by 2a and 12b, the image is divided into two along a detection plane division line (indicated by a broken line) that substantially matches the image of the ridge line of the Foucault prism 109. On this detector 11 and 12, 5A
An image as shown in FIG. 5 or FIG. 5B is formed. Therefore, by processing the output signals of the photodetectors 11 and 12 by the arithmetic circuit 14, a focus error signal and an information reproduction signal can be obtained.

In the optical system shown in FIG. 17, grooves formed concentrically or spirally on the optical disk 7, that is, coincide with the direction in which the image of the track Tr including the tracking guide or pit example extends. As described above, the ridge line of the Foucault prism 109 is determined. Therefore, any one or both of the two images formed on the photodetectors 11 and 12 have a pattern or a dark portion corresponding to the tracking guide due to diffraction caused when the light beam is reflected by the tracking guide. An image is formed. When the light beam is tracking the track, a dark area of equal area is formed on the photodetectors 11 and 12, and when the light beam goes off the track, a dark area is formed on one of the photodetectors 11 and 12. It is formed. Therefore, the seventeenth
As shown in the figure, the detection signals from the detectors 11 and 12 are respectively added to obtain addition signals (A + B) and (C + D), and the difference (A
A tracking error signal is obtained from (+ B)-(C + D). A drive current is supplied from an actuator drive circuit 17 to a tracking coil 18 in response to the tracking signal, and the tracking coil 18 displaces the objective lens 6 in a direction perpendicular to the optical axis of the objective lens, thereby causing the objective lens 6 to move. Is maintained at the track center.

In the optical system shown in FIG. 17, a tilted cylindrical lens 8 is used as a means for converging a light beam in one direction and giving aberration to the light beam, but as shown in FIG. For example, an element 110 having a surface whose curvature continuously changes, for example, a conical surface, for example, a conical lens may be used. Further, the Foucault prism 109 was used as the light beam dividing means, but a double wedge prism as shown in FIG. 19 was used.
115, or an optical element 116 having two cylindrical surfaces for focusing light at different positions as shown in FIG. 20, or a reflecting mirror 117 at a position where the beam is split into two as shown in FIG.
May be used. If only one of the two split beams is used, the splitting means may be a knife edge as shown in FIG. The detection system according to the present invention can be configured by an arbitrary combination of these or an integrated element.

FIGS. 23A, 23B and 24 to 27 show modifications of the optical system shown in FIG. In the optical system shown in FIG. 23A, the reflected light beam from the recording surface of the optical disc 7 guided by the beam splitter 4 is
The first surface is a conical surface, and the second surface is incident on an optical element 118 composed of two cylindrical surfaces. In the optical element 118, the light beam is converged in one direction by the conical surface of the first surface, and as the light beam travels through the optical element, aberration having a directionality in a direction orthogonal to the converging direction is reduced. Granted. Further, in the optical element 118, predetermined spots are formed at different points by the respective lens functions of the two cylindrical surfaces of the second surface. The interval between the two images can be arbitrarily determined by the structure of the second surface, and the two images can be detected by the integrated photodetector 24.

In the optical system shown in FIG. 23B, the first surface of the optical element 119 is formed as a cylindrical surface, and the second surface is formed as two conical surfaces. Therefore, the light beam is converged in one direction by the cylindrical surface of the first surface, is converged in the same direction as the first surface by the two conical surfaces of the second surface, and is directed in the direction orthogonal to the converging direction. An aberration is imparted to the direction, and the light beam is split.

In the optical system shown in FIG. 24, a conical lens 110 and a knife edge 120 are combined. In this optical system, the light beam is converged in one direction by the conical surface of the first surface, and as the light beam travels through the optical element, the directional aberration in the direction orthogonal to the converging direction is reduced. Granted. The light beam emerging from the flat second surface of the optical element 110 is partially blocked by the knife edge 120, and the remaining light beam is directed to the photodetector 11. That is, the knife edge 120 separates the light beam into two. In this embodiment, although one of the two split light beams is not detected, the light use efficiency is reduced.
When the reflection mirror 117 is employed instead of the knife edge 120 as shown in FIG. 25, the light beam utilization efficiency does not decrease. In the optical system shown in FIG. 25, a cylindrical lens 8 is arranged to be inclined with respect to the optical axis in order to focus a light beam in one direction and to give an aberration to the light beam.

In the optical system shown in FIG. 26, a conical lens 110 for converging a light beam in one direction and giving an aberration to the light beam and a Foucault prism 109 for splitting the light beam from the conical prism 110 are used. . As a means for splitting the light beam, the double wedge prism 115 is effective together with the Foucault prism 109 because it can be easily obtained. In the optical system shown in FIG. 27, a conical lens 110 for converging a light beam in one direction and giving an aberration to the light beam, and a double wedge prism 115 for splitting the light beam from the conical prism 110 are used. ing.

As can be seen, FIGS. 17, 23A and 23B and 24
In the optical system shown in FIGS. 27 to 27, conical optical elements 8, 110, 118, and 119 for focusing a light beam in one direction and optical elements 109, 120, 117, and 115 for splitting a light beam.
Is the holographic element shown in FIGS. 14 and 15.
It may be replaced with 51.

FIG. 28 shows a focus error detecting device according to a seventh embodiment of the present invention.

In the optical system shown in FIG. 28, the light beam emitted from the cylindrical lens 8 is split into two light beams by the compound prism 9 and is given an aberration in the direction along the light beam splitting line. The second surface of the composite prism 9 is formed on two reflection surfaces having different reflection angles so as to divide the light beam into two in a direction orthogonal to the converging direction of the cylindrical lens 8. Accordingly, the light beam reflected from the second surface of the composite prism 9 is split into two substantially semicircular light beams traveling in different directions. The third surface of the composite prism 9 is formed on an emission surface that is inclined in directions opposite to each other with respect to the traveling directions of the two light-deficient light beams traveling in different directions. The missing light beam is provided with aberrations in opposite directions along the light beam division line.

The two light beams emitted from the composite prism 9 are condensed in a direction orthogonal to the direction converged by the first lens 8 by a cylindrical lens 120 as a second lens for condensing the light beam. The light is received by a photodetector 32 having a set of two divided detection areas. The light detector 32 has respective detection surfaces 32a, 32b, 32c, 32
As shown by d, the light is split into two along a detection plane division line that substantially matches the image of the light beam division line by the composite prism 9. An image as shown in FIG. 5A or FIG. 5B is formed on the photodetector 32 as described above, and an output signal from this detector 32 is output by an arithmetic circuit in the same manner as in the previously described embodiment. It is processed in 14 and converted into a focus error signal, a tracking error signal and an information reproduction signal.

Here, the operation of the composite prism 9 will be described in some detail.

FIG. 29A shows an example of the shape of the composite prism 9 shown in FIG. FIG. 30 shows, as a reference, the shape of the prism when the reflection surface is formed as a continuous surface without being divided. In both cases, the angle of incidence of the light beam on the exit surface of the prism is the same, and the amount of aberration along the light beam division line given to the light beam emitted from the prism is also the same. As shown in FIG. 29B, the incident angle of the light beam on the exit surface of the prism is θb, the exit angle is θa, the deviation angle of the reflection surface from 45 degrees is α, the inclination angle of the prism exit surface is β, and the refractive index of the prism Let n be n from FIG. 30: θb = β−2α (1) sin θa = n sin θb = n sin (β−2α) (2)

Here, the condition for the exit angle to be parallel to the virtual optical axis turned 90 degrees after reflection is as follows from β = θa (2) to sin β = n sin (β−2α).

That is, when the prism shape is such that tan β = (sin 2α) / (cos 2α−1 / 2) (3), two light beams emitted from the prism are obtained as shown in FIG. 29A. Are parallel to each other, and the two light beams are focused at the same position of the focal point of the cylindrical lens 10. Here, α and β are calculated from the combination of α and β satisfying the expression (3) so that the two light beams split on the surface of the photodetector 32 provide a separation amount necessary for separation and detection. By offsetting, detection with a single packaged photodetector is achieved. Also, since the optical axis of the light beam incident on the cylindrical lens 120 is only slightly inclined with respect to the optical axis of the cylindrical prism 120,
The light can be condensed by the cylindrical lens 10 without greatly disturbing the aberration imparted by the composite prism 9.

The cylindrical lens 8 and the compound prism 9 can be integrally formed by molding with plastics, so that the element cost and the number of assembling steps can be reduced. In the above-described optical system, a cylindrical lens is used as the second lens, but a similar effect can be obtained by using a spherical lens. Further, the focus error was detected by calculating the detection signals from the two photodetectors emitted from the composite prism. However, as shown in FIG. 7, the detection output (AB) or (D) from one photodetector was detected. −
The focus error can also be detected from C).

FIG. 31 shows an optical system of a focus error detecting device according to an eighth embodiment of the present invention. In the optical system shown in FIG. 31, the light beam reflected from the recording surface of the optical disk 7 enters the beam splitter 4 via the objective lens 6 and the mirror 5, and is reflected by the beam splitter 4. The light enters the holographic element 128 having a predetermined function. The holographic element 128
It consists of two parts 128A, 128B, and these two parts 128A, 128B
8B is in contact with a linear boundary line, and each part has a spot where the width of the spot in the direction orthogonal to the boundary line changes continuously along the boundary line with the image of this boundary line as an axis. It has a wavefront conversion function of forming at predetermined intervals on the detection surface. First and second split photodetectors 11 and 12 are arranged for the light emitted from the holographic element 128. The two-part photodetectors 11 and 12 are divided into two parts by a detection plane parting line that substantially matches the image of the boundary line of the holographic element.

Output signals from the two-part photodetectors 11 and 12 are amplified to an appropriate level by an amplifier 13 and then input to an arithmetic circuit 14. The arithmetic circuit 14 sets the output signals corresponding to the two detection surfaces 11a and 11b of the first half-split photodetector 11 to A and B, and the two detection surfaces 12a and 12b of the second half-split photodetector 12. When the output signals corresponding to are represented by C and D, a focus error signal is generated by an operation of (A + D)-(B + C), and (A + B + C +
D) A reproduction information signal is generated by the following operation. When the detection optical system is installed in the direction shown in the figure with respect to the information signal sequence, the tracking error signal is obtained by the calculation of (A + B)-(C + D).

Here, the holographic element 128 gives the holographic element 128 a function of converting the wavefront of the plane wave in the same manner as in the case of the combination of the cylindrical lens and the triangular prism acting on the plane wave as in the above-described embodiment. The shape is determined. Further, the phase transfer function φ (x, y) of the holographic element is represented by φ (x, y) = (λ / 2π) ΣC _pq X ^p y ^q (where λ is the wavelength of the light beam, and x, y is , Represents the coordinates of the X, Y coordinate plane on the holographic element surface.) And selecting some sample points and tracing the ray to obtain a detection area as shown in FIG. 5A or 5B in the detection area. _{Alternatively} , C _pq may be determined so as to obtain an image as shown. In this case, setting the image obtained in the detection area directly leads to designing the detection characteristics, and further increases the design efficiency of the detection system.

According to such an optical system, an image as shown in FIG. 5A or FIG. 5B is detected on the photodetectors 11 and 12, and a signal as shown in FIG. Is output.

By using a holographic element, the size of the optical system can be further reduced. Figures 32 to 35
A modification of the optical system shown in FIG. 31 is shown. No. 32
In the optical system shown in the figure, a holographic element 12 is used in an optical head using a finite conjugate type objective lens 6.
8 are incorporated. According to such an optical system, a light source 1 in which a semiconductor laser as a light source and a photodetector are packaged in one package.
In addition, an integrated element 15 including the photodetectors 11 and 12 can be used, and the size of the optical head can be reduced.

In the optical system shown in FIG. 33, the holographic element 129 for detecting a focus error using a part of the light beam from the optical disk 7 is divided into two based on the extension direction of the image of the tracking guide Tr, Further, each of the two regions is divided into two for focus detection and tracking detection, and divided into four regions 129A to 129D. In this optical system, the light beam incident on the holographic element 129 is focused by the lens action of the holographic element 129 and is separated into four light beams by the four regions. Of the light beams, the light beams from the regions 129C and 129D where the intensity fluctuates most under the influence of the tracking error are detected in the light detection regions 136C and 136D of the photodetector 136, and are converted into tracking error signals. Light beams from the detection areas 129A and 129B are detected by the detection areas 136A and 136B of the detector 136, and are converted into focus error signals. According to the configuration of such an optical system,
Not only is the optical head miniaturized, but also a light beam for detecting a focus signal is separated from a light beam for detecting a tracking signal, so that crosstalk between the focus signal and the tracking signal can be suppressed.

In the optical system shown in FIG. 34, unlike the holographic element 129 shown in FIG. 33, the three regions 130A, 130B, and 130C are divided by a section line orthogonal to the direction in which the image of the tracking guide Tr extends. Divided into areas 130A and
The area 130B between 130C is further divided into two areas 130B1 and 130B2 by a dividing line in the direction in which the image of the tracking guide Tr extends.
The holographic element 128 is divided into four areas 1
30A, 130B1, 130B2, and 130C. Light beams from the central regions 130B1 and 130B2 of the holographic element 130
It is detected in the detection areas 136C and 136D of the photodetector 136, and this detection signal is converted into a tracking error signal. Also,
Light beams from both side regions 130A and 130C of the holographic element 130 are detected by detection regions 136A and 136B of the photodetector 136, and the detection signal is converted into a focus error signal. According to the optical system having such a configuration, the optical head is miniaturized. Furthermore, according to the configuration of such an optical system, the optical system for detecting a focus error is an off-axis optical system, and the intensity distribution of the image is distributed away from the dividing line of the detector. A very small percentage of the received light intensity enters the separation band on the detection surface of the photodetector 136 and is not received.

In the optical system shown in FIG. 35, a detection system is configured by using a reflection type holographic element 132 as a holographic element that focuses a light beam and gives an aberration to the light beam. The optical system having such a configuration has an advantage that the size of the optical head in the direction perpendicular to the optical disk surface is reduced.

[Effects of the Invention] As described above, according to the present invention, since the amount of light incident on the photodetector at the time of focusing is large, it is possible to stably detect a focus error with respect to a deviation in position adjustment of the optical system or a tracking deviation. Therefore, it is possible to provide a focus error detection device capable of setting a large focus error detection range even with a limited optical system size.

According to the present invention, the S / N of the reproduction information signal can be increased, and by setting a large focus error detection range, the initial pull-in of the focusing servo is easy, and even a large defocus is easy. Can be provided.

The focus error detection device of the present invention can be variously modified and implemented without departing from the gist.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an outline of a device for detecting a focus error employing a conventional double knife edge method, and FIGS. 2A to 2C are diagrams showing the focus error shown in FIG. 2B shows a change in the shape of the spot formed on the photodetector of the detection device, FIG. 2B shows the spot when the objective lens is maintained in focus, and FIGS. 2A and 2C respectively show The spots are shown when the objective lens is closer to the optical disk than when it is in focus and when the objective lens is farther away. FIG. 3 is a characteristic diagram showing changes of a focus error signal and a reproduction information signal with respect to a focus error in the focus error detection device of FIG. FIG. 4 is a diagram showing a schematic configuration of an apparatus for detecting a focus error according to the first embodiment of the present invention, FIG.
FIGS. 5A and 5B show the change of the image on the photodetector with respect to the focus error in the apparatus shown in FIG. 4, and FIG.
FIG. 7 is a schematic view of an optical system for explaining a detection principle of a focus error detection method of the optical system of the apparatus shown in FIG. 4, and FIG. FIG. 8 is a characteristic diagram showing a change in the reproduction information signal.
FIG. 9 is a schematic view showing an optical system of a focus error detecting device according to a second embodiment of the present invention, FIG. 9 is a perspective view showing the gradient index optical element shown in FIG. 8, and FIG. 8 is a plan view showing a configuration of the integrated photodetector shown in FIG. 8, FIG. 11 is a schematic diagram showing an optical system of a focus error detecting device according to a third embodiment of the present invention, and FIG. FIG. 13 is a plan view showing a configuration of an integrated photodetector of the device shown in FIG. 11, FIG. 13 is a diagram showing a schematic configuration of a focus error detection device according to a fourth embodiment of the present invention, and FIG.
FIG. 15 is a schematic view showing an optical system of a focus error detecting device according to a fifth embodiment of the present invention. FIG. 15 is an exploded perspective view showing the configuration of the holographic element shown in FIG. Is
FIG. 14 is a diagram showing a change in the cross-sectional shape of a light beam that has passed through the holographic element shown in FIG. 14, and FIG. 17 is a schematic diagram showing an optical system of a focus error detecting device according to a sixth embodiment of the present invention. FIG. 18, FIG. 18 is a perspective view showing a conical lens applicable to the optical system shown in FIG. 17, and FIG. 19 is a perspective view showing a double wedge prism applicable to the optical system shown in FIG.
20 and 21 are schematic diagrams showing optical elements applicable to the optical system shown in FIG. 17, and FIGS. 22 to 27 show modifications of the optical system shown in FIG. FIG. 28 is a schematic view showing an optical system of a focus error detecting device according to a seventh embodiment of the present invention. FIG. 29 is a sectional view of the composite prism shown in FIG.
29 is a sectional view of a composite prism having a shape different from the shape of the composite prism shown in FIG. 29 as a comparative example with respect to the composite prism shown in FIG. 29. FIG. 31 is an eighth embodiment of the present invention. 32 to 35 are schematic diagrams showing an optical system of a focus error detection device according to an example, and FIGS. 32 to 35 are arrangement diagrams of the optical system showing a modification of the optical system shown in FIG. 1 ... light source, 2 ... collimating lens, 3 ... beam shaping prism, 4 ... beam splitter, 5 ... mirror,
6: Objective lens, 7: Optical disk, 8, 62 ... Cylindrical lens, 9, 10, 63 ... Triangular prism, 11, 12
…… Photodetector, 21 …… Wedge prism, 23 ………………………………………………….
… Foucault prism, 118… Optical element, 120… Cylindrical lens

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G11B 7/09

Claims (10)

(57) [Claims] A first optical element for converging a light beam reflected from a recording surface of an optical memory in a first direction; and a first optical element for passing the light beam passing through the first optical element to the first optical element.
Splitting into first and second light beams along a division line extending in a second direction orthogonal to the first direction, and causing the first and second light beams to have aberrations in opposite directions along the second direction. And a second optical element which provides two detection areas separated by a non-detection area extending parallel to the dividing line, and detects the first light beam from the second optical element. A first photodetector that generates first and second detection signals, and two detection regions separated through a non-detection region extending in parallel with the dividing line, A second light detector for detecting the second light beam from the optical element to generate third and fourth detection signals. 2. The optical system according to claim 1, wherein the first optical unit includes a cylindrical lens arranged so that the light beam reflected from the recording surface of the optical memory is incident on the convex surface side and is emitted from the flat surface side. The second optical element has a light beam incident side surface bonded to the flat surface side of the cylindrical lens, and the respective light beam emission side surfaces are arranged to be inclined in directions opposite to each other with respect to the second direction. 2. The optical head according to claim 1, wherein the optical head comprises first and second triangular prisms. 3. A first optical element for dividing a light beam reflected from a recording surface of an optical memory into first and second light beams along a division line extending in a first direction, and a first optical element. The first and second parts divided by an element
A second optical element for converging the light beam in a second direction orthogonal to the first direction, and the first and second light beams from the second optical element along the first direction. A third optical element that gives an aberration, and two detection areas divided through a non-detection area extending parallel to the division line, and the first light beam from the third optical element And a first photodetector for detecting the first and second detection signals, and two detection areas divided through a non-detection area extending parallel to the dividing line, A second photodetector for detecting the second light beam from the third optical element and generating third and fourth detection signals. 4. The first optical element is constituted by a wedge prism, and the second optical element is configured such that the first and second light beams from the first optical element are incident on a convex surface side and are concave surfaces. The third optical element is disposed on a flat surface side of the cylindrical lens, and is divided into first and second sections along the first direction. And the first and second refractive index changing portions are constituted by refractive index distribution type optical elements whose refractive indexes change along the first direction and in directions opposite to each other. 4. The optical head according to claim 3, wherein: 5. A converging function for converging a light beam reflected from a recording surface of an optical memory in a first direction, and a first function of dividing the light beam along a dividing line extending in a second direction orthogonal to the first direction.
A holographic element having a splitting function of splitting the light beam into a second light beam, and a function of giving an aberration along the second direction to the first and second light beams; and a holographic element extending parallel to the split line. A first light detection device for detecting the first light beam from the holographic element and generating first and second detection signals by detecting the first light beam from the holographic element. And two detector regions separated by a non-detection region extending parallel to the dividing line, and detecting the second light beam from the holographic element to generate third and fourth light beams. An optical head, comprising: a second photodetector that generates a detection signal. 6. A light beam reflected from a recording surface of an optical memory is converged in a first direction, and a second beam orthogonal to the first direction is converged.
A first optical element for providing aberration along a direction; and a first optical element for dividing the light beam from the first optical element into first and second light beams along a dividing line extending in the second direction. 2 optical elements, and two detection areas divided through a non-detection area extending in parallel with the dividing line, and detecting the first light beam from the second optical element, A first photodetector that generates first and second detection signals, and two detection regions separated through a non-detection region extending in parallel with the division line, and A second photodetector for detecting the second light beam and generating third and fourth detection signals. 7. The apparatus according to claim 1, wherein the first optical element is constituted by a cylindrical lens arranged to be inclined along the second direction, and the second optical element is constituted by a Foucault prism. The optical head according to claim 6. 8. A light source for generating a light beam, and the light beam generated from the light source is arranged so as to be focused on a recording surface of an optical memory, and forms a minimum beam spot on the recording surface when in a focused state. The light according to any one of claims 1 to 7, further comprising: an objective lens that forms a beam spot larger than the minimum beam spot on the recording surface when out of focus. head. 9. A focus error detection device for detecting a focus error with respect to a recording surface of an objective lens which focuses a light beam generated from a light source on a recording surface of an optical memory, wherein the focus error is detected. A focus error detection device, comprising: an optical head according to claim 1; and processing means for processing the first to fourth detection signals to generate a focus error detection signal corresponding to the focus error. 10. One of the first and second photodetectors is positioned forward with respect to a position on the optical axis where the amount of light incident on the two detection areas is equal when the focus error is zero. 10. The focus error detecting device according to claim 9, wherein the other is arranged rearward.