GB2129931A

GB2129931A - Apparatus for detecting position of optical pick-up

Info

Publication number: GB2129931A
Application number: GB08231479A
Authority: GB
Inventors: Shigeo Kubota
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1982-11-04
Filing date: 1982-11-04
Publication date: 1984-05-23
Also published as: GB2129931B

Abstract

An apparatus for detecting the position of an optical pick-up comprises a lens 7 having lens areas forming four quadrants I to IV sectioned by two planes, each including the optical axis Z and being perpendicular to each other, the focal lengths of the lens areas of the first and third quadrants I, III being equal, the focal lengths of the lens areas of the second and fourth quadrants II, IV being equal to each other, the focal length of the lens areas in the first and third quadrants I, III being different from that of the lens areas in the second and fourth quadrants II, IV, the respective lens areas having symmetry about the optical axis Z and the lens 7 being located within the optical path of the beam reflected from an optical disc and a photodetector having four divided detecting portions in correspondence with the respective lens areas for detecting the reflected beam passed through the lens 7. <IMAGE>

Description

SPECIFICATION Apparatus for detecting the position of an optical pick-up This invention relates to apparatus for detecting the position of an optical pick-up.

Embodiments of apparatus according to the invention can be used with an optical reproducing apparatus for a video disc or a pulse code modulated audio disc.

There has been proposed an optical reproducing apparatus in which a laser beam from a laser light source is converged by an objective lens onto a record surface of an optical disc, the reflected laser beam is impinged on the detecting plane of a photo-detector, and a reproduced signal is derived from the photo-detector.

For such an optical reproducing apparatus, there has been disclosed in U.S. Patent 4,023,033 a focus servo technique in which the light detecting plane of the photo-detector is circular; this light detecting plane is divided equally into four detecting portions forming first to fourth quadrants; a semi-cylindrical lens is located in front of the photo-detector; a focus state detecting output is derived from the difference between the sum of detected outputs from the detecting portions of the first and third quadrants and the sum of detected outputs from the detecting portions of the second and fourth quadrants; and the position of an objective lens along its optical axis is controlled by the focus state detecting output thereby to carry out the focus servo.

This focus servo technique will now be explained with reference to Figures 1 to 6 of the accompanying drawings. In Figure 1, reference numeral 1 designates a semi-cylindrical lens, 2 its semi-cylindrical surface, and 3 its rectangular flat plane surface. Using orthogonal coordinates, an origin 0 is assumed to be at the centre of the plane 3, the X axis is parallel to the generatrix of the semi-cylindrical surface 2 and passes through the origin 0, the Y axis is perpendicular to the generatrix or X axis and passes through the origin 0, and the Z axis perpendicular to the plane 3 and passes through the origin 0. Moreover, a point which is on the semi-cylindrical surface side of the lens 1 and on the Z axis but spaced from the origin 0 by a predetermined distance, is taken as another origin 0'.Using rectangular coordinates, with the origin 0' and within the plane perpendicualr to the Z axis, x and y axes are assumed to intersect the X and Y axes at 450 in their positive sides. Then, the circular detecting plane of the photo-detector exists in the x and y plane (the plane including the x and y axes) and the detecting plane is divided equally into four portions by the x and y axes thereby to provide the photo-detector into light detecting portions of first to fourth quadrants designated I to IV.

Then, a converging beam is assumed to be incident on the plane 3 of the lens 1 in such a manner that, as shown in Figure 2, the optical axis of a beam spot 4 formed by the converging beam is coincident with the Z axis and the spot 4 on the plane 3 becomes a circular shape (Figure 2 shows the spot 4 in a perspective view). Temporarily, axes a, b, c and d, each corresponding to the radius of the spot 4, are provided in the first to fourth quadrants I to IV at the positions intersecting the X and Y axes at 450.

Now, with reference to Figure 3, the loci of light rays 5 and 6 on the cross-section of the lens 1 (referred to as the XOZ cross-section) including the X axis, the origin 0 and the Z axis, and on the cross-section of the lens 1 (referred to as the YOZ cross-section) including the Y axis, the origin 0 and the Z axis when the converging beam is incident on the plane 3 of the lens 1, will be explained. Since the YOZ cross-section is constant in thickness, the ray 6 incident on the YOZ cross-section advances parallel to the ray before being incident thereon after passing therethrough and then passes through a point P on the Z axis. While, since the XOZ cross-section forms a convex lens, the ray 5 incident on the XOZ cross-section is refracted towards the Z axis and hence passes through a point P' nearer to the lens 1 than the point P.

It is assumed that the detecting plane DT of the photo-detector is positioned at the origin 0' (Figure 1) on the Z axis between the points P and P', and when a spot 4' of the laser beam irradiated on the detecting plane DT becomes a circle as shown in Figure 5, the converging beam from the objective lens is focussed on the record surface of the optical disc as a focal point. Thus, the fact that the focal point of the converging beam from the objective lens is formed in front of or behind the record surface of the optical disc is equivalent to the fact that the position of the detecting plane DT of the photo-detector is shifted to a point a in front of the origin 0' or to a point p behind the origin 0', and hence the spot 4' on the detecting plane DT of the photodetector becomes an ellipse as shown in Figure 4 or 6.

That is, the elliptical spot 4 in the case of Figure 4 has its longer diameter in the direction at 450 relative to the x axis in the first and third quadrants I and Ill, while the elliptical spot 4' in the case of Figure 6 has its longer diameter in the direction at 450 relative to the x axis in the second and fourth quadrants II and IV. In Figures 4 to 6, axes a', b', c' and d# correspond respectively to axes, a, b, c and d of Figure 2.

Therefore, if the detecting plane of the photodetector is larger than the spot 4' in area, based upon the difference between the sum of the detected outputs from the detecting portions of the first and third quadrants I and Ill and the sum of the detected outputs from the detecting portions of the second and fourth quadrants II and IV, the converging state of the laser beam on the record surface of the optical record medium such as an optical disc by the objective lens can be detected. Accordingly, focus servo becomes possible by moving the objective lens along its optical axis so as to make the above difference zero.

However, the beam passed through the semicylindrical lens 1 is not symmetrically distributed, so that when the semi-cylindrical lens 1 is combined with the photo-detector, it is difficult to obtain a correct tracking error signal (separated from a focus error signal).

According to the present invention there is provided an apparatus for detecting the position of an optical pick-up, the apparatus comprising: a lens having lens areas of four quadrants sectioned by two perpendicular planes each including an optical axis, the focal lengths of said lens areas of first and third quadrants being equal, the focal lengths of said lens areas of second and fourth quadrants being equal, the focal length of the lens areas in the first and third quadrants being different from that of the lens areas in the second and fourth quadrants, and said respective lens areas having axial symmetry, said lens being located within an optical path of a beam reflected from an otical disc; and a photo-detector having four divided detecting portions in correspondance with said respective lens areas for detecting the reflected beam passed through said lens.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a perspective view showing a semicylindrical lens used in a prior art apparatus; Figure 2 is a perspective view showing a beam spot irradiated on the rectangular plane of the semi-cyindrical lens shown in Figure 1; Figure 3 is a schematic diagram illustrating the loci of the light rays incident on the semicylindrical lens shown in Figure 1; Figures 4, 5 and 6 are each a perspective view showing a beam spot; Figure 7 is a perspective view depicting an example of the lens which forms part of apparatus for detecting the position of an optical pick-up and according to the invention; Figure 8 is a schematic diagram illustrating the loci of the light rays incident on the lens shown in Figure 7;; Figures 9, 10 and 11 are respectively diagrams showing a beam spot; Figure 12 is a graph showing characteristic curves of the beam spot; and Figure 13 is a systematic diagram showing an embodiment of apparatus for detecting the position of an optical pick-up and according to the invention.

First, an example of the lens used inthe embodiment of apparatus for detecting the position of an optical pick-up will be described.

As shown in Figure 7, the example of the lens 7 includes four lens areas 7a, 7b, 7c and 7d of first to fourth quadrants I, íl, Ill and IV which are sectioned by two planes including the optical axis or Z axis and perpendicular to each other, that is the XZ plane (the plane including the X and Z axes) and the YZ plane (the plane including the Y and Z axes).In this case, the focal lengths of the lens areas 7a and 7c at the first and third quadrants I and III are selected to be equal (assumed to be f1); the focal lengths of the lens areas 7b and 7d at the second and fourth quadrants II and IV are also selected to be equal (assumed to be f2); the focal length f, of the lens areas 7a and 7c at the first and third quadrants I and Ill and the focal length f2 of the lens areas 7b and 7d at the second and fourth quadrants II and IV are selected to be different to each other; and the respective lens areas 7a to 7d are located symmetrically with respect to the optical axis or Z axis. Although if the focal lengths f, and ~2 were not equal, they could be either positive or negative, in practice, the focal lengths f, and f2 are both positive and include infinity.

In the illustrated example, the lens 7 is a planoconvex lens. And, the above-mentioned rectangular coordinates consisting of the X and Y axes (where O is its origin) is assumed to be formed on a plane 7A of the lens 7, and the X, Y and Z (which coincides with the optical axis of the lens 7) axes form the orthogonal coordinates.

To manufacture the lens 7, there are proposed three different methods. The first method is one in which the elements of the four quadrants are made of two kinds of lens material such as glass or plastics material having different refractive indexes, which are bonded together by an adhesive agent and then polished to make the lens 7.

The second method is as follows. The same lens material such as glass or plastics material is polished to make lenses of different curvature, that is different focal length, the respective lenses thus made are divided into lenses of four quadrants, and the respective divided lens areas are bonded together by an adhesive agent two by two to make the lens 7.

The third method is as follows. A metal mould having different curvatures is used to form the lens 7 from plastics material with the structure mentioned above. This third method has the advantage compared with the first and second methods that no bonding of the lens elements is required.

A rectangular coordinate is assumed at the spherical surface side of the lens 7 with its origin 0' spaced from the origin 0 by a predetermined distance and its x and y axes are respectively parallel to the x and y axes within the plane perpendicular to the Z axis and parallel to the plane 7A of the lens 7. Then, the circular detecting plane of the photo-detector is made coincident with the x and y plane (on which the x and y axes exist), and the detecting plane of the photo-detector is divided equally into four areas by the x and y axes thereby to divide the photodetector into the light detecting portions of four quadrants.

Then, onto the plane 7A of the lens 7 is incident a parallel beam in such a manner that its optical axis coincides with the Z axis, and its crosssectional area on the plane 7A becomes a circle.

In the case that the parallel beam is projected onto the plane 7A of the lens 7, the loci of the rays 5 and 6 incident on the lens areas 7a and 7b of the first and second quadrants I and II in the cross-sectional area on the x axis will be explained with reference to Figure 8. The ray of the ray 5 incident on the lens area 7a of the lens 7 and then passed therethrough is refracted towards the Z axis and then passes through a focal point F, of the lens area 7a on the Z axis, while the ray of the ray 6 incident on the lens area 7b of the lens 7 and then passed therethrough is also refractive towards the Z axis and then passes through a focal point F2 on the Z axis of the lens area 7b more distant from the lens 7 than the focal point F,.

In Figure 8, it is assumed that when the detecting plane DT of the photo-detector is positioned at the point 0' (Figure 7) between the focal points Ft and F2, and the spot 4' of the irradiated beam on the detecting plane DT becomes a circle as shown in Figure 10, the converging beam from the objective lens is just focussed on the record surface of the optical disc (not shown). With this assumption, the fact that the focal point of the converging beam from the objective lens is focussed in front of or behind the record surface of the optical disc is equivalent to the fact that the position of the detecting plane DT of the photo-detector is shifted to a point a nearer to the lens 7 than the point 0' or to a point more spaced from the lens 7 than the point 0'.

Thus, in such a case the spot 4' on the detecting plane DT of the photo-detector becomes as shown in Figure 9 or 11, in which each of the spots 4' is formed of the combination of T circles of different radius. The spot 4' in Figure 9 is formed of T circles in the second and fourth quadrants II and IV which are large in radius and + circles in the first and third quadrants I and Ill which are smaller than the former in radius.

While, the spot 4' in Figure 1 1 is formed of T circles in the first and third quadrants I and Ill which are large in radius and X circles in the second and fourth quadrants II and IV which are smaller than the former in radius.

In each of Figures 9 to 1 the amounts of light of the spot 4' in the respective quadrants are equal. However, when the radii of the l circles are 4 different to one another, the light amount per unit area is different and hence there appear bright and dark patterns.

In this embodiment, therefore, as shown in Figures 9 to 11 , the configuration of a detecting plane 8 of the photo-detector is formed as a circle whose radius is a little smaller than that of X circles which are smaller than the other T circles whose radius is larger than the former, and the detecting plane 8 is divided equally into four portions to form detecting portions 8a, 8b, 8c and 8d of the first to fourth quadrants I to IV. As a result, the detected outputs from the respective detecting portions are in proportion to the light amount of the spot 4' per unit area thereof.

When a converging beam is irradiated on a certain plane, the relation between the amount of light (%) within a circular area of the converging beam spot and the converging state of the beam is shown in the graph of Figure 12 by curves Si, S2 and 83. The curve S, represents the case that the converging beam is just focussed on the plane, and the curves S2 and S3 represent the case that the converging beam is focussed in front of and behind the plane. Accordingly, if the amount of light in a circular area of a predetermined radius PO (encircled energy) is detected, the converging state of the converging beam on that plane can be detected.

Therefore, with this embodiment, based upon the difference between the sum of the detected outputs from the light detecting portions in the first and third quadrants I and Ill and the sum of the detected outputs from the light detecting portions in the second and fourth quadrants II and IV, the converging state of the laser beam by the objective lens on the record surface of the optical record medium such as an optical disc can be detected. Accordingly, if the objective lens is moved along its optical axis so as to make the above difference zero, focus servo becomes possible.

Moreover, since the beam passed through the lens 7 is kept symmetrical in beam distribution, when the lens 7 is combined with the photodetector, a correct tracking error signal owing to the symmetry of light distribution can be obtained. One way of obtaining the correct tracking error signal is as follows. From the above-mentioned difference between the detected outputs, are provided the focus error signal (low in frequency) and tracking error signal (high in frequency) by frequency separation.By the difference between the sum of the detected outputs from the light detecting portions of the first and second (or first and fourth) quadrants I and 11(1 and IV) and the sum of the detected outputs from the light detecting portions of the third and fourth (or second and third) quadrants Ill and IV (or II and Ill), the tracking state can be detected. Accordingly, it is sufficient for carrying out the tracking servo that, for example, a tracking mirror is controlled to make the above difference zero. In this case, the reproduced signal can be obtained from the sum of the detected outputs from all the light detecting portions.

Now, with reference to Figure 13, an embodiment of apparatus for detecting the position of an optical pick-up and in which the lens 7 explained above is employed will be described.

In the embodiment shown in Figure 13, the diverging laser beam emitted from a laser light source 10 is introduced into a collimator lens 1 1 to form a parallel beam. The parallel beam from the collimator lens 1 1 is introduced through a polarizing beam splitter 12 and a quarter-wave plate 13 onto a galvano-mirror (tracking mirror) 14 to be reflected thereon. The reflected beam from the galvano-mirror 14 passes to an objective lens 1 5 and is focussed thereby on the record surface of an optical disc 16. The reflected beam on the record surface of the optical disc 1 6 passes through the objective lens 15 to the galvano-mirror 14 and via the quarter-wave plate 13 to the beam splitter 12. The beam reflected on the beam splitter 12, which is an approximately parallel beam, is focussed by the lens 7 on the light detecting plane of a photo-detector 17. The detected outputs from the respective detecting portions 8a, 8b, 8c and 8d of the photo-detector 17 are supplied to adders, AD1, AD2 and AD3 and a subtractor SB, and then a lowpass filter F, and a high pass filter F2 so as to produce the focus error signal, the tracking error signal and the reproduced signal as described above.

Thus in the embodiment is provided an optical element (lens) which has different powers dependent upon the directions within the plane perpendicular to the optical axis which are employed, so that the lens has axial symmetry and the beam emitted from the lens has a similar beam distribution at the four quadrants.

Claims

1. An apparatus for detecting the position of an optical pick-up, the apparatus comprising: a lens having lens areas of four quadrants sectioned by two perpendicular planes each including an optical axis, the focal lengths of said lens areas of first and third quadrants being equal, the focal lengths of said lens areas of second and fourth quadrants being equal, the focal length of the lens areas in the first and third quadrants being different from that of the lens areas in the second and fourth quadrants, and said respective lens areas having axial symmetry, said lens being located within an optical path of a beam reflected from an optical disc; and a photodetector having four divided detecting portions in correspondence with said respective lens areas for detecting the reflected beam passed through said lens.

2. An apparatus according to claim 1 wherein based upon the difference between the sum of outputs from divided detecting portions corresponding to the lens areas of first and third quadrants and the sum of outputs from divided detecting portions corresponding to the lens areas of second and fourth quadrants, the distance between said optical disc and said optical pick-up is detected.

3. An apparatus for detecting the position of an optical pick-up, the apparatus being substantially as hereinbefore described with reference to Figures 7 to 11 and 13 of the accompanying drawings.