JPS59186144A - Focus error detector - Google Patents

Abstract

PURPOSE:To make an optical system compact and to improve the accuracy of detection by setting an angle of two cylindrical lenses apart from a right angle to decrease the distance between the two cylindrical lenses. CONSTITUTION:The axis of the 1st cylindrical lens 14 and the 2nd cylindrical lens 15 having an identical focus (f) has an angle of 90 deg.+ or -theta (thetanot equal to 0) and the lenses are arranged at a distance d2 between the 2nd major plane 18 of the lens 14 and the 1st major plane 19 of the lens 15. Further, a photodetector 22 is placed at a position d3m (expressed in an equation I ) from the 2nd major plane 25 of the lens 15, and split lens 23, 24 of the photodetector 22 are tilted by nearly an angle of theta3 (expressed in an equation II) to the axis of the 2nd lens 15. A focus error signal is detected with high accuracy via a differential amplifier 6 by utilizing the change in the pattern of astigmatism images 20, 21 being in orthogonal relation on the photodetector through the constitution above. Further, since the distance betwen the two cylindrical lenses is decreased, the optical system is made compact.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus error detection device for detecting whether a light beam from a light source is correctly focused on the information surface of an optical disc in an optical video disc player or the like.

[Background of the invention]

In an optical disc device, during playback, it is necessary to always correctly focus the light beam from a light source such as a laser onto the information surface of the disc using an optical system as a minute spot, and a focus error detection device is provided for this purpose. It is being

In detecting such a focus error signal, a method is known in which a cylindrical lens is used to generate astigmatism images, and the position of these astigmatism images is monitored using a light receiver divided into four regions. ing. In such a conventional focus error detection device, two cylindrical lenses having approximately the same focal length and arranged orthogonally at a distance from each other are used. This cylindrical lens forms first and second astigmatic images.

In order to obtain a sufficient focus error detection range, it is necessary to make the interval between these images sufficiently large, and the interval between these images is equal to the interval between both cylindrical lenses.

is equal to , so the distance between both cylindrical lenses increases,
The disadvantage is that the optical system becomes larger.

Furthermore, since both cylindrical lenses are spatially separated, there is a drawback that it is difficult to increase the accuracy of the straight alignment and spacing.

Furthermore, in order to solve the above-mentioned drawbacks, an integrated lens has been proposed, which is a cylindrical lens with a large distance in the optical axis direction and whose incident and exit surfaces are perpendicular to each other. Even when molded using this method, the distance between the two surfaces fluctuates greatly due to temperature, humidity, etc., and is not suitable for practical use.

[Purpose of the invention]

An object of the present invention is to provide an improved focus error detection device that can obtain a sufficient focus error detection range even if the distance between two cylindrical lenses is reduced.

[Summary of the invention]

As a result of deriving a general solution for an astigmatism optical system using two cylindrical lenses, the following points become clear4.

It was. That is, there are two astigmatism line images, the interval between the astigmatism line images is determined by the focal length of the cylindrical lens, the interval between the cylindrical lenses, and the angle of the cylindrical lens, and the angle of the cylindrical lens is 90°. If the distance between the astigmatism line images deviates from this, the distance between the astigmatism line images will increase.If the distance between the cylindrical lenses is small, the distance between the astigmatism line images will become smaller.The angle of the astigmatism line images and the angle of the cylindrical lens will be Focal length of cylindrical lens, Focal length of cylindrical lens 2 Distance between cylindrical lenses and 2
This is determined by the relative angles of the two cylindrical lenses, and the relative angle between the two astigmatism line images is always 90°.

In the present invention, in order to reduce the distance between the two cylindrical lenses, the relative angle between the two cylindrical lenses is set to be shifted from 90°.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. Divergent light 3 emitted from a light source 2 such as a semiconductor laser 1 is converted into a parallel light beam 5 by a collimating lens 4 and projected along a first optical path 8 that reaches an information surface 7 of an optical disk 6 .

The objective lens 9 is located within the optical path 8 and focuses the light beam emitted from the semiconductor laser 1 onto a reading point 10 on the information surface 7 . The beam subric 11 is contained within the optical path 8 and establishes a second optical path 13 for the reflected light beam 12 reflected at the information surface 7 . The first cylindrical lens 14 and the second cylindrical lens 15 have almost the same focal length f, and the axis 16 of the first cylindrical lens 14 and the axis 17 of the second cylindrical lens 15 are 90' from each other. ±θ (θ 0°)
, and the distance d between the second major surface of the first cylindrical lens 14 and the first major surface 19 of the second cylindrical lens 15
, located along the optical path 13. Although each of these cylindrical lenses 14 and 15 is a single-convex cylindrical lens in the embodiment shown in FIG. 1, a double-convex cylindrical lens may also be used. These cylindrical lenses 14.
15 is a first astigmatism line image 2 along the optical path 13
0 and a second astigmatism line image 21 are formed. These images 20° 21 are images of the reading point 10. Generally statue 2o
The distance Δ between the image 21 and the image 21 is expressed as f + 0+ d2. From equation (1), generally Δ≧d2 is υ,
θ=0. That is, it can be seen that Δ=d, 2 only when both cylindrical lenses 14 and 15 are in an orthogonal relationship.

On the other hand, the image 2o and the image 21 are lateral to the reading point 10.

Here, f8 is the focal length of the objective lens 9.

As shown in FIG. 2, the light receiver 22 has two orthogonal dividing lines 23. ．． 24 and four light receiving elements 22a.

It has a divided shape of 22b, 22c, and 22d ic,
It is located approximately midway between the image 2o and the image 21.

Generally, images 20 and 21 are formed by the second cylindrical lens 15
Since the light receiver 22 is located from the second main surface 25 of the second cylindrical lens 1
5 from the second main surface.

Further, the relative angle between the images 20 and 21 and the axial direction 17 of the second cylindrical lens 15 is m-1α, where: It has been well known that when the two cylindrical lenses 14.15 are in an orthogonal relationship, the images 20.21 are also in an orthogonal relationship; Regardless of the angle of 90°±θ, the images 20.21 are orthogonal.

Therefore, with respect to the axial direction 17 of the second cylindrical lens 15, the dividing line 23.24 of the light receiver 22 is approximately θ3”ta.
If the installation is tilted by an angle of n-'α±45°, the changes in the pattern of the astigmatism image on the light receiver 22 can be effectively used to align the images in the first diagonal direction, as described below. Two light receiving elements 22α, 2
The sum signal of 2C and the sum signal of the two light receiving elements 221) and 22d arranged in the second diagonal direction are input to the differential amplifier 26, and the focus error signal can be detected from the output signal thereof.

That is, with respect to the moving distance Δ' of the information surface 7 in the vertical direction, the reading light spot changes by 2 Δ' in the same direction.

Therefore, the images 20.21 vary in the same direction over 2Δ'M. Image 2 that is orthogonal to the button on the photoreceiver 22
During the change from 0 to image 21, the focus error detection signal changes almost linearly, and this is the most effective signal for the fohecus control servo device. The moving distance Δ' of the information surface 7 from which such a signal is obtained is generally called the focus error detection Δ range, and is Δ'-□2M'' according to the above explanation.

As mentioned above, if the values of f, θ, d, .
m, M = 18.4, θ = 0°, 1°, 2°
, 3°, 4°, and 5°.

- As an example, in Table 1, in Figure 3, Δ' is 12
．． The relationship between θ and d2 for setting it to 5 μm is shown. As can be seen from Table 1, as the pressure θ increases, d2 decreases.

When θ=0・, d2=8.1 m is a large value, but by setting θ=3°, d2=
It can be reduced to approximately -2.7 tm.

Table 1 θ d2 0° 8.1 108.0L11++1 2° 6.7- 3' 2.7- In this way, the second main surface 18 of the first cylindrical lens 14 and the second cylindrical lens 15 are Since the distance d2 between the first main surfaces 19 can be made small, there is an advantage that the distance between the two cylindrical lenses 14, 15 can be set small.

Furthermore, as shown in FIG. 4, two single-convex cylindrical lenses 1
It is also possible to have a configuration in which the 4.15 planes are brought into close contact with each other. By bringing the planes into contact with each other, there is an advantage that variations and fluctuations in the distance between the two cylindrical lenses can be suppressed to a smaller level than when they are not brought into close contact.

Furthermore, the planar shape of the cylindrical lenses 14 and 15 shown in FIG. , the two opposing sides 29.30 are parallel to the axial direction 31 of the cylindrical lens 27, and the two opposing internal angles 32.33 are relative angles of 90° 10 (
It has a rhombic shape equal to θth). In this way, when the planes of both cylindrical lenses 27 and 28 are brought into close contact with each other, the relative angles in the axial directions 31 and 34 of both cylindrical lenses 27 and 28 can be set simply by aligning the sides of both cylindrical lenses. Therefore, there is an advantage that the angle setting becomes easy and the angle variation can be kept small.

Also, instead of using two cylindrical lenses, 1.11.

A biconvex cylindrical lens 35 as shown in FIG. 6 may also be used. In this case, it is possible to directly process both sides of the optical glass to form a cylindrical surface, but it is also possible to form an optical resin material such as acrylic resin as one piece using various molding methods. .

〔Effect of the invention〕

As described above, according to the present invention, by setting the angles of the two cylindrical lenses to be shifted from 90°,
While maintaining the orthogonal relationship between the two astigmatism line images, the interval between the astigmatism line images can be made larger than the interval between the cylindrical lenses.
The distance between the two cylindrical lenses can be set small,
Furthermore, they can be brought into close contact and integrated. In this case, since the cylindrical lens can be made thin, even if an optical resin or the like is used, the deformation due to temperature or heat is minute and there is no problem in practical use.

[Brief explanation of drawings]

1 is a front view and a side view of a focus error detection device constructed according to the present invention, FIG. 2 is a view showing the optical receiver and differential amplifier shown in FIG.
The figure shows θ, d2. Graphs 4 and 5 showing the relationship between Δ'
6 are a front view, a top view, and a side view of a cylindrical lens. Explanation of symbols 2・・・・・・・・・・・・・・・・・・Light source 6
・・・・・・・・・・・・・・・－・・・・・・Divergent light 5・
・・・・・・・・・・・・・・・・・・・・・Parallel light flux 6
・・・・・・・・・・・・・・・・・・・・・Optical disc 7・・・・・・・・・・・・・・・・・・ Information side 12...・・・・・・・・・・・・Reflected light flux 14・・・・・・・・・・・・・−・First cylindrical lens 15・・・・・・・・・・・・・・・・・・・Second
Cylindrical lens 16・・・・・・・・・・・・・・・
- Axis 17 of the first cylindrical lens 14...
...Axis 18 of second cylindrical lens 15...
...... Second main surface 19 of the first cylindrical lens 14 ......... Second main surface 19 of the second cylindrical lens 15 First principal surface 20......First astigmatic convergent line image 21......・Second astigmatism line image 22...
・・Photo receiver 23, 24・・・・・・・・・・・Dividing line 22α, b, C, d・・Photo receiving element 25・・・・・・・・・・・・・・・・Second main surface 26 of second cylindrical lens 15・・・・・・・・・・・・・・・Differential amplifier 27.28・・・・・・・・・Cylinder Lens 29.
30・・・・・・・・・Two opposing sides 31.34
・・・・・・・・・Axis direction 32.33 of cylindrical lens 27.28・・・・・・・・・Opposing internal angle 3
5・・・・・・・・・・−・・・・・・・・・Patent attorney for biconvex cylindrical lens Akio Takahashi No. 3 OL2 (mm) To Fig. 4 5 Fig. 2 9 6th line 242-

Claims (1)

[Claims] 1. In order to detect whether the light beam emitted from the light source is correctly focused on the information surface of the optical disk, the light beam reflected on the information surface is converged and astigmatism is eliminated. and a light receiver divided into four light receiving elements by two orthogonal dividing lines, into which the light beam reflected on the information surface enters, and a first diagonal of the light receiver. A focus error detection device that detects a focus error signal based on a difference signal between a sum signal of the two light receiving elements arranged in a diagonal direction and a sum signal of two light receiving elements arranged in a second diagonal direction of the light receiver. The optical means is composed of a first cylindrical lens and a second cylindrical lens having the same focal length, and the axial directions of the first and second cylindrical lenses are at an angle of 90·±θ( The second main surface of the first cylindrical lens and the first main surface of the second cylindrical lens are arranged with a distance d2 between the second main surface of the first cylindrical lens and the second main surface of the second cylindrical lens. is installed at a position from the second main surface of the cylindrical lens, and the dividing line of the light receiver is approximately θB = tan-'α±45° with respect to the axial direction of the second cylindrical lens. A focus error detection device characterized by being tilted by an angle. 2. The first and second cylindrical lenses are each a single-convex cylindrical lens, and the flat surfaces of the first and second cylindrical lenses are in close contact with each other. The focus error detection device described in . 3. The first and second cylindrical lenses are each a single-convex cylindrical lens, and the planar shape of the single-convex cylindrical lens is as follows:
A rhombic shape with two opposing sides parallel to the axial direction of the cylindrical lens and two opposing internal angles equal to the angle 90°±θ (θNO) formed by the axial direction of the first and second cylindrical lenses. A focus error detection device according to claim 1, characterized in that: 4. The optical means has both surfaces each having a convex cylindrical surface,
The focus error detection device according to claim 1, wherein the focus error detection device is a biconvex cylindrical lens integrally molded from an optically transparent polymeric material.