JPS6233659B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for adjusting focus by measuring the distance between an object to be photographed, read, etc. and an optical system;
It is about the method.

For example, in a video disk recording or reproducing device, it is necessary to focus a recording light beam or a reading light beam on the disk surface. For this purpose, it is necessary to always focus the optical system on the disk surface for converging the above-mentioned light beam onto the disk surface, that is, to perform automatic focus adjustment. Various automatic focus adjustment devices have been devised for this type of operation, but one method is to reflect the light beam for focus adjustment on a disk surface and detect the shape of the reflected image. Some optical systems intentionally provide astigmatism to adjust the focus by utilizing the fact that the shape of the reflected image changes as the position of the disk surface changes. For example, JP-A-50-10539, JP-A-51-141651, IEEE Chicago Spring Conference On Consumer Electronics,
Consumer Electronics) describes an automatic focusing device that uses a cylindrical lens to generate astigmatism. These principles will be explained with reference to FIG.

When it is desired to focus light from a light source 1 on a disk surface 4 as a reading beam through a half mirror 2 and an objective lens 3, the reflected light is focused on a detector 6 through a cylindrical lens 5 with astigmatism. As described above, when the position of the disk surface 4 changes, the shape of the image on the detector 6 changes due to the astigmatism, so if this is detected, the change in the position of the disk surface 4 can be detected. I will discuss this in more detail below.

In Figure 2, the optical systems 2, 3, and 5 shown in Figure 1 are
Although it is depicted as two convex lenses, as is well known, when the reflected light beam 10 from the disk surface enters an optical system with such astigmatism, this light beam does not converge to one point, but instead focuses on the meridian plane 11 and the sphere. At the image-defective plane 12, the light beams each have a linear cross section. Further, the cross section of the light beam before and after these surfaces becomes elliptical. These situations are shown in Figure 3. In FIG. 3, a cross-sectional view of the light beam on each surface indicated by broken lines is shown on the right side. Meridian image plane 11, spherical image plane 1
In No. 2, it can be seen that the cross section of the light beam has a line segment shape. Of course, in reality, it does not necessarily become a line segment due to other aberrations and the like. Further, in the middle of these two surfaces, there is a surface 13 in which the cross section of the light beam is approximately circular. This plane 13 will hereinafter be referred to as the best imaging plane. Here, if the distance between the optical systems 2, 3, 5 and the disk surface 4 changes in FIG. 1, the spread of the reflected light 10 from the disk surface changes,
The positions of each of the above-mentioned surfaces move. Therefore, for example, if the detector 6 is placed at the best imaging plane 13 when the disk surface 4 is at a prescribed position with respect to the optical systems 2, 3, and 5, that is, at a position where the optical image of the disk surface is a point, the disk When the surface 4 is at the specified position, the image on the detector 6 is approximately circular, and the image on the disk surface 4 is approximately circular.
When the position of the detector 6 shifts, the shape of the image on the detector 6 changes as shown in FIG. Therefore, if the optical system is operated so as to capture this change and always obtain a circular image, the disk surface will always be at a specified position with respect to the optical system.

As a detector 6 for detecting such a change in the optical image, for example, one as shown in FIG. 4 can be considered.
This detector 6 has four light receiving areas A to D and separately detects the amount of light incident on each area. If the center of the detector 6 is aligned with the optical axis of the optical system, it will become an ellipse as shown in Figure 4a if the disk surface is shifted in one direction with respect to the lens, and as shown in Figure 4c if it is shifted in the other direction. A shaped image is formed, and a difference occurs in the amount of light irradiated to areas A, C and areas B, D. When the disk surface is at a specified position, a circular image is formed, and the amounts of light irradiated to areas A and C and areas B and D are approximately equal. FIG. 5 shows a block diagram of a circuit for processing the output signal of the detector 6. Detector 6
The outputs of the light receiving areas A to D are respectively represented by the same numbers A to D. Signals A and C enter the amplifier 21, and A+C
An output corresponding to is obtained from the amplifier 21. Similarly, signals B and D are added by an amplifier 22 to obtain an output corresponding to B+D. These signals are supplied to the differential amplifier 23, and the output of the differential amplifier 23 is (A
+C)-(B+D). This output is supplied to the lens driving section 24. When the lens 3 is driven to change the distance from the disk surface 4, the detector 6
The image above changes as shown in Figure 4, and the output (A+C) - (B+D) of the differential amplifier 23 becomes positive in the state a of Figure 4, negative in the state c, and zero in the state b. . Therefore, feedback is applied to the system shown in FIG. 5, and if this is configured to be negative feedback, the lens driving section 24 will cause the disk surface 4 to come to a specified position.
When the best image forming plane 13 comes to the position of the detector, that is, it stops in the state shown in FIG. 4b. Further, if desired, an offset may be provided in the differential amplifier 23 to obtain (A+C)-(B
It is also possible to stop the lens drive when +D) reaches a certain level.

Figure 6 shows the value of (A+C)-(B+D) and lens 3.
The relationship between and the distance of the disk surface 4 is shown.

In this way, the disk surface can always be placed at a prescribed position with respect to the optical system, and the light beam can be focused on the disk surface. However, this conventional configuration uses a cylindrical lens to generate astigmatism, which increases the size of the lens.
It has the disadvantage of being expensive.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an automatic focusing device which eliminates the above-mentioned drawbacks and is compact and inexpensive.

The present invention provides an automatic system comprising a light source, a light receiving element, an optical system that focuses a projected light beam from the light source onto a reflecting object and refocuses the reflected light beam onto the light receiving element, and an astigmatism generating means. The focus adjustment device is characterized in that the astigmatism generating means is constituted by a hologram. Furthermore, if the optical image is too small, the optical image will enter the dead zone of the detector 6, resulting in a lower detection sensitivity. Therefore, as stated in the above literature, the dynamic range of focus adjustment is set to ±30~ on the disk side.
I had no choice but to widen the lens to about 50μ and reduce the sensitivity of the focus adjustment. (For reference, when using a 20x objective lens to read a video disc signal, the focus needs to be within ±2μ).

Another possible configuration is to have astigmatism already present in the light beam that is focused on the disk surface, but in this case, the aberration causes the light image that is focused on the disk surface to diverge, and this light beam is This is a drawback when used for reading.

The object of the present invention is to provide a simple construction, which eliminates the above-mentioned drawbacks, provides high focus adjustment precision, provides a high-quality readout or recording beam, and provides flexible and easy-to-adjust detector position automatically. The object of the present invention is to provide a focusing method.

The method of the present invention focuses an object beam with astigmatism through an optical system having astigmatism, and passes the object beam through a relay lens whose focal plane is the best image plane between the meridian image plane and the spherical image plane, and then passes the object beam through a photoreceptor. A hologram plate is created by making the light beam incident on the hologram plate at an appropriate angle of incidence θ and causing interference and recording the interference fringes on a photoreceptor. The first-order diffracted light beam is focused by a relay lens with an astigmatism difference △l, and then passed through an objective lens that has the best image formation plane and the video disk surface at a nearly conjugate position onto the video disk surface. The 0th-order diffracted light beam reproduced from the hologram plate without astigmatism is focused on the focal point on the video disk surface through the relay lens and the objective lens. It is focused as a video signal reading light beam at a position that does not overlap with the adjustment light beam, and the focus adjustment light beam reflected on the video disk surface is again passed through the objective lens and relay lens as a light beam with a very large astigmatism difference for focus detection. The feature is that automatic focus adjustment is performed by making the light enter a light receiving element.

The invention will be explained in detail below with reference to the drawings.

FIG. 7 is a diagram showing the configuration of an embodiment of a video disc playback apparatus that implements the method of the present invention. In this embodiment, a hologram 40 created by the method described later is reproduced using a laser beam 41, and a nearly parallel light beam 42 (for example, +1st-order diffracted light) for focus adjustment having astigmatism is reproduced. A readout parallel light beam 43 (0th order diffracted light) is separately generated.
The focusing light beam 42 is reflected by a half mirror 44 and enters a relay lens 45 . relay lens 4
5 focuses this incident light onto a focal plane 46 on the image side of the relay lens. As mentioned above, since the focusing light beam 42 has astigmatism, this focusing is caused by the astigmatism difference △
However, the focusing light beam 42 is created so that the best imaging plane coincides with the relay lens focal plane 46. This will be described later. This light beam is further reflected by the tracking mirror 47 and passes through the objective lens 48 to reach the video disk surface 49. However, the relay lens image-side focal plane 46 and the video disk surface 49 are placed in advance at a substantially conjugate position with respect to the objective lens 48. If the light beam is placed on the video disk surface 49, the light beam will be focused on the video disk surface 49. Of course, this focused light also involves an astigmatism difference, and the optical image on the video disk surface becomes circular, elliptical, or line segment-shaped. The astigmatism difference of this focusing is Δl/α, where α is the vertical magnification of the objective lens 48.
In this example, α=400 and Δl=1.6 mm. Further, the light beam reflected by the video disk surface 49 is again focused by the objective lens 48 near the focal plane 46 on the image side of the relay lens, and the cross section of the light beam here is circular, elliptical, or linear. Furthermore, this light flux passes through the relay lens 45, becomes a substantially parallel light flux with a cross section corresponding to the cross section at the focal plane 46, that is, a light flux with a very large astigmatism difference, passes through the half mirror 44, and is passed through the focus detection element. 50.

For the same reason as in the conventional example described above, when the distance between the video disk 49 and the optical system, that is, the objective lens 48, etc. changes, the shape of the light image on the video disk surface 49 changes, and accordingly, the light is incident on the focus detection element 50. Since the cross section of the light beam also changes, this can be used to adjust the focus. To be more specific, the position where the reading light beam 43 is focused by the relay lens is of course the relay lens image side focal plane 46, but as mentioned above, the best imaging plane of the focusing light beam 42 is matches this. Similarly, near the disk surface 49, the surface on which the readout light beam 43 is focused is the focusing light beam 42.
coincides with the best imaging plane of (As mentioned above, the focusing light beam is, for example, first-order diffracted light, and has a slight angle with the readout light beam 43, so
The two light spots on the disk surface are slightly shifted and do not overlap, so there is no problem with reading. ) Therefore, if the disk surface 49 is always placed on the above-mentioned best imaging plane, the readout light beam 43 can be focused on the disk. When the disk surface 49 is at the best image formation plane, the optical image formed by the focusing light beam 42 on the disk surface 49 is circular, and at that time, the light image formed by the focusing light beam 42 on the image side focal plane 46 of the relay lens 45 is circular. The cross section is also circular.
As described above, the light flux generated by the relay lens 45 on the focus detection element 50 side is a substantially parallel light flux (more precisely, the astigmatism difference is In this case, the light image formed on the focus detection element 50 has a substantially circular shape. When the disk surface 49 deviates from the above-mentioned desired position, the shape of the optical image created by the focusing light beam 42 on the disk surface 49 naturally changes, and accordingly, the detection element 5
Since the shape of the image on 0 also changes as shown in FIG. 4, this can be utilized in the same way as in the conventional example. Of course, in this case, the readout light beam 43 is received by a video signal readout element 51 made of another light receiving element and used for video signal readout.

A hologram that generates two light beams as described above can be created as described below. FIG. 8 is a diagram showing how the hologram 40 is created. First, the object light beam 60 is made into a parallel light beam by a collimator lens (not shown), and the lens 61
The light is focused through an optical system having astigmatism composed of a cylindrical lens 62. The optical systems 61 and 62 are configured so that the astigmatism difference at this time is Δl. The best imaging plane 63 at this time is the relay lens 64 (same focal length f as the relay lens 45 above).
The relay lens 64 is arranged so as to be on the focal plane of the light beam (hereinafter referred to as the "relay lens 64"), and the light beam is made substantially parallel and is made incident on the photoreceptor 66 at an incident angle θ through the half mirror 65. At the same time, a reference light beam 67 is converted into a parallel light beam by a collimator lens (not shown), and a half mirror 65
The photoreceptor 66 is made perpendicularly incident on the photoreceptor 66 via the photoreceptor 66 . In a space where the object light beam 60 and the reference light beam 67 are superimposed, interference fringes are formed by the two. The interference fringes are irradiated onto the photoreceptor 66 placed in the above space in the form of light intensity. As the photoreceptor 66, for example, a high-resolution dry plate made of a silver salt emulsion is used. The photoreceptor 66 is exposed to light for a certain period of time.
In the case of a high-resolution dry plate, a pattern corresponding to the brightness and darkness of the interference fringes is formed on the high-resolution dry plate after undergoing normal photographic processing processes such as development and fixing. A pattern corresponding to the brightness and darkness of the interference fringes formed on the photoreceptor 66 in this manner is generally called a hologram. When the hologram created in this way is irradiated with a reproduction laser beam, the reproduction laser beam is diffracted by a pattern corresponding to the brightness and darkness of the interference fringes of the hologram, and the diffracted light becomes a focus detection light beam 45, It is clear that the light that is not transmitted and is transmitted as it is becomes the light beam 43 for signal reading.

In the embodiments described above, since a hologram is used to generate astigmatism, a smaller device can be realized than when a cylindrical lens is used. Also, since holograms can be copied,
Very cheap. Furthermore, since focus detection and signal readout are performed using separate light beams, a high-quality signal readout light beam that does not have astigmatism can be used. Furthermore, since the optical path length is shortened, this is also advantageous for miniaturization.

Note that the present invention is not limited to the embodiments described above, and various modifications and changes are possible. For example, a hologram may be used to generate a focusing light beam and a video signal readout light beam, and a tracking light beam may also be generated at the same time. What is shown in FIG. 9 is a method for creating such a hologram.The light beams for focus adjustment and video signal reading are created in the same manner as in the above example, but the reference light beam 70 and focal point are used to create these. A parallel light beam 7 for tracking is formed in a plane substantially perpendicular to the plane formed by the adjustment light beam 73 and has an angle of ±φ with the former plane.
A hologram 74 is created by making a hologram 74 incident. When this is reproduced, a signal reading light beam 75, a focusing light beam 76, and tracking light beams 77 and 78 are obtained as shown in FIG. 9b. An embodiment of a video disk playback device using this is shown in FIG.
As shown in FIG. 0B, the reflected light is focused on both sides of the video signal track 80, and the tracking detection elements 81 and 82 detect the reflected light to drive the optical system so that the signal reading light does not deviate from the track 80. Furthermore, in the embodiment described above, the light flux incident on the focus detection element 50 is made into a substantially parallel light flux by the relay lens, but if the astigmatism difference can be increased without doing this, a certain effect can be obtained. One thing is clear. Further, the configuration of the detection element 50 is not limited to that explained in FIG. 4, and various other configurations are possible.

According to the present invention, a light beam having astigmatism can be generated using a hologram without using a conventional cylindrical lens, so that a compact and inexpensive automatic focusing device can be provided.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the configuration of a conventional automatic focus adjustment device that uses astigmatism, and Figure 2 shows the positions of the meridional image plane and the spherical image plane of a light beam passing through an optical system with astigmatism. A line diagram showing an example; Fig. 3 is a cross-sectional view of a light beam passing through an optical system that also has astigmatism; Fig. 4 is a line diagram showing an example of a detector that detects the shape of an astigmatic image;
The figure is a line diagram showing the electrical configuration of a device for detecting the shape of an astigmatic image, and Figure 6 is a line diagram showing the relationship between the focus detection signal obtained by the circuit shown in Figure 5 and the amount of defocus. 7 is a diagram showing the configuration of an example of an automatic focusing device according to the present invention, FIG. 8 is a diagram showing a method for creating a hologram used in the embodiment shown in FIG. 7, and FIG. FIG. 10a is a diagram showing a method for creating a hologram used in another embodiment of the present invention, and FIG.
Figure b is a diagram showing an example of the position of a light spot on the disk surface in the embodiment shown in figure a. 40... Hologram, 41... Laser light, 42
... Light flux for focus adjustment, 43 ... Light flux for reading video signal, 44 ... Half mirror, 45 ... Relay lens, 46 ... Relay lens image side focal plane, 47
...Tracking mirror, 48...Objective lens,
49... Video disk surface, 50... Focus detection element, 51... Video signal reading element, 60...
Object light flux, 61...Lens, 62...Cylindrical lens, 63...Best imaging plane, relay lens focal plane, 64...Relay lens, 65...Half mirror, 66...Photoconductor, 67...Reference light flux ,7
0... Reference light flux, 73, 76... Focus adjustment light flux, 72, 71, 77, 78... Tracking light flux, 75... Video signal reading light flux, 80...
...Video signal track.

Claims (1)

[Claims] 1. An automatic focus adjustment device having a light source, a light receiving element, an optical system that focuses the projected light flux from the light source onto a reflecting object and refocuses the reflected light flux onto the light receiving element, and astigmatism generating means. 2. An automatic focusing device according to claim 1, wherein the astigmatism generating means is composed of a hologram.