JPS61139936A - Focus detecting device - Google Patents

Abstract

PURPOSE:To detect with a high sensitivity a focal state extending over a wide range, and to enlarge a degree of freedom for arranging a photodetector by constituting an optical element for giving an astigmatism, of two cylindrical lenses for refracting a light in each orthogonal direction. CONSTITUTION:A laser luminous flux is made incident on a recording medium 26 through a polarized prism 22, a 1/4 wavelength plate 24 and an objective lens 25, and its reflected light is led to a photodetector 29. By a diverging action of a cylindrical concave lens 28 for diverging a light in the direction on the paper surface, a focus is formed on a focal plane F1, and by a focusing action of a cylindrical convex lens 27 for focusing a light in the direction vertical to the paper surface, a focus is formed on a focal plane F2. Accordingly, by selecting a refractive power of the lenses 27, 28, an astigmatism being a distance of the focal planes F1, F2 can be set optionally, the sensitivity for detecting a focus is raised, the dynamic range can be widened, and the degree of freedom for arranging and constituting the photodetector 29 can be enlarged.

Description

Detailed Description of the Invention (Technical Field) The present invention focuses and projects a recording beam or a reproduction beam onto an optical recording medium such as a video disc, a compact disc, or an optical data disc so that it is properly focused. This invention relates to a focus detection device for.

(Prior Art) Conventionally, in order to correctly focus a light beam on an optical recording medium, the focusing state of the light beam on the recording medium is detected to detect a focusing error, and the objective lens and the recording medium are adjusted accordingly. It is necessary to adjust the distance between the media and the media. In order to perform this focus adjustment correctly, it is first necessary to correctly detect the focus state. Conventionally, various focus detection methods have been proposed, one of which uses an optical element that imparts astigmatism to the light beam, such as a cylindrical lens, to detect changes in the spot shape of the light beam depending on the focal state. The so-called astigmatism method, which is detected by a light receiver, is shown in Fig.
This figure shows an astigmatic focus detection device disclosed in Japanese Patent No. 39123.

A laser beam emitted from the laser beam 8!1 is made into a parallel beam by a collimator lens 2, and is then made incident on an objective lens 4 via a beam blurter 3.

The objective lens 4 focuses this laser beam and projects it onto the optical recording medium 5 as a spot. Beam 1 reflected by recording medium 5. is the light beam focused by the objective lens 4,
It is reflected by the beam splitter 3 and guided to the focus detection system. This focus detection system includes a convex lens 6 having a uniform focusing effect in all directions, a convex cylindrical lens 7 having a focusing effect only in one direction, and a light receiver 8. Due to the combination of the convex lens 6 and the convex cylindrical lens 7, the shape of the spot of the light beam that has passed through the focus detection system changes depending on the focus state. That is, if the light receiver 8 is placed between the focal plane F+ due to the focusing effect of only the convex lens 6 and the focal plane F2 due to the focusing effect of both the convex lens 6 and the convex cylindrical lens 7, the light beam will not be correctly directed onto the recording medium 5. When the light is focused □, a circular spot enters the light receiver, but as the focus shifts, it becomes an elliptical spot. The major axes of this ellipse differ in direction by 90° between the front bin and the encroachment state. Therefore, for example, by providing a light receiving area divided into four parts in the light receiver 8, the spot shape of the beam projected onto the light receiver can be detected, and thereby the focal state of the beam spot on the recording medium 5 can be detected. can.

In the conventional focus detection device described above, the light beam is incident on the photoreceiver only through a focusing optical system such as a convex lens and a convex cylindrical lens, so the spot diameter on the photoreceiver is generally small and the detection sensitivity is low. There is a small drawback. That is, since it is difficult to accurately detect minute changes in the shape of a small spot formed by a focused light beam, there is a drawback that detection sensitivity is low. Furthermore, since both lenses have a focusing effect, the distance between the focal plane F1 of the convex lens and the composite focal plane F2 of the convex lens and the convex cylindrical lens, that is, the astigmatism difference, is short and the dynamic range is narrow. Furthermore, there is also a drawback that the degree of freedom in the arrangement of the light receiver is small, which is inconvenient in terms of design.

In the conventional focus detection device described above, when the light spot is not correctly incident in the width direction of the information track of the recording medium, that is, when there is a tracking error, or when the depth of the bits making up the information track is incorrect. /
If the number of wavelengths is not four, it may affect the distribution shape of the spot light projected onto the light receiver and may also affect the intensity of the light distribution. That is, crosstalk occurs between the tracking error and the focusing error, making it impossible to perform correct focus detection. In order to eliminate such drawbacks, for example, Japanese Patent Publications No. 53-37722 and No. 54-41883 disclose an optical element that generates astigmatism, the axis of which is approximately 45 degrees with respect to the direction of the information track. It has been proposed to arrange them so that they form an angle of .

FIG. 2 is a diagram showing a conventional focus detection device disclosed in Japanese Patent Publication No. 53-37722. Laser light 11
The light beam emitted from jjll is sent to beam splitter 1.
2, the light enters the objective lens 12, and is further focused by the objective lens and projected onto the optical recording medium 14.

An information track 14a is formed on the recording medium 14, and the information is read by scanning the information track with a beam spot. The light reflected by the recording medium 14 is collected by the objective lens 13, reflected by the beam splitter 12, and guided to the focus detection device. This focus detection device includes a convex cylindrical lens 15 that has a focusing action in only one direction. This convex cylindrical lens 1
an axis 1 extending in a direction perpendicular to the focusing direction of 5;
5a is the information track 1 formed by the objective lens 13.
It is inclined at approximately 45° with respect to the direction T of the image 4a. Although not shown in FIG. 2, a light receiver having a light receiving area divided into four by mutually orthogonal axes is placed between the focal planes F1 and F2 so that the direction of one of the divided axes is the direction of the information track image. Place it so that it matches T. With this configuration, for example, even if the beam spot on the recording medium 14 is shifted in the width direction of the information track 14a, an imbalance in the light intensity distribution will occur in the direction of the dividing line, so the output of the light receiving areas facing each other will be When the focus detection signal is taken as the difference between the sums, the component due to unbalance is canceled out and the fluctuation due to unbalance is not mixed into the focus detection signal.

In the above-described conventional focus detection device shown in FIG.
Since the optical element that provides astigmatism is composed of a convex cylindrical lens, it has the drawbacks of a small astigmatism difference, low detection sensitivity, and a narrow dynamic range. Furthermore, since one focal plane F1 is uniquely determined only by the focal length of the objective lens 13, there is a drawback that the degree of freedom in design is even smaller.

(Objective of the Invention) The object of the present invention is to eliminate the above-mentioned conventional drawbacks, eliminate crosstalk due to tracking errors, detect the focus state with high sensitivity over a wide dynamic range, and furthermore, An object of the present invention is to provide a focus detection device that can increase the degree of freedom regarding the structure and arrangement of the device.

(Summary of the Invention) The present invention irradiates an optical recording medium with a light beam emitted from a light source, and causes reflected light or transmitted light from the recording medium to enter a light receiver through an optical element that provides astigmatism. In the focus detection device, the optical element giving the astigmatism is arranged between a first cylindrical lens surface having refractive power only in a first direction and a second direction orthogonal to the first direction. This lens is characterized in that it is constructed in combination with a second cylindrical lens surface having refractive power.

(Embodiment) FIG. 3A is a diagram showing the configuration of an embodiment of a focus detection device according to the present invention. A laser beam emitted from a laser light source 21 is passed through a polarizing prism 22 . Collimator lens 23
The light is then incident on an objective lens 25 through a quarter-wave plate 24, focused by the objective lens, and projected onto an optical recording medium 26 as a spot.

After the light reflected by the recording medium 26 is focused by the objective lens 25, the 1/4 wavelength plate 24 and the collimator lens 2
2. This light passes through the 1/4 wavelength plate 24.
Since it has passed through 12 times, it is reflected by the polarizing prism 22 and guided to the focus detection device. The focus detection device of this example includes a cylindrical lens 21 that has a focusing action only in a first direction (a direction perpendicular to the plane of the paper in FIG. 3A), and a cylindrical lens 21 that has a focusing action in a second direction (a third Concave cylindrical lens 2 that has a diverging effect only in the direction in the plane of the paper in Figure A)
8 and a light receiver 29.

FIG. 3B is a view seen from the same direction as FIG. 3A; in this direction, the focused beam of light reflected from the polarizing prism 22 is not subjected to the lens action of the convex cylindrical lens 27, but the concave cylindrical lens Under the diverging effect of the lens 28, the light is focused at the distant focal plane F1. Third
Figure C is a view seen from a direction 90' different from Figure 3B; in this direction, the focused light beam is subjected to the focusing action of the convex cylindrical lens 27, but is not affected by the lens action of the concave cylindrical lens 28. Therefore, the focal plane F2 is located nearby. Therefore, by appropriately selecting the refractive powers of the convex cylindrical lens 27 and the concave cylindrical lens 28, the distance between the focal planes F1 and F2, that is, the astigmatism difference can be arbitrarily set. Therefore, compared to conventional focus detection devices, it is possible to increase the detection sensitivity and widen the dynamic range, and also to increase the degree of freedom in the arrangement and configuration of the light receiver 29, depending on the conditions required for each device. Optimum design can be achieved by

In addition, the light receiver 29 has a light receiving area divided into four parts, and by calculating the difference in the sum of the outputs of the opposing light receiving areas, the output becomes zero in the focused state, and the polarity is determined depending on the direction and size of the defocus. and a focus detection signal whose amplitude changes. By using this focus detection signal, for example, by driving an objective lens driving device (not shown) to displace the objective lens 25 in its front axis direction, the light beam spot can always be correctly focused on the recording medium 26. .

FIGS. 4A to 4C show another embodiment of the focus detection device of the present invention, in which the convex cylindrical lens 27 and the concave cylindrical lens 28 are arranged in reverse. In this example as well, as shown in FIG. 4B, when viewed from the direction in which the convex cylindrical lens 27 has a focusing effect,
The focused light beam that enters is subjected to a diverging effect by the concave cylindrical lens 28, but not to a converging effect by the convex cylindrical lens 27, and is focused at the distant focal plane F1, as shown in FIG. 4C. When viewed from the direction in which the concave cylindrical lens 28 has a diverging effect, the incident light beam is not affected by the diverging effect of the convex cylindrical lens 28;
Since the focal plane F2 is focused by the focal plane F2, the focal plane F2 is in the vicinity of the focal plane F2. Therefore, as shown in Figure 4A, 4
A light receiver 29 having divided light receiving areas 298 to 29d is placed between the focal planes F and F2, and the sum of the outputs of the light receiving areas 29a and 290 and the sum of the outputs of the first light receiving areas 1ii 29b and 29d are calculated. By taking the difference, the focus state can be detected.

FIGS. 5A to 5C schematically show other embodiments of the focus detection device according to the present invention. In this example, as shown in FIG. 5A, the first concave cylindrical lens 31 has a diverging effect only in the first direction D1, and the first concave cylindrical lens 31 has a diverging effect only in the second direction D2 perpendicular to the first direction. A second concave cylindrical lens 32 having a function is arranged. When viewed in the second direction D2, the incident light beam receives only the diverging action of the first concave cylindrical lens 31 and focuses at the distant focal plane F1, as shown in FIG. When viewed from above, as shown in FIG. 5C, it is focused on the nearby focal plane E2 only by the divergent action of the second concave cylindrical lens 32. Therefore, these focal planes F, and F
By arranging the light receiver 33 having four divided light receiving areas 33a to 33d in the middle of the two, the focus state can be detected.

In this example, the first and second concave cylindrical lenses 3
The refractive powers of 1 and 32 can be equal to each other,
In this case, the distance between the focal planes F1 and F2, that is, the astigmatism difference, is the difference between these concave cylindrical lenses 31 and 32.
is equal to the interval of Furthermore, when the refractive power of the first concave cylindrical lens 31 is made larger than the refractive power of the second concave cylindrical lens 32, the focal plane F1 becomes further away and the focal plane F2 becomes closer, so that the astigmatism difference becomes even larger. .

If both the first and second lenses are concave cylindrical lenses as in this example, the light receiver will generally be placed further away, so three light beams will be used to read information and focus. When performing detection and tracking state detection, there is a margin in the arrangement position of the light receiver, which facilitates the design.

FIGS. 6A to 6C are miniature diagrams showing the configuration of still another example of the focus detection device of the present invention. In this example, a light beam emitted from a laser light source 41 is made parallel by a collimator lens 42 and then enters an objective lens 44 via a beam splitter 43. The light beam focused by the objective lens 44 is made incident on the optical recording medium 45 as a spot, and the reflected light is focused by the objective lens 44, reflected by the beam splitter 43, and guided to the focus detection device. In this example, the focus detection device includes a first convex cylindrical lens 46 that has a focusing action only in a first direction D+ (a direction in the paper plane of FIG. 6A), and a second direction that is orthogonal to the first direction D1. D
2 (in the direction perpendicular to the plane of the paper in FIG. 6A), a second convex cylindrical lens 41 having a focusing action only, and a light receiver 48 are provided. When viewed in the second direction D2, as shown in FIG. 6B, the light beam receives only the focusing action of the first convex cylindrical lens 46, focuses at the nearby focal plane F+, and moves in the first direction. When viewed from D1, as shown in FIG. 6C, it receives only the focusing action of the second convex cylindrical lens 47, so that it is focused at the distant focal plane F2. Therefore, the focal state can be detected by arranging the light receiver 48 having a light receiving area divided into four parts between the focal planes F+ and F2.

In this example, the first and second convex cylindrical lenses 46
When the refractive powers of and 47 are made equal, the focal plane F1
The astigmatism difference, which is the distance between F2 and F2, is equal to the distance between both cylindrical lenses. Further, the refractive power of the first convex cylindrical lens 46 is changed to the second convex cylindrical lens 47.
By making the refractive power larger than the refractive power of F1, the focal plane F1 becomes closer, and conversely, the focal plane F2 becomes further distant, so that the astigmatism difference becomes larger. Therefore, by appropriately selecting the arrangement and refractive power of these convex cylindrical lenses 46 and 47, the positions of the focal planes F+ and F2 and the magnitude of the astigmatism difference can be set arbitrarily.

In the embodiment described above, the refractive surface having a diverging or converging action only in the first direction and the refractive surface having a diverging or converging action only in the second direction orthogonal to the first direction are separate cylindrical lenses. However, they can also be constructed as a single lens body.

7A to 7C show the astigmatism optical system configured as a single lens body as described above, and FIG. 7A shows a concave cylindrical lens surface 51a having a diverging effect only in the first direction D1. A single lens body 5 having a convex cylindrical lens surface 51b having a focusing action only in the second direction D2
1.

The single lens body 52 shown in FIG. 7B has a concave cylindrical lens surface 52a that has a diverging effect only in the first direction D1.
and a concave cylindrical lens surface 5.2b that also has a diverging effect only in the second direction D2.

In addition, the single lens body 53 shown in FIG.
b. These single lens bodies 51.5
2 and 53 can be used in the focus detection devices shown in FIGS. 3, 4, 5, and 6, respectively, described above. The use of such a single lens body results in a compact configuration, which is advantageous in downsizing the entire focus detection device.

FIG. 8 is a diagram showing still another embodiment of the focus detection device of the present invention. In this example, a light beam emitted from a laser light source 61 is projected as a spot onto an optical recording medium 64 via a beam splitter 62 and an objective lens 63. An information track 64a is formed on the recording medium 64,
The light beam modulated by this information track and reflected is sent to the objective lens 63, □50.1-6 Your'Jy*
62t'! as' lead to the output device. In this example, the focus detection device includes a single lens body 65 as shown in FIG.
It has a convex cylindrical lens surface 65b that has a focusing action only in a second direction D2 perpendicular to the first direction D1. The single lens body 65 is arranged such that the first and second directions D1 and D2 both form an angle of approximately 45° with respect to the direction T of the image of the information track 64a formed by the objective lens 63. In addition, the light receiver 6
6 is arranged so that one of the dividing lines coincides with the direction T of the information track image. With this configuration, the beam spot may be displaced in the width direction of the information track 64a, and the depth of the bits forming the information track may vary from 1/4 wavelength to Even if it's off,
The effects of these on the light distribution shape and the light splitting intensity appear in the direction T of the information track image, and in a light receiver having a dividing line in this direction, no focus detection error occurs, and detection accuracy can be improved.

In FIG. 8, the information track 64a on the recording medium 64 is shown as a straight line, but in the case of a circular information track, the direction T of the image of the information track may be the tangential direction of the circular track. .

The present invention is not limited to the embodiments described above, and can be modified and modified in many ways. For example, in the above-mentioned example, the optical recording medium is a reflective type, but it goes without saying that the present invention can also be applied to a transmissive type recording medium. Furthermore, in the example described above, the photoreceiver is placed midway between the focal planes F+ and F2, but it can also be placed at other positions. Also,
It is also possible to arrange two light receivers at the focal planes F1 and F2, or to arrange them symmetrically about a position midway between the focal planes F1 and F2. Furthermore, in the embodiment shown in FIG. 8, two cylindrical lenses can also be used as an optical system that provides astigmatism.

(Effects of the Invention) As described above, according to the present invention, the cylindrical lens surface has a converging or diverging effect only in the first direction, and the cylindrical lens surface has a converging or diverging effect only in the second direction perpendicular to the first direction. Since the structure is configured to generate astigmatism in combination with a cylindrical lens surface having can.

Furthermore, since the positions of the two focal planes can be arbitrarily set, the degree of freedom in the configuration and arrangement of the light receiver is increased, and the optimum design for each device can be easily achieved. In addition, if the direction having the refraction effect is arranged at an angle of 45' with respect to the direction of the image of the information track, and one of the division lines of the light receiving area is aligned with the direction of the image of the information track, the light Highly accurate focus detection can be performed without being affected by deviations in the width direction of the bits constituting the information track of the spot or variations in the depth of the bits.

[Brief explanation of drawings]

1 and 2 are diagrams showing the configuration of a conventional focus detection device, FIGS. 3A-C are diagrams showing the configuration of an embodiment of the focus detection device of the present invention, and FIGS. 4A-C 5A to 5C are diagrams roughly showing the configuration of other embodiments of the focus detection device of the present invention; FIGS. 6A- C is a diagram roughly showing the configuration of another embodiment of the focus detection device of the present invention, and FIGS. 7A to C show the configuration of three examples of astigmatism optical systems used in the focus detection device of the present invention. The perspective view and FIG. 8 are diagrams showing the configuration of still another embodiment of the focus detection device of the present invention. 21, 41.61. ...Light source 22...Polarizing prism 23, 42...Collimator lens 24...1/4 wavelength plate 25, 44.63...Objective lens 26, 45.64...Recording medium 27, 46 .47...Convex cylindrical lens 28,
31.32... Concave cylindrical lens 29, 33.
66...Receiver F+, F2...Focal plane 43, 62...Beam splitter 51, 52.53...Single lens body 64a...Information track patent applicant Olympus Optical Industry Co., Ltd. No. 1 Figure 4A Figure 5 Figure 6 Figure 7A B

Claims (1)

[Claims] 1. A focus detection device in which a light beam emitted from a light source is irradiated onto an optical recording medium, and reflected light or transmitted light from the recording medium is made to enter a light receiver through an optical element that provides astigmatism, The optical element giving the astigmatism includes a first cylindrical lens surface having refractive power only in a first direction, and a second cylindrical lens surface having refractive power only in a second direction orthogonal to the first direction. What is claimed is: 1. A focus detection device comprising: a combination of a cylindrical lens surface and a cylindrical lens surface.