JPS61199246A - Detection system for focus position - Google Patents

Abstract

PURPOSE:To reduce the occurrence of focus detection error with respect to the deviation of the direction or the position of an optical beam by arranging shielding elements before and after the converging point of a reflected light and forming an approximately rectangular optical pattern on a photodetector. CONSTITUTION:The first and the second shielding elements 10 and 11 are arranged before and after a converging point P1 of an optical beam 2, and an approximately rectangular optical pattern 12 is formed on light receiving surfaces of photodetectors 5a and 5b. In this constitution, boundary lines 13 and 14 of photodetectors 5a and 5b are moved in the vertical direction according as the inclination of a lens 3 is varied. In this case, positional relations among the converging point P1 and shielding elements 10 and 11 are selected properly to equalize extents of movement of boundary lines 13 and 14 on photodetectors. Thus, a device is obtained where the focus position detection error is difficult to occur with respect to the deviation of the direction or the position of the optical beam.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a focus position detection method, and more particularly to a focus position detection method suitable for application to an automatic focusing device in an optical disk device.

[Background of the invention]

In optical disk devices that irradiate a rotating recording medium with a light beam and detect information from changes in reflected light that change according to irregularities on the recording medium, an objective lens is moved according to the vertical movement of the medium to detect information. An autofocus servo system is required to keep the recording surface within the depth of focus of the light beam. As a focus position detection method when configuring such an automatic focus servo system, there are various methods as introduced in "Measurement and Control" Vol. 17, No. 10, pages 21 to 22. The format is known.

FIG. 1 shows one of the methods introduced as asymmetric focus error detection in the above-mentioned literature, in which a shielding element ( 4 is inserted, and two divided detectors 5a and 5b are arranged behind this shielding element.

According to this knife method, when the recording medium 1 is aligned with the focal position, as shown in FIG.
A focal image 6 is formed symmetrically on the two detectors 5a and Sb. On the other hand, when the recording medium moves away from the position 1', the light beam forms an image in front of the shielding element 40, as shown by the broken line 2', and the light traveling toward the detector 5a is blocked by the shielding element 4 and detected. The light receiving pattern 6' on the device is as shown in FIG. 2(a).

On the other hand, when the recording medium approaches the lens side, the light receiving pattern becomes as shown in FIG. 2(C). Therefore, the difference V between the outputs of the two detectors 5a and 5b changes in a figure 8 shape as shown by curve m8 in FIG. 3 with respect to the focus error δ.

However, in the above-mentioned Knifezi type focus position detection device, the positional relationship between the lens 3 and the shielding element 4 is important, and if this positional relationship changes due to temperature changes etc., the recording medium may not be detected even if it is at the correct focal position. Vessels 5a, 5
This results in an imbalance in the appearance of b, resulting in an error in the autofocus servo operation. Moreover, when the amount of shielding of the light beam by the shielding element increases, the S-shaped curve in FIG.
Distortion appears like this.

[Object of the Invention] An object of the present invention is to solve the above-mentioned problems and provide a focus position detection method in which a focus position detection error is less likely to occur due to deviation in the optical axis direction or position of the reflected light traveling toward the detector. It is in.

[Summary of the invention]

In order to achieve the above object, the present invention is characterized in that first rays are provided before and after the focal point of the reflected light focused by the lens. arranging the second shielding elements from the same direction,
The main feature is that a substantially rectangular light pattern is formed on the light receiving surface of the photodetector.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 is a diagram showing one embodiment of the focal position detection system according to the present invention.

When the reflective surface 1 is in the focal position, the reflected light beam 2
is refracted by lens 3 to point P.

move in the direction of convergence. A first shielding element 10 is arranged in front of point P! and behind point P (detector side)
The second shielding element 11 is arranged from the same direction as the first shielding element 10. In this way two shielding elements 10.1
1, the light beam from the lens forms a substantially rectangular light pattern 12 on the surfaces of the detectors 5a and 5b.
form. Boundary l11 of the light receiving pattern on the detector 5b
3 is formed by the shielding element 10, and the boundary line 14 on the detector 5a is formed by the shielding element 11.

The detectors 5a and 5b have boundaries in the longitudinal direction of the light pattern, and are arranged so that their respective outputs at a focused point are equal.

5(a) to 5(e) show how the light receiving pattern of the focal position detection system shown in FIG. 4 changes.

The position of the reflecting surface is from focal point position 1 to 1 shown in Fig. 5(C).
As the distance from b and 1a approaches, the amount of light shielded by the first element 1o decreases and the amount of light shielded by the second element 11 increases. force is 5b
It increases on the 5a side and decreases on the 5a side. On the other hand, the reflective surface is 1d, 1
As shown in FIG. 5(d) and (e)k, the first element 10 approaches the lens side from the focal point 1 as shown in FIG.
Since the amount of light shielded by the second element 11 increases and the amount of light shielded by the second element 11 decreases, the detector output becomes large on the 5a side. Furthermore, the fifth
Comparing Figures (a) and (e), the size of the light receiving pattern on the detector is different, but the amount of total reflected light does not change even if the position of the reflecting surface changes, so for example, Figure 5 (a) The output of the detector 5b in the state shown in FIG. 5(e) can be made equal to the output of the detector 5a in FIG. 5(e).

FIG. 6 is an explanatory diagram when the optical axis shifts due to a change in the tilt of the lens 3. Reflected light 2 changes from broken line to solid line 2
′, when shifted vertically, the light pattern on the detector becomes 13 with the boundary @13°14 expanded
', 14'.

In this case, by appropriately selecting the positional relationship between the concentration point P□ and the shielding elements 10.11, it is possible to design the boundary line movement on each detector to change equally. Note that when the optical axis changes in a direction parallel to the boundary 01.13)17 between the detectors 5a and 5b, there is no effect on the outputs of the detectors 5a and 5b.

FIG. 7 shows an embodiment in which the present invention is applied to an optical disk device. A parallel light beam emitted from a conventional parallel light beam device 18 is sent to a polarizing beam splitter 19 and a quarter wave plate 20.
The light passes through the lens 21 and focuses on the track 23 of the optical disk 22 .

The light reflected by the optical disk 22 is reflected by the polarizing beam splitter 19 and guided to the focal position detection system shown in FIG. Shielding element 10°11 and detector 5
a and 5b are arranged in a direction in which the dark 11117 is perpendicular to the track 23. With this arrangement, the diffraction patterns 24 and 25 caused by the track 23 are each divided into two halves by the dark line 17, so that an imbalance in the intensity of the diffraction patterns 24 and 25 that occurs when the track passes has an adverse effect on the focal position detection signal. I have nothing to give.

In the above description, the lens 3 may be a cylindrical lens.

In addition, in the focal position detection method according to the present invention, the reflective surface 1
The rate of change in the light receiving pattern on the detectors 5a and 5b with respect to the amount of deviation between the focus position and the 1° second shielding element 10 is
It can be changed by changing the interval between and 11, and when this interval is narrowed, the change in the light receiving pattern becomes thicker, and the detection sensitivity can be increased.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, by using two shielding elements, it is possible to obtain a focal position detection device that is less affected by the direction deviation or positional deviation of the light beam.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram showing a conventional focal position detection method, Figs. 2 (a) to (C) are diagrams for explaining changes in the light receiving pattern on the detector in the detection method of Fig. 1, and Fig. 3 The figure shows a defocus signal obtained from the detector, FIG. 4 is a perspective view of an optical system showing an embodiment of the focus position detection method according to the present invention, and FIGS. FIG. 6 is a diagram for explaining the change in the light reception pattern on the detector in the example. FIG. 6 is a diagram for explaining the change in the light reception pattern on the detector when the direction of the light beam changes in the present invention. FIG. 7 1 is a configuration diagram showing an embodiment of the present invention applied to an optical disk device. 1... Position of light reflecting surface, 2... Light beam, 3...
- Lens, 4, 10.11... Shielding element, 5a, 5b... Detector, 6... Light receiving pattern. Figure 1 Figure ¥I2 (α) Figure 3

Claims (1)

[Claims] 1. A lens for focusing the reflected light from the reflective surface, first and second shielding elements arranged toward the optical axis center from the same direction as before and after the convergence point of the reflected light, and the first , second
a light-receiving element having a light-receiving surface divided into two parts for receiving light from the lens whose shape has been changed by the shielding element; A focal position detection device characterized by detecting a positional deviation between the focus position and the focus position.