JPH05174397A - Auto-focus device - Google Patents

Abstract

PURPOSE:To simplify the auto-focus device of an optical pickup which reads the optical record. CONSTITUTION:A sample surface S is irradiated with a light beam I at a fixed incident angle, and the beam reflected on the surface S is made incident on a two-split photodetector element F via the polarizers 2 and 3. The voltage applied on both sides of a liquid crystal element 1 is slopingly changed at the center through both ends of the element 1. The reflected beams move in parallel to each other in accordance with the position of the sample S and have the different passing positions of the element 1. The polarizing angle of the element 1 differs according to the tilt and the place of the applied voltage. Then the incident light quantity of the element F varies and the unbalance of the received light quantity of the element F is increased. Thus the focus detecting sensitivity is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus device for an optical pickup for optical recording and reading which is used for compact discs and the like.

[0002]

2. Description of the Related Art The structure of a conventional autofocus device for an optical pickup as described above is shown in FIG. In this figure, S is the sample surface, L is the light source, and the light from the light source is focused on the sample surface at the regular position through the collimator lens C and the objective lens M. The light reflected from the sample surface travels backward in the irradiation light path and is reflected laterally by the semitransparent mirror H. The optical system on the right side of the semitransparent mirror H is an autofocus device, and an image of the focal point 0 on the sample S at the normal position is formed on the light receiving element F by the projection lens P. Since the sample surface is specularly reflected, when the sample surface is vertically displaced from the normal position, the focal point of the light of the light source L by the objective lens M viewed from the projection lens P is vertically displaced from the normal position 0. , And therefore the image of the focal point on the light receiving element F is blurred and has a large diameter. Therefore, although focus detection may be performed based on the size of the light image on the light receiving element, it is not possible to determine the front focus and the rear focus, and therefore the cylindrical lens A may be provided after the projection lens P for the purpose of further increasing the sensitivity. let me in,
The light beam focused on the light receiving element F is an astigmatic light beam, and the minimum circle of confusion of the astigmatic light beam is formed on the light receiving element when the sample surface is at the normal position. Therefore, as the sample surface shifts vertically from the normal position, the condensed image on the light receiving element F becomes vertically long or horizontally long. Light receiving element F is 4
When the focused image becomes vertically long as shown in FIG. 5B, the amount of light received by the light receiving elements F1 and F3 increases and F2
When the amount of light received by F4 decreases and the image becomes horizontally long, on the contrary, F1, F
The light receiving amount of 3 decreases and the light receiving amounts of F2 and F4 increase. In this way, the magnitude of the sum of the outputs of the light-receiving elements F1 and F3 and the sum of the outputs of F2 and F4 is examined to know the direction of defocus, and the sensitivity is enhanced.

[0003]

The above-mentioned conventional device requires a cylindrical lens for producing an astigmatic light beam, and has a complicated optical structure. The present invention is intended to obtain a highly sensitive autofocus device with a simpler structure.

[0004]

As shown in FIG. 1, means for injecting a light beam I at a non-perpendicular constant angle on a sample surface and two divisions for receiving a reflected light beam R of the light beam specularly reflected on the sample surface. A plane including a light receiving element F, a liquid crystal element 1 inserted between the sample surface and the light receiving element, and polarizers 2 and 3 arranged orthogonally on both sides of the liquid crystal element, and including both the incident and reflected light beams. And the plane perpendicular to the reflected light flux is taken as the reference direction, and this is taken as the left-right direction, and the optical rotation angle of the liquid crystal element is 0 at the central portion of this reference direction, and the optical rotation angle becomes more distant from the central portion to the left and right in the reference direction. The light receiving element F is divided into two parts F1 and F2 in the above-mentioned reference direction so that the focus position of the sample can be detected by comparing the outputs of both the light receiving elements F1 and F2. ..

[0005]

In FIG. 1, the surfaces of the liquid crystal element 1 and the light receiving element F are drawn toward the viewer of the drawing, but in reality they are perpendicular to the light beam R. The sample irradiation point of the incident light flux I when the sample S is at the in-focus position is set to 0. The reflected light flux R moves in parallel with the vertical movement of the sample surface, and the reflected light flux irradiation spot r on the light receiving element F moves in the direction of the arrow in the figure. Since the polarizers 2 and 3 are orthogonal to each other, the amount of light incident on the light receiving element F is zero without the liquid crystal element 1. In the liquid crystal element 1, the optical rotation angle is zero at the center and the optical rotation angle is symmetrically increased toward the left and right ends. In the figure, the arrow of the liquid crystal represents the optical rotation angle, and the direction perpendicular to the reference direction X means the optical rotation angle 0, and the optical rotation angle becomes larger as it becomes parallel to X. The reflected light flux R transmitted through the polarizer 2 is the liquid crystal element 1
Since the deflecting surface rotates inside, some light enters the light receiving element F through the polarizer 3. When the sample surface is at the in-focus position, the reflected light flux R passes through the center of the liquid crystal element 1. At this time, most of the reflected light flux passes through a place where the optical rotation angle of the liquid crystal is small,
The amount of light incident on the light receiving element F is small, and both F1 and F2 have the same amount of incident light. When the sample surface moves slightly above the focus, the reflected light flux moves to the left side of the liquid crystal element 1, the incident light quantity of the light receiving element F1 increases, the incident light quantity of F2 decreases, and the optical rotation of the reflected light flux R by the liquid crystal 1 occurs. When the angle becomes large, the amount of light incident on the light receiving element F1 increases more than the amount of light flux moving on the light receiving element F, and the output of F1 changes as shown by the solid line in FIG. Similarly, the output of the light receiving element F2 when the sample surface goes down changes as shown by the dotted line in FIG. The way of changing the outputs of the light receiving elements F1 and F2 is such that the reflected light flux R directly enters the two-divided light receiving elements F1 and F2 without the liquid crystal element 1 and the polarizers 2 and 3. Compared with the output change (dashed line in FIG. 2) due to the change in the distribution area of the spot to both light receiving elements, the spot is enlarged in the vicinity of the focus and the focus detection sensitivity is improved.

[0006]

FIG. 3 shows an embodiment of the present invention. In the figure, 4 is an objective lens for converging and irradiating the sample surface with light, and a thin partial light beam near the periphery of the sample irradiation light beam is an incident beam I to the sample used for autofocusing. Since the incident light flux of is a parallel light flux, it passes through the focal point 0 of the lens 4. That is, in the case of this embodiment, the objective lens 4 serves as means for irradiating the sample surface with a light beam non-perpendicularly at a constant angle. The incident beam I is specularly reflected on the sample surface S, and the reflected light flux R moves in parallel due to the vertical movement of the sample surface, but all of them pass through a point Q on the focal plane above the lens 4 by the lens 4. These reflected light beams are reflected laterally by the mirror 5 inserted in a part of the light beam incident on the objective lens 4.
In this side optical path, the liquid crystal element 1 and the polarizers 2 and 3 before and after the liquid crystal element 1 are provided.
2 is inserted in the rear of the polarizer 3 and vertically in the figure.
The divided light receiving elements F are arranged. Deflector 2,3
Are orthogonal to each other, and the liquid crystal element 1 is arranged such that the optical rotatory angle increases vertically from the center in a symmetrical manner with respect to the horizontal line Z in the drawing.

FIG. 4 shows a structure for changing the optical rotation angle of the liquid crystal element 1 depending on the location as described above. 1 in the figure
Reference numerals 1 and 12 are transparent electrodes sandwiching the liquid crystal, and lead wires 13 and 14 are attached to the center of the liquid crystal element.
1 and 12 are short-circuited to each other at both ends. Therefore, the voltage difference between the transparent electrodes is maximum at the center of the liquid crystal element and is zero at both ends. A nematic type is used for the liquid crystal, and the surfaces of the transparent electrodes 11 and 12 in contact with the liquid crystal are treated so that the liquid crystal molecules are aligned in the directions orthogonal to each other. For this reason, in this liquid crystal element, the optical rotation angle of light is 90 ° in any part in the state where no voltage is applied between the electrodes 11 and 12, and the polarization direction of the polarized light passing through the polarizer 2 is rotated by 90 °. And send it to the polarizer 3. Since the polarizer 3 is orthogonal to the polarizer 2, light is evenly transmitted through the front surface of the liquid crystal element. When a voltage is applied between the electrodes 11 and 12 here, the liquid crystal molecules are oriented in the direction of the electric field, so that at the central portion where the voltage difference between the electrodes 11 and 12 is large, the rotation is almost 0, and the voltage difference between the two ends. Is 0
Therefore, the optical rotation angle is up to 90 °, and eventually the device is such that the optical rotation angle increases symmetrically from the center to both ends.

The reflected light beam R from the sample surface is a parallel light beam after passing through the lens 4 when the sample surface is at the in-focus position, but becomes a divergent light beam when the sample surface moves up and down, and an irradiation spot on the light receiving element F is obtained. Area increases, but there is no problem in operation. Each divided segment F of the light receiving element F
The output signals of 1 and F2 are applied to both input terminals of the differential amplifier 6. By driving the servomotor 7 by the output of the differential amplifier 6 to move the sample S up and down, the differential amplifier 6
Output becomes 0. When the output of the differential amplifier becomes 0, it is in focus.

[0009]

According to the present invention, since an aspherical optical element such as a cylindrical lens for producing an astigmatic light beam is not required,
The autofocus device can be made inexpensive, and the change in the polarization direction of the illumination spot is added to the configuration that detects the movement of the light flux due to the vertical movement of the sample as the change in the received light amount due to the movement of the illumination spot on the light receiving surface, thus expanding the change in the received light amount This also improved the sensitivity of focus detection.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of the principle of the present invention.

FIG. 2 is a graph showing changes in output of a two-divided light receiving element according to the present invention.

FIG. 3 is a side view of an apparatus according to an embodiment of the present invention.

FIG. 4 is a perspective view of a liquid crystal element in the above embodiment.

FIG. 5 is a side view of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Liquid crystal element 2,3 Polarizer 4 Objective lens F 2 Split light receiving element F1, F2 Two split segment of light receiving element S Sample I Incident beam R Reflected light flux

Claims (1)

[Claims] 1. A means for irradiating a sample surface with a light beam from an obliquely fixed direction, a two-divided light receiving element for receiving a specularly reflected light beam from the sample surface of the light beam, and a light receiving element arranged in an optical path of the reflected light beam. A liquid crystal element and polarizers arranged on both sides of the liquid crystal element and having polarization directions orthogonal to each other, and an intersection line direction of a plane including the sample irradiation light beam and the reflected light flux and a plane perpendicular to the reflected light flux. With the reference direction as the reference direction, the liquid crystal element extends in the reference direction, and the optical rotation angle of the polarization increases from 0 symmetrically toward both ends from the point where the center of the reflected light flux when the sample is at the in-focus position passes. Thus, the two-division light-receiving element is one in which two divided segments are arranged in the above-mentioned reference direction with the incident point of the center of the reflected light beam when the sample is at the in-focus position as a boundary.