JPS61139938A - Focus control device - Google Patents

Abstract

PURPOSE:To execute with a high accuracy a focus control by an inexpensive and light constitution by forming an optical means by placing correspondingly many absorbing parts for absorbing a light by shifting the position, on both faces of a parallel face plate for making a light transmit through. CONSTITUTION:A luminous flux is made incident on a disk 15 through an objective lens 14, and its reflected light is led to a photodetector 17 through an optical means 16. The optical means 16 is formed by placing an absorbing part 16a which does not make a light transmit through by shifting the position, on both faces of a transmitting part 16b of a parallel face plate for making a light transmit through, and a line for connecting the end parts of a pair of absorbing parts 16a is inclined by a prescribed angle against the optical axis. When a disk 15 is focused, parallel rays are made incident on the optical means 16, and when they approach the lens 14, they become a diffused light, a transmitting light is made incident mostly on the right half 17b of the photodetector, and when it goes away from the lens, it becomes a converged light and made incidently mostly on the left half 17a. By detecting this difference by an amplifier 18, a focused state is detected, and the detection accuracy can be raised by an inexpensive and light structure.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a focus control device that can be applied to big devices such as optical video disc players and digital audio disc players.

[Conventional technology]

FIG. 7 is a schematic diagram of a method called the critical angle method among such pickups. In the figure, 1 is a critical angle prism, 2 is a light receiving element divided into two parts, and 3 is a differential amplifier.

In this system, in the focused state, the critical angle prism 1
It is adjusted so that the incident light becomes approximately parallel light and is reflected at an angle close to its critical angle. Therefore, for example, when the disk (not shown) and the objective lens are in the in-focus position, 2
The light irradiated onto the divided light receiving elements 2 becomes approximately equal. However, if the disk and the objective lens become close and the light becomes diffused, the right half of the light with respect to the optical axis in Figure 7 becomes a distance, and if the disk and the objective lens become far apart and the light becomes convergent, the left half of the light becomes the light. , which enters the critical angle prism 1 at an angle smaller than the critical angle, so they are transmitted without being reflected, and the outputs of the upper or lower photodetector 2 become smaller, so that an output appears in the differential amplifier 3. become. Therefore, focus control that controls the position of the objective lens in accordance with the output of the differential amplifier 3 becomes possible.

Further, other focus control devices include an astigmatism method, a Knifezi method, and the like.

[Problem that the invention seeks to solve]

However, since the critical angle method uses a critical angle prism, it is not only expensive, but also has the disadvantage that the entire pickup becomes heavy. In addition, since the astigmatism method and the Naifetsu method need to be used in a state where the light is converged,
The disadvantages are that the device is large and the adjustment is complicated.

[Means for solving problems]

FIG. 1 shows an optical system in which the focus control device of the present invention is applied to a pickup. 11 is a light source such as a semiconductor laser; 12 is a collimator lens that converts the light from the light source 11 into substantially parallel light; 13 is a beam splitter; and 14 is an objective lens that converges the light onto the disk 15. light source 1
1, the incident light enters the disk 15 via the collimator lens 12, the beam splitter 13, and the objective lens 14. The light is incident on the light receiving element 17 via the optical means 16. The light receiving element 17 is divided into two parts 17a, 17)) by a straight line perpendicular to the optical axis of the reflected light, and the error signal of the output of each of the two divided light receiving elements 17a, 17b is sent to a differential amplifier. It is now available at 18.

The position of the objective lens 14 in the optical axis direction is controlled in accordance with this error signal.

FIG. 2 shows the relationship between the optical means 16 and the light receiving element 17 in more detail. The optical means 16 is composed of an absorbing section 16a that absorbs light but does not transmit it, and a transmitting section 16b that transmits the light. The absorbing portion 16a does not have to completely absorb light, but may be formed to absorb or block light to an extent that satisfies the effect described below.1For example, it may be formed by changing the refractive index, reflectance, etc. can do. Further, the transmitting portion 16b can be formed of a transparent or semi-transparent material, or a simple space.

For example, the transmission part 16b is
A parallel plane plate made of transparent glass or the like having two planes can be used, and a plurality of absorption parts 16a can be formed on both sides of the plate by printing or the like. As a result, in one aspect, this
For example, the absorption part 16a and the transmission part 16 have the same width.
b are alternately formed on each surface.

In the absorbing portion 16a, a line connecting the absorbing portion 16a formed on one surface of the parallel plane plate and the absorbing portion 16a formed on the other surface forms a predetermined angle φ with respect to the optical axis. It is formed to be slanted. Assuming that the absorbing portions 16a on one surface and the other surface that overlap when viewed from the optical axis direction correspond to each other, the left and right end portions of the two absorbing portions 16a on one surface that are adjacent to each other and the corresponding 2 mutually adjacent surfaces on the other side
The transmissive part 16b formed by the left end and right end of each of the two absorbing parts 16a has a substantially parallelogram-shaped cross section, and the angle formed by one diagonal with the optical axis of the incident reflected light is θ8, and the other diagonal forms an angle of θ8. The angle that the diagonal forms with the optical axis is θ2 (
θ2×O).

[Effect]

The operation will be explained with reference to FIG.

When the disk 15 is in the focused position with respect to the objective lens 14, the adjustment is made so that parallel light is incident on the optical means 16 as shown in FIG. 3(a).

Assuming that the disk 15 approaches the objective lens 14, diffused light enters the optical means 16 as shown in FIG. 3(b), and as it moves away, convergent light enters the optical means 16 as shown in FIG. I will do it.

Assuming that a light beam is now incident on the optical means 16 at an angle of φ with respect to the optical axis, among the light receiving elements 17 arranged behind the optical means 16, the left light receiving element 1 in FIG.
The output Pa of 7a is expressed by the following equation.

When φ×θ2 or φ>θ, Pa=0 θ2<φ×φ. When P a = 1+2(tanφ−tanφ,)/(ta
nθ□−tanθ2)φ. When Kuφ Ku θ□, P a = 1-2(tanφ-tanφ1ll)/(
tan θ1 - tan θ2) This is because if the thickness of the absorbing portion 16a is 2t, the interval (width of the transmitting portion 16b) is d, and the amount of light incident on the slit with width d is 1, the amount of light reaching the light receiving element 17 is equal to the amount of light reaching the light receiving element 17. It changes according to the vignetting caused by the absorbing portion L6a on the right or left side toward 17.

When the incident angle φ is smaller than the angle θ2 or larger than the angle θ, the amount of light becomes zero. Further, the incident angle φ is larger than the angle θ2, and the angle φ. When it is smaller, the light is eclipsed by the absorption part L6a on the right side toward the light receiving element 17, and the amount of received light Pa is Pa = (d+2ttanφl, -2t, tanφ
)/d=1+2t(tanφ−tanφo)/
d. By the way, t tan θ, + t tan θ2=d2 (t ta
nθ, + t tanφo)” d, so tanθ1−tanθ2=d/1=2tanφ0&r,
c'), #°1゛j Pa ==1+2(t4nφ−tanφo)/(tan
θ, -t, anθ2).

Similarly, the output PI> of the right light receiving element 17b is the angle φ in the above equation. −φ. Since this is the case where it is closed, the following equation is obtained.

When φku-01 or φ〉-02 pb=.

Oneφ. When φku-02, Pb ==1-2(t, anφ+tanφ.)/(ta
When nθ1-tanθ2)-〇, kuφku-φ0, P b = 1+2(tanφ+tanφ.)/(ta
nθ, -tanθ2), that is, φ=0 at the focus position shown in FIG. 3(a).

Pa=Pb,=1-2tanφ,/(tanθ□-t
anθ2). Further, when the objective lens 14 and the disk 15 approach each other as shown in FIG. 3(b), φ is 0, and 1φ1 is φ. In the range of Pa<
It becomes Pb. Furthermore, as shown in FIG. 3(c), when the objective lens 14 and the disk 15 move away from each other, Pa>Pb. Therefore, by taking the difference between the outputs Pa and pb using the differential amplifier 18, a focus error signal can be obtained.

In the above equation, for simplicity, 1 which is incident on the light receiving element 17 is
Although only one angle φ was considered, in reality, φ=0 at the center of the luminous flux (near the optical axis), and the value of 1φ1 increases toward the outer periphery.

Therefore, the change in the amount of light caused by the absorption section 16a differs depending on the position on the light receiving element 17. Angle φ. 2.4 minutes, angle θ,
8.2 minutes, taking into account the loss of light amount due to deviation from the in-focus position, and calculating the outputs Pa and Pb for the amount of defocus from the above equation by simulation, we obtain approximately symmetrical characteristics as shown in Figure 4. becomes. Furthermore, when the focus error signal is determined when the number of absorbing portions 16a on one surface is 500, it has a substantially S-shaped characteristic as shown in FIG.

In the above example, the case where the light becomes parallel at the in-focus position has been described, but the present invention is capable of obtaining an error output according to the angle of the incident light.

It can be applied even when the light is not parallel at the focused position.

〔Example〕

FIG. 6 represents another embodiment of the optical means 16. That is, in the above example, the ratio of the lengths of the absorbing part 16a and the transmitting part 16b in the plane perpendicular to the optical axis was 1:1, and the interval between them was also constant, but in this example, These things are changing. In other words, FIG. 6(a) shows the absorption part 1.6 a
the distance between the light sources 11. This is an example in which the side is wide and the light receiving element 17 side is narrow.

By doing this, the outputs P''a and Pb change in the vicinity of the angle φ so that they remain constant regardless of the incident angle, so
The range in which stable focus control can be performed can be substantially expanded. Further, FIG. 6(b) is an example in which the length and interval near the optical axis of the absorbing portion 16a are made smaller than in the outer peripheral portion. In this example, although the angle of the line connecting the absorbing portions 16a with respect to the optical axis of the reflected light is inclined at a predetermined angle, it is not constant.

〔effect〕

As described above, in the present invention, focus control is performed using an optical means in which absorbing parts are provided on both sides of the transmitting part, and the line connecting the absorbing parts on each surface is inclined with respect to the optical axis. Not only is the optical means easy to form, but also inexpensive, lightweight, compact, and highly accurate pink-up can be realized, and adjustment is easy.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of the optical system of the focus control device of the present invention, Fig. 2 is a sectional view showing the relationship between the optical means and the light receiving element, and Fig. 3 is the in-focus position and the position deviated from the in-focus position. 4 is a characteristic diagram of the output of the light receiving element, FIG. 5 is a characteristic diagram of the error signal, and FIG. 6 is a sectional diagram of another embodiment of the optical means. , FIG. 7 is a schematic diagram of an optical system of a conventional focus control device. 1... Critical angle prism 2.17... Light receiving element 3.18... Differential amplifier 11... Light source 12...
・Collimator lens 13...beam splitter 14...objective lens 15...disc 16...
・Optical means 16a...Absorption part 16b...Transmission part

Claims (5)

[Claims] (1) A light source, a recording medium on which the light emitted from the light source is converged, an objective lens that converges the light emitted from the light source onto the recording medium, and a receiver that receives the reflected light reflected by the recording medium. a light-receiving element, an optical means disposed in the optical path of the reflected light, and a beam splitter that separates the reflected light from the optical path of incidence on the recording medium, and the optical means splits a part of the reflected light. It consists of an absorbing part that absorbs the light and a transmitting part that transmits a part of the reflected light, and the absorbing part is formed on two surfaces of the transmitting part that are substantially perpendicular to the optical axis of the reflected light. Focus control characterized in that a line connecting the absorption section formed on one surface and the absorption section formed on the other surface is inclined at a predetermined angle with respect to the optical axis of the reflected light. Device. (2) The light-receiving element is divided into two parts by a straight line perpendicular to the optical axis of the reflected light, and the reflected light is irradiated via the optical means. 2. A focus control device according to claim 1, wherein the position of the objective lens is controlled accordingly. (3) A plurality of the absorption parts and the transmission parts are formed alternately in a plane substantially perpendicular to the optical axis of the reflected light, as claimed in claim 1 or 2. Focus control device as described. (4) The focus control device according to claim 3, wherein the absorbing section is formed to have a longer length on the light source side than on the light receiving element side. (5) The focus control device according to claim 3 or 4, wherein the length and interval of the absorption portions are smaller in a portion closer to the optical axis than in a portion farther from the optical axis.