JPH0430647B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a focus control device applicable to pickups 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 critical angle prism.
is a differential amplifier. In this system, the light incident on the critical angle prism 1 in a focused state becomes substantially parallel light and is adjusted so that it is reflected at an angle close to the 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 convergent light, 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. are incident on 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, and an output appears in the differential amplifier 3. It becomes like this. Therefore, focus control is possible in which the position of the objective lens is controlled in accordance with the output of the differential amplifier 3.

Further, other focus control devices include an astigmatism method, a knife edge 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. Furthermore, since the astigmatism method and the knife edge method must be used in a state in which the light is converged, they have disadvantages in that the apparatus becomes large and the adjustment is complicated.

[Means for solving problems]

FIG. 1 shows an optical system when the focus control device of the present invention is applied to a pickup. 11
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 a disk 15. Emitted from the light source 11,
collimator lens 12, beam splitter 13,
The incident light that enters the disk 15 through the objective lens 14 is reflected by the disk 15 to become reflected light, separated from the incident light by the beam splitter 13, reflected, and incident on the light receiving element 17 via the optical means 16. It is becoming more and more common. The light receiving element 17 is divided into two parts 1 by a straight line perpendicular to the optical axis of the reflected light.
7a and 17b, and a differential amplifier 18 obtains an error signal of the output of each of the two divided light receiving elements 17a and 17b. 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 includes an absorption section 16 that absorbs light but does not transmit it, and a transmission section 16b that transmits light. Absorption section 16
a 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; for example, it can 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 transmitting portion 16b may be a parallel plane plate made of transparent glass or the like having two planes perpendicular to the optical axis of the reflected light, and the plurality of absorbing portions 16a may be formed on both sides of the plate by printing or the like. As a result, when observing this on one surface, for example, a plurality of absorbing portions 16a and transmitting portions 16b having the same width are alternately formed on each surface. Absorption section 16
a is formed so that 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 is inclined by a predetermined angle φ ₀ with respect to the optical axis. be done. 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 two mutually adjacent absorbent parts 1 on the other side
The transmissive part 16b formed by each left end and right end of 6a has a substantially parallelogram-shaped cross section, one diagonal of which forms an angle with the optical axis of the incident reflected light is θ 1 , and the other diagonal forms an angle of θ ₁ with the optical axis of the incident reflected light. The angle formed with the optical axis is θ ₂ (θ ₂ <0).

[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 optical means 1
6, the disk 15 is adjusted so that parallel light is incident on the objective lens 14.
When the light approaches the optical device 16, the diffused light enters the optical means 16, as shown in FIG.

Now, the light beam is directed to the optical means 1 at an angle of φ with respect to the optical axis.
6, among the light receiving elements 17 arranged behind the optical means 16, the output Pa of the left light receiving element 17a in FIG. 3 is expressed by the following equation.

When φ<θ ₂ or φ>θ ₁ Pa=0 When θ ₂ <φ<φ ₀ Pa=1+2(tanφ−tanφ ₀ ) /(tanφ ₁ − tanφ ₂ ) When φ ₀ <φ<θ ₁ Pa =1-2(tanφ- _tanφ0 )/( _tanθ1 - _tanθ2 ) This is because 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. Then, the amount of light reaching the light receiving element 17 changes according to the vignetting caused by the absorbing portion 16a on the right or left side toward the light receiving element 17. When the incident angle φ is smaller than the angle θ ₂ or larger than the angle θ ₁ , the amount of light becomes zero. Moreover, when the incident angle φ is larger than the angle θ ₂ and smaller than the angle φ ₀ , the light receiving element 1
7, the light is eclipsed by the absorption section 16a on the right side, and the amount of received light Pa is Pa=(d+2t tanφ ₀ −2t tanφ)/d = 1+2t(tanφ−tanφ ₀ )/d. By the way, since t tanθ ₁ −t tanθ ₂ =d 2 (t tanθ ₁ −t tanθ ₀ )=d, tanθ ₁ −tanθ ₂ =d/t=2tanφ ₀ , and therefore Pa=1+2(tanφ−tanφ ₀ ) /(tanθ ₁ −tanθ ₂ ).

Similarly, the output Pb of the right light receiving element 17b is
Since this is the case where the angle φ ₀ in the above equation is set to −φ ₀ , the following equation is obtained.

When φ<-θ ₁ or φ>-θ ₂ Pb=0 When −φ ₀ <φ<-θ ₂ Pb=1-2 (tanφ+tanφ ₀ ) / (tanθ ₁ −tanθ ₂ ) −θ ₁ <φ< -φ ₀ Pb=1+2(tanφ+tanφ ₀ ) /(tanθ ₁ −tanθ ₂ ) In other words, at the focus position shown in Figure 3a, φ
= 0, and Pa=Pb=1-2tanφ ₀ /(tanθ ₁ −tanθ ₂ ). In addition, as shown in FIG. 3b, the objective lens 1
4 and disk 15 are close to each other,
φ<0, and in the range |φ|<φ ₀ , Pa<
Becomes Pb. Further, as shown in FIG. 3c, when the objective lens 14 and the disk 15 move away from each other, Pa>Pb. Therefore, differential amplifier 1
By calculating the difference between the outputs Pa and Pb using 8, a focus error signal can be obtained.

In the above equation, only one angle φ of incidence on the light receiving element 17 was considered for simplicity, but in reality, at the center of the luminous flux (near the optical axis), φ=
0, and the value of |φ| 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 ₀ 2.4
By setting the angle θ ₁ to 8.2 minutes and taking into account the loss of light amount due to deviation from the in-focus position, the output Pa for the amount of defocus is determined by simulation from the above formula.
When Pb is determined, it has approximately symmetrical characteristics as shown in FIG. Further, the absorbing portion 16a on one side
When the focus error signal is calculated when the number of 0 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 was explained, but since the present invention can obtain 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.

[Embodiment] FIG. 6 shows another embodiment of the optical means 16. That is, in the above example, the ratio of the lengths of the absorption part 16a and the transmission part 16b in the plane perpendicular to the optical axis is 1.
1, and the intervals were also constant, but in this embodiment these are changed. That is, FIG. 6a shows an example in which the spacing between the absorbing portions 16a is widened on the light source 11 side and narrowed on the light receiving element 17 side. By doing so, the outputs Pa and Pb change in the vicinity of the angle φ ₀ so as to remain constant regardless of the incident angle, so that the range in which stable focus control can be performed can be substantially expanded. In addition, FIG. 6b shows the absorption part 16a.
This is an example in which the length and spacing near the optical axis are smaller than those at the outer periphery. 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 a 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 an inexpensive, lightweight, compact and highly accurate pick-up can be realized, and adjustment is also 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 shifted 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, 1
2... Collimator lens, 13... Beam splitter, 14... Objective lens, 15... Disk,
16... Optical means, 16a... Absorption section, 16b...
...Transparent part.

Claims (1)

[Claims] 1. A light source, a recording medium on which the light emitted from the light source is focused, an objective lens that focuses the light emitted from the light source on the recording medium, and a light source that is reflected by the recording medium. The optical means includes a light receiving element that receives the reflected light, 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. It consists of an absorbing part that absorbs a part of 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. The line connecting the absorbing portion formed on one surface and the absorbing portion formed on the other surface is inclined at a predetermined angle with respect to the optical axis of the reflected light. Features a focus control device. 2. The light-receiving element is divided into two by a straight line perpendicular to the optical axis of the reflected light, and the light-receiving element corresponds to the difference in output between the two divided light-receiving elements that are irradiated with the reflected light via the optical means. 2. The focus control device according to claim 1, wherein the position of the objective lens is controlled by the following. 3. The method according to claim 1 or 2, characterized in that a plurality of the absorbing portions and the transmitting portions are formed alternately in a plane substantially perpendicular to the optical axis of the reflected light. Focus control device. 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 close to the optical axis than in a portion farther from the optical axis.