JPS58200442A - Focus detection device of optical pickup, or the like - Google Patents

Abstract

PURPOSE:To prevent detection error without increasing the sperture degree of a cylindrical lens, by arranging two cylindrical optical system lenses for generating the astigmatism so that their buses cross orthogonal to each other. CONSTITUTION:Instead of a condenser lens and cylindrical lens of conventional optical system, two cylindrical lenses 10 and 11 having the same focal length are arranged so that their buses cross orthogonal to each other. As a result, each of the cylindrical lenses acts in a face including an optical axis and buses of lenses, and a detected difference signal becomes the one shown in the figure so that an error signal is prevented from being detected that shows as if a recording surface and an objective lens are getting closer although they are actually getting away. Since incident light keeps to be parallel in the acting surface of the convergence effect of the cylindrical lens so that a convergent luminous flux is not made to be incident, it is not necessary to widen the aperture angle of outgoing light.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus detection method in an optical pickup or the like, and particularly to an improvement of an astigmatic SI method using a cylindrical lens.

2. Description of the Related Art The development of a device that records and reads information by forming an image on an optical recording medium as a minute spot with a laser beam beam diameter of about 1.2 μm is underway. In these & poor, I%!
! In order to record and reproduce information at once, it is necessary to make the spot size as small as possible and maintain its size during tracking.

For this purpose, the imaging lens facing the recording unit must be placed at the focal point k.
In order to maintain the distance to the recording medium within the tO width of 114 (in the optical axis direction), it is necessary to make ceramics.For this reason, the focal ejection method that is generally used for focus control uses astigmatism. This is a method called the 211 method.

The outline of this method is shown in Figure 0II1 shown in Figures Tsuru 1 to Yuki 4. In Figure 0II1, light from a semi-S body laser 1 is made into parallel light by a 1XVi coupling lens 2, and (a) T beams 1 liter 3. /41F4 through objective lens 5.
On the other hand, this reflected light is turned into a traveling light by the objective lens 5, and by the aerial root 4, it becomes polarized light whose plane of polarization has been rotated by 9 seconds, and by the beam splitter 3. 1 to a different optical path from the incident light, condenser lens 7
, enters the light receiving element 9 via the cylindrical lens 8f. At this time, in the plane <r*> diagram that includes the optical axis and the generatrix of the cylindrical lens 8, the cylindrical lens 8
It is not affected by the convergence effect of 7. Kuto V'1P1Vc conduit. In a plane perpendicular to this (Figure (b)), the light is focused at 22 points under the action of the cylindrical lens 8.

Focus detection is performed by detecting a second lens at a position where the beam width is the same in both of the above 〒Im.
As shown in the figure, the light receiving element 9f is arranged with the light receiving surface divided into four parts, and the output of each partial element A, B, C%D is compared. Now, it is assumed that the plane including the optical axis and the generatrix of the cylindrical lens passes over the light receiving element and over the A%C area. Let the outputs of each partial element A, B, C, and D be a, b, c, and d, respectively, and (a + c ) − (b + d
)of! ! If IF5 is detected, Tsuru 2
The above O difference signal is 0 in an incident state such as 1[a].

When the recording unit 6 and the objective lens 5 approach, the condensing point P1 moves away from the light receiving element 9, and the 22 points approach, so the incident state to the light receiving element becomes as shown in Figure 1b), (&+C) -
(1)+d) Vi(+) Sends a signal similar to fi1 and the objective lens 5tiJl to the control device. On the other hand, if the recording medium 6 and the objective lens 5 move away from each other, the incident situation on the light receiving element 9 becomes as shown in Figure 1, C) (breath + e) - (
b+d) Vi(-) t! Then, a signal called IIIJtf1@ is emitted, which brings the objective lens closer.

The objective lens moves further away from the disk.

When the reflected light becomes focused on the cylindrical lens B, the convergence effect of the cylindrical lens 8 is no longer smooth, and the incident state on the light receiving element 9 becomes as shown in the figure -d), and it is ejected! No. 1 becomes zero and passes No. 11. As it moves further away, #IIK of the cylindrical lens 8 occurs at the focal point P1tj, and the @ridge light enters the cylindrical lens 8, resulting in a happy 3! As shown in IK, the spread of the original thickness in figure b) becomes smaller, and even though the recording surface and the objective lens are moving away, the detected difference signal becomes (-), and the detection unit determines that they are approaching. Then, I ended up going to Mari'.
Mau. The relationship between this eruption tM and the out-of-focus wrinkles is shown in Figure 4. The d position where the (+) detection signal becomes zero is Fig. 2 1d
), and the optical system t'i@
*Target Y (It is not possible to have a larger ejection range. And...

Raise the detection ellipse! lL5wk opening angle θ of optical lens 7
If 1 is increased, this ejectable range becomes smaller and smaller.

Further, the aperture 11362 of the cylindrical lens 8 is larger than the aperture angle θ1 of the condensing lens 7, and if this is too large, a positive beam cannot be formed, which becomes a constraint on the optical system design.

According to the present invention, a condensing system in which two cylindrical lenses are used with their generatrix orthogonal to each other is used instead of the condensing lens 7 and the cylindrical lens 80 for producing astigmatism of 1!7 kt. ◎ In other words, as shown in 151jAK, instead of the condenser lens 7 and the cylindrical lens 8 in the optical system shown in Figure 1, 2
Two cylindrical lenses 1O111 are arranged so that their generatrix lines are perpendicular to each other.

If we do this, only one cylindrical lens will act in the plane including the optical axis and the matrix at of each cylindrical lens, and the detected bell No. 1i will be as shown in Figure IIE7, and the recording surface and Since the objective lenses are moving away from each other, there is no risk that a single signal that appears to be approaching will be detected.

Moreover, the convergence effect of the valley cylindrical lens is such that the incident light remains as a converging light flux within the plane, and no convergent light flux enters, so the aperture 02 of the output light does not become particularly large. Two cylindrical lenses 10.11.

Even if the lenses have the same shape and the same focal length, astigmatism occurs only at the ζ and 11 corners, so there is no need to use a particularly special cylindrical lens.

As described above, the *invention has a simple configuration for the first time, and it is possible to achieve an I.
This effect can be achieved without increasing costs by preventing @detection and preventing the detection limit from becoming particularly short even if electric shock is increased.

[Brief explanation of drawings]

Fig. 1 is an original schematic diagram of a conventional optical pickup device. Fig. 2 is an explanatory diagram of the light-receiving function of the light-receiving element. Fig. 3 is an explanatory diagram of the Vi-signal generation. The optical path diagram of the pickup acid, Figure 6 is the output curve of the light receiving signal, λ l: light * 3: beam splinter 4. /4 board 5:
Opposite lens 6: Arrangement surface 7: Condensing lens 8 Nijilin trical lens 9; Light receiving element 10.11 Nijilin trical lens
d) Figure 3 Figure 4 Figure 5 Figure 6

Claims (1)

[Claims] (1) Tawara astigmatism where the light flux from the light source is made into parallel light by a coupling lens, condensed onto the recording medium via the objective lens L7, and the reflected light is converted into f-line light via the objective lens. In a focus ejection device that has an optical system that condenses light and a detector is distributed near the astigmatism forming position.
Focus point 11 for light pickup, etc., which is composed of 92 cylindrical lenses arranged so as to intersect with each other.
1 in Wt.) A focusing device such as an optical pickup as claimed in claim 4, characterized in that the two cylindrical lenses have the same shape.