JPS62203111A - Focusing state detecting device - Google Patents

Abstract

PURPOSE:To obtain a focusing state detection signal which has only one zero zero point corresponding to a focusing point and to facilitate focusing control by arranging the 2nd photodetector including the part which becomes the shadow of the 1st photodetector as to luminous flux converged by an optical system. CONSTITUTION:Divergent luminous flux from a body 22 is made parallel by an objective and converged by a condenser lens 26. Luminous flux which is nearby the optical axis in the convergent luminous flux is made incident on the 1st photodetector 28 and luminous flux which is distant from the optical axis is reflected by a reflecting surface 32 and made incident on the 2nd photodetector 30. The size of the photodetectors is so determined that the luminous flux converged by the condenser lens 26 is about three times as large as the diameter of the 1st photodetector at the photodetector; and the output of the 1st photodetector 28 is set equal to the output of the 2nd photodetector 30 in this state. Then the signal obtained by subtracting the output of the 2nd photodetector 30 from the output of the 1st photodetector 28 is a detection signal indicating the focusing state of the objective 24 and body 22.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a focus state detection device, and more particularly to a focus state detection device using a beam size method. Such a device is suitably used, for example, for detecting a focus state in an optical information recording/reproducing device.

[Conventional technology]

In an optical information recording/reproducing device, a radar beam emitted from a light source is focused by an optical system and irradiated onto a TW information recording medium (for example, a disk-shaped or card-shaped medium) in the form of a spot to record or reproduce information. will be carried out.

By the way, optical information recording and reproducing is characterized by the ability to operate at high density and high speed, and for this reason, the beam spot formed on the recording medium is required to be extremely small and have little variation in its diameter. Ru. Therefore, in optical information recording and reproducing devices, focusing control is performed to maintain the beam spot diameter formed on the recording medium within a predetermined range. This focusing control focuses a beam spot formed on a recording medium on a photodetector, determines the focused state of the beam spot on the recording medium from the imaged state, and then sets a predetermined focus based on this. This is done by slightly displacing the beam spot imaging optical system so as to maintain the focused state.

The beam size method is used as a method for detecting the focused state as described above.

FIG. 4 is a diagram showing an optical system of a magneto-optical information recording and reproducing apparatus equipped with a focusing state detection device using a beam size method.

In Fig. 4, 2 is a semiconductor radar as a light source;
6 is a collimator lens, 6 is a beam shaping prism, 69.8 is a polarizing beam sinter, 10 is an objective lens, 65 and 12 are condensing lenses, and 14 is a photodetector. Further, 16 is a recording medium, 16ati is its recording surface, and 16b is its protective glass.

The radar beam emitted from the semiconductor radar 2 is collimated by the collimator lens 2, and the beam shaping prism expands the circular intensity distribution in only one direction to form a circular intensity distribution. A part of the light passes straight through the beam splitter, is focused by the objective lens 10, passes through the protective glass 16b of the recording medium 16, and forms a spot on the recording surface 16m. The reflected beam from the beam spot on the recording surface 16m is focused by the objective lens 10 and made almost parallel, and enters the polarizing beam sinter 8, where a part of it is reflected by the beam sinter and passes through the condenser lens. 12
The light beams are bundled and incident on the photodetector 14.

FIG. 5 is an enlarged view of the vicinity of the photodetector 14.

In FIG. 5, reference numeral 13 indicates the focal position of the condenser lens 12, and the photodetector 14 is placed slightly closer to the condenser lens 12 than the focal position 1113.

FIG. 6 is an enlarged plan view of the photodetector 14 seen from the beam incident direction.

As shown in FIG. 6, the photodetector 14 has a circular first light receiving section 14& at the center and a second annular light receiving section 14b around the light receiving section. These two light receiving sections are separated by a dead zone 14c.

When the recording medium 16 is in the reference position and a beam spot of a predetermined diameter is formed on the medium.

In the photodetector 14, a beam spot is formed spanning the light receiving sections 14& and 14b as shown by 15 in FIG. 6, and the output of the light receiving section 14m and the output of the light receiving section 14b are set to be equal. -Thus, this light receiving section 14
The in-focus state can be detected by comparing the output of the light receiving section 14b with the output of the light receiving section 14b, and based on the detection signal, the objective lens 10 can be controlled to move in the direction of its optical axis.

[Problem that the invention seeks to solve]

Therefore, in the conventional focusing state detection device described above, even when the position of the recording medium 16 with respect to the river object lens 10 deviates from the reference position, the same focusing state detection signal as when moving to the reference position IIC is generated. The problem is that you can get it. That is,
This is the case when a beam indicated by a dotted line in FIG. 5 is incident on the photodetector 14.

FIG. 7 shows the focus state detection signal from the photodetector 14 (i.e., the light receiving unit 1
This is a diagram showing the output of the light receiver 11s14b subtracted from the output of the light receiver 11s14b. Point A in FIG. 7 corresponds to the case of beam incidence (i.e., in true focus) shown by the solid line in FIG. 5, and point B in FIG. 7 is shown by the dotted line in FIG. This corresponds to the case where the beam is incident on the

In this way, in the focus state detection device as described above, the focus state detection signal obtained from the photodetector 14 has two zero points.
Therefore, in order to determine which zero point corresponds to the true focus position, for example, the objective lens 10
At the time of initial positioning, it is necessary to perform troublesome operations such as limiting the direction of movement and counting the number of times a zero point signal is obtained to set the correct position.

[Means for solving problems]

According to the present invention, in order to solve the problems of the prior art as described above, there is provided an optical system that focuses a light beam from an object,
A first photodetector and a second photodetector are arranged in sequence along the traveling path of the light beam focused by the optical system, and the second There is provided a focus state detection device, characterized in that a photodetector is arranged including a portion that is a shadow of the first photodetector.

〔Example〕

Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.

FIGS. 1(a), 1(b), and 1(c) are diagrams showing an optical system of an embodiment of the in-focus state detection device of the present invention.

In FIG. 1, 22 is an object such as an information recording medium, 24 is an objective lens, 26 is a condensing lens, 28 is a first photodetector, and 30 is a second photodetector. 32 is a reflective surface. The objective lens 24 and the condensing lens 26 constitute an optical system. The first photodetector 28 is arranged perpendicularly to the optical axis of the optical system, and has a circular shape, and has a light receiving surface on the optical system 9111 (i.e., on the left side 1141). . Second photodetector 30
has substantially the same shape and size as the first photodetector 28, and is disposed adjacent to the right side of the first photodetector 28 and perpendicular to the optical axis of the optical system. Additionally, a reflective surface 32 having a light-receiving surface on the right side is arranged on the optical axis of the optical system on the right side of the second photodetector, separated by an appropriate distance, and perpendicular to the optical axis. . The reflective surface 32 is circular.

FIG. 1(a) shows the state of the light flux when the object 22 is at the focal position of the objective lens 24, and FIG.
Fig. 1(c) shows the state of the light flux when the object 22 is on the right side of the focal position of the objective lens 24. .

In FIG. 1(,), a diverging light beam from an object 22 is collimated by an objective lens and converged by a condensing lens 26. In FIG. Among the focused beams, the beam near the optical axis enters the first photodetector 28, and the beam away from the optical axis enters the reflective surface 3.
2 and enters the second photodetector 30. In the state shown in FIG. 1 (&), the photodetector is adjusted so that the light beam focused by the condenser lens 26 is about three times the diameter of the first photodetector 28 at the position of the photodetector 28. The size of the photodetector 28 is determined, and the output of the first photodetector 28 and the output of the second photodetector 30 are set to be equal in this state. The output of the first photodetector 28 minus the output of the second photodetector 30 is the detection signal of the in-focus state between the objective lens 24 and the object 22, which is shown in FIG. In this case, the detection signal is O.

In FIG. 1(b), the diverging light beam from the object 22 passes through the objective lens 24 and the condensing lens 26 and is focused, as shown by the solid line, and is incident only on the first photodetector 28. Therefore, there is no output from the second photodetector 30.

In addition, in FIG. 1(b), the diverging light flux from the object 23 is reflected by the objective lens 24 and the condenser lens 2, as shown by the dotted line.
6, a minimum spot 25 is formed before reaching the first photodetector 28, and the light beam near the optical axis passes further to the right from the spot and is focused on the first photodetector 28. The light beam incident on the optical axis 30 and away from the optical axis travels further to the right without entering the first photodetector.

In FIG. 1(,), a diverging light flux from an object 22 passes through an objective lens 24 and a condensing lens 26 and is converged, and a light flux near the optical axis enters a first photodetector 28, and the light flux near the optical axis The light flux departing from the first photodetector reaches the reflective surface 32 without being incident on the first photodetector, is reflected by the reflective surface, and a portion thereof enters the second photodetector 30.

FIG. 2 is a diagram of a focus state detection signal obtained in the above embodiment.

In FIG. 2, the horizontal axis represents the distance between the objective lens 24 and the object 22. In addition, in Fig. 2, A is the state in Fig. 1 (,), B is the state in the case of the object 22 in Fig. 1 (b), B/ is the state in the case of the object 23 in Fig. 1 (b), C is shown in Figure 1 (C
), respectively.

According to this embodiment as described above, only one zero point appears in the focus state detection signal. Furthermore, according to this embodiment, since the light beam reflected by the reflecting surface 32 is focused and incident on the second photodetector, the same small detectors are used as the first and second photodetectors on opposite sides. It can be used by pasting them together facing the same direction.

FIG. 3 is a diagram of an optical system showing an example of a magneto-optical information recording/reproducing apparatus using the in-focus state detection device of the present invention. In FIG. 3, the same members as in FIG. 4 are given the same reference numerals, and their explanations will be omitted here.

In FIG. 3, the polarizing beam splitter 8 is, for example, P
It has the characteristics of transmitting 80% of polarized light and reflecting 98% of S-polarized light. Further, 40 is a polarizing plate, 28 is a first photodetector, 30 is a second photodetector, and 32 is a reflective surface.

Note that the recording surface 16a of the recording medium 16, the objective lens 10, and the condensing lens 12 in FIG. 3 correspond to the object 22, the objective lens 24, and the condensing lens 26 in FIG. 1, respectively. The second photodetector 28,30 and the reflective surface 32 have a similar arrangement as in FIG.

When reproducing information recorded on the recording medium 16, the polarized beam focused by the objective lens 10 and forming a spot on the recording medium 16 is polarized by the magnetic Kerr effect according to information on the direction of magnetization recorded on the recording medium. It is reflected after being modulated by the rotation of the surface, is made almost parallel by the objective lens lO, is reflected by the polarizing beam splitter 8, is further focused by the condensing lens 12, passes through the polarizing plate 40, and passes through the first polarizing plate 40. A first photodetector 28 as shown in the figure.
and further enters the second photodetector 30.

In this way, it is possible to obtain a focus state detection signal similar to that described with reference to FIGS. 1 and 2 above.

Note that the reproduced information signal is obtained by the sum of the output of the first photodetector 28 and the output of the second photodetector 30.

In the embodiment described above, the light beam reflected by the reflective surface 32 is incident on the second photodetector 3o, but in the present invention, the same diameter name as that of the reflective surface is provided at the position of the reflective surface 32. A second photodetector can also be arranged.

〔Effect of the invention〕

According to the present invention as described above, since an improved nine-focus state detection signal having only one zero point corresponding to the focus position is obtained, one-focus control can be easily performed. Further, according to the present invention, since the photodetector does not have a dead zone, a stable signal can be obtained even when interference fringes occur on the light receiving surface of the photodetector and the interference fringes move.

[Brief explanation of drawings]

FIGS. 1(,) to (e) are diagrams showing the optical system of the apparatus of the present invention, and FIG. 2 is a diagram of a focus state detection signal obtained by the apparatus. FIG. 3 is a diagram of an optical system of an optical information recording/reproducing apparatus using the apparatus of the present invention. FIG. 4 is a diagram of the optical system of a conventional magneto-optical information recording/reproducing device. ? , FIG. 5 is a partially enlarged view of the device, FIG. 6 is an enlarged plan view of the photodetector, and FIG. 7 is a diagram of a focus state detection signal obtained in the device. 10.24... Objective lens, 12.26... Condensing lens, 16... Information recording medium, 22, 23-... Object, 28.30... Photodetector, 32-... Reflection surface. Agent Patent Attorney Yohei Yamashita Figure 1 Figure 2 Figure 3 Figure 4

Claims (2)

[Claims] (1) It has an optical system that focuses a light beam from an object, and a first photodetector and a second photodetector that are arranged in order along the traveling path of the light beam focused by the optical system, A focusing state detection device, characterized in that the second photodetector is arranged to include a portion that is a shadow of the first photodetector with respect to the light beam focused by the optical system. (2) A reflective surface is arranged in the traveling path of the light beam from the optical system, and a part of the light beam that does not enter the first photodetector is reflected by the reflective surface and becomes a second light beam. The focusing state detection device according to claim 1, wherein the traveling path of the light beam is set so as to reach the detector.