JPS598146A - Focus detector of optical pickup - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus detection device such as an optical pickup device for reproducing information from an optical information recording medium such as a video disk.

An optical system used for recording information on an optical information recording medium and reproducing information generally performs focus control of an objective lens while performing focus detection so that the focus is always on the recording medium.

This focus detection method uses an astigmatism method, which focuses the reflected beam from the recording medium by giving it a large astigmatism, and detects the shift in the position of the beam waist using a detector placed between the two beam waists. Most commonly used.

However, this detection method 1) Increasing the detection accuracy will narrow the detection range.

2) It has a fixed detection range, and if the defocus exceeds this detection range, a detection error occurs.

3) The aperture angle of the cylindrical lens that provides astigmatism becomes large, which limits the range of use.

Such disadvantages arise.

For this reason, a method has been proposed that does not use a cylindrical lens and utilizes the point where the light beam is tilted in opposite directions on both sides of the optical axis, but this also requires 1) a highly precisely machined prism.

2) It is necessary to assemble and hold the prism with high precision.

The problem of the hat arises.

The present invention provides illumination with a ring-shaped light beam with a narrow cross section, and takes advantage of the fact that the reflected light beam deviates to the outside or inside of the incident light beam depending on the focusing state, thereby increasing the detection accuracy without being limited by the detection width. It is an object of the present invention to obtain a newer focus detection device even though the processing precision of the optical elements used is not as high as warpage.

A detailed description will be given below with reference to the drawings.

Figure @1 is an optical path diagram of one embodiment of the focus detection device of the present invention, in which the light beam from the light source 1 passes through the transmitting part of the mirror 2, which also has a transmitting part in the center. The light is reflected by the mirror 3 and focused onto the recording surface 5 by the objective lens 4.

The mirror 2.3 having a transmitting part in the center is shown in FIG. 2(a) (
As shown in b), in the center of the mirror surface 21.31, there is an elliptical transmission area 2 whose projection onto a plane perpendicular to the optical axis is circular.
2.32 and passes through the transmissive region 22 by -114, only its peripheral portion is reflected at the peripheral portion of the surface 31 adjacent to the transmissive region 32. Therefore, the light beam reflected by the reflecting mirror 3 becomes a ring-shaped light beam with a narrow cross section.

If 2, the reflected light flux passes through the same optical path as the incident light,
Return to light source 1. If the recording surface 5 is close to the objective lens 4 as shown in FIG. 3(a), the reflected light beam will spread as shown by the dotted line and pass outside the incident light beam. On the other hand, if the recording surface 5 moves away from the objective lens 4 as shown in FIG. 4B, the reflected light beam will pass inside the incident light beam, as shown by the dashed line.

Therefore, as shown in FIG.
When the beam approaches , the cross section of the beam expands, so that the beam enters the mirror surface portion 21 of the mirror 2 and enters the detector 6 . On the other hand, when the recording surface 5 moves away from the objective lens 4, the cross section of the reflected light beam decreases, passes through the transmission area 32 at the center of the mirror 3, and enters the detector 7. Therefore, depending on whether the output from the detector 6 is obtained, the output from both detectors are not obtained, or the output from the detector 7 is obtained, the objective lens may be too close to the recording surface. Detection of whether the 7 points are in focus or too far away is performed, and can be used as a focusing correction device of the pickup optical system.

The Atsushi example shown in Fig. 4 is an example in which the light beam from the light source J is made parallel to the objective lens 4 by the coupling lens 8, and this configuration improves detection accuracy and shortens the total optical path. You can. in this case,
Needless to say, condensing lenses 1] and 12 for converging the incident light beam onto the detectors 6 and 7 may be provided as necessary.

Furthermore, in this embodiment, a half mirror 9 is disposed between the coupling lens 8 and the mirror 2, and a detector 1o is provided for detecting the light beam when in focus. In this case, the relationship between the distance from the distribution surface 50 and the output of the detector is as shown in the following table. In the table, 1.0 indicates the presence or absence of detection by the detector.

These detectors 6, 7, 10, etc. may be of a single type when only detecting focus, but when performing tracking detection at the same time, detectors 6, 7, 10 etc. may be of a single type, but when performing tracking detection at the same time, detectors 6, 7, 10, etc. *! A two-part type with +86 is used.

The detection signal processing in this case is shown in FIG. 6, where fs=(A
+rl)-(C-D) From K, it is determined that the focus is M<O far, and this judgment statement may be directly fed back to the servo mechanism for focusing correction of the pickup optical system. On the other hand, the tracking signal (is (A-
R), (CD>, etc.).

The present invention has the following features: 1. The mirror having a transmission part in the center has a large allowance for positioning and tilting in the direction of the optical axis, and is easy to maintain.

2. A mirror having a transmitting portion in the center can be manufactured easily and inexpensively with high precision using conventional thin film patterning technology.

3. The detectable defocus range is very narrow.

4. The amount of light incident on each detector is in a reciprocal relationship such that when one increases, the other decreases, so the detection signal ratio is good and sensitive focusing detection is possible.

[Brief explanation of the drawing]

Fig. 1 is an optical path diagram of one embodiment of the focus detection device of the present invention, Fig. 2 is a plan view and a projection view in the optical axis direction of a mirror having a transparent part in the center, and Fig. 3 is an explanatory diagram of the optical path when out of focus. , FIG. 4 is an optical path diagram of another embodiment, FIG. 5 is a diagram of the configuration of the photodetector,
FIG. 6 is a circuit diagram of pickup information processing 1: Light source 2, 3: Mirror with a transmission part provided in the center 4: Objective lens 5: Focusing position N 6, 7. to: light detection? ?
, 8: Coupling lens 9: Half-mi patent issuer Ricoh Co., Ltd. Applicant's agent Patent attorney Fumio Sato (and 1 other person) Asahi 3 [Figure 5 Figure 6

Claims (1)

[Claims] 2 having a light transmitting part in the center between the light source and the objective lens;
A photodetector is equipped with two mirrors, the light beam transmitted through the first mirror and reflected by the second mirror is incident on the objective lens, and the light beam transmitted and reflected by each mirror in the reflected light beam is detected. A focus detection device for an optical pickup, characterized by the following: