JPH07107742B2 - Optical disc head focus shift detector - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to an apparatus for detecting a focus shift of an optical disc head, and in particular, information from an optical information recording medium such as a digital audio disc or a video disc in a non-contact manner. The present invention relates to an apparatus for detecting a focus shift of an optical disk head for reading or writing.

[Prior art]

A conventional apparatus of this type will be described with reference to FIG. 1. A laser beam 2 emitted from a light emitting source 1 passes through a beam splitter 3, is converted into a parallel beam 5 by a collimator lens 4, and a quarter wavelength plate 6 is emitted. The objective lens 7 is then used to form a minute condensing spot 9 on the surface of the optical disk 8. The reflected light beam from the light-condensing spot 9 re-enters the objective lens 7,
Then, the optical path is returned by passing through the quarter-wave plate 6 and the collimator lens 4. The reflected light beam 10 that has returned is reflected by the beam splitter 3 and can be changed in direction by 90 °. The reflected light beam whose direction is changed by 90 ° is split into two light beams by a half mirror 11 installed in the middle of the optical path. Here, the first light flux 12a that has passed through the half mirror 11 is focused on the point P _1, and the second light flux 12b reflected by the half mirror 11 is focused on the point P ₂ . The arrow x indicates the displacement direction of the optical disk 8.

A _first photodetector 13 is arranged in front of the condensing point P _{1, and} a second photodetector 14 is arranged behind the condensing point P ₂ , and these photodetectors 13 and 14 are arranged in the second position. As shown in FIGS. 3A and 3B, the image is divided into two circular inner sensitive areas I ₁ and I ₂ and outer sensitive areas II ₁ and II ₂ .

The first photodetector 13 is incident on the inner sensitive area I ₁ and the outer sensitive area II ₁ as shown in FIG. 3A when the optical disk 8 is at the focus position of the lens 7. Light spot 15
The position and the size of the inner sensitive area I ₁ are set so that the light amounts of the two are equal to each other. Therefore, the sensitive area
The output difference of I ₁ and II ₁ (II ₁ −I ₁ ) becomes zero at the focal point.
On the other hand, when the optical disc 8 moves away from the in-focus position, the focal point P ₁ of the first light beam 12a approaches the first light detector 13, and the optical spot 15 on the surface of the first light detector 13 moves to the first position. As shown in FIG. 3 (b), the amount becomes small, the amount of light incident on the sensitive region I ₁ becomes large, and the output difference (II ₁ −I ₁ ) becomes negative. On the contrary, when the optical disk 8 comes closer to the focus position, the optical spot 15 becomes larger as shown in FIG. 3 (c), and the optical detector output difference (II
_1- I ₁ ) is positive. Therefore, the horizontal axis represents the deviation x from the in-focus position of the optical disk 8 (the direction in which the optical disk 8 moves away is positive), and the vertical axis represents the photodetector output difference (II ₁ -I ₁ ) as shown in FIG. A characteristic curve is obtained. It has been confirmed both experimentally and theoretically that the characteristic curve has the shape shown in FIG. As described above, the characteristic obtained from only the first photodetector 13 is that the detection sensitivity near the in-focus point is non-linear and that there are two zero crosses (points intersecting the x axis). Is unsuitable as Second photodetector
14 is intended to compensate for the above points, when the optical disc 8 in the same manner as the first optical detector 13 is in the focused position, the inner sensible region I ₂ as FIG. 3 (a), the outer Yes The position of the second photodetector 14 and the size of the region I ₂ are set so that the light amounts of the light spots 16 incident on the sensitive region II ₂ are equal. Therefore, the output difference (I ₂ −II ₂ ) between the sensitive areas I ₂ and II ₂ at the time of focusing becomes zero, but when the optical disc 8 moves away from the focusing position, the condensing point P of the second light flux 12b is generated. ₂ is away from the second photodetector 14 and the optical spot 16 on the surface of the second photodetector 14 is shown in FIG. 5 (b).
As shown in, the output difference (I ₂ −II ₂ ) becomes negative. On the contrary, when the optical disk 8 approaches the focus position, the optical spot 16 becomes small and the photodetector output difference (I ₂ −II ₂ ) becomes positive as shown in FIG. 5 (c). Therefore, the horizontal axis shows the deviation x from the in-focus position of the optical disk 8 (positive in the away direction), and the vertical axis shows the photodetector output difference (I ₂ −II ₂ ).
By taking, the characteristic curve as shown in FIG. 6 is obtained. This characteristic is in the form of the characteristic shown in Fig. 4 rotated 180 ° around the origin. Therefore, as shown in Fig. 7, a photodetector
If the output of the subtracter 17 connected to 13, 14 is added by the adder 18, the characteristic curve shown in FIG. 8 can be obtained by adding the characteristic curve of FIG. 4 and the characteristic curve of FIG. . The characteristics obtained in this way have a linear detection sensitivity in the vicinity of the in-focus point, and since there is only one zero cross, they are optimal as focus shift detection signals.

This focus shift detection signal is used to control a mechanism (not shown) that drives the objective lens 7 or the entire optical system in the optical axis direction. More specifically, it is returned to a mechanism (for example, a voice coil) for driving the objective lens 7 or the entire optical system so that the detection signal is always zero.

However, in the conventional device having the above configuration, the positions of the first and second photodetectors 13 and 14 must be independently adjusted in three axes, and the adjustment is complicated and the adjustment cost is high. Had. In addition, the half mirror 11 and the photo detectors 13,1
Since 4 are separately arranged, there is a possibility that they may be affected by the displacement of these elements due to aging and the like, and there is also a drawback that reliability is low.

[Outline of Invention]

The present invention aims to eliminate the above-mentioned drawbacks of the conventional beam splitter, and in addition to the conventional beam splitter, a constant second beam splitter for separating the reflected beam into two beams is provided. Install the second beam splitter and the first,
By integrating the second photodetector, it is possible to provide an optical disc head focus shift detection device which is easy to adjust, has a low adjustment cost, and has high reliability against aging.

Example of Invention

FIG. 9 shows a first embodiment of the present invention, and the same reference numerals as those in FIG. 1 indicate the same or corresponding portions. In the figure, 19
Is a cylindrical half prism forming a second beam splitter, and the reflected light beam 10 is separated from the first light beam 12a reflected by the reflecting surface 19a and the second light beam 12b which is transmitted as it is. First,
The first light beam 12a travels toward the condensing point P ₁ outside the half prism 19 and enters the first photodetector 13 placed on the optical path before the converging point P ₁ at the time of focusing. The contact surface of the first photodetector 13 is adapted to coincide with one outer surface 19b of the half prism 19, and is fixed to the outer surface 19b by adhesion or the like. In addition, the second light flux 1 reflected by the reflecting surface 19a
2b travels through the half prism 19, converges on the condensing point P ₂ in the half prism 19, and enters the second photodetector 14 placed behind the condensing point P ₂ at the time of focusing. Like the first photodetector 13, the second photodetector 14 has its abutting surface fixed so as to match the other outer surface 19c of the half prism 19. Therefore, if the half prism 19 is formed with a predetermined accuracy and the photodetectors 13 and 14 are fixed to the outer surfaces 19b and 19c with appropriate accuracy, the photodetectors can be adjusted halfway with the detectors 13 and 14. Prism 1
All that is required is to adjust the three integrated 9 axes.

As described above, the number of adjustment points is remarkably reduced as compared with the conventional one, the adjustment is easy, and the adjustment cost is low. Further, since the half prism 19 and the photodetectors 13 and 14 are integrated, the influence of undesired displacement of the element due to aging etc. is less than that of the conventional one in which these are separately arranged, and the reliability is also improved. To do.

It goes without saying that the focus shift detection signal in the above is obtained by the same arithmetic circuit as in FIG.

In the first embodiment, the two outer surfaces 19 of the half prism 19 are used.
The first and second photodetectors 13 and 14 are fixed to b and 19d, but as a second embodiment, a half prism 20 having a shape as shown in FIGS. 10 and 11, that is, two reflecting surfaces 20a. , 20b, two photodetectors can be arranged on the same outer surface of the half prism 20. Therefore, if one 4-division photodetector 21 having a pattern as shown in FIG. 11 is used, it is only necessary to fix one photodetector 21 to one outer surface of the half prism, which simplifies further adjustment. Is achieved.

FIG. 12 shows a third embodiment in which a quarter-wave plate 22 is provided at one end.
The use of the second beam splitter 23 in which the above is joined makes it possible to make the optical system more compact. In this case, the reflected light flux
The light beam 10 travels from the reflecting surface 23a toward the external condensing point P ₁ and passes through the quarter-wave plate 22 to enter the first photodetector 13.
Here, the outer surface 23b of the quarter-wave plate 22 is provided with a half mirror coating, and half of the light flux incident on the first photodetector 13 is reflected by the outer surface 23b. This reflected light beam passes through the quarter-wave plate 22 again and travels through the beam splitter 23. Since this light has passed through the quarter-wave plate 22 twice, the polarization direction of the linearly polarized light is rotated by 90 °, and the beam splitter
The light is not reflected by the reflecting surface 23a of 23 and is transmitted without being reflected.
_The light is condensed at 2 and reaches the second photodetector 14. Light detector 13,14
The contact surfaces of the outer surface 23b and the outer surface 2 opposite to the outer surface 23b
It matches 3c.

In addition to the simplification of adjustment and high reliability, the effect of making the optical system compact can also be expected in this embodiment.

FIG. 13 shows a fourth embodiment of the second beam splitter.
Half of one outer surface 24b of 24 is provided with a mirror coating 25, and a first photodetector 26 is disposed on the remaining portion of the outer surface 24b.
The second photodetector 27 is disposed on the outer surface 24c facing b, and has a sensitive area as shown in FIGS. 14 (a) and 14 (b). Thus, the reflected light beam 10 has a semi-circular part partially in the photodetector 26
And the remaining semicircular portion is reflected by the mirror coating 25, travels through the second beam splitter 24, and is incident on the second photodetector 27.

In the case of this embodiment, the same effect as that of the third embodiment can be expected. Here, in each of the above-mentioned embodiments, two 2-division photodetectors or one 4-division photodetector having a circular or semi-circular sensitive area are used, but a fifth embodiment is shown in FIG. Such two 3-division photodetectors 28, 29 or one 6-division strip photodetector composed of these two may be used. Reference numeral 17 is a subtracter and 18 is an adder, and the focus shift detection signal E _f in this case is E _f = (I ₂ + II ₁ + III ₁ ) − (I ₁ + II ₂ + III ₂ ). If the dividing line direction of the photodetectors 28 and 29 is set so as to be parallel to the direction in which the optical spots on the photodetector surface move with tracking (r direction in the figure), the focus deviation error due to tracking will occur. Has the advantage of not causing

〔The invention's effect〕

As is clear from the above description, the present invention is
It has an excellent effect that the focus shift detection with good characteristics can be performed by simple adjustment, the adjustment cost is low, and the entire apparatus can be inexpensive. Further, since the optical components are integrated, the influence of the displacement of the element due to aging etc. is minimized, and the reliability is improved.

[Brief description of drawings]

1 to 8 show a conventional device, FIG. 1 is a side view of an optical arrangement, FIGS. 2 (a) and 2 (b) are plan views of a photodetector, and FIGS. 3 (a) and 3 (b). ), (C) are plan views for explaining the operation of the first photodetector, FIG. 4 is a characteristic diagram of the first photodetector,
5 (a), (b), and (c) are plan views for explaining the operation of the second photodetector, FIG. 6 is a characteristic diagram of the second photodetector, and FIG.
FIG. 8 is a connection diagram of the photodetector, and FIG. 8 is a characteristic diagram according to FIG. 9 to 15 show an embodiment of the present invention, FIG. 9 is a side view of the optical arrangement of the first embodiment, and FIGS. 10 and 11 are partial side sectional views of the second embodiment. And a partial front view, FIG. 12 is a side view of the optical arrangement of the third embodiment, FIG. 13 is a partial side sectional view of the fourth embodiment, and FIGS. 14 (a) and 14 (b) are respectively 13
FIG. 15 is a plan view of the photodetector in the figure, and FIG. 15 is a photodetector of the fifth embodiment and its wiring diagram. 1 ... Emission source, 2 ... Emitted light flux, 3 ... First beam splitter, 4 ... Collimating lens, 5 ... Parallel light flux,
6 ... Quarter wave plate, 7 ... Objective lens, 8 ... Optical disk, 9 ... Focusing spot, 10 ... Reflected light flux, 12a, 12
b ... first and second luminous flux, 13,14 ... first and second photodetector,
17 …… subtractor, 18 …… adder, 19,20 …… half prism (second beam splitter), 21 …… photodetector, 22,2
4 …… Second beam splitter, 25 …… Mirror coating, 26,27 …… First and second photodetectors, 28,29 …… First and second photodetectors. In each figure, the same reference numerals indicate the same or corresponding parts.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinsuke Kama, No. 1 Baba Institute, Nagaokakyo, Kyoto Prefecture Mitsubishi Electric Corporation Electronic Product Development Laboratory (72) Teruo Fujita, No. 1 Baba Institute, Nagaokakyo, Kyoto Mitsubishi Electric Electronic Product Development Laboratory Co., Ltd. (56) Reference JP-A-58-128032 (JP, A) JP-A-56-87241 (JP, A) JP-A-58-211721 (JP, A)

Claims (7)

[Claims] 1. A light emitting source, an objective lens for converging a light beam emitted from the light source on a track of an optical disk, a light beam emitted from the light source and a reflected light beam passing through the objective lens from the track. And a second beam splitter for separating the separated reflected light beam into two light beams having different traveling directions, and a light beam separated by the second beam splitter. In the defocus detection device for an optical disc head, the two photodetectors are integrally formed on the surface of the second beam splitter and the separated light beams are collected. One is farther from the condensing point where light is emitted, and the other is arranged near the other, and the position between the second beam splitter and the light emitting source is adjustable. Tsu and has a focus deviation detecting system of the optical disk head. 2. The defocus detecting device for an optical disk head according to claim 1, wherein the second beam splitter is a half prism. 3. A second beam splitter divides a reflected light beam into two light beams having different traveling directions, and a first reflecting surface, and one of the separated two light beams having different traveling directions is the other light beam. A second reflection surface reflecting in a parallel direction, and the two photodetectors are integrally formed on one end surface of the second beam splitter in the traveling directions of the two light beams. A focus shift detection device for an optical disk head according to item 1. 4. The two photodetectors are a photodetector integrally formed on one end face of a second beam splitter through a quarter-wave plate whose surface is provided with a half mirror coating, and The focus shift detection device for an optical disk head according to claim 1, which is a photodetector integrally formed on a surface of the second beam splitter facing the one end surface. 5. The second beam splitter is mirror-coated on one half of its one end face, and the two photodetectors are:
2. The focus shift detecting device for an optical disk head according to claim 1, wherein one is integrally formed on the remaining half of the one end surface and the other is integrally formed on an end surface facing the one end surface. 6. An optical disk head defocus detection device according to claim 1, wherein each of the two photodetectors is formed by dividing a sensitive area into three rectangular areas. 7. An optical disk head defocus detection device according to claim 6, wherein the two photodetectors are formed integrally with each other.