JPH09212885A - Focus detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized focus detector without being affected by a disturbance noise by executing focus detection with a knife edge method using a non-division photodiode and an optical fiber. SOLUTION: A light beam 1a from a laser light source 1 is divided into transmission light 2a and reflection light 2b by a beam splitter 2. The transmission light 2a is converged on an optical disk 4 by an objective lens 3, and the reflection light 4a is divided into the reflection light 2c and the transmission light 2d through the objective lens 3 again. A knife 5 is arranged on the rear convergent position (b) of the objective lens 3 through the beam splitter 2. The optical fibers 6a, 6b are arranged at equal interval so that the light intensity of the incident reflection light 2c become equal to each other. Photodetectors 7a, 7b use the non-division photodiodes, and on which outgoing light from the optical fibers 6a, 6b are made incident, and are arranged closely on a main substrate so as to make the distance electrically shortest, and output electric signals in proportion to the light intensity of the optical fibers 6a, 6b, and a comparator 8 obtains a focus error signal by comparing the electric signals from the photodetectors 7a, 7b with each other to amplify the result.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus detection device for detecting a focus error signal of an optical pickup device used in an optical disc drive.

[0002]

2. Description of the Related Art The structure of a focus detecting device according to the prior art is shown in FIGS. 7, 8 and 9 and will be described below. Reference numeral 1 in the figure denotes a laser light source, which emits coherent light. Reference numeral 2 is a beam splitter having a half mirror therein, 3 is an objective lens, 4 is an optical disk as a detection target whose position is confirmed, 5 is a knife, 8 is a comparator, 13 is a photodetector, and 14 is a flexible substrate. (Hereinafter referred to as FPC), 15 is a main substrate, and 16 is an optical unit.

A light beam 1a emitted from a laser light source 1 in FIG. 7 is transmitted by a beam splitter 2 to a transmitted light 2a and a reflected light 2b.
Fork into two. Transmitted light 2 transmitted through the beam splitter 2
A is focused on the optical disc 4 by the objective lens 3. Further, the reflected light 4a reflected by the optical disk 4 passes through the objective lens 3 again, and is reflected by the beam splitter 2 as the reflected light 2a.
It is split into two, c and transmitted light 2d. The knife 5 is arranged at the rear focusing position b of the objective lens 3 via the beam splitter 2. The photodetector 13 receives the reflected light 2 from the beam splitter 2.
It is arranged on the same optical axis as c. At this time, if the optical disc 4 arranged at the front focusing position a of the objective lens 3 fluctuates in the optical axis direction due to surface wobbling generated during rotation, the reflected light 2c
The image formation point P of ## EQU3 ## fluctuates with the fluctuation of the surface wobbling.

For example, the surface wobbling displacement of the optical disc 4 is
x, the focal length of the objective lens 3 is f, the front focus position is a,
Assuming that the rear focus position is b, the displacement amount X of the image forming point P of the reflected light 2c due to the surface wobbling is X = f {a / (af)-(a + 2Δx) / (af +).
2Δx)}. Therefore, a part of the reflected light 2c entering the photodetector 13 is blocked by the knife 5 according to the displacement amount X of the image forming point P. As shown in FIGS. 8 and 9, the photodetector 13 includes the photodetectors 13a and 13b that are divided into two parts, and the electric signals output from the photodetectors 13a and 13b are carried by the FPC 14 connected to the main board 15. Be done. The photodetector 13 outputs an electric signal proportional to the intensity of the light beam focused on the photodetectors 13a and 13b on each divided area. The comparator 8 can obtain a focus error signal by comparing and amplifying the electric signal from the photodetector 13a and the electric signal from the photodetector 13b, and this method is generally called a knife edge method.

[0005]

In order to perform the focus detection apparatus by the knife edge method, a photodiode divided by the photodetector is used to detect the change in the amount of light shielded by the knife due to the fluctuation of the surface fluctuation of the optical disk. There is a need. However, in general, a multi-segment photodiode is complicated in shape and difficult to manufacture, and since the outer shape is constituted by a lead frame package, it is about 20 to 30 mm, which is an order of magnitude smaller than that of a light detecting unit of about 200 to 300 μm. There has been a problem that the optical part is inevitably large in size when it is arranged near the optical disk due to the large external dimension. Furthermore, the optics part equipped with the photodetector and the main board are several tens of mm
The spaces must be connected by a wiring material such as FPC, and at that time, there is a problem that they are easily affected by disturbance noise. Further, when the semiconductor laser is arranged near the optical disk, there is a problem that the optical section becomes large due to the same reason as described above. It is an object of the present invention to provide a focus detection device that is compact and that can obtain a strong focus error signal against disturbance noise.

[0006]

In order to achieve this object, according to the present invention, the intensity of the reflected light of the light spot exposed on the surface of the object to be measured whose position is detected is measured to measure the measured portion. In a focus detection device for detecting the position of the optical lens system for forming a reflected light image of a light spot exposed on the surface of the object to be measured at a focus position, and a knife arranged at the focus position of the optical lens system. And first and second light derivation means arranged at a position farther from the optical lens system than the knife and at an edge of the reflected light image;
A first light detecting section for detecting the light emitted from the second light deriving means, a second light detecting section for detecting the light emitted from the second light deriving means, and the first and second Provided is a focus detection device comprising: a comparison unit that compares electric signals output from a photodetection unit and outputs a difference between the electric signals. More specifically, the output light from a laser light source is used as incident light. A beam splitter that splits the transmitted light into a transmitted light and a reflected light, an objective lens that focuses the transmitted light from the beam splitter on the optical disc as the DUT, and an objective lens that reflects from the optical disc as the DUT. The optical fibers arranged at equal intervals in the direction perpendicular to the optical axis of the reflected light reflected by the beam splitter via the beam splitter, the knife arranged at the rear focusing position of the objective lens via the beam splitter, and closely arranged on the main substrate. A non-split type photodetector that outputs an electrical signal proportional to the light intensity from the optical fiber, a comparator that amplifies the electrical signal from the photodetector and outputs a focus error signal, and a holding that holds the optical fiber. The optical disc is equipped with a table, and the function of detecting the change in the amount of light shielded by the knife due to the fluctuation of the surface of the optical disc and converting the optical signal to the electrical signal is performed using two thin optical fibers with an outer diameter of approximately 125 μm and a non-split type. The photodetector is made to individually perform this.

Further, a semiconductor laser is arranged as a laser light source, and the reflected light from the semiconductor laser is made incident on the beam splitter. Further, an optical fiber is arranged as a laser light source, the light emitted from the semiconductor laser closely arranged on the main substrate is made incident on the optical fiber, and the light emitted from this optical fiber is made incident on the beam splitter. . Also, as a beam splitter, a prism having an exit end face at the rear focus position of the objective lens and a metal reflection film that expects a knife edge effect is arranged on a part of the exit end face, and the light emitted from the prism is output by an optical fiber. It is designed to be incident on.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Next, an embodiment according to the present invention is shown in FIGS. 1, 2 and 3, and a detailed description thereof will be given. The same parts as those of the conventional example shown in FIGS. 7 to 9 are designated by the same reference numerals, and the description thereof will be omitted except for necessary parts.
In the figure, 6a and 6b are optical fibers. The tips of the optical fibers 6a and 6b are arranged farther from the objective lens 3 than the knife 5 arranged near the focal point of the reflected light 2c, and the edge portion of the circular optical image formed by the reflected light 2c ( (See a and b in FIG. 1) and guides the reflected light 2c incident on these tips. As shown in FIG. 3, the semiconductor laser 11, the beam splitter 2, the knife 5, and the objective lens 3 are housed in an integrated case to form an optical section 12. Further, the optical fibers 6a and 6b whose tips are arranged at the edges of the circular optical image produced by the reflected light 2c are led out to the outside from the side wall of the optical section 12, and the other tips are provided with the photodetectors 7a and 7b. The optical fiber 6 is fixed to a holding table 9 arranged in the vicinity of the main substrate 10 that has been cut.
Optical signals emitted from the other ends of a and 6b are configured to be incident on photodetectors 7a and 7b facing these ends, respectively.

Next, the operation of the first embodiment of the present invention will be described. In FIG. 1, a light beam 1a emitted from a laser light source 1 is split into a transmitted light 2a and a reflected light 2b by a beam splitter 2. The transmitted light 2a transmitted through the beam splitter 2 is focused on the optical disc 4 by the objective lens 3. Further, the reflected light 4a reflected by the optical disk 4
Passes through the objective lens 3 again and is split into the reflected light 2c and the transmitted light 2d by the beam splitter 2. The knife 5 passes through the beam splitter 2 and the rear focus position b of the objective lens 3
To place. The two optical fibers 6a and 6b are arranged at equal intervals in the direction perpendicular to the optical axis of the reflected light 2c so that the incident reflected lights 2c have equal light intensities (A in FIG. 1). At this time, when the optical disc 4 arranged at the front focus position a of the objective lens 3 changes in the optical axis direction due to surface wobbling or the like generated when the optical disk 4 rotates, the image forming point P of the reflected light 2c becomes P ′ due to the surface wobbling fluctuation. Fluctuates up to. When the image forming point of the reflected light 2c changes to P ′, the beam diameter at the point P where the knife 5 is arranged expands, so that part of the beam is shielded by the knife 5 (see B in FIG. 1). .

For example, the surface wobbling displacement of the optical disk 4 is expressed by Δ
x, the focal length of the objective lens 3 is f, the front focus position is a,
Assuming that the rear focus position is b, the displacement amount X of the image forming point P of the reflected light 2c due to the surface wobbling is X = f {a / (af)-(a + 2Δx) / (af +).
2Δx)}. Further, when the converging angle of the reflected light 2c is set to θ, the beam spot radius d at the point P where the knife 5 is arranged is: d = X · tan θ = f {a / (a−f) − (a + 2Δ
x) / (af−2Δx)} tan θ. Therefore, a part of the reflected light 2c incident on the two optical fibers 6a or 6b is shielded by the knife 5 according to the displacement amount X of the image forming point P, and finally according to the surface wobbling displacement of the optical disc 4. Thus, the incident light amount ratio of the two optical fibers 6a and 6b changes.

The photodetectors 7a and 7b use non-split type photodiodes to enter the light emitted from the two optical fibers 6a and 6b, and closely contact the main substrate 10 so that the distance is electrically shortest. Optical fibers 6a, b
It outputs an electric signal proportional to the light intensity of. The comparator 8 is
The focus error signal can be obtained by comparing and amplifying the electric signals from the photodetectors 7a and 7b.

Next, a second embodiment according to the present invention is shown in FIG. 4 and will be described in detail. 1 to 3
The same parts as those of the first embodiment of the present invention shown in are given the same reference numerals, and the description thereof will be omitted except for necessary parts.
In the second embodiment of the present invention shown in FIG. 4, a semiconductor laser 11 is used as the laser light source 1.
The light emitted from the semiconductor laser 11 is incident on the beam splitter 2 for use.

Next, a third embodiment according to the present invention is shown in FIG. 5 and will be described in detail. 1 to 3
The same parts as those of the first embodiment of the present invention shown in are given the same reference numerals, and the description thereof will be omitted except for necessary parts.
In the third embodiment of the present invention shown in FIG. 5, an optical fiber 6c is used as the laser light source 1, and the emitted light from the semiconductor laser 11 closely attached to the main substrate 10 is supplied to the optical fiber 6c. Incident on this optical fiber 6
The emitted light from c is incident on the beam splitter 2 for use. As the optical fiber 6c, a short wavelength single mode fiber having a core diameter of about several μm is used.

Next, a fourth embodiment according to the present invention is shown in FIG. 6 and will be described in detail. 1 to 3
The same parts as those of the first embodiment of the present invention shown in are given the same reference numerals, and the description thereof will be omitted except for necessary parts.
In the fourth embodiment of the present invention shown in FIG. 6, 17 is a prism. That is, in FIG. 6, the prism 17 is used as the beam splitter 2, and the exit end face of the prism 17 is arranged so as to coincide with the backward focusing position b of the objective lens 3. Further, by providing a metal reflection film on a part of the emission end face, a part of the reflected light incident on the optical fibers 6a and 6b at the time of surface wobbling is shielded by the metal reflection film without using the knife 5 for use. It should be noted that a gold vapor deposition film or the like is used as the metal reflection film, and the reflected light 2c when the optical disc 4 is located at the front focusing position a of the objective lens 3, that is, when surface wobbling does not occur.
Is applied to the area that is not shaded. For example, the beam spot diameter on the optical disc 4 focused by the objective lens 3 is φ
If it is 1.6 μm, the non-metal film region on the exit end face of the prism 17 is also set to φ1.6 μm. If the beam spot diameter is φ0.9 μm, the non-metal film region is similarly set to φ0.9 μm.
Used as μm.

[0015]

According to the present invention, the conventional optical detection and optical
The electrical signal conversion was performed by a lead frame package type photodiode having an outer shape of about 20 to 30 mm and having a light detecting portion of about 200 to 300 μm.
Two thin optical fibers of about μm and a non-split type photodiode are used. The change in beam shading amount is detected by the optical fiber, and the non-split type photodiode closely attached to the main board performs optical-electrical signal conversion. Therefore, the optical unit is small, and focus detection that is strong against disturbance noise can be performed. For example, assume that the outer shape of the conventional optical section is limited by the outer shape of the lead frame package type photodetector, and the outer shape of the optical section devised this time is limited by the outer shapes of the two optical fibers and the optical path branching circuit. It is possible to reduce the size of the optical unit to about 1/3 as a whole.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of a focus detection device according to the present invention.

FIG. 2 is a schematic diagram showing a photodetector and its circuit according to the present invention.

FIG. 3 is a connection diagram between an optical unit and a main substrate according to the present invention.

FIG. 4 is a configuration diagram of a second embodiment according to the present invention.

FIG. 5 is a configuration diagram of a third embodiment according to the present invention.

FIG. 6 is a configuration diagram of a fourth embodiment according to the present invention.

FIG. 7 is a configuration diagram of an embodiment of a focus detection device according to the related art.

FIG. 8 is a schematic diagram showing a conventional photodetector and its circuit.

FIG. 9 is a connection diagram between an optical unit and a main substrate according to a conventional technique.

[Explanation of symbols]

 1-Laser light source 2--Beam splitter 3--Objective lens 4--Optical disk 5--Knife 6a, b, c-Light Fibers 7a, b ... Photodetector 8 ... Comparator 9 ... Holding stage 10 ... Main substrate 11 ... Semiconductor laser 12 ... ... Optical part 17 ... Prism

Front page continuation (72) Inventor Tokunori Chino 1743-1, Asana-cho, Iwata-gun, Shizuoka Minebea Co., Ltd. Development Technology Center (72) Inventor Riki Tsuchiya 1743-1, Asana-cho, Iwata-gun, Shizuoka Minebea Co., Ltd. Development Technology Center

Claims (7)

[Claims] 1. A focus detection device for detecting the position of a measured portion by measuring the intensity of reflected light of a light spot exposed on the surface of the measured object, the position of which is detected. An optical lens system for forming a reflected light image of a light spot that is exposed at a focal position, a knife arranged at the focal position of the optical lens system, and farther from the optical lens system than the knife. First and second light derivation means that are located at the edge of the reflected light image, and first that detects the light that is derived from the first light derivation means
And a second light detecting section for detecting light emitted from the second light deriving means.
2. A focus detection device comprising: the photodetector section and the comparator section for comparing the electric signals output from the first and second photodetector sections and outputting the difference therebetween. 2. The focus detecting apparatus according to claim 1, wherein the light source for generating a light spot on the surface of the object to be measured whose position is to be detected is a laser light source. 3. The focus detection device according to claim 2, wherein the laser light source is a semiconductor laser. 4. The focus detecting apparatus according to claim 2, wherein the laser light source is light emitted from a laser by being guided by an optical fiber and emitted from the tip thereof. 5. The focus detection device according to claim 1, wherein the object to be measured is an optical disk. 6. A focus detection device for detecting the position of a measured portion by measuring the intensity of reflected light of a light spot exposed on the surface of the measured object for position detection, wherein A beam splitter that splits incident light into incident light and transmitted light and reflected light; an objective lens that forms transmitted light from the beam splitter into incident light and forms an image on the surface of an optical disc as the DUT; and an optical disc as the DUT. An optical fiber that is reflected from the surface, passes through the objective lens again, and is reflected by the beam splitter at equal intervals in the direction perpendicular to the optical axis of the reflected light; and a knife that is placed at the rear focus position of the objective lens through the beam splitter. A non-split type photodetector, which is closely arranged on the main board and outputs an electric signal proportional to the light intensity from each optical fiber, and the electric signal from the photodetector. Focus detecting apparatus characterized by comprising a comparator for outputting a focus error signal to compare amplification,. 7. A prism as a beam splitter having an exit end face at a rear focus position of an objective lens, and a metal reflection film provided on a part of the exit end face is arranged, and light emitted from the prism is incident on an optical fiber. The focus detection device according to claim 6, wherein