JP2000298852A - Focus detecting device and optical head apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a focus detecting device whose constitution is simplified and which is comparatively low-cost. SOLUTION: In this focus detecting device, two kinds of refractive-index bodies (a first prism and a second prism) 6a, 6b are installed in the halfway part of the optical path of light 13 which is reflected from the recording face of an information recording medium, the difference of a light beam diameter (a spot size) due to the difference of an optical path length by the two kinds of refractive-index bodies is detected by the light receiving face 17 of a light receiving element 7, and a focus is detected on the basis of the difference. Thereby, the focus is detected by using only the two kinds of refractive-index bodies 6a, 6b, the constitution of the focus detecting device is simplified, and the focus detecting device can be made low-cost.

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 and an optical head device using the same, and more particularly, to a focus detection method for controlling the light emitted from a light source on a recording surface of an information recording medium. .

[0002]

2. Description of the Related Art There are various examples of such a focus detection method, and there is a so-called spot size detection (SSD) method as an example, and its principle and operation will be described. FIG. 7 shows a basic optical system for explaining the SSD method.
1 is converted into parallel light by a condenser lens 42, passes through a beam splitter 43, and passes through an objective lens 4.
The light is condensed on the recording surface of an optical disk 45 as an information recording medium by 4.

The light reflected by the optical disk 45 is reflected by the beam splitter 43 through an opposite optical path, and the optical path is bent and guided to a detection optical system. The light split by the beam splitter 43 is converted into convergent light by the refocusing lens 46, and is separated into transmitted light and reflected light by the half mirror surface 48 in the microprism 47. The transmitted light is received by the photodetector 50 before focusing, and the reflected light is reflected on the mirror surface 4.
9 and is received by the photodetector 50 after being collected.

FIG. 8 is a diagram showing a light receiving state on the photodetector 50. As shown in FIG. FIG. 8B shows a light receiving state when the light spot is focused on the optical disk 45 (at the time of focusing). Outputs from the light receiving surfaces 51 to 56 are V (51) and V
(52), V (53), V (54), V (55), V
Assuming (56), the focus error signal FE is: FE = {V (51) −V (52) + V (53)} − ΔV
(54) −V (55) + V (56)}.

In the state shown in FIG. 8B, the light receiving surfaces 52, 5 are set such that the focus error signal FE becomes zero.
It is assumed that the width of 5 and the size of the light beam are set. FIG. 8A shows a case where the optical disk 45 has moved away from the objective lens 44, and at this time, the focus error signal FE has a positive value. FIG. 8C shows a case in which the vehicle approaches, and FE is a negative value. FIG. 8 (d)
Represents a so-called well-known S-shaped curve of the focus error signal in this method, which is a symmetric curve centered on the in-focus state.

[0006] In addition, a method using first-order diffracted light and -1st-order diffracted light of a hologram is disclosed in Yasuyuki Kawauchi et al., National Technic.
al Report Vol.41 No.6, December 1995, PP.667-668 (Fig. 4).

[0007]

The prior art shown in FIG. 7 has the following disadvantages. First, in order to convert the beam into two beams, a microprism, a hologram, or the like having a half mirror function is required, and it is difficult to simply construct the beam. However, there is a disadvantage that the price of the optical head device increases. Was.

SUMMARY OF THE INVENTION The present invention has been made in order to improve the disadvantages of the prior art, and has as its object to provide a focus detection device capable of simplifying the configuration and making it relatively inexpensive, and a focus detection device therefor. An optical head device using the same is provided.

[0009]

According to the present invention, there is provided a focus detecting device for controlling the light emitted from a light source to converge on a recording surface of an information recording medium, wherein the focus detecting device detects light passing through the recording surface. Different light path length setting means for setting first and second light paths having different light path lengths, and light receiving means for receiving light passing through the first and second light paths, the first and second light paths A focus detection device is provided, wherein focus detection of the light source is performed in accordance with a spot size of light passing through an optical path on a light receiving surface of the light receiving unit.

The different optical path length setting means has a first prism forming the first optical path, and a surface facing the reflection surface of the first prism, and has a refractive index of the first prism. And a second prism having a different refractive index from the second prism forming the second optical path.

Further, when the focus state of the light source is on the recording surface, the spot sizes of the first and second convergent lights are set to be the same. The refractive index of one prism is set to be smaller than that of the second prism, and the light incident on the first prism is totally reflected by the reflection surface, so that the first optical path is the first light path. It is characterized by being set longer than the second optical path.

Furthermore, the light source for generating light emitted to the recording surface is built in, and the light emitted from the built-in light source is recorded on the light incident surface of the first and second prisms passing through the recording surface. It is characterized in that the light is reflected toward the surface. Further, an air region is used instead of the first prism.

[0013] Further, the present invention is characterized in that there is a cover with an opening on the entrance surface of the first and second prisms.

The operation of the present invention will be described. Two types of refractive index bodies (first and second brhythms) are provided in the middle of the optical path of the light reflected from the recording surface of the information recording medium, and the light beam diameter (due to the optical path length difference between the two types of refractive index bodies) Spot size)
Is detected on the light receiving surface of the light receiving element, and focus detection is performed based on this difference. Thus, focus detection can be performed only by using two types of refractive index bodies, and the configuration is simplified and the cost is reduced.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of a first embodiment of the present invention, and is a schematic configuration diagram showing a state in which an optical disk device is playing back an optical disk using a focus detection device according to the present invention.

As shown in FIG. 1, a laser diode 1 is provided as a light source used for reproducing information on an optical disk 5. A prism 2 that transmits the light 8 emitted from the laser diode 1 and reflects the light 13 reflected from the optical disk 5, a condensing lens 3 that converts the light transmitted through the prism 2 into parallel light 9, An imaging optical system comprising an objective lens 4 for converting the parallel light 9 into a convergent light 10 and condensing it on the information recording surface 5a of the optical disk 5, extending the optical path of the light 13 reflected by the optical disk 5 and the prism 2, and A light receiving module 18 for performing focus detection is provided.

As shown in FIG. 2, the light receiving module 18 of this embodiment includes first and second prisms 6a and 6b having different refractive indices, and a photodetector 7. The refractive index of the prism 6a is selected to be smaller than that of the prism 6b, and one surface of the prism 6b is opposed to the reflecting surface of the prism 6a with the joint surface 23 interposed therebetween. The prism 6b has an action of extending the optical path of the light 13, and the prism 6a has an action of hardly changing the optical path length of the light 13. That is, the light 13 from the incident surface of the prism 6a travels almost straight and is totally reflected by the joining surface, which is a reflecting surface (as transmitted light 14).
7, the light 13 from the incident surface of the prism 6b travels straight inside (as transmitted light 15), and the light receiving surface 1 of the photodetector 7
7 is reached. The transmitted light 14 of the prism 6a has a focal point 21 on the joint surface 23.

FIG. 3 shows an example of the arrangement of the light receiving portions on the light receiving surface 17 of the photodetector 7. When the information recording surface 5a of the optical disk 5 is located at the focal point 22 of the objective lens 4, the light receiving module 18 is configured to form a light spot 16a drawn on the light receiving surface 17 by the light 14 transmitted through the prism 6a and a light 15 transmitted The light spots 16b drawn on the light receiving surface 17 are arranged so as to have the same size.

Next, the reproducing operation of the optical disk will be described. As shown in FIG. 1, the light 8 emitted from the laser diode 1 passes through the prism 2, is converted into parallel light 9 by the condenser lens 3, then enters the objective lens 4, and enters the information recording surface 5 a of the optical disk 5. Is collected. The divergent light 11 reflected on the information recording surface 5a of the optical disk 5 passes through the opposite direction of the above optical path, is converted into convergent light 13 by the condenser lens 3, reflected by the prism 2, and enters the light receiving module 18. .

As shown in FIG. 2, the light 13 incident on the light receiving module 18 becomes light 14 and light 15 transmitted through the prisms 6a and 6b, respectively, and is converted into an electric signal by the photodetector 7. As shown in FIG. 3, the light receiving surface 1 of the photodetector 7
7 has light receiving portions 7a to 7f. The electric signal generated from the light receiving unit is turned into a focus error signal, a reproduction signal, and a track error signal by an arithmetic circuit (not shown).
Assuming that the electric signals generated by the photodetector are Ia to If, respectively, the focus error signal, the reproduction signal, and the track error signal can be expressed by the following equations (1) to (3). An arithmetic circuit generates a focus error signal, a reproduction signal, and a track error signal, and reproduces the optical disk.

In the light receiving module 18, when the information recording surface 5a of the optical disk 5 is located on the focal point 22 of the objective lens, the light 14 transmitted through the prism 6a passes through the focal point 21 located on the bonding surface of the prism 6a and the prism 6b. The condensed and reflected light is incident on the photodetector 7 to form a light spot 16a on the light receiving surface 17, and the light 15 transmitted through the prism 6b
Are preferably arranged such that the size of the light spot 16b drawn on the light receiving surface 17 is equal to the size of the light spot 16a.

The details of the focus detection device according to the present invention will be described with reference to FIGS. 2, 3 and 4. FIG. In the present invention, as shown in FIG.
To the light receiving module 18. The light receiving module 18 includes prisms 6a and 6b and the photodetector 7. The prisms 6a and 6b are joined at the boundary surface 23 with a reflection film interposed therebetween, or are close to each other with an air layer interposed therebetween.

The convergent light 13 incident on the light receiving module 18
Transmits through the prism 6a or the prism 6b. The refractive index of the prism 6a is smaller than the refractive index of the prism 6b. The light 14 transmitted through the prism 6a is condensed at the focal point 21 on the boundary surface 23, is totally reflected, and enters the light receiving surface 17 of the photodetector 7.

Similarly, the transmitted light 1 transmitted through the prism 6b
5 enters the light receiving surface 17 of the photodetector 7. As shown in FIG. 3, the light receiving surface 17 of the photodetector 7 includes light receiving portions 7a to 7f, and the transmitted light 14 transmits light to the light receiving portions 7a, 7b, and 7b.
e and is converted into an electric signal. The prism 6
The transmitted light 15 transmitted through the light receiving portions 7c, 7d and 7f
And is converted into an electric signal. The electric signal is converted into a focus error signal, a track error signal, and a reproduction signal by an arithmetic circuit. Assuming that the electric signals from the light receiving units 7a to 7f are Ia to If, respectively, the focus error signal, the reproduction signal, and the track error signal are expressed by the formula (1), respectively.
It is detected from (2) and (3). The track error signal uses a phase difference method or a push-pull method.

Focus error signal = (Ia + Ib + If)-(Ic + Id + Ie) (1) Reproduction signal = (Ia + Ib + Ic + Id + Ie + If) (2) Track error signal = (Ia + Ic)-(Ib + Id) or (Ia + Id)-(Ib + Ic) (3) Here, as shown in FIG. 1, when the information recording surface 5a is at the focal position 22 of the objective lens 4, the light receiving module 18
The light spots 16a and 16b on the light receiving surface 17 of the light transmitted through the prisms 6a and 6b are arranged so as to have the same size. As a result, the amounts of electric signals from the light receiving units 7a to 7d are equal, and similarly, the amounts of electric signals from the light receiving units 7e and 7f are equal. Therefore, the focus error signal of the arithmetic circuit becomes zero.

FIG. 4 shows the relationship between the focus error signal 20 and the spots 16a and 16b on the light receiving surface 17 when the optical information recording medium is moved while the position of the objective lens is fixed. FIG. 4A shows an objective lens 4 and an information recording surface 5.
The output value of the focus error signal 20 according to the distance a is shown. 4B shows a case where the objective lens 4 is very far from the information recording surface 5a, FIG. 4C shows a case where the focus error signal 20 is minimized, and FIG. When the information recording surface 5a is at the position of the focal point 22 of
FIG. 4E shows the spots 16a and 16b when the focus error signal 20 is maximized, and FIG. 4F shows when the objective lens 4 and the information recording surface 5a are very close.

In the case of FIG. 4B, since the focal point 21 moves largely toward the objective lens, the light spots 16a and 16b are much larger than the light receiving portions. As a result, the electric signal (Ia + Ib) is substantially equal to Ie and (Ic + Id) is equal to If, and the output of the focus error signal becomes zero.
In FIG. 4C, the spot 16a is larger than the photodetector,
The spot 16b is almost a point. For this reason,
(Ic + Id) increases, and the focus error signal becomes the minimum value. In the case of FIG. 4D, the size of the spot 16a is equal to that of the spot 16b, so that the focus error signal becomes zero.

In the case of FIG. 4E, the spot 16a is almost a point, and the spot 16b is larger than the photodetector. For this reason, (Ia + Ib) increases, and the focus error signal becomes the maximum value. In the case of FIG. 4F, since the spot 16a and the spot 16b are sufficiently larger than the photodetector, (Ia + Ib) and Ie are almost equal, and (Ic + Id) and If are also equal. As a result, the focus error signal becomes zero. Using the focus error signal 20, the relative position between the objective lens 4 and the information recording surface 5a can be adjusted well.

In the prior art, in order to realize focus error detection by the two-beam SSD method, two beams are required.
Although special optical components such as a microprism and a hologram with a half mirror function were necessary to make the beam, according to the present embodiment, the conventional single-function photo Since a simple configuration in which two prisms having different refractive indices are arranged in the diode, a special optical component can be omitted and the cost can be reduced.

As an example, the incident angle of the convergent light 13 on the refractive index body is 5.7 °, the refractive index of the prism 6a is 1.2, the refractive index of the prism 6b is 2.2, and the thickness of the prisms 6a and 6b. Is set to 2.1 mm. At this time, the radius of each of the spots 16a and 16b on the light receiving surface 17 is 40 μm.

As described above, according to the optical head device using such a focus detection device, even if the signal detection optical system has a simple configuration of two prisms having different refractive indices and a photodetector, the focus can be obtained. Since the optical disk can be reproduced by detecting the error signal, the size and cost of the optical head device can be reduced.

FIG. 5 is a diagram showing an optical head device using a focus detection device according to a second embodiment of the present invention.
2 are indicated by the same reference numerals. In this example, instead of the laser diode 1, prism 2, and light receiving module 18 of the optical head device shown in FIG.
This is an example in the case where 9 is provided.

As shown in FIG. 5, the optical module 19 includes a light source 26, and prisms 6a and 6b for reflecting the light 8 emitted from the light source 26 and transmitting the light 13 reflected from the optical disk and returned. , A photodetector 7 are provided. The refractive index of the prism 6a is smaller than the refractive index of the prism 6b.

The light 14 transmitted through the prism 6 a of the optical module 19 is condensed and reflected at the converging point 21 on the joint surface 23, and is incident on the photodetector 7. The light 15 transmitted through the prism 6b enters the photodetector 7. Although astigmatism occurs in the light incident on the photodetector 7, the same effect as the light receiving module 18 in the embodiment shown in FIG. 1 can be obtained.

FIG. 9 shows another example of the light receiving section in the focus detection devices according to the first and second embodiments of the present invention. In this case, the light receiving surface 17 of the photodetector 7 includes light receiving sections 7g to 7l as shown in FIG. In this case, the detection of the focus error signal, the reproduction signal, and the track error signal is also possible. That is, assuming that the electric signals from the light receiving units 7g to 7l are Ig to Il, respectively, the focus error signal, the reproduction signal, and the track error signal are expressed by the equations (4) and (5), respectively.
It is detected from (6). The track error signal uses a phase operation method or a push-pull method. The light receiving module 1 in the embodiment shown in FIG.
The same effect as that of No. 8 can be obtained.

Focus error signal = (Ig + Ik + Il)-(Ih + Ii + Ij) (4) Reproduction signal = (Ig + Ih + Ii + Ij + Ik + Il) (5) Track error signal = (Ii + Il)-(Ij + Ik) or (Ii + Ik)-(Ij + I) (6) FIG. 10 is a view showing another example of a light receiving module as a third embodiment of the present invention, and the same parts as those in FIGS. Convergent light 1 due to cover 25 with opening
Light near the center of 3 is incident on the prisms 6a and 6b.
The cover 25 with an opening has a shape that is open near the center, that is, a shape obtained by hollowing out a small column portion near the center of the cylinder. In FIG. 10, the converging lights 13a and 13b, which are a part of the incident light, are
However, the transmitted lights 14 and 15 pass through the opening of the cover 25 with an opening, and cannot enter.
a, 6b, and performs the same operation as in FIG. Thereby, the diameters of the spots 16a and 16b on the light receiving surface can be made constant regardless of the effective diameter of the objective lens and the numerical aperture of the condenser lens 3, and even if the condenser lens 3 having a large numerical aperture is used. This has the effect of preventing instability of the focus error signal. This is because, if a condenser lens 3 having a large diameter and a large numerical aperture is used, the diameter of the incident light beam becomes large and the diameter of the spots 16a and 16b becomes large without the cover 25 having the opening. This is because the shape of the focus error signal 20 in FIG.

As described above, the optical head device using the focus detection device of the present embodiment can
Accordingly, the configuration of the optical head device is simplified, and the number of assembling steps and the like of the optical head device can be reduced. In addition, further miniaturization can be achieved by this optical head device. In addition, the prism 6a of the optical axis of the light source 26 of the present embodiment
The angle of incidence on the reflecting surface 24 of the prism 6b is preferably 45 °, but may be any value as needed. Further, the reflectance of the reflection surface 24 may be set to an arbitrary value as needed. The cross section of each of the prisms 6a and 6b is desirably a right triangle as shown in FIG.
The shape may be a parallelogram, a diamond, or the like.

Instead of the condenser lens 3 and the objective lens 4 of the optical head devices of the first and second embodiments, a single finite objective lens may be used as the imaging optical system. This allows
The configuration of the optical head device is simplified, and the number of assembling steps and the like of the optical head device can be reduced. In addition, it is possible to eliminate the space between the condenser lens 3 and the objective lens 4, whereby the cost and size of the optical head device can be further reduced. In the above description, the wavelength of the light-emitting means is not specified, but any wavelength can be used as long as the optical disc can be reproduced.

In the optical head devices of the first and second embodiments, the material of the prisms 6a and 6b is desirably optical glass having high transparency.
It may be a crystal made of liquid crystal, crystal, or the like. Further, the transmittance and the reflectance may be set to arbitrary values as needed. Further, it is desirable that the difference between the thicknesses of the prisms 6a and 6b does not exist in order to reduce the cost due to the number of manufacturing steps and to stabilize the focus error signal, but it may be changed as necessary.

In the light receiving module 18 of the optical head device of the first embodiment, the prisms 6a and 6b are desirably rectangular parallelepipeds, but may be triangular prisms, trapezoidal prisms or the like as necessary. FIG. 6 shows a fourth embodiment.
As shown in FIG. 7, the prism 6a may be air.

[0041]

As described above, according to the present invention, an optical head device using the SSD method as a focus error detecting method can be obtained with a simple configuration using only two refractive index bodies. Therefore, there is an effect that the cost can be reduced by reducing expensive optical components.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment of an optical head device according to the present invention.

FIG. 2 is a diagram showing a configuration of a light receiving module 18 of FIG.

FIG. 3 is a schematic configuration diagram illustrating an example of an arrangement of a light receiving surface 17 of the photodetector 7 in FIG.

FIG. 4 is a diagram for explaining the principle of focus detection according to the present invention.

FIG. 5 is a diagram illustrating a schematic configuration of a focus detection device used in an optical head device according to a second embodiment of the present invention.

FIG. 6 is a diagram illustrating a schematic configuration of a focus detection device used in an optical head device according to a fourth embodiment of the present invention.

FIG. 7 is a schematic configuration diagram of a conventional optical head device.

8 is a diagram illustrating an example of a spot on a light receiving surface of the device in FIG. 7;

FIG. 9 is a schematic configuration diagram showing another example of the arrangement of the light receiving surface 17 of the photodetector in FIG.

FIG. 10 is a diagram illustrating a schematic configuration of a focus detection device used in an optical head device according to a third embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser diode 2, 6a, 6b Prism 3 Condensing lens 4 Objective lens 5 Optical disk 5a Information recording surface 7 Photodetector 7a-7l Light receiving part 8 Laser emitting light 9 Outgoing parallel light 10 Outgoing convergent light 11 Incoming diverging light 12 Incoming light paralleling 12 Light 13 Return path convergent light 14 Transmitted light of prism 6a 15 Transmitted light of prism 6b 16a Spot by transmitted light 16b Spot by transmitted light 15 Light receiving surface 18 Light receiving module 19 Optical module 20 Focus error signal 21 Focus 22 Objective lens focus 23 Joining Surface 25 Cover with opening 26 Light source

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Shochi Oba 5-7-1 Shiba, Minato-ku, Tokyo F-term in NEC Corporation (Reference) 5D118 AA04 BA01 BB02 BF02 BF03 CC03 CD02 CF08 DA12 DA17 DA21 DA46 DC02 5D119 AA04 BA01 CA09 EA02 EA03 EC38 FA05 JA16 JA17 JB03 KA02

Claims (8)

[Claims] 1. A focus detection device for condensing light emitted from a light source on a recording surface of an information recording medium, comprising:
Different light path length setting means for setting first and second light paths having different light path lengths with respect to light passing through the recording surface, and light receiving means for receiving light passing through the first and second light paths. A focus detection device for detecting the focus of the light source according to a spot size of a light passing through the first and second optical paths on a light receiving surface of the light receiving unit. 2. The optical path length setting means, comprising: a first prism forming the first optical path; and a surface facing a reflection surface of the first prism, and a refractive index of the first prism. The focus detection device according to claim 1, further comprising a second prism having a different refractive index and forming the second optical path. 3. The spot size of the first and second convergent light beams is set to be the same when the focus state of the light source is on the recording surface. The focus detection device according to any one of the preceding claims. 4. The refractive index of the first prism is set smaller than that of the second prism, and the light incident on the first prism is totally reflected by the reflection surface, thereby 4. The focus detecting device according to claim 2, wherein the first optical path length is set longer than the second optical path length. 5. A light source for generating light emitted to the recording surface, wherein the light emitted from the built-in light source is recorded on an incident surface of the light passing through the recording surface of the first and second prisms. The focus detection device according to claim 1, wherein the light is reflected toward a surface. 6. The focus detection device according to claim 1, wherein an air region is used instead of the first prism. 7. An optical head device comprising the focus detection device according to claim 1. 8. The light-emitting device according to claim 1, further comprising a cover with an opening that allows light to pass only through an opening on an incident surface of the light passing through the recording surface of the first prism and the second prism. 7. The focus detection device according to 6.