JP2513237B2 - Optical head device - Google Patents

Description

The present invention relates to an optical head device used for recording or reproducing information in an optical disc device, a digital audio disc device, a video disc device or the like, and particularly uses a lattice type optical element. Optical head device.

[Conventional technology]

In a video disc device, a digital audio disc device, an optical disc device, etc. (hereinafter collectively referred to as an optical disc device), a conventional optical head device used for recording or reproducing information on a recording medium is a second optical head device. It has a configuration as shown in the figure. That is,
Light source 1 and collimated beam 3 of emitted light 2 from light source 1
, A collimating lens 4 for converting the collimated beam 3 into a convergent beam 15 and irradiating the recording medium 14 as a convergent beam 15, an actuator 6 for driving the convergent lens 5, a light source 1, and a collimating lens 4 In addition to the beam splitter prism 7 disposed between and, a focus error detecting means and a tracking error detecting means are provided.

There are various types of focus error detection means,
As a system most closely related to the present invention, there is a wedge prism system. As shown in FIG. 2, the focus error detecting means using the wedge prism system includes wedge prisms 8 and 9, a first two-divided photodetector having photodetectors 10 and 11, and photodetectors 12 and 13. And a second two-divided photodetector. The focus error detecting means configured as described above, when the convergent beam 15 is correctly focused on the recording medium 14 (disk surface), the light beam 16 from the wedge prism 8 or 9
Or 17 is focused between photodetectors 10 and 11 or between photodetectors 12 and 13, respectively, but focused beam 15
When is focused at a position offset with respect to the disk surface 14, the light beams 16 and 17 are offset towards or away from each other. Therefore, the focus error signal can be obtained by detecting the differential output of the photodetectors 10 and 11 or the differential output of the photodetectors 12 and 13.

Based on the focus error signal obtained in this way,
The actuator 6 is driven in a direction parallel to the optical axis, and servo control is performed so that the convergent beam 15 is correctly focused on the disk surface 14.

There are various types of tracking error detecting means, but the push-pull type is the most closely related to the present invention. The push-pull type tracking error detecting means is a method of detecting the reflected light from the disk surface by using a two-division photodetector, and the sum of the outputs of the photodetectors 10 and 11 shown in FIG. A tracking error signal is obtained by detecting the difference between the sum of the outputs of the devices 12 and 13. Based on this tracking error signal, the actuator 6 is driven in the direction of the adjacent track in a plane perpendicular to the optical axis to perform tracking control. (Philips Technical Review (Philips
Technical Review) Volume 40 (Published in 1982) Issue 6 Issue 15
(See pages 1 to 156) [Problems to be solved by the invention] The above-mentioned conventional optical head device currently in practical use has a size of about 40 × 40 × 30 mm ³ and is therefore heavy. However, it is an obstacle to downsizing and weight reduction of the entire optical disk device, and is an obstacle to realizing a stack type large capacity optical disk device. One of the causes of this is that the reflected light from the recording medium (optical disk) is bent from the light source by bending its optical axis 90 ° with a half prism or a polarizing beam splitter prism, and a photodetector is arranged behind it. Since it is used, it is difficult to make the optical system uniaxial.

For such a problem, a small-sized optical head has been proposed that utilizes the so-called SCOOP effect in which the oscillation output increases due to the self-coupling effect when the light is returned to the light emitting portion of the semiconductor laser light source.

However, it has been pointed out that the self-coupling effect is due to the instability of the oscillation phenomenon of the semiconductor laser, and in the digital audio disc device and the video disc device which have been put into practical use in the last few years, the reproduction signal and the positioning As a noise that leaks into a signal, a technology for suppressing this self-coupling effect is being developed. The self-coupling effect of a semiconductor laser is
A reflection surface called an optical disk is added to the resonator of the semiconductor laser itself, and the structure must be considered as a resonator composed of three mirrors. While the optical disc is spinning,
Due to flutter (positional fluctuation) in the optical axis direction of the optical disc,
Even when the focus servo control is applied, the distance between the semiconductor laser and the optical disk fluctuates within a width of about 1 μm, resulting in a resonator structure with extremely poor stability. Therefore, it is difficult to reproduce the signal on the optical disk by the SCOOP effect.

Further, when the push-pull method is adopted as the tracking error detection method, when the focusing lens 5 is moved in the direction perpendicular to the optical axis by the actuator 6 based on the tracking error signal, as shown in FIG. A deviation occurs between the optical axis and the dividing line 19 of the two-divided photodetector 18. Since the reflected light from the disk surface 14 is incident on the two-division photodetector 18 with the optical axis of the converging lens 5 as the center, the light quantity is not correctly divided into two at the division line 19 of the two-division photodetector 18, and In the example shown in FIG. 3, the amount of light from the photodetector 20 is greater than the amount of light from the photodetector 21. As a result, DC offset occurs in the tracking error signal, and the control range of tracking error control is narrowed.

Further, the above-mentioned conventional optical head has many drawbacks in that adjustment is complicated, weight is heavy and therefore expensive, and volume is also large, because many optical components that require optical polishing are used.

An object of the present invention is to solve the above-mentioned drawbacks of the conventional optical head device, and to provide a small-sized and lightweight optical head device which is excellent in mass productivity and low in cost.

[Means for solving problems]

An optical head device according to the present invention includes a laser light source, an imaging lens that converges a light beam emitted from the laser light source and advances straight onto a recording medium, and between the laser light source and the imaging lens. A grating coupler formed on the transparent substrate, the grating coupler having two regions having different lattice directions with a central boundary line as a boundary, and the reflected light from the recording medium is the two regions of the grating coupler. In the transparent substrate, two pairs of photodetectors are provided corresponding to the waveguide for guiding the diffracted light when diffracted in and the two regions for receiving the diffracted light guided by the waveguide. In the optical head device provided, the point symmetry is provided on the boundary line of the two regions in the grating coupler, and the point at which the reflected light intersects the boundary line is the center of symmetry. Two tracking error detection regions that are provided at different positions and have different grating directions, a waveguide that guides diffracted light from each of the two tracking error detection regions, and two waveguides that are provided corresponding to the waveguides. A plurality of photodetectors are provided on the transparent substrate.

Further, a tracking error detection method for an optical head device according to the present invention includes a laser light source, an imaging lens that converges a light beam emitted from the laser light source and advances straight onto a recording medium, the laser light source, and the laser light source. A grating coupler which is arranged between the imaging lens and a transparent substrate and which has two regions having different lattice directions with a central boundary line as a boundary, and reflected light from the recording medium is reflected by the grating. Two pairs of light provided corresponding to the waveguide for guiding the diffracted light when diffracted in the two regions of the coupler and the two regions for receiving the diffracted light guided by the waveguide, respectively. In an optical head device in which a detector is provided on the transparent substrate, the reflected light is provided on the boundary line between the two regions in the grating coupler, The diffracted light of the Waru point of two regions having a lattice directions different provided at a position of the center and the point symmetry of symmetrical guiding the photodetector in waveguide, and performs a tracking error detection.

[Operation] The operation and the operating principle of the present invention are as follows. That is, in order to guide the reflected light from the recording medium to the photodetector,
An optical waveguide formed on the transparent substrate and a grating coupler formed on the transparent substrate for coupling the reflected light with the optical waveguide are used. When light is incident on the grating coupler, transmitted light that is transmitted without being coupled to the optical waveguide and diffracted light that is diffracted by coupling with the optical waveguide are generated. Therefore, the portion of the grating coupler of the optical waveguide having a photodetector is arranged between the light source of the semiconductor laser and the imaging lens, and the light passing from the semiconductor laser to the recording medium is transmitted through the grating coupler. Use light. At this time, the transmitted light has the same effect as when a transparent plate having a thickness substantially equal to the thickness of the substrate is simply inserted in the optical path width.

On the other hand, diffracted light from the grating coupler is used for the reflected light from the recording medium. As a result, the reflected light can be taken out of the optical axis without using a half prism or a polarization beam splitter prism as in the conventional case. That is, the grating coupler acts as a beam splitter. As a result, a compact and lightweight optical head device can be constructed.

Further, in the present invention, in order to extract the focus error signal in addition to the information signal from the reflected light that is the diffracted light extracted outside the optical axis, a line that intersects the grating axis of the grating coupler with the optical axis of the imaging lens. With the grating coupler having a weak focal power (the function of the lens) so that the grating coupler has an effect equivalent to that of the wedge prism in the conventional optical head device shown in FIG. Among them, it is converted into a light beam almost equivalent to the wedge prism method.

In addition, in order to extract the tracking error signal, two regions having different lattice directions with point symmetry are located in positions slightly apart from each other with the point where the optical axis of the imaging lens intersects with the grating coupler as the center of symmetry. By forming the gratings in these two regions in different directions and comparing the intensities of the guided light from the two regions, the tracking error signal is obtained by the principle of the push-pull method.

〔Example〕

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

FIG. 1 is a perspective view showing the basic structure of the optical system of the first embodiment of the present invention.

In FIG. 1, a radiated light 2 from a light source 1 such as a semiconductor laser passes as a transmitted light through a grating coupler 22 having a focal power (a function of a lens), is converged by an imaging lens 23 and is converged on a recording medium 14. To be done. The reflected light from the recording medium 14 is converged by the imaging lens 23, is coupled with the optical waveguide in the grating coupler 22, and becomes guided light 24 and 25 and 26 and 27, which are installed in the optical waveguide. Fourth and fifth photodetectors 31 and 32 and sixth photodetector, which are arranged adjacent to the second and third photodetectors 29 and 30, which are arranged adjacent to the photodetector 28. Reach vessel 33 and respectively.

The grating coupler 22 has four different regions. That is, a region (A) 37 and a region (B) having different waveguide directions with a boundary line 35 including an intersection of the optical axis 34 of the imaging lens 23 and a transparent substrate (substrate) 36 forming an optical waveguide as a boundary. 38, and further on a straight line perpendicular to the track direction of the recording medium 14 including the intersection of the optical axis 34 and the substrate 36, the waveguide direction at the symmetrical position with the intersection as the center of symmetry. Two different circular areas (C) 39 and areas (D) 40 are formed. Area (C) 39 and area (D) 40
The distance between and is larger than or equal to the maximum movable distance of the imaging lens 23 in the direction perpendicular to the track. The area (A) 37 reflects the reflected light toward the light emitting point of the light source 1,
The second photodetector 29 and the third photodetector 30 have a grating pattern which is guided on the contact line. The area (B) 38 has a grid pattern that guides the reflected light toward the light emitting point of the light source 1 onto the contact line between the third photodetector 31 and the fourth photodetector 32. Similarly, the area (C) 39 is for reflecting the reflected light to the first photodetector.
In FIG. 28, the area (D) 40 has a lattice pattern that guides the reflected light to the sixth photodetector 33.

FIG. 4 is a diagram for explaining the operation principle of focus error detection in the above-described embodiment. (In the figure, area (C) 39
The area (D) 40 and the first photodetector 28 and the sixth photodetector 33 are omitted. ) FIG. 4 (a) is a diagram showing a focused state in which the light beam is focused on the recording medium. In this state, the guided light 25 from the area (A) 37 and the area (B) 38 are shown.
Guided light 26 from the second and third photodetectors, respectively.
29 and 30 contact lines and 4th and 5th photodetectors
It converges on the contact lines of 31 and 32. FIG. 4B is a diagram showing guided light in a defocused state (a state in which the focus is not properly focused) in which the recording medium 14 is displaced and approaches the imaging lens 23. In this state, the guided lights 25 and 26 enter the second photodetector 29 and the fifth photodetector 32, and do not enter the third and fourth photodetectors 30 and 31. FIG. 4C is a diagram showing guided light in a state where the recording medium 14 is displaced and moved away from the imaging lens 23. In this state, the guided lights 25 and 26 enter the third and fourth photodetectors 30 and 31, respectively, and do not enter the second and fifth photodetectors 29 and 32. Therefore, if the outputs of the second to fifth photodetectors 29 to 32 are S2 to S5, (S2 + S5) − (S3
The focus error signal can be obtained from + S4).

On the other hand, the tracking error signal utilizes that the intensity distribution of the reflected light becomes unbalanced when the focused spot of the radiated light 1 converged and irradiated on the recording medium 14 deviates from the center of the track.

That is, when a track error occurs, the area (C) 39
There is a difference between the amount of light incident on the area (D) and the amount of light incident on the area (D) 40. This light intensity difference is due to the two photodetectors 28 and 33.
Can be detected as the output difference, and the direction of the tracking error can also be detected based on whether the signal indicating the output difference is positive or negative. The reproduction signal from the recording medium 14 can be obtained as the sum of the output signals of the first to sixth photodetectors 28 to 33.

When the push-pull method is adopted as the tracking error detection method and only the converging lens or the imaging lens is moved to correct the error, there is a gap between the optical axis of the error detection optical system and the optical axis of the lens side optical system. Occurs, and a difference occurs in the amount of light incident on the photodetector for detecting a track error, so that an offset occurs in the error signal. In order to solve this problem, the embodiment shown in FIG. 1 is provided with the following means. FIG. 5 is an explanatory diagram for explaining the track error detection operation in the embodiment of FIG. 1, and is a diagram showing the distribution of reflected light on the grating coupler. Fig. 5 (a)
FIG. 3 is a diagram showing a case where the optical axis of the imaging lens and the optical axis of the error detection optical system coincide with each other. Area (C) 39 and area (D) 40
Detects light that is symmetric about the center line B of the reflected light beam and is incident on the same area, so that no tracking error offset occurs. 5 (b) and (c),
It is a figure which shows the case where the optical axis of an imaging lens has each shifted | deviated to the left or the right with respect to the optical axis of an error detection optical system. In any of these cases, a tracking error offset is generated to obtain a tracking error signal from the difference in the amount of light incident on the area (C) 39 and the area (D) 40 on the left and right of the center line B of the reflected light beam. Is suppressed.

FIG. 6 is a perspective view showing the basic structure of the optical system of the second embodiment of the present invention.

In the first embodiment, the area (C) 39 and the area (D) 40 have their centers on the boundary line 35 between the area (A) 37 and the area (B) 38 so as to straddle these areas. However, in this embodiment, the region (C) 59 is arranged inside the region (A) 57, and the region (D) 60 is arranged inside the region (B) 58. Further, in the first embodiment, the first to sixth photodetectors 28 to 33 are provided inside the optical waveguide, but in the present embodiment, they are provided at the end faces of the optical waveguide. The other structure is the same as that of the first embodiment. The operation of this embodiment is also the same as that of the first embodiment.

〔The invention's effect〕

As described above in detail, since the optical head device of the present invention is composed of only the imaging lens and the transparent substrate having the grating coupler formed on the surface as components constituting the optical system, the conventional optical head device has many components. It is possible to drastically reduce the number of parts of the optical system as compared with the case of using. Therefore, there is an effect that a small-sized and lightweight optical head device can be obtained. In addition, since the optical waveguide and the grating coupler can be manufactured with high precision by the planar technique, there is an effect that an optical head that can be easily optically adjusted can be obtained.

[Brief description of drawings]

FIG. 1 is a perspective view showing the basic structure of the optical system of the first embodiment of the present invention, FIG. 2 is a sectional view showing an example of a conventional optical head device, and FIG. 3 is a track in the example of FIG. FIG. 4 is an explanatory view showing the distribution of the light quantity on the error detector, FIG. 4 is an explanatory view for explaining the operation principle of the focus error detection in the embodiment of FIG. 1, and FIG. 5 is a track in the embodiment of FIG. FIG. 6 is an explanatory view for explaining the error detecting operation, and FIG. 6 is a perspective view showing the basic configuration of the optical system of the second embodiment of the present invention. 1 ... Light source, 2 ... Radiant light, 3 ... Collimated beam,
4 ... Collimating lens, 5 ... Converging lens, 6
...... Actuator, 7 …… Beam splitter, 8.9
...... Wedge prism, 10/11/12/13/20/21/28
29 ・ 30 ・ 31 ・ 32 ・ 33 ・ 48 ・ 49 ・ 50 ・ 51 ・ 52 ・ 53 …… Photodetector, 14 …… Recording medium, 15 …… Focused beam, 16 ・ 17 ……
Refracted light, 18 …… 2 split photodetector, 19 …… split line, 22 ・ 62
...... Grating coupler, 23 …… Imaging lens, 24 ・
25 ・ 26 ・ 27 …… Guided light, 34 …… Optical axis, 35 ・ 55 …… Boundary line, 36 ・ 56 …… Substrate, 37 ・ 57 …… Area (A), 38 ・ 58…
Area (B), 39/59 area (C), 40/60 area (D).

Claims (2)

(57) [Claims] 1. A laser light source, an image forming lens for converging a light beam emitted from the laser light source and traveling straight, and projecting the light beam onto a recording medium. The laser light source is provided between the laser light source and the image forming lens. And a grating coupler having two regions having different lattice directions on the transparent substrate with a central boundary line as a boundary, and reflected light from the recording medium is diffracted in the two regions of the grating coupler. At this time, two pairs of photodetectors are provided on the transparent substrate so as to respectively correspond to the waveguide for guiding the diffracted light and the two regions for receiving the diffracted light guided by the waveguide. In the head device, provided in the grating coupler on the boundary line of the two regions, and provided in a point-symmetrical position with the point at which the reflected light intersects the boundary line as the center of symmetry. And two tracking error detection regions having different lattice directions is, a waveguide for guiding the diffracted light from each of the two tracking error detection area,
An optical head device comprising: two photodetectors provided corresponding to the waveguides and provided on the transparent substrate. 2. A laser light source, an image forming lens for converging a light beam emitted from the laser light source and propagating straight, and projecting the light beam onto a recording medium. The laser light source is arranged between the laser light source and the image forming lens. And a grating coupler having two regions having different lattice directions on the transparent substrate with a central boundary line as a boundary, and reflected light from the recording medium is diffracted in the two regions of the grating coupler. At this time, two pairs of photodetectors are provided on the transparent substrate so as to respectively correspond to the waveguide for guiding the diffracted light and the two regions for receiving the diffracted light guided by the waveguide. In the head device, provided in the grating coupler on the boundary line of the two regions, and provided in a point-symmetrical position with the point at which the reflected light intersects the boundary line as the center of symmetry. Is by guiding the photodetector in waveguide diffracted light from the two regions with different lattice directions, a tracking error detection method for an optical head device which is characterized in that the tracking error detection.