JP2616018B2 - Optical head device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk device for recording and reproducing information signals on an optical disk medium.

(Prior Art) An optical head device used for optical recording using an optical disk has a focus error detection function for forming a minute light spot on the optical disk, and a track error detection for accurately tracing a desired track. It is required to have functions. In the read-only, write-once, or phase-change optical discs, the reproduction of recorded information is optically performed by changing the intensity of the reflected light. Therefore, means for detecting the intensity of the reflected light is also required.

FIG. 2 is a diagram showing a basic configuration of an optical head according to the prior art. Linearly polarized light emitted from a semiconductor laser 1 as a light source is converted into parallel light 3 by a collimator lens 2 and enters a polarization beam splitter 4. Here, the polarization direction of the incident light is set to a direction that passes through the polarization beam splitter 4. The light passing through the polarizing beam splitter 4 passes through a quarter-wave plate 5 and is converged by a converging lens 6 to an optical disk 7.
Focused on top. The reflected light from the optical disk 7 is again 1/4
The light passes through the wave plate 5 and becomes linearly polarized light having a polarization direction orthogonal to the light emitted from the semiconductor laser 1 and enters the polarization beam splitter 4. The light incident on the polarization beam splitter 4 is guided to a signal detection system after the optical path is bent. The light extracted by the polarization beam splitter 4 is focused by a beam splitter 9 for detecting a focus error 10 and for detecting a track error 11.
The focus error is detected by the knife edge method, and the track error is detected by the push-pull method. The readout signal is obtained from the sum of the outputs of the focus error detection photodetector and the track error detection photodetector.

(Problems to be Solved by the Invention) In the optical head device according to the prior art described above, it is necessary to arrange a large number of optical components such as a polarizing beam splitter quarter-wave plate and a knife edge. It was difficult to reduce the size and weight. Further, it is difficult to reduce the cost because an expensive polarizing beam splitter is used and the number of components is large. The large number of assembly steps also hindered price reduction. SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a small, lightweight, and low-cost optical head device.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a light source that emits substantially linearly polarized light, and a light source for condensing light emitted from the light source onto an optical disc. Image optical means, 1 /
A four-wavelength plate, an optical diffraction element, a first photodetector receiving + 1st-order diffracted light from the optical diffraction element, and a second light receiving -1st-order diffracted light from the optical diffraction element A focus error signal and a track error signal are detected from the first photodetector, and an information signal is obtained from at least a sum of outputs of the first photodetector and the second photodetector. The light diffraction type element is formed of a birefringent optical crystal, and is disposed such that the polarization direction of the substantially linearly polarized light emitted from the light source and the optical axis of the birefringent optical crystal are substantially orthogonal to each other. It is a birefringent diffraction grating element in which a phase compensation grating is formed on a phase grating formed on a refractive optical crystal so as to refract extraordinary light and transmit ordinary light.

(Operation) Hereinafter, the operation of the present invention will be described in detail. The grating element used in the present invention has the function of combining the functions of the information signal detection optical system and the focus error / track error detection optical system according to the prior art.

When proton exchange with, for example, benzoic acid is applied to an X plate or a Y plate of lithium niobate, the refractive index for extraordinary rays increases by about 0.13, and the refractive index for ordinary rays decreases by about 0.04. Here, by canceling the phase difference between the ordinary ray passing through the exchange area and the ordinary ray passing through the non-exchange area by means such as providing a phase compensation film, the refractive index change due to proton exchange is as if the extraordinary ray. Can only occur in Therefore, periodic exchanges,
If a non-exchange region is provided and the above-described phase difference canceling means is employed, the ordinary ray does not feel the refractive index difference and the extraordinary ray feels the refractive index difference, so that a diffraction grating acting only on the extraordinary ray can be manufactured. At this time, if the phase difference between the exchanged area and the non-exchanged area of the extraordinary ray is set to II, the transmittance of the extraordinary ray is almost 0% and the transmittance of the ordinary ray is almost 100%. Can be.

The application of such a birefringent diffraction grating as a polarizer has been studied in the past. The report is stated.

3 and 4 are diagrams for explaining the operation of the present invention. In the present invention, since the polarization direction of the linearly polarized light 12 emitted from the laser and the Z-axis 14 of the lithium niobate crystal 13 are orthogonal to each other, the laser emission light is, as shown in FIG. Is not affected by the lattice pattern. On the other hand, as shown in FIG. 4, the polarization direction of the reflected light 16 from the optical disk that has passed through the quarter-wave plate twice has been rotated in a direction orthogonal to the polarization direction at the time of emission, so that the effect of the grating pattern was obtained. As a result, most of the light amount becomes the diffracted lights 17 and 18. Therefore, the laser emission light hardly returns to the laser, a stable oscillation operation without return light noise can be continued, and the reproduction quality of the reproduction read signal is not impaired. Further, in order to improve the reproduction quality of the reproduction read signal, at least two diffracted lights are received by the photodetector to compensate for a decrease in the amount of light.

Further, by using a hologram as the lattice pattern formed on the lithium niobate crystal, it is possible to perform a focus error detection operation and a track error detection operation using the diffracted light.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram for explaining a first embodiment of the present invention.

The linearly polarized light 20 emitted from the semiconductor laser 19 is incident on a birefringent diffraction element 21 made of a lithium niobate crystal having a Z axis in a direction perpendicular to the linearly polarized light. As described above, the birefringent diffraction element has no influence on the incident light because it becomes ordinary light. The light that has passed through the birefringent diffraction element 21 is exchanged by the collimate lens 22 into parallel light,
After passing through 3, the light is converged on the optical disk 25 by the converging lens 24. The reflected light from the optical disk 25 is exchanged again by the converging lens 24 into parallel light, enters the quarter-wave plate 23, is converted into linearly polarized light in a direction orthogonal to the laser emission light, and is transmitted to the birefringent diffraction element 21. Incident. The reflected light from the optical disk is extraordinary light with respect to the lithium niobate crystal and is diffracted by the action of the lattice pattern. In the present embodiment, the + 1st-order diffracted light 26
A focus error signal and a track error signal are detected from the output of the first photodetector 27 that receives the light, and read from the sum of the outputs of the first photodetector 27 and the second photodetector 29 that receives the −1st-order diffracted light 28. I'm getting the signal.

FIG. 5 is a diagram showing the correspondence between the grating pattern of the birefringent diffraction element 21 and the segment division pattern of the first photodetector 27. In this embodiment, the birefringent diffraction element is divided into four regions, and the first photodetector is divided into six segments in order to detect a focus error and a tracking error using the + 1st-order diffracted light. . In the first lattice pattern section 30,
When the light spot is correctly formed on the optical disk, a grating is formed such that the light incident on this area is converged on the point A 31 on the first photodetector 27. Just like,
The second grating pattern part 32, the third grating pattern part 33, and the fourth grating pattern part 34 each receive light incident on each region,
A grating is formed so as to converge at points B, C and D on the first photodetector. The focus error signal is obtained from the difference signal of the diagonal sum of the central four segments of the first photodetector 27, and the track error signal is obtained from the difference signal between the outputs of the fifth segment 38 and the sixth segment 39.

In this embodiment, the lattice pattern of the birefringent diffraction element is set so that the ± 1st-order diffracted light is generated on the left and right sides of the semiconductor laser in FIG. 1, but the lattice pattern of the birefringent diffraction element and the crystal of lithium niobate crystal are used. Since the relationship with the axis is arbitrary,
For example, a configuration in which ± first-order diffracted light is generated above and below the plane of the paper is of course also possible.

FIG. 6 is a diagram for explaining a second embodiment of the present invention. In the present embodiment, the semiconductor laser 40, the photodetector 41, the birefringent diffraction element 42, and the quarter-wave plate 43 are housed in the same package, and the size is reduced by forming an optical module 44.

The birefringent grating element described above can be easily manufactured by a planar batch process. For example, after a titanium diffusion layer is formed on a Y-plate lithium niobate substrate, proton exchange of a desired lattice pattern is performed to form a lattice. Nb ₂ O ₅ having a thickness that compensates for the difference in the refractive index of ordinary light is formed on the proton exchange region. Further, an SiO ₂ film may be formed to reduce the reflectance over the entire surface of the substrate.

(Effect of the Invention) According to the present invention described above, it is possible to provide a small, lightweight, and low-cost optical head device.

[Brief description of the drawings]

1 and 5 are views for explaining a first embodiment of the present invention, FIG. 2 is a view for explaining a conventional technique, and FIG.
FIG. 4 is a view for explaining the operation of the present invention, and FIG. 6 is a view for explaining a second embodiment of the present invention. In the figure, 1 ... semiconductor laser, 2 ... collimating lens, 3 ...
Parallel light, 4 ... Polarizing beam splitter, 5 ... 1/4 wavelength plate, 6 ... Converging lens, 7 ... Optical disk, 8 ... Lens, 9 ... Beam splitter, 10 ... Light beam for focus error detection Reference numeral 11 denotes a light beam for detecting a track error, 12 denotes laser emission light, 13 denotes a lithium niobate crystal, and 14 denotes Z.
Axis, 15: transmitted light, 16: reflected light, 17: diffracted light, 18 ...
… Diffracted light, 19… semiconductor laser, 20… linearly polarized light, 21
... birefringent diffraction element, 22 ... collimating lens, 23 ...
... 1/4 wavelength plate, 24 ... convergent lens, 25 ... optical disk, 2
6 + 1st-order diffracted light, 27 first photodetector, 28 -1
Next-order diffracted light, 29... Second photodetector, 30... First grating pattern portion, 31... Point A, 32.
Third lattice pattern part, 34 ... Fourth lattice pattern part, 35 ...
... Point B, 36 ... Point C, 37 ... Point D, 38 ... Fifth segment, 39 ... Sixth segment, 40 ... Semiconductor laser, 41 ...
... a photodetector, 42 ... a birefringent diffraction element, 43 ... a quarter-wave plate, 44 ... an optical module.

Claims (1)

(57) [Claims] A light source that emits substantially linearly polarized light; imaging optical means for condensing light emitted from the light source onto an optical disk; a quarter-wave plate; a light diffraction element; A first photodetector that receives + 1st-order diffracted light from the diffractive element, and a second photodetector that receives -1st-order diffracted light from the light diffractive element, wherein the first light A focus error signal and a track error signal are detected from a detector, and an information signal is detected from at least a sum of outputs of the first photodetector and the second photodetector. The birefringent optical crystal is made of a crystal, and the polarization direction of the substantially linearly polarized light emitted by the light source and the optical axis of the birefringent optical crystal are arranged so as to be substantially orthogonal to each other. Birefringent diffractometer with phase compensation grating formed to refract and transmit ordinary light An optical head apparatus which is a type element.