JP2646782B2 - Optical head device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head device used for recording and reproduction of an optical disk.

[Conventional technology]

Conventional optical head devices have various configurations according to a track error signal or focus error signal detection method.
An example is shown in FIG. 2, which is configured using a number of lenses and prisms.

In this conventional optical head device, a three-beam method is used for detecting a track error signal, and an astigmatism method is used for detecting a focus error signal. Radiation light 2 from a semiconductor laser 1 as a light source is diffracted by a diffraction grating 16 and divided into a +1 order light and a −1 order light sub-beam in addition to a 0 order light main beam. After passing through the beam splitter 13, the converging lens 4 forms a main beam spot 26 and sub-beam spots 27 and 28 on the surface of the optical disk 8 as shown in FIG. In FIG. 3, reference numeral 24 denotes a pit. The light reflected by the optical disk 8 passes through the opposite optical path, is reflected by the beam splitter 13, becomes light having astigmatism by the convex lens 17 and the cylindrical lens 14, and enters the six-divided photodetector 15.

FIG. 4 is a view showing a state of condensing light incident on the six-divided photodetector 15. 4B shows the light focusing state when the light is focused on the optical disk surface, and FIG. 5C shows the light focusing state when the optical disk approaches and away from the converging lens 4 from the focused state. From this, the focus error signal is detected by the photodetectors 21, 22, 23, 2
If the output of 4 is V (21), V (22), V (23), V (24), it is detected from V (21) + V (24) -V (22) -V (23). On the other hand, the optical disk 8 is used for detecting a track error signal.
This is based on the fact that when the main beam spot 26 deviates from the pit 24 on the surface, the reflected light amounts of the two sub beam spots 27 and 28 become unbalanced. That is, the sub beams 27 and 28 on the surface of the optical disk 8 are condensed on the photodetectors 19 and 20 on the six-segment photodetector 15, respectively. Therefore, if their outputs are V (19) and V (20), The error signal is detected as V (19) -V (20). The recording signal is the total light quantity of the main beam, that is, V (21) + V (22)
+ V (23) + V (24).

[Problems to be solved by the invention]

In the above-mentioned conventional optical head device, it is difficult to reduce the size and weight of the device due to the configuration of the biaxial system for returning the return light from the optical disk to the outside of the optical axis.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical head device that is small and lightweight and has a high light utilization rate.

[Means for solving the problem]

An optical head device according to an aspect of the invention includes a light source, a diffraction grating that divides light emitted from the light source into substantially three beams, an imaging lens system that narrows an image of the light source onto a recording medium, and A hologram element that diffracts and separates the returning light out of the optical axis of the imaging lens system; a quarter-wave plate disposed between the diffraction element and the hologram element and the recording medium; A focus error signal, a track error signal, and a photodetector that obtains a recording signal by receiving the separated diffracted light outside, wherein the diffraction grating and the hologram element are a birefringent diffraction grating type comprising a birefringent crystal. Elements, and their crystal optical axes are orthogonal to each other, and the hologram element hardly diffracts the light emitted from the light source and almost diffracts the return light from the recording medium. axis Is set.

[Action]

The operation and principle of the present invention are as follows. In the present invention, in order to reduce the size of the apparatus, the beam splitter and the optical system for detecting a focus error signal of the conventional optical head apparatus use a single hologram element and the configuration of the optical head is a uniaxial system. In the case of this uniaxial configuration, the light emitted from the light source passes through the hologram element and the diffraction grating for generating three beams used for detecting the track error signal, so that unnecessary diffracted light components are also generated.
Therefore, when realizing an optical head with a high light utilization rate,
The hologram element and the diffraction grating for generating three beams have a birefringent diffraction grating type structure having polarization.

As a method of realizing a birefringent diffraction grating, there is a method using a birefringent crystal described below. As shown in FIG. 5, an X plate of lithium niobate 52, which is a birefringent crystal, or Y plate
When the plate is subjected to proton exchange with benzoic acid, the refractive index for extraordinary light, which is polarized light parallel to the crystal optical axis, increases by about 0.09, and the refractive index for ordinary light, which is polarized light perpendicular to the optical axis, increases by about 0.04. Decrease. Therefore, a grating in which the exchange region 53 subjected to proton exchange and the non-exchange region 54 not subjected to proton exchange are periodically arranged to function as a diffraction grating.

In this grating, if a phase compensation film 55 is formed on the exchange region as shown in FIG. 5 to cancel the phase difference between ordinary light passing through the exchange region 53 and ordinary light passing through the non-exchange region 54, On the other hand, this grating does not act as a diffraction grating and can be transmitted without diffraction. In other words, this lattice looks like a mere transparent substrate. On the other hand, by changing the depth of the exchange region 53 while satisfying the above-described phase difference canceling condition for ordinary light, the phase difference for extraordinary light can be set to an appropriate value. Efficiency is obtained. For example, when the phase difference is π, the extraordinary light is completely diffracted. The application of such a birefringent diffraction grating as a polarizer is being studied, and is described in BINORING et al., BINORINGENT GRATING, published in the Technical Digest of the Second Optoelectronics Conference, pp. 167-169.
POLARIZER ”.

In the above, the birefringent diffraction grating type structure of the diffraction grating and the hologram element has been described in which the birefringent crystal material lithium niobate is made to have anisotropy in the diffraction efficiency of the grating by a proton exchange method. It is also possible to fill the birefringent medium having a lattice groove described in the specification with a substance having the same refractive index as one of ordinary light and extraordinary light of the medium in the lattice groove. Furthermore, a birefringent diffraction grating type structure in which a birefringent material such as a liquid crystal is filled in an isotropic medium having a lattice groove described in Japanese Patent Application No. 62-98854 can be realized.

〔Example〕

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

FIG. 1 is a diagram showing a first embodiment of the present invention, in which the same components as those in the prior art shown in FIG. 2 are denoted by the same reference numerals.
The optical head device includes a semiconductor laser 1 as a light source, a diffraction grating 11 having a birefringent diffraction grating structure for substantially splitting outgoing light from the semiconductor laser into three beams, and an image of the light source focused on an optical disk 8. A collimating lens 3 and a converging lens 4 forming an image lens system; a hologram element 5 having a birefringent diffraction grating type structure for diffracting and separating return light from the optical disk 8 out of the optical axis of the imaging lens system; A quarter-wave plate (λ / 4 plate) 6 disposed between the hologram element 5 and the optical disk 8, a six-segment photodetector 7 for receiving diffracted light separated off the optical axis, and a photodetector It consists of 12 and.

In this optical head device, the crystal optic axis of the hologram element 5 is set perpendicular to the polarization plane of the radiated light 2 which is linearly polarized light from the semiconductor laser 1 as a light source, and the crystal optic axis of the diffraction grating 11 is set parallel. doing. Therefore, the radiated light 2 enters the hologram element 5 as ordinary light, enters the diffraction grating 11 as extraordinary light, passes through the hologram element 5 without being diffracted, and only passes through the diffraction grating 11 as described above. Diffracted. The phase difference of the extraordinary light in the diffraction grating 11 is set so as to be slightly diffracted for a sub beam used for detecting a track error signal. The radiated light 2 with the sub-beam is converted into a collimated light 10 by a collimating lens 3 and converted into a circularly polarized light by a λ / 4 plate 6. And
The light is converged on the surface of the optical disk 8 by the converging lens 4 as shown in FIG.

The return light reflected by the optical disk 8 is converted to a convergent lens 4, λ
It passes through the / 4 plate 6 and the collimating lens 3 in the reverse route. λ / 4 plate 6
Then, the polarization state of the return light is converted from circular polarization to linear polarization perpendicular to the polarization plane of the radiated light 2. Therefore, the return light enters the diffraction grating 11 as ordinary light and enters the hologram element 5 as extraordinary light, passes through the diffraction grating 11 without being diffracted, and is diffracted only by the hologram element 5. The phase difference of the extraordinary light of this hologram element is π so that it is completely diffracted.
Is set to Of the diffracted light, the + 1st-order diffracted light is 6
The light enters the split photodetector 7 and is used for error signal detection and recording signal detection. The -1st-order diffracted light of the main beam enters the photodetector 12 and is used for recording signal detection.

As shown in FIG. 6, the hologram element 5 is formed of two A hologram areas 36 and B hologram areas 37 having different diffraction directions separated by a dividing line 44 perpendicular to the track direction 45. As shown in FIG.
The diffracted light is focused on 12. FIG. 7B shows an optical disk 8.
When the convergent light is focused on the A hologram area 36
The diffracted light of the main beam out of the return light from the optical disk that has entered the B hologram area 37 converges to points 40 and 41 on the division line of the six-division photodetector 7. A hologram area 36
Has a hologram pattern corresponding to an interference fringe between a spherical wave emitted and diverged from the semiconductor laser 1 and a spherical wave diverged from the convergence point 40 of the diffracted light. On the other hand, the B hologram region 37 has a hologram pattern corresponding to an interference fringe of a spherical wave diverging from the semiconductor laser 1 and a spherical wave diverging from a convergence point 41 of the diffracted light. FIGS. 7A and 7C show a case where the optical disc 8 is displaced and moves away from the converging lens 4 and a case where it comes close.

Therefore, the four photodetectors 31, 32,
Assuming that the outputs of 33 and 34 are V (31), V (32), V (33) and V (34), the focus error signal is (V (31) + V (34))-(V
(32) + V (33)). On the other hand, the tracking error signal uses a three-beam method as in the conventional technique, and the sub-beams returned from the optical disk 8 along with the main beam converge on the photodetectors 29 and 30, respectively.
Assuming that the outputs of the two photodetectors 29 and 30 are V (29) and V (30), they can be obtained from V (29) -V (30). The recording signal is the sum V of the return light of the main beam, that is, the sum V (31) of the sum of the outputs of the photodetectors 31, 32, 33, and 34 in the six-split photodetector 7 and the output of the photodetector 12. + V (32) + V (33) + V
(34) + V (12).

As described above, in the optical head device of the present embodiment, the light utilization rate is increased by preventing the hologram element from diffracting incoming light and the diffraction grating from returning light.

FIG. 8 shows a second embodiment of the present invention, in which the collimating lens 3 and the converging lens 4 of the first embodiment are replaced by a single imaging lens 9.

FIG. 9 shows a third embodiment of the present invention, in which a hologram element 5, a diffraction grating 11 and a λ
This is an optical head device in which the / 4 plate 6 is bonded to form one element.

FIG. 10 shows a fourth embodiment of the present invention. An optical head in which the six-segment photodetector 7, photodetector 12, and semiconductor laser 1 of the third embodiment are housed in one package. Device.

In each of the above embodiments, the diffraction grating and the hologram element have a birefringent diffraction grating type structure in order to increase the light utilization rate. As another embodiment of the present invention, the light utilization is low,
A normal diffraction grating and a hologram element having a birefringent diffraction grating structure may be used in the same configuration as the above embodiments.

〔The invention's effect〕

In the optical head device of the present invention, since the hologram element is used, the configuration can be simplified, and further, a uniaxial configuration can be adopted, so that the device can be reduced in size and weight. Also,
By making the hologram element and the diffraction grating have a birefringent diffraction grating type structure, it is possible to reproduce the signal of the optical disk with high efficiency.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention, FIG. 2 is a basic configuration diagram of a conventional optical head device, and FIG. 3 is a view showing a state of a condensed spot on an optical disk surface of the conventional optical head device. FIG. 4 is a diagram for explaining a state of light incident on a six-segment photodetector in a conventional optical head device, and FIG. 5 is a diagram for explaining a structure of a hologram element and a diffraction grating used in the present invention. FIG. 6 is a diagram showing a configuration of a hologram element used in the optical head device of the present invention. FIG. 7 is a light focusing state of diffracted light focused on a photodetector of the optical head device of the present invention. FIG. 8 is a diagram showing a second embodiment of the present invention, FIG. 9 is a diagram showing a third embodiment of the present invention, and FIG. 10 is a fourth embodiment of the present invention. It is a figure showing an example. DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser 2 ... Emitted light 3 ... Collimate lens 4 ... Convergent lens 5 ... Hologram element 6 ... λ / 4 plate 7,15 ... Six division photodetector 8 ... Optical disk 9 ... Conclusion Image lens 10 Collimated light 11, 16 Diffraction grating 12 Photodetector 13 Beam splitter 14 Cylindrical lens 17 Convex lens 24 Pit 26 Main beam 27, 28 Sub beam 19 , 20,21,22,23,24,29,30,31,32,33,34… Photodetectors 36,37… Lattice area 40,41… Convergence point 44… Partition line 45… Track Direction 52 Lithium niobate 53 Exchange area 54 Non-exchange area 55 Phase compensation film

Claims (1)

(57) [Claims] A light source; a diffraction grating for substantially dividing light emitted from the light source into three beams; an imaging lens system for focusing an image of the light source on a recording medium; and returning light from the recording medium. A hologram element for diffracting light off the optical axis of the imaging lens system, a quarter-wave plate disposed between the diffraction element and the hologram element and the recording medium, and separating the light off the optical axis. A focus error signal, a track error signal, and a photodetector that obtains a recording signal by receiving the diffracted light, wherein the diffraction grating and the hologram element are birefringent diffraction grating elements made of a birefringent crystal, The crystal optic axes are set so that the crystal optic axes are orthogonal to each other, and the hologram element hardly diffracts the light emitted from the light source and almost diffracts the return light from the recording medium. hand The optical head device according to claim Rukoto.