JPH0810495B2 - Light pickup - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical information processing for recording and reading information stored in an optical storage medium such as an optical disk or a magneto-optical disk using laser light. It relates to an optical pickup.

2. Description of the Related Art Conventionally, an optical pickup that records and reads information stored in an optical storage medium such as an optical disk or a magneto-optical disk using a laser beam is a semiconductor laser as a light source, and a light emitted from this semiconductor laser is a light source. An optical lens for converging on the storage medium, a beam splitter for guiding the laser light reflected by the optical storage medium to a light receiving element, and astigmatism etc. to the reflected light, and light converged on the optical storage medium. The defocusing of the light is detected, the cylindrical lens for forming an image on the light receiving element and the reflected light are received to detect the defocus tracking deviation and the information stored in the optical storage medium. It is configured to include.

An ordinary optical pickup gives an astigmatism or the like to a beam splitter or reflected light for guiding the laser light reflected by the optical storage medium to the light receiving element in this way, and defocuses the light converged on the optical storage medium. Since a cylindrical lens or the like for detecting the light and forming an image on the light receiving element is required, the number of optical components increases, the optical axis alignment becomes difficult, the manufacturing cost becomes expensive, and the optical pickup is downsized. It was a big problem.

Therefore, in order to realize miniaturization, an optical pickup has been developed in which a hologram element capable of simultaneously realizing the functions of both the beam splitter and the cylindrical lens is developed. (Kimura et al., The 22nd Micro-Optics Research Group Vol 14,228,1986) Figure 6 shows the outline of this hologram type optical pickup. The light emitted from the semiconductor laser 61 passes through the hologram lens 62 and is focused on the surface of the optical disc 64 by the imaging lens 63. The light reflected and spread on the surface of the optical disc 64 with the intensity according to the recorded information is converged again by the imaging lens 63 and part of it returns to the semiconductor laser 61, but part of it is in two regions (R1, R2) The divided hologram element 62 divides the image in two directions and forms an image on the four-divided light receiving element 65, and the so-called knife edge method detects the focus deviation tracking deviation and the signal of the information stored in the optical storage medium.

Problems to be Solved by the Invention However, in this hologram type optical pickup,
The light emitted from the semiconductor laser 61 is once a hologram element.
Since the light passes through 62 and is focused on the surface of the optical disc 64 by the imaging lens 63, it is diffracted once when passing through the hologram element 62, and the light transmission efficiency is low when the diffraction efficiency of the hologram element is high. Therefore, there is a problem in that the amount of light condensed on the surface of the optical disc 64 decreases, and sufficient light intensity cannot be obtained on the disc surface. On the contrary, when a hologram element having a low diffraction efficiency is used, there is not only a problem that the intensity of the light reaching the light receiving element 65 is weak and it is impossible to detect a signal with less noise, and the semiconductor laser 61 is Since the amount of returning light increases, there is a big problem that the oscillation of the semiconductor laser becomes unstable and noise is generated.

The present invention overcomes such problems and provides a hologram type optical pickup which is highly efficient, has low noise, is small and lightweight, and is inexpensive.

Means for Solving the Problems In order to solve the above problems, the present invention provides a laser emitting polarized light, an optical lens for converging light emitted from the laser onto an optical storage medium, and the optical recording medium. A phase plate for rotating the polarization direction of the laser light reflected by the optical disk in the direction substantially perpendicular to the polarization direction of the light at the time of emission, and a predetermined wavefront arranged in the optical path of the light reflected by the optical recording medium. In an optical pickup including a diffractive element for generating light and a light receiving element for detecting light diffracted by the light diffractive element, the diffractive element is a uniaxial uniaxial optical element having a refractive index anisotropy. One optical crystal, and the optical axis of the optical crystal is set to be parallel to the surface of the diffraction element, further, the refractive index in the optical axis direction inside the optical crystal, periodically. With distributed optical media The present invention provides an optical pickup formed as described above.

Action The present invention is based on the fact that a diffraction element such as a hologram element using an optical medium having a refractive index anisotropy causes anisotropy in diffraction efficiency depending on the direction of polarized light entering the diffraction element. The application of the fact that the polarized light of the emitted laser light is not diffracted by selecting the polarized light direction of the emitted laser light, and only the reflected light rotated in the direction perpendicular to the polarized light direction can be diffracted. An efficient optical pickup is realized.

Examples Details of the present invention will be described with reference to examples.

The outline of the first embodiment of the present invention is shown in FIG. Light emitted from the semiconductor laser 1 that emits polarized light passes through a hologram element (diffraction element) 2 using liquid crystal, and an imaging lens 3
Is focused on the surface of the optical disk 5 via the quarter-wave plate 4. In this case, the laser light passing through the hologram element 2 is not diffracted, and almost all the emitted light from the semiconductor laser 1 is focused on the surface of the optical disc 5. The laser light reflected on the surface of the optical disk 5 passes through the quarter-wave plate 4 again and is converged by the imaging lens 3.

At this time, the reflected light passes through the quarter-wave plate twice, so that its polarization direction is rotated by 90 degrees. Therefore, a part of the reflected light returns to the semiconductor laser 1, but a part of the reflected light is diffracted by the hologram element 2 having two regions (R1, R2), divided into two directions, and the information on the disc surface is divided into four. The focus deviation, the tracking deviation, and the signal of the information stored in the optical storage medium are detected. At this time, the amount of light guided to the light receiving element is substantially determined by the diffraction efficiency of the hologram element 2. However, unlike the conventional example, the diffraction efficiency of the hologram element does not affect the light utilization efficiency of the outward path.

The principle of the hologram element (diffraction element) of the present invention is shown in FIGS. 2 and 3. FIG. 2 shows a refractive index ellipsoid of a uniaxial optical crystal. (A) shows a refractive index ellipsoid in the case where the optical axis is in the Z direction, in which case the refractive index is Ne for light (extraordinary light) having a polarization direction in the Z direction, and XY The refractive index is No for light (ordinary ray) with a polarization direction in the plane. Further, (b) shows a refractive index ellipsoid when the optical axis of the uniaxial optical crystal is tilted from the Z direction, and in this case, the refractive index is smaller than Ne for light having a polarization direction in the Z direction. (That is, Ne × cos θ, where θ is the average inclination of an elliptic pair), and for light having a polarization direction in the X direction, the refractive index is No. Therefore, comparing (a) and (b), the refractive index is always No for light propagating in the Y direction and having the polarization direction in the X direction, and the polarization direction is present in the Z direction and propagating in the Y direction. The refractive index is different for the light to be emitted. Therefore, the optical axis direction is changed by some method, for example, by applying a voltage. Alternatively, it is possible to form a hologram element having polarization direction dependency by spatially distributing the tilted area.

FIG. 3 (a) shows a schematic cross section of a hologram element using the liquid crystal used in the first embodiment. 31 and 32 are transparent glass substrates, 33 is a transparent electrode having a spatial periodic structure corresponding to a hologram, 34 is a transparent common electrode, and 35 and 36 are uniformly arranged in parallel with the substrate (homogeneous orientation) It is a nematic liquid crystal layer having a positive dielectric anisotropy, and in particular, 36 is a region in which a voltage is applied between the transparent electrodes to form an inclined array. In this case, the direction of the optical axis of the liquid crystal layer is the horizontal direction (Z direction) on the paper surface. In such a liquid crystal hologram element
When light enters from the direction of 37 (Y direction), the light whose polarization direction is perpendicular to the paper surface is always ordinary light and no diffraction phenomenon occurs. That is, it does not operate as a hologram element. However, with respect to light whose polarization direction is the horizontal direction (Z direction) of the paper surface, the refractive index differs between the region where no voltage is applied and the region where liquid crystal molecules are tilted due to voltage application, and as a result, the diffraction phenomenon occurs. Then, the liquid crystal layer acts as a hologram element. In the above embodiment, the polarization direction of the emitted light from the semiconductor laser 1 and the direction of the optical axis of the liquid crystal layer are arranged at right angles to each other.

FIG. 3B shows the outline of the shape of the transparent electrode 33 having a spatial periodic structure formed in the liquid crystal hologram element which can be observed from above in FIG. This shape is formed by the holographic exposure method, but it can also be formed by the electron beam exposure method that numerically processes the same shape. The optical axis direction of the liquid crystal layers arranged corresponding to this figure is the Z direction, and laser light polarized in the X direction is emitted, passes through this element, and is reflected by the disk and rotated by 90 degrees in the polarization direction. Are diffracted by this liquid crystal hologram element.

Further, when the liquid crystal hologram element is used, the tilt of the liquid crystal molecules can be controlled according to the applied voltage, and as a result, the diffraction efficiency of the hologram element can be arbitrarily controlled, and the amount of light returning to the semiconductor laser can be controlled. Since it is possible, the optical pickup can be operated in a state where the return light noise of the semiconductor laser is minimized.

In the example, the case where a nematic liquid crystal layer having a positive dielectric anisotropy uniformly arranged in parallel to the substrate (homogeneous orientation) was used as a hologram element was shown, but it was arranged vertically to the substrate (homeotropic orientation). It is also possible to use a nematic liquid crystal layer having a negative induced anisotropy as a hologram element. It is also possible to use a twisted nematic type liquid crystal layer which is parallel to the substrate and has a twisted structure as a hologram element, and in this case it does not matter even if cholesteric liquid crystal is included.

Although the case where the liquid crystal is used as the hologram element has been described above, it is also possible to use an optical crystal having a uniaxial refractive index anisotropy such as lithium niobate.

FIG. 4 shows an outline of an anisotropic hologram element formed by using a lithium niobate substrate. A mask is formed by photolithography or the like (including holographic exposure) on a predetermined area of the surface of the Y-direction-cut lithium niobate substrate 41, and ion exchange is performed on the exposed area 42 of the surface using benzoic acid or the like. , A region 42 having a high refractive index is formed.
The region formed by ion exchange on the surface of the Y plate does not change its refractive index for polarized light in the X direction and increases its refractive index for polarized light in the Z direction. Therefore, this element does not function as a hologram element for polarized light in the X direction, but functions as a hologram element for polarized light in the Z direction. Therefore, it can be applied to the practice of the present invention similarly to the above-mentioned liquid crystal hologram element. Further, in this element, the diffraction efficiency can be controlled by the thickness of the surface layer subjected to ion exchange.

Although FIG. 4 shows the case where a Y-cut plate is used,
The same effect can be provided by using an X-cut plate. In this case, it does not function as a hologram element for polarized light in the Y-axis direction, but functions as a hologram element for polarized light in the Z-axis direction.

In the first embodiment, the optical pickup using the 1/4 wavelength plate is shown, but when the optical recording medium is a magneto-optical disk, the polarization direction of the reflected light rotates according to the recording information. It is also possible to omit the / 4 wave plate.

Further, in the first embodiment, the case where the reflected light is divided into two directions by the anisotropic hologram element was shown, but since the hologram element as described above has a constant thickness, it becomes a convergent beam. Astigmatism can be created by a method that uses the stigma generated in response to this (Nakamura et al., National tech. Report, vol32, 490 (1986)). By utilizing this, it is possible to construct an optical pickup having a simpler structure.

In addition, the astigmatism wavefront is recorded on the hologram element itself,
It is also possible to give the same effect by the reproduced wavefront.

FIG. 5 shows an embodiment of a hologram type optical pickup using this stigma method. The light emitted from the semiconductor laser 51 emitting polarized light passes through the hologram element 52 using liquid crystal, and is focused on the surface of the optical disc 55 by the imaging lens 53 through the quarter wavelength plate 54. In this case, the laser light passing through the hologram element (diffraction element) 52 is not diffracted as in the first embodiment, and almost all the emitted light from the semiconductor laser 51 is focused on the surface of the optical disc 55.
Then, the laser light reflected on the surface of the optical disk 55 is again 1 /
It is converged by the condenser lens 53 through the four-wave plate 54. At this time, the reflected light passes through the quarter-wave plate twice, so that its polarization direction is rotated by 90 degrees. Therefore, a part of the reflected light returns to the semiconductor laser 51, but a part of the reflected light is diffracted by the diffractive element 52 and imaged on the ordinary four-division light receiving element 56, which is defocused and tracking deviated and stored in the optical storage medium. A signal of information is detected. The point that a highly efficient optical pickup can be realized, and the tilt of liquid crystal molecules can be controlled according to the applied voltage, and the diffraction efficiency of the hologram element can be arbitrarily controlled.
As in the first embodiment, the amount of light returning to the semiconductor laser can be controlled, so that the optical pickup can be operated in a state where the returning light noise of the semiconductor laser is minimized. Although the liquid crystal is used as the diffractive element in this embodiment, it is obvious that a crystal having uniaxial optical anisotropy such as lithium niobate can be used.

The hologram shape of the diffractive element in this embodiment may be an ordinary diffraction grating, and it is also a great feature of this embodiment that it can be easily realized by a two-beam interference exposure method or the like.

In the examples, the anisotropic hologram type optical pickup by the knife edge method and the astigma method is shown, but it is clear that the anisotropic hologram type optical pickup based on other detection methods can be configured. is there.

Further, although the semiconductor laser is used as the light source in the embodiments, it is obvious that a gas laser such as a He-Ne laser, a dye laser, or a solid laser can be used.

Further, in the present embodiment uses a liquid crystal or lithium _{niobate, KD 2 PO 4, β-} BaB 2 O 4, it is also possible to use a uniaxial crystal having an electrooptic effect or the like such as PLZT Further, it is possible to exert the effect by using a medium having a refractive index anisotropy as a diffractive element, including a biaxial optical crystal such as KTiPO ₄ .

EFFECTS OF THE INVENTION As described above, the present invention overcomes the problems of the conventional optical pickup, has high light utilization efficiency, can reduce the return light noise of the semiconductor laser, and is a small-sized, lightweight, low-cost hologram. Type optical pickup,
It is considered to have great value.

[Brief description of drawings]

FIG. 1 is a schematic perspective view of a first embodiment of an anisotropic hologram type optical pickup of the present invention, and FIG. 2 shows a refractive index ellipsoid of a uniaxial optical crystal, which shows the principle of the hologram element of the present invention. Explanatory drawing, FIG. 3 is a schematic view of an anisotropic hologram element using the liquid crystal used in the first embodiment, FIG. 4 is a schematic view of an anisotropic hologram element formed using a lithium niobate substrate, FIG. 5 is a schematic view of a main part of an embodiment of an anisotropic hologram type optical pickup using the Astigma method, and FIG. 6 is a schematic view of a conventional hologram type optical pickup. 1 ... Semiconductor laser emitting polarized light, 2 ... Liquid crystal hologram element, 3 ... Imaging lens, 4 ... Quarter wave plate, 5 ...
… Optical disc, 6… Light receiving element.

Claims (6)

[Claims] 1. A laser emitting polarized light, an optical lens for converging light emitted from the laser onto an optical storage medium, and a polarization direction of the laser light reflected by the optical recording medium. A phase plate for rotating in a direction substantially at right angles to the polarization direction, a diffractive element disposed in the optical path of the light reflected by the optical recording medium, and generating a predetermined wavefront, and diffracted by the optical diffractive element. In the optical pickup including a light receiving element for detecting light, the diffractive element is a uniaxial single optical crystal having refractive index anisotropy, and the light of the optical crystal is The axis is set to be parallel to the surface of the diffractive element, and the refractive index in the optical axis direction inside the optical crystal is formed by using an optical medium that is periodically distributed. Optical pick up . 2. An optical axis direction of a single uniaxial optical crystal forming a diffractive element is parallel to a polarization direction of laser light reflected by an optical recording medium and incident on the diffractive element. The optical pickup according to claim 1. 3. The optical pickup according to claim 2, wherein the diffractive element is a light transmissive type and has a function of generating a converging wavefront. 4. The diffractive element includes a liquid crystal mainly composed of uniformly arranged nematics to which an electric field having a spatial distribution is applied, and the diffractive element is constituted. Optical pickup described in. 5. The X of lithium niobate having a spatial refractive index formed in the Z direction on the surface of the diffractive element by an ion exchange method.
The optical pickup according to claim 1, wherein the optical pickup is formed by using an azimuth or Y azimuth cut plate. 6. The optical pickup according to claim 1, wherein the laser is a semiconductor laser.