JPS63241735A - Optical pickup - Google Patents

Abstract

PURPOSE:To obtain an optical pickup with high efficiency, by using an optical medium having an aeolotropic refraction index as a diffraction element, and giving diffraction only on reflected light rotated in a perpendicular direction to polarized light without giving the diffraction on laser polarized light. CONSTITUTION:A semiconductor laser beam which emits the polarized light passes through the diffraction element 2 using liquid crystal, and is condensed on an optical disk 5 via an image-forming lens 3 and a 1/4-wavelength plate 4. A beam of light which passes through the element 2 is not diffracted, and the reflected light passes through the 1/4-wavelength plate 2 for two times, and a polarizing plane is rotated by 90 deg., and a part of it is returned to the laser 1, and another part is bi-sected in two directions by the diffraction element 2 having two areas R1 and R2, and information on a disk plane is image-formed on a quartered photodetector 6, and focus deviation, tracking deviation, and a storage information signal are detected. A light quantity introduced to the photodetector is decided by the diffractive efficiency of the diffraction element 2. The element 2 is the liquid crystal arranged uniformly and is nematic mainly, and it can be controlled by applying an electric field having spatial distribution, and a return light quantity can be controlled.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical information processing device for recording and reading information stored in an optical storage medium such as an optical disk or a magneto-optical disk using a laser beam. This relates to optical pickups.

Conventional technology Conventionally, optical pickups that use laser light to record and read out information stored on optical storage media such as optical disks and magneto-optical disks use a semiconductor laser as a light source and a light source that uses the light emitted from the semiconductor laser. An optical lens for converging the laser beam onto the optical storage medium; a beam splitter for guiding the laser beam reflected by the optical storage medium to a light receiving element; A cylindrical lens for detecting defocus of light and forming an image on a light receiving element; and a light receiving element for receiving reflected light and detecting defocus, tracking deviation, and information stored in the optical storage medium. It is composed of:

A normal optical pickup has a beam splitter that guides the laser light reflected by the optical storage medium to the light receiving element, and a beam splitter that applies astigmatism to the reflected light, causing a focal shift of the light converged on the optical storage medium. A cylindrical lens or the like is required to make it detectable and form an image on the light receiving element.
The number of optical components increases, making it difficult to align the optical axes, making manufacturing expensive, and posing a major problem in miniaturizing optical pickups.

Therefore, in order to achieve miniaturization, an optical pickup has been developed in which the optical pickup is replaced with a hologram element that can simultaneously perform the functions of both the beam splitter and the cylindrical lens. (Kiku et al., Paper of the 22nd Micro-Optics Study Group Vol. 14, 228.1986) The outline of this hologram type optical pickup is shown in FIG. The light emitted from the semiconductor laser 61 passes through the hologram lens 62 and is directed to the optical disk 6 by the imaging lens 63.
The light is focused on the 40 surface. The light that is reflected and spread by the surface of the optical disk 64 with an intensity corresponding to the recorded information is converged again by the imaging lens 63 and a part of it returns to the semiconductor laser 61.
A part of the image is divided into two directions by a hologram element 62 divided into two regions (R1, R2), and is imaged on a quarter-divided light receiving element 65, and is detected by the so-called Knifezi method to detect defocus, tracking, and the optical storage medium. A signal of information stored in is detected.

Problems to be Solved by the Invention However, in this hologram type optical pickup, the light emitted from the semiconductor laser 61 once passes through the hologram element 62 and is focused on the surface of the optical disk 64 by the imaging lens 63. Once the light passes through the hologram element 62, it is diffracted, and if the diffraction efficiency of the hologram element is high, the light transmission efficiency is low, so the amount of light focused on the surface of the optical disk 64 is reduced. There was a problem in that sufficient light intensity could not be obtained on the disk surface.

Conversely, when using a hologram element with low diffraction efficiency, there is a problem that the intensity of light reaching the light receiving element 65 is weak and it becomes impossible to detect a signal with low noise. As the amount of light returning increases,
There was a major problem in that the oscillation of the semiconductor laser became unstable and generated noise.

The present invention overcomes these problems and provides a holographic optical pickup that is highly efficient, has low noise, is small, lightweight, and inexpensive.

Means to solve problems The present invention provides the following optical system.

(1) A laser that emits polarized light, an optical lens that focuses the light emitted from the laser onto an optical storage medium, and a diffraction element that is placed in the optical path of the light reflected by the optical storage medium and that generates a predetermined wavefront. and an optical pickup including a light receiving element for detecting light diffracted by the diffraction element, wherein the diffraction element is formed using an optical medium having refractive index anisotropy. The present invention provides an optical pickup characterized by:

■ The present invention also provides a laser that emits polarized light, an optical lens for converging the light emitted from the laser onto an optical storage medium, and a polarization direction of the laser light reflected by the optical storage medium when the light is emitted. a phase plate for rotating in a direction perpendicular to the direction; a diffraction element disposed in the optical path of the reflected light to generate a predetermined wavefront; and a light receiving element for detecting the light diffracted by the diffraction element. In the optical pickup configured, the diffraction element is formed using an optical crystal having refractive index anisotropy, and the optical axis direction of the optical crystal is parallel to the polarization direction of the emitted light from the laser. It is located in

(3) Moreover, the optical pickup is such that the diffraction element is formed using a uniaxial optical crystal having refractive index anisotropy.

(4) Further, the diffraction element is formed using a uniaxial optical crystal having refractive index anisotropy, and the optical axis direction of the uniaxial optical crystal is the polarization direction of the emitted light of the laser. is placed parallel to.

(5) Furthermore, the diffraction grating has a function of being disposed in the optical path of light reflected by the optical storage medium, generating a predetermined wavefront, and transmitting and converging itself.

(6) The diffraction element includes a uniformly arranged liquid crystal mainly composed of nematic to which an electric field having a spatial distribution is applied.

σ) The diffraction element is formed using a cut plate of lithium niobate in the X direction or the Y direction, on the surface of which a spatial refractive index is formed by an ion exchange method.

(8) The laser is a semiconductor laser.

The present invention is based on the fact that a diffraction element such as a hologram element using an optical medium having refractive index anisotropy has anisotropy in diffraction efficiency depending on the direction of polarized light incident on the diffraction element. This is an application of the fact that by selecting the polarization direction of the emitted laser beam, diffraction is not imparted to the polarized light of the emitted laser beam, and diffraction can be imparted only to the reflected light rotated at right angles to the polarization direction. An efficient optical pickup is realized.

Example The details of the present invention will be explained using examples.

A first embodiment of the present invention is schematically shown in FIG.

Light emitted from a semiconductor laser 1 that emits polarized light passes through a hologram element (diffraction element) 2 using liquid crystal, and is focused by an imaging lens 3 onto the surface of an optical disk 5 via a quarter-wave plate 4. . In this case, the laser light passing through the hologram element 2 is not diffracted, and almost all of the light emitted from the semiconductor laser 1 is focused on the surface of the optical disk 5.

The laser beam reflected from 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 its polarization direction is rotated by 90 degrees. Therefore, part of the reflected light returns to the semiconductor laser 1, but part of it returns to the two regions (R1, R
2) is diffracted by the hologram element 2 and divided into two directions, and the information on the disk surface is imaged on the four-divided light-receiving element 6, and the focal shift, tracking shift, and signal of the information stored in the optical storage medium are detected. be done. At this time, the amount of light guided to the light receiving element is approximately 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 utilization efficiency of light on 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 the index ellipsoid of a uniaxial optical crystal. (a) shows a refractive index ellipsoid when the optical axis is in two directions; in this case, the refractive index is Ne for light with a polarization direction in the Z direction (extraordinary light), and The refractive index is No for light whose polarization direction exists in a plane (ordinary light). Also, (b) shows the refractive index ellipsoid when the optical axis of a uniaxial optical crystal is tilted from the Z direction. , in this case, the refractive index is smaller than Ne for light with a polarization direction in the Z direction (i.e., Nex cos θ, θ is the average slope of the ellipse pair), and for light with a polarization direction in the X direction, , the refractive index is NO.Therefore, (a),
Comparing (b), the refractive index is always No for light that has a polarization direction in the X direction and propagates in the Y direction, and
The refractive index increases for light that has a polarization direction in the Y direction and propagates in the Y direction. Therefore, the optical axis direction is changed by some method, for example, by applying a voltage. Alternatively, by spatially distributing the tilted regions, it is possible to form a hologram element having polarization direction dependence.

FIG. 3(a) shows a schematic cross-section of the hologram element using liquid crystal used in the first embodiment. 31.32 is a transparent glass substrate, 33 is a transparent electrode having a spatial periodic structure corresponding to a hologram, 34 is a transparent common electrode, 35.3
6 is a nematic liquid crystal layer having positive dielectric anisotropy that is uniformly aligned parallel to the substrate (homogeneous alignment); in particular, 36 is a region where a voltage is applied between the transparent electrodes and the liquid crystal layer has an inclined alignment; be. In this case, the direction of the optical axis of the liquid crystal layer is the horizontal direction (Z direction) on the paper surface. When light enters such a liquid crystal hologram element from direction 37 (Y direction), the light whose polarization direction is perpendicular to the plane of the paper always becomes ordinary light and no diffraction phenomenon occurs. That is, it does not operate as a hologram element. However, for light whose polarization direction is horizontal to the plane of the paper (Z direction), the refractive index is different between the region where no voltage is applied and the region where the liquid crystal molecules are tilted due to the application of voltage, resulting in a diffraction phenomenon. The liquid crystal layer acts as a hologram element. In the embodiment described above, 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. 3(b) schematically shows the shape of a transparent electrode 33 having a spatial periodic structure formed in the liquid crystal hologram element that can be observed from above in FIG. 3(a). This shape is formed by holographic exposure, but it can also be formed by electron beam exposure, which numerically processes the same shape.

The optical axis direction of the liquid crystal layer arranged according to this figure is the Z direction, and a laser beam polarized in the X direction is emitted, passes through this element, is reflected by the disk, and the polarization direction is rotated by 90 degrees. is diffracted by this liquid crystal hologram element.

Furthermore, by using a liquid crystal hologram element, 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 returned to the semiconductor laser can be controlled. Since this is possible, the optical pickup can be operated in a state where the return optical noise of the semiconductor laser is minimized.

In the example, a case was shown in which a nematic liquid crystal layer having positive dielectric anisotropy that was uniformly aligned parallel to the substrate (homogeneous alignment) was used as a hologram element. ) and has negative dielectric anisotropy can also be used as a hologram element. It is also possible to use a twisted nematic type liquid crystal layer parallel to the substrate and having a twisted structure as a hologram element, and in this case there is no problem even if cholesteric liquid crystal is included.

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

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

In addition, although Fig. 4 shows the case where a Y-cut plate is used,
A similar effect can be achieved by using a 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, an optical pickup using a 1/4 wavelength plate was shown, but when the optical recording medium is a magneto-optical disk, the polarization direction of the reflected light rotates according to the recorded information, so this method It is also possible to omit the /4 wavelength plate.

Furthermore, in the first embodiment, the reflected light is split into two directions by the anisotropic hologram element, but since the hologram element described above has a certain thickness, the reflected light is split into two directions by the anisotropic hologram element. A method using astigma that occurs against people (Nakamura et al., Nat 1ona 1Tech, Report
, vol. 132, 490 (1986)), it is possible to create astigmatism. By utilizing this, it is possible to construct an optical pickup with a simpler structure.

Note that it is also possible to record an astigmatic wavefront on the hologram element itself and provide the same effect with the reproduced wavefront.

FIG. 5 shows an embodiment of a hologram type optical pickup using this astigma method. Light emitted from a semiconductor laser 51 that emits polarized light passes through a hologram element 52 using liquid crystal, and is focused by an imaging lens 53 onto the surface of an optical disk 55 through a quarter-wave 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 of the light emitted from the semiconductor laser 51 is focused on the surface of the optical disk 55. The laser beam reflected from the surface of the optical disk 55 passes through the quarter-wave plate 54 again and is converged by the condenser lens 53. At this time, the reflected light passes through the quarter-wave plate twice, so its polarization direction is rotated by 90 degrees. Therefore, part of the reflected light returns to the semiconductor laser 51, but
A portion of the light is diffracted by a diffraction element 52 and imaged onto a conventional quadrant light receiving element 56 to detect defocus, tracking deviation, and signals of information stored on the optical storage medium.

It is possible to realize a highly efficient optical pickup, and the tilt of the liquid crystal molecules can be controlled according to the applied voltage, the diffraction efficiency of the hologram element can be arbitrarily controlled, and the amount of light returned to the semiconductor laser can be controlled. This is similar to the first embodiment in that the optical pickup can be operated in a state where the feedback noise of the semiconductor laser is minimized. Although this example also shows a case where liquid crystal is used as a diffraction element, it is obvious that it is also possible to use a crystal having uniaxial optical anisotropy such as lithium niobate.

The hologram shape of the diffraction element in this embodiment may be a normal diffraction grating, and a major feature of this embodiment is that it can be easily realized by a three-beam interference exposure method or the like.

In the examples, an anisotropic hologram optical pickup using the Naifetsu method and the Astigma method was shown.
It is clear that it is also possible to construct an anisotropic holographic optical pickup based on other detection methods.

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

In addition, in this example, it is possible to use liquid crystals, lithium niobate, etc., uniaxial crystals having electro-optical effects such as KD2Po4, β-BaB2O4, PLZT, etc., and biaxial crystals such as KTiP04. It is possible to achieve this effect by using a medium having refractive index anisotropy, including optical crystals, as a diffraction element.

Effects of the Invention As shown above, the present invention overcomes the problems of conventional optical pickups, has high light utilization efficiency, reduces return light noise of a semiconductor laser, and provides a small, lightweight, and low-cost hologram. It provides a type optical pickup and is considered to be of great value.

[Brief explanation of drawings]

FIG. 1 is a schematic perspective view of a first embodiment of the anisotropic hologram optical pickup of the present invention, and FIG. 2 shows a refractive index ellipsoid of a uniaxial optical crystal, and illustrates the principle of the hologram element of the present invention. Explanatory drawings, FIG. 3 is a schematic diagram of an anisotropic hologram element using liquid crystal used in the first example, and FIG. 4 is a schematic diagram of an anisotropic hologram element formed using a lithium niobate substrate. FIG. 5 is a schematic diagram of a main part of an embodiment of an anisotropic hologram type optical pickup using the astigma method, and FIG. 6 is a schematic diagram of a conventional hologram type optical pickup. DESCRIPTION OF SYMBOLS 1... Semiconductor laser that emits polarized light, 2... Liquid crystal hologram element, 3... Imaging lens, 4... 1/4 wavelength plate, 5... Optical disk, 6... Light receiving element. Name of agent: Patent attorney Toshio Nakao and one other person Figure 1 Figure 2 Figure 3 Figure 33 Transparent to the extreme Figure 4 Figure 5 Figure 55
Nainug Figure 6 Otsu 4 ≠ Inuk 654 minutes 9 "PD

Claims (14)

[Claims] (1) A laser that emits polarized light, an optical lens that focuses the light emitted from the laser onto an optical storage medium, and a diffraction element that is placed in the optical path of the light reflected by the optical storage medium and that generates a predetermined wavefront. and an optical pickup including a light receiving element for detecting light diffracted by the diffraction element, wherein the diffraction element is formed using an optical medium having refractive index anisotropy. An optical pickup featuring (2) The optical pickup according to claim 1, wherein the diffraction element is formed using a uniaxial optical crystal having refractive index anisotropy. (3) The diffraction element is formed using a uniaxial optical crystal having refractive index anisotropy, and the optical axis direction of the uniaxial optical crystal is arranged parallel to the polarization direction of the emitted light of the laser. The optical pickup according to claim 2, characterized in that: (4) Claim 3, characterized in that the diffraction grating is arranged in the optical path of the light reflected by the optical storage medium and has the function of generating a predetermined wavefront, and transmitting and converging the diffraction grating itself. optical pickup. (5) Claim 1, characterized in that the diffraction element includes a uniformly arranged nematic-based liquid crystal to which an electric field having a spatial distribution is applied. optical pickup. (6) The diffraction element is in the X direction or Y direction of lithium niobate on which a spatial refractive index has been formed by an ion exchange method on the surface.
The optical pickup according to claim 1, wherein the optical pickup is formed using an azimuth cut plate. (7) The optical pickup according to claim 1, wherein the laser is a semiconductor laser. (8) A laser that emits polarized light, an optical lens for converging the light emitted from the laser onto an optical storage medium, and a polarization direction of the laser light reflected by the optical storage medium relative to the polarization direction of the light when emitted. a phase plate for rotating in a right angle direction; a diffraction element disposed in the optical path of the reflected light to generate a predetermined wavefront; and a light receiving element for detecting the light diffracted by the diffraction element. In optical pickup,
The diffraction element is formed using an optical crystal having refractive index anisotropy, and the optical axis direction of the optical crystal is arranged parallel to the polarization direction of the emitted light of the laser. optical pickup. (9) The optical pickup according to claim 8, wherein the diffraction element is formed using a uniaxial optical crystal having refractive index anisotropy. (10) The diffraction element is formed using a uniaxial optical crystal having refractive index anisotropy, and the optical axis direction of the uniaxial optical crystal is arranged parallel to the polarization direction of the emitted light of the laser. The optical pickup according to claim 8, characterized in that: (11) Claim 8, characterized in that the diffraction grating is arranged in the optical path of the light reflected by the optical storage medium and has the function of generating a predetermined wavefront, and transmitting and converging the diffraction grating itself. optical pickup. (12) Claim 8, characterized in that the diffraction element includes a uniformly arranged nematic-based liquid crystal to which an electric field having a spatial distribution is applied. optical pickup. (13) Claim 8, characterized in that the diffraction element is formed using a cut plate of lithium niobate in the X direction or the Y direction, on the surface of which a spatial refractive index has been formed by an ion exchange method. Optical pickup described in section. (14) The optical pickup according to claim 8, wherein the laser is a semiconductor laser.