JP2001273640A - Optical information recording/reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce crosstalk from the other layer during the placing of a multiplayer optical disk. SOLUTION: A reflected light from the other layer is detected by photodetectors 131, and 121 or the like having proper sizes, installed in an image surface, and a signal from a reading layer is processed. Crosstalk is reduced, and the reading reliability of the multiplayer optical disk is enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

The field of application (object) of the invention described in all claims is described in a form supplementing the title of the invention. The present invention relates to an optical information recording / reproducing apparatus, and more particularly to a reading optical system of an optical disk drive.

2. Description of the Related Art A general outline of an optical system of an optical disk drive will be described with reference to FIG. The laser light emitted from the semiconductor laser 1 is converted into a parallel and circular beam by the collimator lens 23 and the triangular prism 2. After that, the laser beam is reflected by the polarization beam splitter 31, and λ /
The light is converted into circularly polarized light by the four-wavelength plate 21. This circularly polarized light is focused on the optical disk 50 by the 41 objective lens. A guide groove is provided inside the optical disk, and recording marks having different reflectances are written along the groove. Since the optical disc is rotating, the recording mark located at the irradiation position of the laser beam moves, and the amount of reflected light changes with time. The length and interval of the recording mark are decoded according to the information to be recorded. The reflected light having information returns to the objective lens 41 and is converted from circularly polarized light into linearly polarized light by the λ / 4 wavelength plate 21. Since this polarization direction is orthogonal to the polarization direction of the light emitted from the semiconductor laser, it can be transmitted through the polarization beam splitter 31. This transmitted light is split into two light beams by the half prism 32. The light passing through the condenser lens 42 is converged by the condenser lens, but the knife edge 22 is placed in front of the focal position, so that half of the light beam is blocked. A two-segment detector 51 is installed at the focal position, and if the focus of the irradiation light on the optical disc shifts, an imbalance occurs in the amount of light entering the two photodetectors of the two-segment photodetector 51. This imbalance is detected as a differential signal by the electronic circuit 60 and is set as a focus error signal 71. The focus error signal is used to adjust the position of the objective lens 41 by using the 63 actuators so that defocus does not occur so that the focal position of the light emitted from the objective lens 41 always matches the recording mark. On the other hand, the light reflected by the half mirror 32 is irradiated on the two-divided photodetector 52 by the condenser lens 43. Electronic circuit 6
Reference numeral 1 denotes a differential circuit whose output is a tracking error signal 72. The signal added by the electronic circuit 62 becomes a data signal 73. The optical disk drive described above is an example, and the focus error signal is detected by a Foucault method and the tracking error signal is detected by a method called a diffracted light differential type. In addition to these, various methods have been proposed. As typical examples, a focus error signal detection method includes an astigmatism method,
Image rotation method, and the like. As the tracking error signal detection method, there are a three spot type, a wobbling type, and the like. These methods have advantages and disadvantages such as optical adjustment and light use efficiency, and are properly used depending on the type of drive. These details are described in detail in "Basics and Applications of Optical Disk Storage" (edited by Yoshito Tsunoda, edited by the Institute of Electronics, Information and Communication Engineers). At present, it is becoming possible to transfer large-capacity, high-speed data, so that an optical disk as an information storage medium is required to have a higher density in order to record large-capacity data such as moving image information.
To achieve this, the interval between recording marks and the track pitch of the optical information medium must be reduced. The spot size of the laser beam for reading and writing on the optical information medium is also reduced to the diffraction limit of the lens aperture and is very small. In recent years, an objective lens having a large numerical aperture has been used, and the spot size has been reduced to as small as the wavelength of a semiconductor laser to be used. When reading data while making full use of the autofocus technique and the tracking technique, the laser light emitted from the objective lens 41 is applied mainly to the target track. However, in the design of the optical information recording medium, the track pitch is shortened to the extent that crosstalk does not occur in order to achieve high density. At this time, both adjacent tracks are irradiated with the laser beam although the intensity is low. When the tracking slightly deviates for some reason or when the spot shape becomes abnormal, the reflected light from the recording mark of the adjacent track increases. For this reason, the reflected light from the recording mark existing in the center track may not be accurately read. In order to avoid such crosstalk between tracks, as described in, for example, JP-A-6-243473, a light beam is narrowed down to two adjacent tracks, reflected light of each of the three beams is detected, and a central track is detected. It has been proposed to remove a crosstalk by adding and subtracting an adjacent signal with respect to the above signal.

As described above, it is desired to increase the density of an optical information storage medium.
There is a fact that the wavelength of a semiconductor laser that can be used is limited, and the maximum numerical aperture of an objective lens cannot be increased to about 1 or more in air. Therefore, there is a limit to miniaturization of the spot size of the laser beam. Therefore, it is impossible to reduce the size of the recording mark to a certain degree or more, and it is not possible to expect improvement of the recording density in the plane. Therefore, it has been considered to increase the information density per area by multiplying the optical information recording film. Reference numeral 501 in FIG. 3 is a cross-sectional view of a two-layer optical information medium, where the first optical information medium layer is denoted by 511 and the second optical information medium layer is denoted by 512. It is assumed that the irradiation light 80 from the semiconductor laser is converged by the objective lens 41 and is focused on the first optical information medium layer 511. The reflected light containing information is transmitted through the objective lens 4
1. The light passes through the condenser lens 43 and enters the two-divided detector 52 in the state of 81. This signal is added up by the electronic circuit 62 to become a data signal as in FIG. On the other hand, since the optical disk 501 has a multilayer structure, the laser light 82 that has also transmitted through the second optical information medium layer 512 is irradiated in a defocused state. After the reflected light 83 from the recording mark on the second optical information medium layer 512 passes through the objective lens 41 and the condenser lens 43, the reflected light 84 in a spread state irradiates the photodetector 52. The reflected light 84 causes crosstalk to the originally intended reflected light 81 from the first optical information medium layer, and there is a possibility that reading is not performed accurately.
An object of the present invention is to reduce noise due to reflected light caused by other layers, so-called crosstalk, when reading a multilayer optical recording medium, and to reduce the probability that data cannot be read.

In order to achieve the above object, an object from which signal light, which is noise from an adjacent layer, is detected at a position deviating from a focal position of a condenser lens in front of a photodetector and data is to be read out. A method of subtracting from the signal of the information medium layer at the focal position of the lens is adopted. The principle will be described with reference to FIG. The laser light is applied to the first optical information medium layer 5 by the objective lens 41.
It is assumed that focus is on 11. The second optical information medium layer 512 is out of focus as shown in FIG.
It is assumed that a range of the spot size a is irradiated. After transmitting this image through the half mirror 33, the image is formed on the photodetector 53 by the condenser lens 43, and the spot size becomes 2
ωb. The detection area of the photodetector 53 is approximately 2ωb. The image forming position is different from the image forming position of the reflected light from the first optical information medium, and the position can be calculated from the layer interval and the focal length of the lens. Under such conditions, reflected light mainly enters the photodetector 53 from the second optical medium layer 512. On the other hand, the light reflected by the half mirror 32 is narrowed down by the condenser lens 43, and a photodetector 54 is provided at the focal position. This photodetector 54 has a first
Although the reflected light from the optical information medium layer 511 mainly enters,
Since the detection range of the detector is sufficiently wider than the narrowed spot size, reflected light from the second optical medium layer 512 is also mixed. Here, a signal obtained by multiplying the signal 732 of the photodetector 53 by an appropriate magnification and subtracting the result from the signal 731 of the photodetector 54 is calculated by the electronic circuit 63, and the output is calculated as
Assuming that the signal is 33, the signal has less crosstalk from the adjacent second optical information medium layer, and the reliability of data reading can be improved. The solution in this patent is also applicable when a third optical media layer is present. For example, if the third optical information medium layer is located closer to the objective lens 41 than the first optical medium layer,
The reflected light from the recording mark of the optical information medium layer focuses farther than the focal position of the condenser lens 43, and this image range is detected at the image forming position of this image. As in the case of the signal 732, by subtracting this signal from the signal 731, it becomes possible to reduce the crosstalk from the third optical information medium layer from the data information.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. The laser light emitted from the semiconductor laser 1 is converted into parallel light by the collimator lens 23, and then converted into a circular beam by the triangular prism 2. This laser light is reflected by the polarizing prism 31, converted into circularly polarized light by the λ / 4 wavelength plate 21, and then narrowed down to a minute spot by the objective lens 41. The multi-layer optical disc 50 is located at the focal position of the spot.
1 is rotating, and the optical information medium layers 511, 512, 51
From 3 and the like, reflected light having an intensity change corresponding to the length and interval of the recording mark is generated. It is assumed that the recording mark has been written in both the land and groove structures. The reflected light returns to the objective lens 41, is converted into linearly polarized light by the λ / 4 wavelength plate 21, and transmits through the polarizing prism 31. In this embodiment, a hologram optical element 90 is used. The reflected light is incident on the hologram optical element in a state of 88 as shown in FIG. Hologram optical elements are upper and lower 91,9
It is divided into two, and the shape is a circular grating having a condensing power. The diffracted lights of the +1 order, 0 order, and −1 order light by the upper and lower holograms are collected by the condenser lens 4 shown in FIG.
4 collects light. Although FIG. 1 shows six condensed light beams, only three light beams by the hologram diffractive optical element 91 are shown in the figure. The zero-order diffracted light is 851 and irradiates the two-divided photodetectors 111 and 112 formed on the substrate 100. The focal position of the -1st-order diffracted light 861 is
0 and illuminates the photodetector 121. Also, 1
The next-order diffracted light is 871, the focal position is located beyond the surface of the substrate 100, and irradiates the photodetector 131 just before the focal point. FIG. 6 shows a plane pattern of the photodetector 100. The first-order diffracted light by the hologram optical element 91 is 8
The portion indicated by the dotted line 61 is irradiated, and the -1st-order diffracted light by the hologram optical element 92 irradiates the portion indicated by the dotted line 862. A light detection area 11 in each irradiation area
1,112. It is assumed that the size of the area corresponds to the image of the laser light irradiation area of the information medium layer closer to the objective lens 41 than the optical information medium layer to be read by the optical drive device. Reference numerals 131 and 132 in FIG. 6 detect + 1st-order diffracted light from the hologram optical elements 91 and 92, respectively. The focal position of the diffracted light is farther than the detector surface. The detection area size of the photodetectors 871 and 872 is
This corresponds to an image of the laser light irradiation area of the information medium layer located farther from the objective lens 41 than the optical information medium layer to be read by the optical drive device. The zero-order diffracted light from the hologram optical elements 91 and 92 is focused on the surface of the photodetector 100 as indicated by 851 and 852. The diffraction light 851 irradiates the center of the split detectors 111 and 112. The information recording medium has land and groove grooves, and diffracted light is generated from the groove structure.
The 1,112 differential signals become tracking error signals. The zero-order diffracted light from the hologram optical element 92 is in a focused state as shown by 852 by the condenser lens 44.
The reflected light at 852 is only due to the hologram optical element 92, and it can be considered that the portion of the hologram optical element 91 is in a shielded state. This reflected light 852 is divided into upper and lower photodetectors 113,
At 114, the difference between the signals of the two detectors is taken and becomes the focus error signal. That is, when the medium layer is in focus, the same amount of light is incident on the photodetectors 113 and 114, and the focus error signal indicates zero, but when the focus is out of focus, an error signal is generated.
The servo is applied to the focal position adjusting device of the objective lens, and the objective lens 41 is moved so that the focus error signal becomes zero. In this way, the focal position of the light emitted from the objective lens 41 can always be matched with the optical information medium layer. The data signal is a waveform equalizer, shaper, discriminator,
The signal passes through a decoder or the like and becomes a reproduced signal. A second embodiment of the present invention will be described with reference to FIG. In this embodiment, the present invention is added to the general conventional optical disk drive shown in FIG. The added portions are half prisms 34 and 35 for splitting the reflected light, and condenser lenses 44 and 4.
5, photodetectors 55 and 56 and their substrates 101. The condenser lenses 44 and 45 have different focal lengths, and the condenser lens 44 is shorter. The photodetector 55 is for detecting the reflected light from the adjacent medium layer on the objective lens side with respect to the medium layer to be read, and is installed at the image forming position. It corresponds to the illuminated area. The photodetector 56 is for detecting the reflected light from the adjacent medium layer on the surface away from the objective lens with respect to the medium layer to be read, is installed at the image forming position, and the detection area is This corresponds to the range illuminated by the incident laser light. Both detectors are formed on the same substrate 101. In the above two embodiments, writing to the optical information medium layer is not mentioned because it is not directly related to the present invention, but this function is added to the configuration of the actual optical information recording / reproducing apparatus.

As described above, the effect of the first aspect of the present invention is that when data is read from a multi-layer optical disc, crosstalk from other optical information medium layers can be reduced, and the read reliability is improved. There is an effect of improving. Conversely, this effect makes it possible to reduce the distance between the optical information medium layers. As the layer spacing of the optical information medium is increased, the crosstalk is reduced, but it is necessary to realize multiple layers within the limited disk thickness. At this time, according to the present invention, since the crosstalk can be reduced, an optical disk drive capable of reading an optical disk having more layers can be realized. The effects of the invention of claim 2 are, in addition to the effects of the invention of claim 1, the downsizing of the apparatus and the ease of adjusting the optical system. A photodetector can be manufactured on a semiconductor substrate with accurate pattern accuracy, and an amplifier can be manufactured as an integrated circuit on the same substrate. Therefore, the space occupied by the signal processing circuit and the like can be saved, and the size of the device can be reduced. In addition, the use of the hologram optical element simplifies the optical system and facilitates adjustment of the optical system.

[Brief description of the drawings]

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

FIG. 2 is a schematic diagram of a conventional three-dimensional optical information recording / reproducing apparatus.

FIG. 3 is a schematic diagram for explaining a problem of a conventional three-dimensional optical information recording / reproducing apparatus.

FIG. 4 is a schematic diagram showing a solution of the present invention.

FIG. 5 is a schematic view of a pattern of a hologram optical element used in an example.

FIG. 6 is a schematic diagram showing an arrangement of photodetectors in the embodiment.

FIG. 7 is a schematic view of a second embodiment of the present invention.

[Explanation of symbols]

 1. Semiconductor laser 2. Triangular prism 21. λ / 4 wavelength plate 22. Knife edge 23. Collimator lens 31. Polarization beam splitter 32. , 34. , 35. Half prism 41. Objective lens 42. , 43. , 44. , 45. Condensing lens 50. , 501. Optical disk 52.53. , 54. , 55. , 56. Photodetector 501 511. First optical information medium layer 512. Second optical information medium layer 513. Third optical information medium layer 60.61. , 62. Electronic circuit 80. Irradiation light 81. 82. Light reflected from first optical information medium layer Irradiation light to second optical information medium layer 83. , 84. 871. Reflected light from the second optical information medium layer , 872.1 order diffracted light 851. , 852.0 order diffracted light 861. , 862. -1st-order diffracted light 90. , 91. , 92. Hologram optical element 100. , 101. Substrate for forming photodetector 111. , 112.113. , 114.2 split detector 121. , 122. , 131. , 132. Light detector.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takeshi Maeda 1-280 Higashi Koigakubo, Kokubunji-shi, Tokyo F-term in Central Research Laboratory, Hitachi, Ltd. 5D090 AA01 BB12 CC04 DD03 DD05 EE17 EE18 FF45 LL03 LL05 5D119 AA13 BA01 BB13 CA15 DA05 EA02 EA03 JA14 KA02 KA04 LB02

Claims (2)

[Claims] 1. An optical disc reading apparatus comprising: an optical unit for focusing a laser beam on an optical information storage medium; and an optical unit for detecting reflected light from the optical information medium, wherein the optical information medium has a multilayer structure, and An optical element that partially divides the reflected light into at least three parts, a condenser lens that collects the divided reflected light, a first photodetector that detects substantially the total amount of the first reflected light, The detection position in the axial direction substantially coincides with the image formation position of the image of the optical information recording film closest to the optical information recording film, and the reflected light detection region coincides with the whole or a part of the image formation of the irradiation region of the adjacent optical information medium. The second photodetector and the displacement direction of the second photodetector are opposite to the optical axis direction about the focal position,
It is installed at the optical image position of the closest optical information recording film at a position opposite to the closest optical information recording film detected by the second detector, and the reflected light detection area is the same as the irradiation area of the closest optical information medium. Consisting of a third detector that matches the whole or a part of the image formation, and a signal processing circuit that processes a signal from the first photodetector installed at the focal point using the first and second signals An optical information recording / reproducing apparatus characterized by being performed. 2. The optical information recording / reproducing apparatus according to claim 1, wherein the first, second, and third photodetector groups are on the same substrate plane, and the reflected light is divided by a hologram optical element having a lens function. apparatus.