JP2002367211A - Optical information recording / reproducing device - Google Patents

Abstract

(57) Abstract: By using at least two critical angle prisms, light rays that are not parallel to the optical axis are transmitted, and only light rays that are nearly parallel to the optical axis are detected by the optical detector of the optical disk drive. To reach. [Effect] Since reflected light as stray light from an adjacent layer is reduced, focus position control and track position control are accurately performed, and read reliability of a multilayer optical disc is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical information recording / reproducing apparatus, and more particularly to an optical system of an optical disk drive for reading a multilayer medium.

[0002]

2. Description of the Related Art A general outline of an optical system of an optical information recording / reproducing apparatus will be described with reference to FIG. The laser beam 11 emitted from the semiconductor laser 10 is converted into a parallel and circular beam by the collimator lens 21 and the triangular prism 31. Thereafter, the light is reflected by the polarization beam splitter 32 and the galvanometer mirror 35 and passes through the λ / 4 wavelength plate 34.
After being converted into circularly polarized light by the wavelength plate, the light is condensed on the optical disk 40 by the objective lens 22. A guide groove is provided inside the optical disk, and recording marks having different reflectances are written along the groove. The length and interval of the recording mark are decoded according to the information to be recorded. 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 reflected light with information returns to the objective lens 22,
The light is converted from circularly polarized light into linearly polarized light by the λ / 4 wavelength plate 34. Since this polarization direction is orthogonal to the polarization direction of the light emitted from the semiconductor laser, the polarization beam splitter 3
2 is transmitted. This transmitted light is split into two light beams by the half prism 33. After half of the light transmitted through the half prism 33 is blocked by the knife edge 36, the light is focused on the two-segment photodetector 12 at the focal position of the condenser lens 24. If the optical disk is displaced in the optical axis direction, that is, if the focus of the irradiation light is displaced from the layer having the recording mark, an imbalance occurs in the amount of light entering the two photodetectors of the two-part photodetector 12. This imbalance is detected as a differential signal by the electronic circuit 13 and is used as a focus error signal 14.
By using the focus error signal 14, the position of the objective lens 22 is adjusted by the lens actuator 37 so that the focal position of the light emitted from the objective lens 22 does not become defocused. On the other hand, the light reflected by the half prism 33 is irradiated on the four-divided photodetector 15 by the condenser lens 23 in a defocused state. An electronic circuit 16 that processes a signal from the quadrant photodetector 15 generates a tracking error signal 17 by a push-pull method or a phase difference method. The signal 17 adjusts the deflection angle of the galvanomirror 35 to control tracking. The signal added by the electronic circuit 16 becomes a data signal 18.

An optical configuration different from the optical disk drive optical system described above is described in detail in "Basics and Application of Optical Disk Storage" (edited by Yoshito Tsunoda, edited by the Institute of Electronics, Information and Communication Engineers). There are various methods for detecting a focus error signal or a tracking error signal, and there are advantages and disadvantages in optical adjustment, light use efficiency, and the like. For this reason, the optical disk drive is properly used depending on the type of the target disk.

[0004] At present, it is becoming possible to transfer large-capacity, high-speed data, and optical discs as information storage media are required to have higher densities in order to record large-capacity data such as moving images. To achieve this, the interval between recording marks and the track pitch of the optical information medium must be reduced.

[0005] In order to read and write on a high-density optical information medium, a laser beam is miniaturized to a spot size of about a wavelength by an objective lens having a large numerical aperture.
However, the size of the spot size has a lower limit determined by the numerical aperture of the objective lens, the wavelength used, and the diffraction limit, and cannot be reduced to any extent. For this reason, even if the size of the recording mark is reduced beyond the limit determined by the lower limit of the spot size, reading cannot be performed, so that the readable in-plane recording density is limited. Therefore, in order to overcome this limitation, it has been considered to improve the information density per area of the optical disk by forming the optical information recording film into a multilayer structure.

When reading a multi-layer optical disc, a large error may occur in a read signal in a conventional optical system. This is because, when light is applied to other than the target optical information recording film, reflected light is generated and becomes stray light. As a result, the focal position of the laser beam may deviate from the optical information medium layer to be read.

A technique for eliminating interference in a focus error signal is disclosed in Japanese Patent Application Laid-Open No. Hei 10-222867, entitled "Optical Pickup Device". This technique is devised to provide a small area auxiliary light receiving area on both sides of a two-segment detector. The basic idea is to narrow the spread of the single layer focus error signal.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to reduce stray light due to an adjacent optical information medium layer in a reading optical system of an optical disk drive for reading a multilayer optical disk. This stray light adversely affects not only the focus error signal but also the tracking error signal and the data signal. Here, the mechanism of the influence of stray light on the focus error signal will be described in detail. 4 in FIG.
Numeral 0 indicates a cross-sectional view of a two-layer optical disk, in which the first optical information medium layer is denoted by 401 and the second optical information medium layer is denoted by 402. It is assumed that the irradiation light 111 from the semiconductor laser is collected by the objective lens 22 and is focused on the first optical information medium layer 401. The reflected light from the first optical information medium layer 401 becomes parallel light after passing through the objective lens 22, and a part of the parallel light is blocked by the knife edge 36 provided in the middle of the optical path. The light transmitted through the knife edge 36 is incident on the two-segment photodetector 12 by the condenser lens 24 with the focus of 112. The incident light on the detection surface of the two-segment photodetector 12 is condensed as a small spot at the division position of the two-segment photodetector as shown in FIG. In this state, the same amount of light is incident on the two-segment photodetector. Since the focus error signal 14 is obtained as a difference signal of the two-segment photodetector, the difference signal when the object is in focus is zero. In the case of a single-layer disc without an adjacent information medium layer,
Servo can be applied so as to optimize the focal position by this signal. However, in the problem of the present invention, the case where the adjacent information medium layer exists is considered, and the light reflected by the adjacent layer becomes stray light.

Since the optical disk 40 has a multilayer structure,
The laser beam 114 also transmitted through the second optical information medium layer 402
Are irradiated in a defocused state. Objective lens 22
Of reflected light 11 from the second optical information medium layer 402 passing through
5 is not parallel light but convergent light. Therefore, the position where the size of the light flux of the reflected light 113 after passing through the condenser lens 24 is minimized approaches the condenser lens 24 and spreads on the detector surface of the two-divided photodetector 12. FIG. 5 shows an irradiation state of the reflected light 113 onto the two-divided photodetector 12. The reflected light 113 is incident on one of the two split photodetectors 12 with a large intensity. As described above, the first optical information medium layer 401
And 2 for the reflected light of the second optical information medium layer 402 separately.
Although the state of incidence on the split photodetector 12 has been described, they actually enter simultaneously. For this reason, the difference signal from the two-segment photodetector 12 does not become zero even though the subject is in focus, and the balance is lost. If the servo is applied in this state, the focal position will be shifted. This also occurs when the second optical information medium layer is closer to the objective lens 22 than the first optical information medium layer, and the balance is shifted in the opposite direction.

The light reflected by the half prism 33 includes light reflected from an adjacent layer. in this case,
The light enters the quadrant photodetector 15 in FIG. 2 as stray light, adversely affects the tracking error signal 17 and the data signal 18 and causes a bit error.

As described above, a problem to be solved by the present invention is to reduce crosstalk due to light reflected from an adjacent layer when reading a multilayer optical disc.

[0012]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, a method for reducing the incidence of reflected light from an adjacent information medium layer on a photodetector is adopted. As shown in FIG. 3, the reflected light from the optical information medium layer at the focal position is
After passing through, the reflected light from the adjacent optical information medium layer 402 becomes convergent light instead of parallel light, and conversely, from the optical information medium layer (not shown) near the objective lens 22. Is reflected light. That is, most of the reflected light from the adjacent optical information medium layer has an angle with respect to the optical axis after passing through the objective lens 22. In the present application, the problem is solved by removing reflected light from an adjacent optical information medium layer having an angle with respect to a parallel light beam and causing only a light beam close to the parallel light beam to enter a photodetector.

When light is incident from a substance having a high refractive index to a substance having a small refractive index, total reflection occurs at an incident angle of a certain angle (critical angle) or more. In FIG. 6, a substance having a large refractive index and an object having a small refractive index are separated from each other at an interface 500, and light 116 is incident on the substance having a large refractive index at an incident angle 501 to generate transmitted light 117 and reflected light 118. It shows how to do. The plane of incidence is parallel to the page. When the material on the incident side is BK7 (refractive index 1.5168) which is an optical glass, the material on the transmitting side is air (refractive index 1.0), and the polarization direction of the incident light is parallel to the incident surface. FIG. 7 shows the incident angle dependence of the reflectance. It can be seen that total reflection occurs at an incident angle of 41.245 ° or more. In the optical system shown in FIG. 8, critical angle prisms 50 and 51 are used. The incident light 116 first enters the first critical angle prism 50 at an incident angle 501, and the reflected light is reflected by a normal reflecting mirror 38 and then enters the second critical angle prism 51 at an incident angle 502. The angle of the reflecting mirror 38 is set such that when the incident angle 501 is the critical angle, the incident angle 502 is also the critical angle. Second critical angle prism 5
The reflected light from 1 is reflected by the reflecting mirror 39 in the same direction as the incident light 116 and becomes the outgoing light 118. Assuming that the reflectance of the reflecting mirrors 38 and 39 is 1, the dependence of the intensity of the emitted light 118 on the incident angle 501 in FIG. 8 is shown in FIG. It can be seen that the reflectance increases near the critical angle. That is, the incident angle 5
When 01 is smaller than the critical angle of the critical angle prism 50, the light is transmitted through the critical angle prism 50, so that the outgoing light 118
Becomes smaller. Similarly, when the incident angle 501 is larger than the critical angle, the incident light is reflected by the critical angle prism 50,
Since the incident angle 502 to the critical angle prism 51 becomes smaller than the critical angle, the light passes through the critical angle prism 51. Therefore, as shown in the calculation result of FIG. 9, only the incident light having an angle near the critical angle becomes the output light 118. Therefore, it can be seen that the optical system shown in FIG. 8 using two critical angle prisms can transmit only incident light having a certain angle. The range of reflection is the critical angle prism 5
It can be changed by adjusting the angles of 0 and 51. Further, by increasing the number of critical angle prisms or using a multilayer film for the critical angle prism, the angle discrimination function becomes sharper, and only light in a narrow angle range is emitted as emitted light 118.

The optical system shown in FIG. 8 is set between the objective lens 22 and the knife edge 36 shown in FIG. At this time, the direction of the critical angle of incidence of the critical angle prism 50 in FIG. 8 is made to coincide with the optical axis direction in FIG. This prevents light that is not parallel to the optical axis direction from reaching the optical system after the knife edge 36. As described above, components that are not parallel to the optical axis are included in the reflected light from the adjacent optical information medium layer. In addition, nonparallel components increase as the information medium layer interval increases. These non-parallel components will not reach the photodetector 12, and stray light from adjacent optical information medium layers will be reduced, thus reducing crosstalk between layers.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG.

Laser light 1 emitted from semiconductor laser 10
1 is converted into parallel light by a collimator lens 21 and then converted into a circular beam by a triangular prism 31. This laser light is reflected by the polarization beam splitter 32, is circularly polarized by the λ / 4 wavelength plate 34, is guided to the objective lens 22 by the galvanometer mirror 35, and is narrowed down to a minute light spot. The optical disc 40 having a multilayer optical information medium is rotating at the focal position of the spot, and focuses on the optical information medium layer 401 having recording marks. 4
02 and 403 are optical information medium adjacent layers. The reflected light returns to the objective lens 22, is converted into linearly polarized light by the λ / 4 wavelength plate 34, and transmits through the polarizing prism 32. The transmitted light is
The light enters the critical angle prism 52 whose critical incident angle direction coincides with the optical axis. The critical angle prism 52 includes 521, 522, 52
It has four critical angle reflecting surfaces 3, 524, and only light substantially parallel to the optical axis is transmitted. The critical angle prism 52 has more reflective surfaces having a critical angle than the optical configuration of FIG. 8, and the change in reflected light intensity near the critical angle is steeper. Light substantially parallel to the optical axis is transmitted through the critical angle prism 52, and the transmitted light is split into two by the half prism 33. After the reflected light of the half prism passes through the condenser lens 23,
As shown in the conventional example of FIG.
7 and data signal 18. The tracking error signal is sent to the galvanomirror 35 and used as a control signal for tracking the track. After the transmitted light of the half prism 33 is blocked by the knife edge 36,
The light is condensed by the condensing lens 24 and is irradiated onto the two-divided photodetector 12. The signal from each photodetector is an electronic circuit 1
The difference signal becomes a difference signal at 3, and the actuator 37 in the optical axis direction of the objective lens 22 is controlled as a focus error signal.

FIG. 10 shows a second embodiment. In this embodiment, a critical prism 53 is used. 531 of critical prism
And 534 are critical reflection surfaces. On the other hand, 532
Is a total reflection surface having no incident angle dependence. Also,
By installing the prism 54, the surface 533 is set so that about 5% of the incident light is transmitted. The light transmitted through the prism 54 is incident on the two-divided photodetector 12, and becomes a differential signal output from both detectors by the electronic circuit 13.
Used as a focus error signal. The light reflected by the critical reflection surface 534 is detected by the four-division photodetector 15. These signals are processed by the signal processing circuit 16, and the tracking error signal 17 and the data signal 1 are processed.
8 is generated.

FIG. 11 shows an embodiment in which an optical disk drive as the optical information recording / reproducing device 608 is constructed using the optical head 602 based on the above-described embodiment. Laser light is generated from the semiconductor laser in the optical head 602 driven by the laser drive circuit 600 and the objective lens 22
Through the optical disk 40. The optical disk is rotated by a spindle motor 601, and the light irradiation position changes with time. The light reflected from the optical disk 40 returns to the optical head 602, and a control signal is generated by the optical head. The focus error signal 14 is supplied to the focus detection circuit 603.
And is fed back to the lens actuator of the objective lens 22 to perform focus control. The tracking error signal 17 is converted into a tracking signal by the tracking circuit 604, and is fed back to the galvanomirror in the optical head. The controller circuit 6
05 read address instruction and tracking
A tracking circuit 604 that determines the amount of movement of the optical head from the information of the error signal drives the feed motor 606 to position the optical head 602 at an appropriate disk radial position.
The signal 18 from the photodetector becomes a reproduction signal in the controller circuit 605 and is output as user data 607.
The operation at the time of recording is as follows. The user data 607 is input to the controller circuit 605, and the controller circuit 605 controls the laser driving circuit 600 according to the user data 607, controls the light emission of the semiconductor laser chip, and records information on the optical disk.

[0019]

As described above, according to the present invention for a multilayer optical disc, it is possible to reduce the influence of the reflected light from the adjacent optical information medium layer and to improve the reliability at the time of reading. effective. Conversely,
With this effect, it is possible to reduce the distance between the optical information medium layers. As the distance between the optical information medium layers is increased, the crosstalk is reduced. However, it is necessary to increase the density by increasing the number of layers within the limited disk thickness. At this time, the present invention can reduce the influence of other adjacent layers, and thus can realize an optical disk drive capable of reading a multilayer optical disk having a smaller layer interval.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a first embodiment.

FIG. 2 is a schematic diagram of a conventional optical information recording / reproducing device.

FIG. 3 is a schematic diagram for explaining a problem in a focus error signal when reading a multilayer disc.

FIG. 4 is a schematic view showing an image on a photodetector of light reflected from an optical information medium layer at a focal point.

FIG. 5 is a schematic diagram illustrating an image on a photodetector of light reflected from an adjacent optical information medium layer.

FIG. 6 is a diagram showing light incidence, transmission, and reflection at an interface separating different substances.

FIG. 7 is a diagram showing the dependence of the reflectance on the incident angle.

FIG. 8 is a schematic diagram showing an optical system having two critical angle reflecting surfaces.

FIG. 9 is a diagram showing the incident angle dependence of the reflectance when there are two critical angle reflecting surfaces.

FIG. 10 is a schematic diagram of a second embodiment.

FIG. 11 is a schematic view of a third embodiment.

[Explanation of symbols]

 10. Semiconductor laser 11. Laser light 12.2 split photodetector 13. Electronic circuit 14. Focus error signal 15.4 split photodetector 16. Electronic circuit 17. Tracking error signal 18. Data signal 21. Collimator lens 22. Objective lens 23.24. Condensing lens 31. Triangular prism 32. Polarization beam splitter 34. λ / 4 wavelength plate 33. Half prism 35. Galvanomirror 36. Knife edge 37. Lens actuator 38-39. Total reflection surface 40. Optical disk 401. First optical information medium layer 402. Second optical information medium layer 403. Third optical information medium layer 50-53. Critical angle prism 521-524. Critical angle reflecting surface 531.534. Critical angle reflecting surface 532. Total reflection surface 533. Reflective surface 111. Irradiation light 112. Reflected light from first optical information medium layer 113. Reflected light from the second optical information medium layer 114. Transmitted laser light 115. Reflected light from the second optical information medium layer 116. Incident light 117. Transmitted light 118. Reflected light 500. Interface 501. Incident angle 600. Laser drive circuit 601. Spindle motor 602. Optical head 603. Focus detection circuit 604. Tracking circuit 605. Controller circuit 606. Feed motor 607. User data 608. Optical information recording / reproducing device.

Claims (3)

[Claims] 1. A laser light source, a photodetector for detecting laser light from the laser light source, and light incident outside the critical angle between the laser light source and the photodetector, and reflected within the critical angle. A first reflecting surface that transmits the light incident at the first reflection surface, and a second reflection surface that reflects light incident outside the critical angle and transmits light incident within the critical angle out of the light reflected by the first reflecting surface. An information recording / reproducing apparatus, comprising: a prism having a reflecting surface. 2. A laser light source; a laser light from the laser light source; a condensing optical system for condensing the laser light; and a photodetector for detecting return light reflected on an optical disk having a multilayer recording film. Having a control mechanism for guiding the return light to the photodetector and attenuating light other than light parallel to the optical axis, and a control mechanism for controlling a condensing position of the laser light on the optical disc. Optical information recording and reproducing device. 3. A detection optical system, comprising: a first reflection surface that transmits light incident within a critical angle and reflects light incident outside a critical angle; and a direction substantially the same as the incident surface of the first reflection surface. Of the light reflected by the first reflecting surface,
3. The optical information recording / reproducing apparatus according to claim 2, further comprising a second reflection surface that transmits light incident within the critical angle and reflects light incident outside the critical angle.