Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-06-15
2002-05-14
Hudspeth, David (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S094000, C369S044370, C369S103000, C369S122000
Reexamination Certificate
active
06388977
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup device for use in an optical disk apparatus for optical recording/reproduction of information on recording media such as optical disks, and a hologram laser unit to be incorporated in such an optical pickup device. More particularly, the present invention relates to an optical pickup device which is capable of performing accurate recording/reproducing operations for a plurality of types of optical disks having different recording densities; and a hologram laser unit to be incorporated in such an optical pickup device.
2. Description of the Related Art
In recent years, optical disks capable of recording signals representing a large amount of information at a high density have been utilized in a number of fields including audio, video, and computer applications. Compact disks (CDs), video disks, mini-disks (MDs), and magneto-optical disks for computers, which are widely commercially available now, generally employ a substrate which is 1.2 mm thick. Accordingly, optical pickup devices for use in performing recording and reproducing operations of information for these optical disks are designed so as to correct the aberration due to the 1.2 -thick substrate by utilizing an objective lens incorporated in the optical pickup devices.
A variety of attempts have been made to enhance the recording capacity of optical disks. Among these attempts is a method of improving the optical resolution by employing an objective lens having an increased numerical aperture (NA), and a method of recording or reproducing information by employing light (laser light) having a shorter wavelength.
The diameter &phgr; of a converged beam spot on an optical disk can be represented by the following formula:
&phgr;=
K×&lgr;/NA
where K is a constant; NA is the numerical aperture of the objective lens; and &lgr; is the wavelength of a laser beam used. According to the above equation, the converged beam diameter &phgr; decreases in inverse proportion to the numerical aperture NA. On the other hand, tolerance for a tilt of the optical disk decreases in proportion to the numerical aperture of the objective lens to the power of 3 (i.e., NA
3
). Therefore, in order to increase the numerical aperture NA of the objective lens while maintaining the same level of tolerance for the tilt of the optical disk, it is necessary to reduce the thickness of the substrate of the optical disk. Specifically, when the numerical aperture NA of the objective lens is increased from 0.5 to 0.6, for example, the thickness of the substrate (i.e., the substrate thickness) of the optical disk is required to be reduced to approximately 0.6 mm in order to maintain the same level of tolerance for the tilt of the disk as that of an optical disk having a 1.2 -thick substrate.
Thus, in order to realize a higher-density optical disk, it is necessary to optimize both the substrate thickness of the optical disk and the numerical aperture (NA) of the objective lens used in the optical pickup device.
However, if the substrate thickness of the optical disk is reduced as described above, the optical disk is no longer compatible with optical disks having a substrate of a conventional thickness. Accordingly, it becomes necessary to adjust the numerical aperture (NA) of the objective lens in the optical pickup device in accordance with the substrate thickness of the optical disk.
In this connection, for example, Japanese Laid-Open Publication No. 8-45105 discloses a method of adaptably changing the numerical aperture NA of the objective lens.
Specifically, the above laid-open publication describes means for selectively changing an aperture of an objective lens (hereinafter referred to as “selective aperture changing means”) which is provided integrally with a moving unit for the objective lens. The selective aperture changing means is configured so that the aperture is effectively changed by inserting into or retracting from the optical path a plate for restricting the aperture, or by partially varying the transmittance of a liquid crystal plate. The use of such selective aperture changing means makes it possible to adaptably change the numerical aperture NA of the objective lens in the optical pickup device according to different substrate thicknesses of the optical disk.
Furthermore, Japanese Laid-Open Publication No. 6-124477 discloses a method of changing the numerical aperture NA using a liquid crystal filter and a polarizing filter.
Specifically, as described in the above laid-open publication, a predetermined pattern of electrodes are attached to a liquid crystal filter. For example, the electrodes may be patterned into an inner portion and an outer portion defining concentric circles. By applying a voltage to the respective portions of the patterned electrodes with appropriate timing, some portions of the filter are imparted with a polarization direction which is rotated by 90 degrees from that of the other portions. As a result, light which has passed through the liquid crystal filter is selectively divided into portions having different polarization states. When light having such portions of selectively varied polarization states is allowed to pass through a polarization beam splitter, the light is divided into a reflection light component and a transmittance light component depending on the polarization states. By configuring the optical system so that only the transmittance light component enters the objective lens, the diameter of the light beam entering the objective lens can be varied. As a result, the numerical aperture NA of the objective lens can be effectively changed.
However, the various aforementioned conventional methods of changing the numerical aperture NA have the following problems.
The method in which the aperture restriction plate is placed in and out of the optical path requires a mechanism capable of highly precisely performing the insertion and retraction of the aperture restriction plate, resulting in an increase in the size and manufacturing cost of the device thereby adversely affecting mass production. Moreover, since the insertion/retraction mechanism is required to be integrated and driven with the moving unit for the optical pickup, the mass of the moving unit is inevitably increased so as to degrade the servo performance of an actuator used for driving the moving unit.
The method using the liquid crystal plate does not require any additional moving mechanism. However, in the case where the liquid crystal plate is integrated with the moving unit for the optical pickup device, the whole size of the moving unit is increased. Furthermore, a movable mechanism for supplying operating power is required to securely supply a necessary voltage to the electrodes attached to the liquid crystal plate as the liquid crystal plate is moved along with the moving unit.
Alternatively, in the case where the liquid crystal plate is provided separately from the moving unit, the optical axis of the lens may be misaligned with respect to the center of the pattern of the liquid crystal plate as the objective lens is driven. Moreover, regardless of whether the liquid crystal plate is integrated with the moving unit or not, the liquid crystal materials contained in the liquid crystal plate are susceptible to a change in their characteristics (e.g., refractive index) due to a change in temperature. Therefore, such a device may not exhibit the desired performance due to changes in the environmental conditions.
In addition, all of the above-described methods require an additional mechanism in the optical pickup device, likely causing drawbacks such as complicated configuration and increase in cost.
SUMMARY OF THE INVENTION
A hologram laser unit of the present invention is to be used for performing recording and reproducing operations of information for an optical disk. The hologram laser unit includes a light source, a photodetector, and a hologram element which are formed integrally with each other. The hologram element includes
Nakata Yasuo
Sakai Keiji
Chu Kim-Kwok
Conlin David G.
Edwards & Angell LLP
Hudspeth David
Jensen Steven M.
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