Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2000-02-10
2001-11-20
Ben, Loha (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S648000, C359S738000, C359S741000, C359S743000, C359S831000, C359S832000, C369S044230, C369S044320
Reexamination Certificate
active
06320699
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an optical apparatus, and relates to an optical apparatus for use in an optical information recording/reproducing apparatus for writing and/or reading a signal to/from an optical disc used as an optical information recording medium.
2. Description of Related Art
An optical disc has an information recording surface and a transparent substrate to protect the information recording surface. Information is written or read out to or from the information recording surface of an optical disc by irradiating a laser beam onto the information recording surface through the transparent substrate.
As for the optical disc, the improvement in its recording density is being pursued. In order to write or read out information to/from the information recording surface of an optical disc having improved recording density, it is necessary to converge the laser beam to a sufficiently small beam on the information recording surface. As a method to converge the laser beam into a small beam, it is attempted to raise the numerical aperture of the objective lens. Hitherto, the numerical aperture of the objective lens has generally been equal to or about 0.45. It is now contemplated to raise the value to a range between 0.6 and 0.85.
Since the laser beam has been designed so as not to cause an aberration for a transparent substrate having a predetermined thickness, a problem was that a spherical aberration occurs when the transparent substrate has a thickness error. Particularly, when using an objective lens with a large numerical aperture, as compared with a case of using an objective lens with a small numerical aperture, it is easily influenced by the thickness error of the transparent substrate and a large spherical aberration occurs even with a small thickness error.
When reproducing an optical disc having a multilayer structure, since a thickness of light transmitting substrate differs in dependence on the layer, a problem was that the spherical aberration occurs so that it is made difficult to reproduce the signal in a preferable way. On the other hand, a method has been deviced in which a liquid crystal device divided concentrically is used, a refractive index of the concentric portion is changed by applying a voltage to the liquid crystal devices, so as to effect the correction of a spherical aberration of the optical disc having multilayer structure. Such a method is disclosed in Japanese Patent Kokai No. 10-269611.
The liquid crystal device, however, has a problem such that its response speed is low and the device cannot respond to a spherical aberration which fluctuates in one circumference of the substrate thickness of the optical disc. The problem of the occurrence of the spherical aberration occurs typically in the case where a numerical aperture of an objective lens is increased. In this case, preferable recording and reproduction cannot be performed unless thickness precision of the optical disc is set severely.
OBJECT AND SUMMARY OF THE INVENTION
The invention has been made in consideration of the problems and it is an object of the invention to provide an aberration correcting apparatus which can remove preferably a spherical aberration based on a thickness error of an optical disc substrate and widen a thickness error permission range of the optical disc substrate even if an objective lens with a high numerical aperture is used, and to also provide an optical pickup apparatus using the aberration correcting apparatus.
According to the invention, there is provided an aberration correcting apparatus which is arranged on an optical axis of an optical path in an optical pickup apparatus for irradiating a laser beam onto an optical disc, the aberration correcting apparatus comprises: a pair of light transmitting substrates each of which is arranged so as to intersect perpendicularly to the optical axis, wherein at least one of the light transmitting substrates is held to be rotatable about the optical axis, the light transmitting substrates have distributions of optical path lengths which mutually complementarily decrease or increase in a predetermined range on a concentric circle in which the optical axis is set to a center, and a distribution of the total optical path length of the pair of light transmitting substrates is changed uniformly within a predetermined range on the concentric circle in which the optical axis is set to the center in accordance with the rotation of the light transmitting substrate, thereby applying a phase difference to the transmitted laser beam.
In one aspect of the invention, the pair of light transmitting substrates have mutually complementary thicknesses, and the thickness of each light transmitting substrate gradually decreases or increases in a predetermined range on a circumference of the same radius in which the optical axis is set to the center. The term “gradually” has a meaning including both a state where the thickness changes smoothly and a state where it changes in a stairway manner.
In another aspect of the invention, the thickness of each light transmitting substrate gradually decreases or increases on the same radius vector from the optical axis.
In another aspect of the invention, the surfaces of the light transmitting substrates form mutually complementary curved surfaces so that the total thickness of the pair of light transmitting substrates is proportional to a phase deviation amount to be corrected.
In another aspect of the invention, the thickness of each light transmitting substrate is constant within a predetermined range on the same radius vector in which the optical axis is set to the center.
In another aspect of the invention, a refractive index of each light transmitting substrate is distributed so as to gradually decrease or increase in a predetermined range on a circumference of the same radius in which the optical axis is set to the center.
In another aspect of the invention, the distribution of the refractive indices gradually decreases or increases on the same radius vector from the optical axis.
In another aspect of the invention, the pair of light transmitting substrates provide a quartic phase difference onto the same radius vector from the optical axis of the transmitted laser beam.
In another aspect of the invention, the light transmitting substrates have mutually complementary distributions of refractive indices so that the total distribution of the refractive indices of the pair of light transmitting substrates is proportional to a phase deviation amount to be corrected.
In another aspect of the invention, the distribution of the refractive indices of each light transmitting substrate is constant on the same radius vector in which the optical axis is set to the center.
In another aspect of the invention, a transparent flow medium whose refractive index differs from that of the light transmitting substrate is filled in a gap between the pair of light transmitting substrates.
According to the invention, there is provided an optical pickup apparatus which writes or reads out a signal to/from an optical disc and has a light source for emitting a laser beam, an objective lens for converging the laser beam toward an information recording surface of the optical disc, an irradiating optical system for leading the laser beam to the objective lens, and a detection optical system which includes photodetecting means and leads reflection light from the information recording surface to the photodetecting means through the objective lens, the optical pickup apparatus comprises: an aberration correcting apparatus which is arranged on an optical axis of an optical path of the irradiating optical system and comprises a pair of light transmitting substrates that face each other, wherein at least one of the light transmitting substrates is held rotatably around the optical axis, the light transmitting substrates have distributions of optical path lengths which mutually complementarily and gradually decrease or increase on a circumference of a same radius in which
Kikuchi Ikuya
Maeda Takanori
Ben Loha
Pioneer Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
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