Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2001-11-02
2004-09-28
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S641000, C369S112240, C369S044320
Reexamination Certificate
active
06798581
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical pickup apparatuses which read/write information signals from/to optical recording media, such as optical disks, magnetic optical disks, optical cards, etc., and to read/write apparatuses which contain the optical pickup apparatuses and which read/write information signals from/to optical recording media.
2. Description of the Related Art
Optical pickup apparatuses which read/write information signals from/to optical recording media, such as optical disks, etc., have been suggested. In such optical pickup apparatuses, in order to increase the storage capacities of the optical recording media, a laser diode (LD) having a short emission wavelength is often used as a light source.
In addition, in the optical pickup apparatuses, a lens having a high numerical aperture (NA) is used as an objective lens.
For example, compared with optical pickup apparatuses for “Compact Discs (CDs)” (trademark), in which the emission wavelength of the light source is 780 nm, in optical pickup apparatuses for “Digital Versatile Discs (DVDs)” (trademark), the emission wavelength of the light source is 650 nm. Thus, the emission wavelength of the light source is reduced as the recording density is increased. In addition, the numerical aperture (NA) of the objective lens used in the optical pickup apparatuses for Compact Discs (CDs) (trademark) is 0.45, and the numerical aperture (NA) of the objective lens used in the optical pickup apparatuses for Digital Versatile Discs (DVDs) is 0.60. Thus, the numerical aperture (NA) of the objective lens is increased as the recording density is increased.
Recently, an optical pickup apparatus using a blue-violet laser diode, of which the emission wavelength is 405 nm, as the light source and an objective lens having a high numerical aperture (NA) of 0.85 has been suggested.
However, in the case in which the emission wavelength of the light source is reduced and the numerical aperture (NA) of the objective lens is increased, there is a problem in that a wavefront aberration is easily increased due to various manufacturing errors, and the optical performance is degraded.
Accordingly, in order to correct a spherical aberration which occurs due to a thickness error of an optical recording medium and manufacturing errors in the lens, an optical pickup apparatus in which an expander lens unit including two lenses is inserted between the light source and the objective lens has been suggested. In this optical pickup apparatus, the distance between the two lenses included in the expander lens unit can be adjusted.
In addition, the spherical aberration can also be corrected by moving a collimator lens unit, which is used for making a light beam emitted from the light source parallel, along the optical axis.
However, the above-described construction in which the expander lens unit is used for correcting the spherical aberration has the following problems.
In the above-described expander lens unit, when the distance between the two lenses is changed from a designed value, that is, a value at which the light beam emitted from the expander lens unit is made parallel, the light beam emitted from the expander lens unit is either diffused or converged. The total quantity of light incident to the objective lens is limited by an aperture diaphragm disposed near the objective lens. Accordingly, compared with the case in which the light beam is parallel, the total quantity of light incident to the objective lens is increased when the light beam is converged and is reduced when the light beam is diffused.
When the total quantity of light incident to the objective lens changes, the quantity of light radiated on an optical recording medium in the process of reading/writing information signals from/to the optical recording medium also changes. In addition, the quantity of light that is reflected from the optical recording medium and detected by a detector also changes. Accordingly, there is a risk that error signals for implementing various servo operations cannot be adequately detected. In addition, even if the servo operations are stable, in the process of reproducing so-called “RF signals”, that is, information signals read out from the optical recording medium, the “RF amplitude” also changes so that data may not be read accurately.
Furthermore, in the case in which the optical recording medium is a so-called “recordable disc”, when the quantity of light radiated on the optical recording medium is too high, there is a risk that the information signals recorded on the optical recording apparatus will be destroyed in the reproducing process. On the contrary, when the quantity of light radiated on the optical recording medium is too low, there is a risk that the information signals cannot be recorded on the optical recording medium in the recording process.
The quantity of light is not the only problem. Generally, the light beam incident to the aperture diaphragm of the objective lens is a so-called Gaussian beam, in which the cross-sectional distribution of light intensity has the form of a Gaussian distribution. However, when the light beam incident to the aperture diaphragm is converged or diffused, the used area of the light beam changes. Thus, the so-called “rim intensity (RI)”, that is, the light intensity of the peripheral region of the light beam relative to that of the central region thereof, also changes, so that the numerical aperture (NA) of the optical system changes substantially. In such a case, there is a risk that it will be difficult to read/write the information signals from/to the optical recording medium.
The above-described problem also occurs in the case in which the spherical aberration is corrected by moving the collimator lens unit along the optical axis.
SUMMARY OF THE INVENTION
In view of the above-described situation, an object of the present invention is to provide an optical pickup apparatus in which the quantity of light incident to the aperture diaphragm of the objective lens and the “rim intensity (RI)” is maintained constant, even when the spherical aberration is corrected by passing the light beam through an expander lens unit before it reaches the objective lens, and which is able to adequately read/write information signals from/to optical recording media and to provide a read/write apparatus which includes the optical pickup apparatus.
In order to attain the above-described object, according to one aspect of the present invention, an optical pickup apparatus includes a light source, an objective lens which condenses a light beam emitted from the light source on an optical recording medium, and an expander lens unit which includes a first lens group and a second lens group, which is disposed on the optical axis between the objective lens and the light source, and in which the distance between the first and second lens groups is changed so as to correct a spherical aberration which occurs on the optical recording medium. The expander lens unit is disposed such that the focal position of the second lens group, which is closer to the objective lens than the first lens group, at the side closer to the objective lens is on the aperture diaphragm of the objective lens.
Thus, in this optical pickup apparatus, the distance between the expander lens unit and the aperture diaphragm of the objective lens is determined such that the focal position of the second lens group, which is closer to the objective lens than the first lens group, is on the aperture diaphragm of the objective lens. Even when the distance between the first and second lens groups is changed and the light beam emitted from the expander lens unit is thereby diffused or converged, the diameter of the light beam is always the same at the focal position of the second lens group.
At positions away from the focal position of the second lens group, the diameter of the light beam changes as the distance between the first and second lens groups is changed. Accordingly, when the aperture diaphragm of the objec
Kato Yoshiaki
Osato Kiyoshi
Yamamoto Kenji
Epps Georgia
Harrington Alicia M
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
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