Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2001-12-06
2002-11-12
Hindi, Nabil (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112010
Reexamination Certificate
active
06480455
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical pickup device and, more particularly, to an optical pickup device adapted to. handle two kinds of optical information recording media.
2. Description of the Related Art
Conventionally, a compact disc (hereinafter, referred to as CD-R) is known as an optical information recording medium. A digital videodisc (hereinafter referred to as DVD) having a higher recording density has been put into practice. The DVD has a thickness different from the thickness of the CD-R. Additionally, a frequency of a beam used for the DVD is different from that of the CD-R. Thus. a conventional optical pickup device for the CD-R cannot be used for recording information on or reproducing information from the DVD. Similarly, an optical pickup device designed for the DVD cannot be used for recording information on or reproducing information from the CD-R.
In order to enable an optical disc apparatus to record information on or reproduce information from both the CD-R and the DVD, both the optical pickup device for the CD-R and the optical pickup device for the DVD may be provided in the optical disc apparatus so that one of the optical pickup devices is selectively used in response to a disc to be used. However, this method increases size and cost of the optical disc apparatus since two separate optical pickup devices must be provided in the optical disc apparatus.
From this point of view, an attempt was made to share a part of an optical system of each of the optical pickup devices by providing two light sources emitting light beams having different wavelengths. In this case, if an objective lens is shared by the two optical pickup devices, the following problem may occur. That is, if an objective lens, which is designed to provide a desired performance for the DVD, is used for the CD-R, a wavefront aberration is deteriorated due to a difference between wavelength or thickness when the objective lens is used for the CD-R. The deterioration of the wavefront surface may cause side lobes in a beam spot, which results in a decrease in a peak strength and expansion of a spot diameter. Accordingly, an operation on the CD-R cannot be performed in a good condition.
In order to solve the above-mentioned problem, in Japanese Laid-Open Patent Application No.8-55363, a method is suggested for changing an object distance in response to a selected optical information recording medium, the object distance being related to a beam incident on an objective lens from a light source. That is, a parallel beam is incident on the objective lens from the light source when information is recorded on the DVD. On the other hand, a divergent beam is incident on the objective lens when the CD-R is selected. In such a method, an appropriate beam spot can be formed on a recording surface of an optical recording medium irrespective of a thickness of the optical recording medium.
However, the above-mentioned patent document does not refer to a specific structure of a condenser lens that changes a light beam to a parallel beam or a divergent beam. Additionally, there is no consideration as to the deterioration of a beam spot associated with an offset generated in an optical axis of an objective lens.
It is a very important issue for an optical pickup device to obtain a high optical efficiency when the optical pickup is used for the DVD or the CD-R. Additionally, it is a very important issue to control the deterioration of a beam spot associated with an offset of an optical axis of an objective lens within an allowable range.
BACKGROUND OF THE INVENTION
It is a general object of the present invention to provide an improved and useful optical pickup device in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an optical pickup apparatus which shares an objective lens for two different light sources so as to handle two kinds of optical information recording media and which has a high optical efficiency and decreases deterioration in a beam spot caused by an offset of an optical axis of an objective lens.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention an optical pickup device applicable to both a first optical recording medium and a second optical recording medium, the second optical recording medium having a thickness larger than a thickness of the first optical recording medium, the optical pickup device comprising:
a first light source used for the first optical recording medium, the first light source. emitting a first beam having a first wavelength;
a second light source used for the second optical recording medium, the second light source emitting a second beam having a second wavelength;
an objective lens converging the first beam and projecting the converged first beam onto the first optical recording medium, the objective lens also converging the second beam and projecting the converged second beam onto the second optical recording medium,
an optical path guiding unit guiding the first beam and the second beam to the same optical path directed to the objective lens;
a signal extracting unit for extracting signals from the first and second beams reflected by a selected one of the first optical recording medium and the second optical recording medium; and
a second coupling lens coupling the second beam emitted from the second light source, the second coupling lens alone having a wavefront aberration substantially proportional to a field angle thereof so that the second beam after passing through the second coupling lens has a wavefront different from a wavefront of the first beam.
In the above-mentioned invention, the first beam emitted by the first light source toward the objective lens can be either a parallel beam or a convergent beam. Since the objective lens is designed to match the first optical recording medium having a smaller thickness, the objective lens can focus the first beam onto the first optical recording medium with an appropriate beam spot. On the other hand, the second beam is changed by the second coupling lens so that the second beam has a wavefront different from a wavefront of the first beam. That is, for example, if the first beam is incident on the objective lens as a parallel beam, the second beam is incident on the objective lens as a divergent beam. If the first beam is incident on the objective lens as a divergent beam, the second beam is incident on the objective lens as a divergent beam having a stronger divergence.
Thus, according to the above-mentioned invention, an appropriate beam spot can be formed on both the first optical recording medium having a smaller thickness and the second optical recording medium having a larger thickness, by changing the second beam to a beam which can be appropriately focused on the second recording medium by a combination of the second coupling lens and the objective lens.
In the optical pickup device according to the present invention, the objective lens may have a performance optimum for at least one of a recording operation, a reproducing operation and an erasing operation performed on the first optical recording medium.
Accordingly, a recording operation, a reproducing operation, or an erasing operation can be appropriately performed with respect to both the first optical recording medium having a smaller thickness and the second optical recording medium having a larger thickness.
In one embodiment of the present invention, the second coupling lens may provide a predetermined divergence to the second beam. Additionally, the optical pickup device according to the present invention may further comprise a first coupling lens changing the first beam into a parallel beam.
The second coupling lens may be a single lens, and one of opposite surfaces of the second coupling lens may be an aspherical surface and the other may be one of a spherical surface and a flat surface. According to this structure, the second coupling lens can be eas
Dickstein Shapiro Morin & Oshinsky LPP
Hindi Nabil
Ricoh & Company, Ltd.
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