Objective lens for optical pick-up

Details Objective lens for optical pick-up Objective lens for optical pick-up

2003-04-01

2004-10-19

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

Including a nonspherical surface

C359S708000, C369S044230, C369S112230

Reexamination Certificate

active

06807018

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an objective lens employed in an optical pick-up to be employed in an optical disc device capable of reading/writing from/to two or more types of optical discs having different cover layer thicknesses and data densities.
There exist many types of optical discs according to various standards having different thicknesses of the cover layer (transparent substrate covering the recording surface) and different data densities. For instance, the cover layer thickness of CD (Compact Disc) and CD-R (Compact Disc Recordable) having relatively low data density is 1.2 mm, while that of DVD (Digital Versatile Disc) having relatively high data density is 0.6 mm (½ of that of CD/CD-R).
For the reading/writing of DVDs having high data density, a laser beam having a relatively short wavelength (635-660 nm) is necessary in order to realize a small beam spot diameter. On the other hand, a laser beam having a relatively long wavelength (approximately 780 nm) is necessary for the reading/writing of CD-Rs due to their reflection characteristics.
Since these optical discs are often used in the same optical device, optical disc devices (CD players, DVD recorders, etc.) are desired to support all the optical disc standards. In order to use both DVDs and CD-Rs, the optical disc device is required to have at least two laser diodes for emitting laser beams of wavelengths suitable for the characteristics of the optical discs. Meanwhile, for downsizing the optical disc device, the optical system of its optical pick-up has to be as compact as possible. Therefore, it is preferable that optical elements of the optical pick-up (objective lens, etc.) be shared between two wavelengths and a module containing two laser diodes in one package be employed as a light source unit.
However, if one objective lens is used for each of two types of optical discs which have different cover layer thicknesses as above and spherical aberration is corrected for both discs, it becomes impossible to correct coma for both discs. Therefore, wavefront aberration has its minimum value when the laser beam's incident angle to the objective lens is 0° and increases as the incident angle deviates from 0° for both optical discs. Meanwhile, since DVDs with high data density require a large NA (Numerical Aperture), astigmatism changes greatly as the incident angle changes, by which allowable range of the incident angle (in which required performance can be attained) for DVDs becomes narrower than that for CDs. Therefore, the allowable incident angle range in which the wavefront aberration for DVD can be kept within an allowable limit is included in an allowable incident angle range for CD, overlapping with the center of the range for CD.
When a module containing two laser diodes in one package is employed for the light source unit as mentioned above, light emitting points of the two laser diodes are aligned in a direction perpendicular to the optical axis of the objective lens. Therefore, there exists a certain difference of incident angle (to the objective lens) between the two laser beams having different wavelengths, and it is impossible to set both the incident angles (of the two laser beams having different wavelengths) to 0° in order to minimize the wavefront aberrations. For the above reasons, the incident angles of the two laser beams have to be properly set in a balanced manner so that the wavefront aberrations for the two optical discs will be within the allowable limit. However, since the allowable incident angle range (in which the wavefront aberration can be kept within the allowable limit) stretches around 0° for each laser beam, in order to maintain a preset incident angle difference between the two laser beams, the incident angles to DVD and CD will generally have opposite signs (+/−) and each incident angle is necessitated to be set close to the edge of each allowable incident angle range. Therefore, due to the narrow range of settable incident angle for each laser beam, precise setting is required for the assembly of the elements of the optical pick-up, taking time and trouble. Further, even if the assembly is completed perfectly according to design values, the whole optical system has to be maintained with high accuracy since the wavefront aberration is already close to the allowable limit.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above problems. According to the present invention, there is provided an optical disc device for an optical pick-up of an optical disc drive by which the aberrations can be reduced and signal reading/writing from/to multiple types of optical discs of different standards can be realized even in the case where a module containing two laser diodes in one package is employed for the light source unit and the optical elements (objective lens etc.) are shared between two wavelengths.
In accordance with a first aspect of the present invention, there is provided an objective lens of an optical pick-up which converges a shorter wavelength laser beam on a first optical disc having a thin cover layer and high data density and converges a longer wavelength laser beam on a second optical disc having a thick cover layer and low data density. The objective lens is formed to provide axial astigmatism corresponding to wavefront aberration of 0.01&lgr; [rms] or more when the shorter wavelength laser beam is converged on the first optical disc, and to provide abaxial third order coma so that its sign when the shorter wavelength laser beam is converged on the first optical disc will be opposite to its sign when the longer wavelength laser beam is converged on the second optical disc in a virtual state in which the optical axis of the objective lens coincides with the normal of the optical disc.
Since the objective lens is configured to provide the axial astigmatism (astigmatism on the optical axis), an allowable incident angle range (allowable range of the incident angle of a laser beam to the optical disc in which wavefront aberration can be kept within an allowable limit) can be widened for each optical disc even though the minimum value of the wavefront aberration increases. Further, since the objective lens is configured to provide comas of opposite signs for the first optical disc (for which the shorter wavelength laser beam is used) and the second optical disc (for which the longer wavelength laser beam is used), both the wavefront aberrations for the first and second optical discs can be corrected at different incident angles to the first and second optical discs by tilting the optical axis of the objective lens relative to the normal of the optical disc. In other words, two incident angles (to the first and second optical discs) capable of minimizing the wavefront aberrations for the first and second optical discs shift in opposite directions relative to 0°, and the allowable incident angle ranges (in which the wavefront aberrations can be kept within an allowable limit) for the first and second optical discs also shift in opposite directions relative to 0°.
Therefore, the allowable incident angle range for the first optical disc and that for the second optical disc can be set in different angle ranges (even if part of them overlapped with each other), differently from the conventional cases where the allowable incident angle range for the first optical disc is totally included in that for the second optical disc. By this, even if a laser module having two light emitting points is employed (that is, even if two laser beams emitted from two light emitting points and incident on the objective lens had a certain incident angle difference), each incident angle to each optical disc can be set in a wide range, by which aberrations can be reduced for both the first and second optical discs and an excellent beam spot can be formed on both optical discs, without the need of precise assembly and setting which used to be necessary for conventional optical pick-ups.
Preferably, the

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