Optical pickup device

Details Optical pickup device Optical pickup device

2000-04-27

2003-09-02

Tran, Thang V. (Department: 2653)

Communications, electrical: acoustic wave systems and devices

Echo systems

Distance or direction finding

C369S044230, C369S112060, C369S112180

Reexamination Certificate

active

06614720

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical pickup devices forming a microspot on an information recording medium to optically densely record and reproduce information.
2. Description of the Background Art
In recent years, large capacities of data such as digital still pictures and moving pictures are increasingly used as multimedia advances. In general, such data is stored in a large-capacity recording medium such as optical discs and randomly accessed and reproduced as required. An optical disc is randomly accessible and has a recording density higher than magnetic recording media such as floppy discs. Furthermore, magneto-optical discs, which are rewritable, can be used as a recording medium as above. Most of such magneto-optical discs have an information recording layer with convex and concave portions referred to as lands and grooves, respectively, used as a tracking guide.
Such recording media as optical discs and magneto-optical discs are increasingly adapted to record data densely to be able to record larger capacities of data. For example a medium would have a track pitch reduced to increase a linear density in the direction of the track or have a minimal mark length reduced to enhance its recording density in its tangential direction to record data densely.
Furthermore, even if a highly efficient data compression system such as MPEG2 is used to record and reproduce moving pictures of high quality, the system is still required to transfer data at as high a rate as 10 Mbps to 20 Mbps. To record such a moving picture at real-time while an impact or the like has caused a servo to be displaced, such system must record it at at least 1.5 times the above data transfer rate. Accordingly the optical disc of interest must be rotated faster to increase linear velocity. A higher linear velocity entails a higher recording power and this requires that light availability, i.e., an optical output from an objective lens that originates from an optical output from a light source, be maximized.
Furthermore, for potable applications, recording media are increasingly adapted to have smaller sizes and optical pickup devices are accordingly required to have smaller sizes (in weight and volume).
Thus to accommodate a recording medium of large capacity and high transfer rate an optical pickup device is required to minimize in size a spot converged and thus formed on the recording medium and ensure a recording power of high output. Furthermore, as recording media are miniaturized optical systems are also required to be miniaturized.
Such optical pickup devices in general use semiconductor laser as their optical source. As shown in
FIG. 14
, a semiconductor laser
101
currently put to use varies in angle of divergence in the y direction in the x-y plane parallel to a surface
101
c
joining laser chips
101
a
and
101
b
together and in the z direction in the x-z plane perpendicular to joint surface
101
c
. An AlGaInP-based semiconductor laser of approximately 650 nm in wavelength provides an angle of divergence of approximately eight degrees in the y direction and an angle of divergence of approximately 24 degrees in the z direction, as represented in full width half maximum, and a region having a uniform optical intensity in a cross section in the y-z plane perpendicular to an optical axis of a light beam has an elliptic pattern with its shorter and longer axes corresponding to the y and z directions, respectively. If ellipticity is defined by a ratio of a diameter in the longer axis's direction to that in the shorter axis's direction, then the AlGaInP-based semiconductor laser would have an ellipticity of three.
Furthermore a GaN-based semiconductor laser of approximately 400 nm in wavelength that is currently being developed would have a further increased ellipticity of approximately four.
Furthermore, while a light emitted from semiconductor laser
101
in general polarizes in a direction parallel to joint surface
101
c
or the y direction, a light emitted from semiconductor laser
101
for example of approximately 635 nm in wavelength can polarize in a direction perpendicular to joint surface
101
c
or the z direction.
Recording a larger capacity of data on a recording medium can be achieved simply by minimizing the area of a spot converged on the recording medium and optimizing the recording medium's track pitch and shortest mark length to match the shape of the converged spot. A converged spot has its area minimized when it is a round spot with its diameter corresponding to diffraction limited. Two techniques can be used to obtain such round, converged spot from a light beam having an elliptic cross section.
The first technique uses beam shaping means such as a shaping prism to allow a light beam incident on an objective lens to have an isotropic intensity distribution. Reference will now be made to
FIG. 15
to describe the shaping prism's operation. Shaping prism
105
has a receiving side, with the
FIG. 14
xyz coordinate system considered, and an outputting side, with an x′y′z coordinate system considered. The x′ axis is adapted to be parallel to an optical axis of a light beam emerging from shaping prism
105
. One coordinate system corresponds to the other coordinate system with the x and y axes rotated around the z axis by a predetermined angle.
When shaping prism
105
, in the form of a wedge, receives a collimated light beam having a diameter Din (Y) in the y direction and a diameter Din (Z) in the z direction, incident on a plane of incidence
105
a
at an angle of &thgr;
1
, the light beam is refracted at an angle of refraction &thgr;
2
. If shaping prism
105
is formed of a material having an index of refraction n then the following relationship is established:
sin(&thgr;
1
)=
n
×sin(&thgr;
2
).
The refracted light beam is incident on a plane of emergence
105
b
perpendicularly and it is thus not refracted at the plane of emergence
105
b
and emerges in the form of a collimated beam having a diameter Dout (Y′).
Thus, diameter Din (Y) in the y direction is increased by shaping prism
105
by Dout (Y′).
In contrast, diameter Din (Z) in the z direction is not shaped by shaping prism
105
and thus emerges from shaping prism
105
as it is, i.e., Dout (Z)=Din (Z).
Furthermore the beam's direction of emergence is polarized relative to its direction of incidence in the x-y plane by a predetermined angle.
Herein the ratio of Dout to Din defines shaping-ratio. For example an elliptic cross section of an ellipticity of three can be converted to a round cross section by setting the shape and index of refraction of shaping prism
105
and the angle of incidence &thgr;
1
and the angle of refraction &thgr;
2
to set a shaping ratio equal to the ellipticity of three to increase its diameter in the shorter axis's direction three times.
More specifically, a light beam having a wavelength of 655 nm and shaping prism
105
formed of BK
7
result in an index of refraction of 1.51389. As such, with a &thgr;
1
of 75.13 degrees and a &thgr;
2
of 39.68 degrees, shaping prism
105
having the plane of incidence
105
a
and the plane of emergence
105
b
forming an angle of 39.68 degrees can provide the shaping ratio of three.
The second technique uses only a portion of a light beam having an elliptic cross section that is close to the optical axis and has an isotropic intensity distribution. This can be implemented by increasing a focal length of a collimator lens to reduce an effective NA (i.e., a radius of an effective aperture of an objective lens that is divided by a focal length of a collimator lens) to 0.1 or therebelow.
Optical pickup devices corresponding to the first and second techniques are configured as will be described below:
As a first conventional example,
FIG. 16
shows an optical pickup device employing a shaping prism corresponding to the first technique to shape a beam. In the figure, semiconductor laser
101
emits an anisotropic lig

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