Optical pickup using an optical phase plate

Details Optical pickup using an optical phase plate Optical pickup using an optical phase plate

2000-11-30

2001-08-07

Edun, Muhammad (Department: 2651)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

C369S112050, C369S094000

Reexamination Certificate

active

06272096

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup apparatus which can record information on and read information from a digital video disk (DVD) and a recordable compact disk (CD-R), respectively.
Recording media for recording and reading the information such as video, audio or data, are a disk, a card, or a tape. Among them, the disk type is chiefly used. Recently, in the field of the optical disk apparatus, a laser disk (LD), a compact disk (CD) and a digital video disk (DVD) have been developed. Such an optical disk includes a plastic or glass medium having a certain thickness along an axial direction to which light is incident, and a signal recording surface on which information is recorded and located on the plastic or glass medium.
So far, a high-density optical disk system enlarges a numerical aperture of an objective lens in order to increase a recording density, and uses a short wavelength light source of 635 nm or 650 nm. Accordingly, the high-density optical disk system can record or read signals on or from a digital video disk, and can also read signals from a CD. However, to be compatible with a recent type of a CD, that is, a recordable CD (CD-R), light having a wavelength of 780 nm should be used, due to the recording characteristic of the CD-R recording medium. As a result, using the light beam wavelengths of 780 nm and 650 nm in a single optical pickup becomes very important for compatibility of the DVD and the CD-R. A conventional optical pickup which is compatible with the DVD and the CD-R will be described below with reference to FIG.
1
.
FIG. 1
shows an optical pickup using two laser diodes as light sources for a DVD and a CD-R and a single objective lens. The
FIG. 1
optical pickup uses laser light having a wavelength of 635 nm when reproducing a DVD, and uses laser light having a wavelength of 780 nm when recording and reproducing a CD-R. Light having the 635nm wavelength emitted from a laser diode light source
1
passes through a collimating lens
2
and a polarization beam splitter
3
and then goes to an interference filter type prism
4
. Light having the 780 nm wavelength emitted from a laser diode light source
11
passes through a collimating lens
12
, a beam splitter
13
and a converging lens
14
and then goes to the prism
4
, which converges the light having the 780 nm wavelength. An optical system having such a structure is called a “finite optical system.” The prism
4
transmits the light beam having a wavelength of 635nm reflected from the polarization beam splitter
3
, and reflects the light beam converged by the converging lens
14
. As a result, the light beam from the light source
1
is incident to a quarter-wave plate
5
in the form of a parallel beam by the collimating lens
2
, while the light from the light source
11
is incident to the quarter-wave plate
5
in the form of a divergent beam by the convergent lens
14
and the prism
4
. The light transmitting through the quarter-wave plate
5
is incident to an objective lens
7
.
The light of the 635 nm wavelength emitted from the light source
1
is focussed by an objective lens
7
on a signal recording surface in a DVD
8
having a thickness of 0.6mm. Therefore, the light reflected from the signal recording surface of the DVD
8
contains information recorded on the signal recording surface. The reflected light transmits through the polarization beam splitter
3
, and is then incident to a light detector
10
for detecting optical information.
If the finite optical system described above is not used, when the 780nm wavelength light emitted from the light source
11
is focussed on a signal recording surface in the CD-R
9
having 1.2 mm thickness using the above-described objective lens
7
, spherical aberration is generated due to a difference in thickness between the DVD
8
and the CD-R
9
. In more detail, the spherical aberration is due to a fact that the distance between the signal recording surface of the CD-R
9
and the objective lens
7
is farther than that between the signal recording surface of the DVD
8
and the objective lens
7
, along an optical axis. To reduce such a spherical aberration, a construction of a finite optical system including a convergent lens
14
is required. By using a variable aperture
6
to be described later with reference to
FIG. 2
, the 780 nm wavelength light forms an optimized beam spot on the signal recording surface of the CD-R-
9
. The 780 nm wavelength light reflected from the CD-R
9
is reflected by the prism
4
and then the beam splitter
13
, so as to be detected in the light detector
15
.
The variable aperture
6
of
FIG. 1
has a thin film structure as shown in
FIG. 2
which can selectively transmit the rays of the light incident to the region if not more than the numerical aperture (NA) of 0.6 which coincides with the diameter of the objective lens
7
. That is, the variable aperture
6
is partitioned into two regions based on the NA of 0.45 with respect to an optical axis. Among the two regions, a first region
1
transmits both 635nm and 780 nm wavelength light and a second region
2
totally transmits the 635 nm wavelength light and totally reflects the 780 nm wavelength light. The region
1
has the numerical aperture of 0.45 or below, and the region
2
is an outer region of the region
1
and is made by coating a dielectric thin film. The region
1
is comprised of a quartz (SiO
2
) thin film in order to remove any optical aberration generated by the dielectric thin film coated region
2
. By using the variable aperture
6
, the 780 nm wavelength light transmitting the region
1
having the 0.45 NA or below forms a beam spot appropriate to the CD-R
9
on the signal recording surface thereof. Thus, the
FIG. 1
optical pickup uses an optimum light spot when a disk mode is changed from the DVD
8
to the CD-R
9
. Accordingly, the
FIG. 1
optical pickup is compatible for use with the CD-R.
However, the
FIG. 1
optical pickup as described above should form a “finite optical system” with respect to the 780 nm wavelength light in order to remove any spherical aberration generated when changing a DVD compatibly with a CD-R. Also, due to the optical thin film, that is, the dielectric thin film, which is formed in the region
2
having the NA of 0.45 or above, an optical path difference between the light transmitting the region
1
having the NA of 0.45 or below and that transmitting the region
2
having the NA of 0.45 or above, is generated. To eradicate this difference, it is necessary to form an optical thin film in the region
1
. Due to this reason, a quartz coating is formed in the region
1
and a multi-layer thin film is formed in the region
2
. However, such a fabricating process does not become only complicated but also adjustment of the thickness of the thin film should be performed precisely in units of “&mgr;m.” Thus, it has been difficult in mass-producing the optical pickup.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical pickup apparatus which is compatible with a digital video disk and a recordable compact disk by removing a spherical aberration using a phase plate.
To accomplish the above object of the present invention, there is provided an optical pickup apparatus for at least two optical recording media, which are different in distance from an optical pickup to information recording surfaces and uses light beams of different wavelengths for recoding and reading information, the optical pickup apparatus comprising:
a plurality of laser light sources for emitting a first light beam having a relatively shorter wavelength and a second light beam having a relatively longer wavelength, respectively;
an objective lens having a predetermined focal length in which the focal point of the objective lens according to the first light beam coincides with the position of the information recording surface in a first optical recording medium having the information recording surface closer to the objective lens;
optical detection means

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