Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reissue Patent
2002-07-25
2004-06-22
Neyzari, Ali (Department: 2655)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S124110, C369S044230, C369S053220
Reissue Patent
active
RE038538
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for and a method of recording information on or reproducing information from a recording medium and, more particularly, to an apparatus for and a method of recording information on or reproducing information from a plurality of disk-shaped recording mediums having different substrate thicknesses.
2. Description of the Related Art
Compact discs (CDs) have been widely used as a recording medium from which recorded information is reproduced with light. In recent years, a new recording medium, known as digital video disc (DVD), has been used for digitally recording video images over a long period of time.
For reading recorded digital information from an optical recording medium, a laser beam is applied to the optical recording medium, light reflected from the optical recording medium is detected, and the level of the reflected light is converted into binary data.
FIG. 1
of the accompanying drawings shows an optical pickup device for use with a CD. As shown in
FIG. 1
, a laser diode (LD)
1
emits a laser beam having a wavelength of 780 nm. The laser beam emitted from the LD
1
is divided into a plurality of laser beams, e.g., three laser beams, by a grating
2
. One of the three laser beams is used to read recorded information and to control the optical pickup device in focusing servo operations. The remaining two laser beams are used to control the optical pickup device in tracking servo operation. The three laser beams will also be referred to collectively as light.
A beam splitter
3
that comprises a transparent planar plate reflects the laser beams from the grating
2
toward an objective
4
. Light (converged light) reflected from a CD
10
and transmitted through the objective
4
passes through the beam splitter
3
toward a photodiode (PD)
5
that serves as a photodetector. While the reflected light is passing through the beam splitter
3
, the reflected light is given astigmatism by the beam splitter
3
.
The objective
4
converges the laser beams onto an information recording layer
12
that comprises minute pits on the CD
10
. The objective
4
also converges light reflected from the information recording layer
12
of the CD
10
through the beam splitter
3
onto the photodiode
5
.
The larger the numerical aperture (NA) of the objective
4
, the larger the angle through which the objective
4
converges light into a smaller spot. In
FIG. 1
, the objective
4
has an NA of 0.45.
The photodiode
5
detects the returning light reflected from the CD
10
. Since the laser beam emitted from the LD
1
is divided into three laser beams, the photodiode
5
has three corresponding photodetector units. One of the photodetector areas serves to detect the laser beam that is used to read recorded digital information. The remaining two photodetector areas serve to detect the two laser beams for tracking servo control. Specifically, based on the difference between the optical energy quantities of the two tracking laser beams, the objective
4
is controlled to apply the laser beam used to read recorded digital information to a predetermined track on the CD
10
in tracking servo operation.
Since the light reflected from the information recording layer
12
and applied to the photodiode
5
passes as converged light through the beam splitter
3
, the light is subject to astigmatism. The objective
4
is controlled in focusing servo operation based on the astigmatism thus produced.
The CD
10
has a transparent substrate
11
having a thickness t of 1.2 mm with the information recording layer
12
disposed thereon and a protective film
13
disposed on the information recording layer
12
. The laser beams from the LD
10
are converged by the objective
4
and pass through the transparent substrate
11
to the information recording layer
12
that has minute pits representative of recorded information. When the laser beams are applied to pits, they are diffracted, causing the returning light that is reflected by the recording medium and applied to the photodiode
5
to be reduced in intensity. When the laser beams are applied to a pit-free area of the information recording layer
12
, they are reflected, and hence the returning light has a high intensity. The returning light from the CD
10
is detected by the photodiode
5
, which converts higher and lower intensities of the returning light into respective binary levels of “1” and “0”, thereby reading the digital information recorded as pits on the CD
10
.
While the objective
4
is being thus controlled in tracking and focusing servo modes, the laser beams are applied to a given position on the CD
10
, and the returning light is detected to read the recorded digital information from the CD
10
.
FIG. 2
of the accompanying drawings illustrates a digital video disc (DVD)
20
that has been proposed recently. The DVD
20
has digital information recorded in a double-sided structure, whereas the CD
10
has digital information recorded in a single-side structure. Specifically, the DVD
20
includes a first disc member comprising a substrate
21
, an information recording layer
22
disposed on the substrate
21
, and a protective film
23
disposed on the information recording layer
22
, and a second disc member comprising a substrate
31
, an information recording layer
32
disposed on the substrate
31
, and a protective film
33
disposed on the information recording layer
32
, the first and second disc members being bonded to each other through the protective films
23
,
33
. Therefore, the DVD
20
is symmetrical with respect to a median plane thereof.
Since digital information is recorded with high density on the DVD
20
, the substrates
21
,
31
are thinner than the substrate
11
of the CD
10
in order to minimize skews and errors of substrate thicknesses. Specifically, while the substrate
11
of the CD
10
has a thickness of 1.2 mm, each of the substrates
21
,
31
of the DVD
20
has a thickness of 0.6 mm. The length of and intervals between the pits on the DVD
20
are smaller than those of the CD
10
.
Inasmuch as the recording density of the DVD
20
is greater than the recording density of the CD
10
, an LD
41
of an optical pickup device for use with the DVD
20
emits a laser beam having a shorter wavelength of 650 nm than the LD
1
of the optical pickup device for use with the CD
10
. The optical pickup device for use with the DVD
20
has other components including a grating
42
, a beam splitter
43
, an objective
44
, and a photodiode (PD)
45
that are identical to those of the optical pickup device for use with the CD
10
.
However, because the DVD
20
has pits smaller than the CD
10
due to the larger recording density, the objective
44
has a numerical aperture (NA) of 0.6, which is greater than the objective 4 (NA=0.45) of the optical pickup device for use with the CD
10
. The objective
44
with the larger numerical aperture is capable of converging a laser beam into a smaller spot to read smaller pits.
As described above, the CD
10
and the DVD
20
are structurally different from each other. Usually, therefore, it is necessary to use different optical systems (optical pickup devices) for reading recorded information from the CD
10
and the DVD
20
.
If the optical pickup device for use with the DVD
20
is applied to the CD
10
, for example, as shown in
FIG. 3
of the accompanying drawings, then since the optical pickup device for use with the DVD
20
is designed to read the recorded information from the DVD
20
under optimum conditions, it suffers spherical aberration due to the difference between the thicknesses of the substrate
11
of the CD
10
and the substrates
21
,
31
of the DVD
20
and the difference between the numerical apertures of the objectives
4
,
44
when reading the recorded information from the CD
10
.
For example, when a CD whose substrate has a thickness of 1.2 mm is played back using an objective having a numerical aperture of 0.6 which i
Uemura Kamon
Yamakawa Akio
Kananen Ronald P.
Neyzari Ali
Rader & Fishman & Grauer, PLLC
LandOfFree
Apparatus and method for recording and reproducing... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus and method for recording and reproducing..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus and method for recording and reproducing... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3349150