Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-04-21
2002-04-23
Hudspeth, David (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044410
Reexamination Certificate
active
06377520
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical drives used to read information from optical discs, and more particularly, to a focus/tracking method and system for use on an optical drive, which is capable of detecting both the focusing error and the tracking error of the pickup head to thereby control the focus/tracking of the same during read or write operation on an optical disc. This invention allows the focus/tracking method and system for the optical drive to be simplified in structural complexity, thereby saving manufacturing cost.
2. Description of Related Art
Pickup heads for optical discs must produce signals that indicate whether the optical stylus is in focus on the disc surface and the position of the optical stylus with respect to the information track besides just reading the coded information from the disc. As information is being recorded on the discs with ever-increasing density and in multiple layers, and as the number of styles of optical discs that a single pickup is expected to read is also increasing, more robust and versatile methods for producing these signals are called for.
It is well known that single-beam tracking performs better than three-beam tracking (which is commonly used for CD drives) when the tracks are spaced more closely together and there are multiple layers of information on the discs as, for example, in DVD discs. Single-beam tracking, which includes the methods of pushpull tracking, heterodyne tracking, and differential phase detection (DPD), also has the advantage over three-beam tracking of being generated directly from the disc information track rather than requiring critical alignment of tracking spots. Heterodyne and DPD tracking further have an advantage over pushpull tracking in that the pit depth which maximizes these tracking signals is the same as the depth which maximizes the information signal; whereas for pushpull tracking, its signal is at maximum at the pit depth which minimizes the information signal.
Multiple wavelength sources are required in pickups which are used to handle a wide variety of disc media. For example, red lasers of around 650 nm wavelength are required for reading DVD discs, while write-once CD-R media must be read using an infra-red laser with a wavelength around 780 nm. In order to avoid multiplying the number of components in the pickup, one set for each wavelength, a new means for generating the focus/tracking signals is needed which can be aligned properly for all of the wavelengths simultaneously. The differential spot-size detection method for focus-error signal generation is such a system that can be aligned for multiple wavelengths simultaneously but it is incompatible with heterodyne and DPD single-beam tracking methods.
Prior art for this invention includes a description of differential spot-size detection disclosed in Japanese Laid-Open Patent Document Number 63-229640 dated Sep. 26, 1988. The essential information processing scheme is reproduced in
FIG. 1A
, which includes a laser source
10
, a holographic beamsplitter element
11
, an objective lens
12
, and a pair of 3-element photodetectors
16
,
17
. The holographic beamsplitter element
11
is used to divide the beam returning from the disc
13
into two beams
14
and
15
, which are incident respectively on two 3-element photodetectors
16
,
17
. The holographic beamsplitter element
11
further has a focusing effect which causes the first beam to focus in front of one of the 3-element photodetectors and causes the second beam to focus behind the other 3-element photodetector. The spots on the photodetectors are diagrammed in
FIGS. 1B-1D
for various cases of the focus of the optical stylus on the information surface of the disc. The focus error signal (FES) indicating the focus error of the optical stylus with respect to the information surface in the disc is given by combining the electrical signals generated by the photodetector elements as follows:
FES=(S
A″
+S
C″
−S
B″
)−(S
A′
+S
C′
−S
B′
)
In the case shown in
FIG. 1B
, the stylus is focused behind the information surface which causes the spots from the two beams to have different sizes on their respective 3-element photodetectors and FES to be positive. For the case shown in
FIG. 1C
, the stylus is focused properly on the information surface, the spots from the two beams have the same size on their respective 3-element photodetectors, and FES=0. For the case shown in
FIG. 1D
, the stylus is focused in front of the information surface causing the spots from the two beams vary in a complementary manner to the case shown at the top and FES to be negative. For different wavelengths, the diffraction angles of the two beams from the holographic element vary, causing the spots to move along the photodetectors parallel to lines dividing the detector into three elements. This does not affect the resulting FES. Other prior art disclosing similar differential spot-size detection is found in U.S. Pat. No. 5,111,448 (May 1992). The drawback of these methods is that, since the complete beam area is incident on both of the 3-element photodetectors, there is no way to access the heterodyne and DPD tracking information which is embedded in an interference pattern in the beam.
An example of the interference pattern embedded in the beam is given in
FIG. 2. A
beam after experiencing diffraction from the information surface of the disc is shown centered on a coordinate system with quadrants labeled I, II, III, and IV. The arcs drawn within the main circular beam represent the overlap of the main circular beam and diffraction orders created by diffraction from the disc information surface. There is interference between these diffracted orders and the main beam. As the optical stylus moves on and off the information track, the intensity of these interference regions changes. The shaded areas indicate the interference regions that contribute to heterodyne and DPD tracking signals. Signals from each of the four quadrants must be available separately in order to generate these tracking signals. As stated above, the prior art for differential spot-size focus detection does not provide separate signals from these four quadrants and therefore cannot be used to generate these tracking signals. The astigmatic focus detection method is described in any introductory text to optical disc technology (e.g. A. B Marchant, Optical Recording, Addison Wesley Publishing, Reading, Mass., *990) and is a method which does provide separate access to the signals in each of the four quadrants. Moreover, for instance, U.S. Pat. No. 4,731,772 (Mar. 1988) uses a quadrant detector to provide separate signals from each of the four quadrants as shown in
FIGS. 3A-3D
. However, since the spot must remain centered on the quadrant photodetector, this approach is not tolerant of position shifts that will occur with multiple wavelength sources.
SUMMARY OF THE INVENTION
This invention uses a new method to combine some the best features of previously incompatible differential spot-size-detection focus-error and signal-beam tracking-error signal generation techniques to create a focus/tracking system that is well suited to multiple layer, high density and multiple wavelength optical disc systems while requiring a minimum number of components to implement.
This invention is compatible with all of the above-mentioned tracking methods, however the preferred embodiments utilize its special ability to produce heterodyne and DPD tracking signals in a multiple-wavelength system.
In accordance with the foregoing and other objectives of the present invention, a focus/tracking method and system for the pickup head of an optical drive is provided. The method of the invention includes the following steps of: generating a laser beam; focusing the laser beam on the optical disc; splitting the reflected light from the optical disc in half into a first half part and a second half part; guiding the first half part of the
Freeman Mark O.
Shih Hsi-Fu
Wang Jinn-Kang
Blakely & Sokoloff, Taylor & Zafman
Chu Kim-Kwok
Hudspeth David
Industrial Technology Research Institute
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