Optical disk reading method for multi-layered optical disk system

A multi-layered optical disk reading method which makes possible to reproduce encoded data from each data layer by focusing laser beam precisely onto a data layer of the multi-layered optical disk. After a multi-layered optical disk is loaded, a central processing unit (CPU) receives an operator's signal to access certain data layer. Then the CPU refers to stored data in the CPU's memory for a diffraction angle to focus laser beam onto a certain data layer. The CPU retrieves an operational coded signal, and transmits the signal to a liquid crystal diffraction device to modulate the liquid crystal diffraction device to deflect and focus the laser beam onto the certain data layer. After the liquid crystal diffraction device is modulated by the operational signal from the CPU, the CPU also operates a routing circuit to transmit read-out signal from a detector which receives reflected laser beam from the certain data layer, to a selected decoding circuit in a data processor.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
This invention relates to an optical data reproducing method. More 
specifically, this invention relates to an optical disk reading method 
which is able to reproduce encoded data from each data layer of a 
multi-layered optical disk. 
2. DESCRIPTION OF THE PRIOR ART 
Initialized by the vast increase in information that needs to be processed, 
optical data storage systems have become very important systems 
particularly because of their high storage density per unit area. Most of 
the recent optical information storage systems, which rotate a single 
optical disk are used on which the information is digitally stored in 
concentric circular tracks in an ordered, predefined manner to allow 
chronological fast reading and fast random access to desired pits of data. 
In order to accomplish even more storage capacity of optical disk systems 
for enormous information processing, such as video or picture 
communication like the so called video-on-demand service, multiple disk 
systems have been proposed. An optical disk system having two or more data 
layers may in theory be accessed at different disks by changing the focal 
point with a moving lens. Examples of this type of state-of-the-art are 
described in U.S. Pat. No. 5,202,875 issued Apr. 13, 1993 to Rosen, et 
al.; and Japanese Published Application, 63-276732 published Nov. 15, 1988 
by Watanabe, et al. 
Such a multiple disk reading system is applied to varied optical disk 
information storage systems. For example, a digital video disk (DVD) 
system for home entertaining is one typical application. The mentioned 
advantage of vast storage capacity may contribute especially to video 
signal transmission. In order to reproduce encoded data from each data 
layer, a laser beam emitted from a laser source has to be focused 
precisely onto each data layer. To focus the laser beam onto each data 
layer, the methods such as to change a focusing lens or to move the 
focusing lens by an actuator are applied to the conventional system. 
However, such method is not appropriate to apply to the multi-layered 
optical disk systems which has more than two data layers. Because the 
number of the changeable focusing lens or the focal point interval 
modulated by the actuator is limited, a multi-layered optical disk reading 
method is needed to overcome the mentioned disadvantage of the 
conventional method. 
3. SUMMARY OF THE INVENTION 
The present invention has for its object to provide a multi-layered optical 
disk reading system which is able to reproduce encoded data from each data 
layer by focusing laser beam precisely onto the data layer and detecting a 
reflected laser beam from each data layer selectively without employing 
any mechanical moving component. 
The object of the present invention can be achieved by an optical disk 
reading method, the method comprising the steps of: to load a 
multi-layered optical disk which has M data layers (wherein M is an 
integer greater than 1), to receive an operator's signal a to access an 
Nth data layer of the multi-layered optical disk (wherein N is an integer 
greater than 1 and not greater than M), to refer to stored, data in a 
central processing unit (CPU) for a diffraction angle to focus 
(hereinafter "operation signal") laser beam onto Nth data layer, to 
modulate a liquid crystal diffraction device to focus the laser beam onto 
the Nth data layer, and to operate a routing circuit to transmit a readout 
signal from selected detector to a decoding circuit. 
After a multi-layered optical disk is loaded, the total number of the data 
layers of the multi-layered disk is identified by reading a total of 
contents (TOC) data or by any discrimination means. Then the operation 
signal to access a certain data layer is transmitted to the CPU, and the 
CPU retrieves stored data on a modulation signal to let the liquid crystal 
diffraction device to focus the laser beam onto the certain data layer. 
For example, two operational modulation signals to modulate the liquid 
crystal diffraction device to focus the laser beam onto each of two data 
layers, must be stored in the memory of the CPU for a double-layered 
optical disk reading system. The CPU sends the modulation signal to the 
liquid crystal diffraction device. According to the modulation signal, the 
liquid crystal diffraction device modulates each of its liquid crystal 
segments to change numerical aperture between any two of the liquid 
crystal segments. By changing the numerical aperture between the liquid 
crystal segments, the transmitted laser beam is focused onto a preferred 
data layer. The reflected laser beam from the data layer is transmitted to 
a detector array circuit. Each detector of the array is best placed at a 
separate position in order to detect selectively the reflected laser beam 
from selected data layer, and to minimize the effect of an interference. 
The CPU operates a routing circuit to transmit a readout signal from the 
selected detector to a selected decoding circuit. 
For a fuller understanding of the nature and advantages of the present 
invention reference should be made to the following detailed description 
taken in conjunction with the accompanying drawings.

5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Embodiments of the present invention will be explained with reference to 
the drawings. 
FIG. 1 is a block diagram of an example of a multi-layered optical disk 
reading apparatus to which the optical disk reading methods of the present 
invention can be applied. A multi-layered optical disk 10 which has two or 
more data layers is mounted on and secured by a turntable 11 to be rotated 
by a spindle motor 12. Encoded pits on the multi-layered optical disk 10 
are read by a pickup 13 which includes a laser diode, a focusing lens, a 
tracking actuator, a photo-detector array, and a focusing lens actuator 
cooperating with a liquid crystal diffraction device 14. The movement of 
the pick-up 13 and the modulation of the liquid crystal diffraction device 
14 are controlled by a focusing servo circuit 15 and a tracking servo 
circuit 16. The readout signal from the pickup 13 is transmitted to a 
pre-amplifier 17. According to a focusing error signal, the focusing servo 
circuit 15 modulates the focusing lens actuator and the liquid crystal 
diffraction device 14 to move the focal point of the laser beam emitted 
from the laser diode to access certain data layer of the multi-layered 
optical disk 10. And according to a tracking error signal, the tracking 
servo circuit 16 modulates the tracking actuator to move the pickup 13. 
The spindle servo circuit 18 modulates the spindle motor 12 in order to 
track linear velocity of the multi-layered optical disk 10. 
The detected readout signal applied to the pre-amplifier 17 from one of the 
detectors in the detector array of the pickup 13, is transmitted to a data 
processor 19. The data processor 19 consists of at least one decoding 
circuit to decode the readout signal. In order to reproduce encoded data 
from two types of optical disk format, for example a compact disk (CD) and 
a digital video disk (DVD), the data processor must be composed of two 
decoding circuits. And the routing from the detector to the decoding 
circuit in the data processor 19 is operated by a central processing unit 
20 (CPU). Part of the readout signal reproduced by the data processor 19 
is transmitted to the CPU 20 to observe a data reproduction procedure, and 
to operate a servo control circuit 21 which modulates the focusing servo 
circuit 15, the tracking servo circuit 16 and the spindle servo circuit 
18. And the decoded readout signal from the data processor 19 is 
transmitted to an audio processor 22, a sub-picture processor 23 and a 
video processor 24. The audio processor 22, the sub-picture processor 23 
and the video processor 24 are controlled by the CPU 20. The CPU 20 is 
operated by an operation signal from a key operating unit 25 which 
transmits all operating signals of an operator. The CPU 20 also controls a 
display unit 26 to show the data reproducing status to the operator. 
FIG. 2(a) shows a first exemplified structure of a pick-up 13 described in 
the FIG. 1 to which the present invention can be applied. A laser beam is 
emitted from a laser diode 30, and the laser beam is collimated by a 
collimate lens 31. The collimated laser beam is deflected by a half-mirror 
32. Then, the deflected laser beam is focused by a liquid crystal 
diffraction device 33. By modulating each liquid crystal segment of the 
liquid crystal diffraction device 33, the produced focal point can be 
moved to focus the laser beam onto a preferred data layer. Also the 
position of the focal point is precisely moved by a focusing lens 34 which 
is driven by a dual actuator 35-1.about.2. When the laser beam is 
reflected from the data layer of a multi-layered optical disk 36 it is 
transmitted to a lens 37 and focused on a detector 38-1.about.2. The 
detector 38-1 is positioned to detect the laser beam from reflect lower 
data layer, and the detector 38-2 is to detect laser beam from reflected 
upper data layer, selectively. The readout signal from the detector 
38-1.about.2 is transmitted to a routing circuit 39 to be selectively sent 
to a decoding circuit of a data processor. 
FIG. 2(b) shows a second exemplified structure of a pick-up 13 described in 
the FIG. 1 to which the present invention can be applied. A laser beam is 
emitted from a laser diode 40, and the laser beam is collimated by a 
collimate lens 41. The collimated laser beam is deflected by a half-mirror 
42. Then, the deflected laser beam is focused by a liquid crystal 
diffraction device 43. By turning on or off each liquid crystal segment of 
the liquid crystal diffraction device 43, the distance between the liquid 
crystal diffraction device 43 and the focal point can be modulated to 
focus the laser beam onto a preferred data layer. In this example 
described in FIG. 2(b), the liquid crystal segment shuts outer spectrum of 
the laser beam to extend the distance to focal point. By modulating the 
numerical aperture of the liquid crystal diffraction device 43, the focal 
point can be moved. Also the position of the focal point is precisely 
moved by focusing lens 44 which is driven by a dual actuator 45-1.about.2. 
When the laser beam is reflected from the certain data layer of a 
multi-layered optical disk 46 it is transmitted to a grating 47 and 
transmitted to a detector 48-1.about.2. The grooves of the grating 47 are 
fabricated to deflect the inner spectrum of the reflected laser beam from 
the upper data layer of the multi-layered optical disk 46. The detector 
48-1 is positioned to detect the laser beam reflected from lower data 
layer, and the detector 48-2 is to detect the portion of the laser beam 
reflected from upper data layers selectively. The readout signal from the 
detector 48-1.about.2 is transmitted to a routing circuit 49 to 
selectively sent to a decoding circuit of a data processor. 
FIG. 3(a) shows a first pattern of an example of a liquid crystal 
diffraction device to which the present invention can be applied. And the 
example of a liquid crystal diffraction device described in FIG. 3(a) is 
suitable for the first exemplified structure of a pick-up described in the 
FIG. 2(a). According to an operation signal from a central processing 
unit, a driver circuit 50 modulates each of a gate circuit 51-1.about.2. 
Electrodes 52 are fabricated to connect to one of the liquid crystal 
segment 53. The array of the liquid crystal segments 53 forms a grating, 
and by selectively turning on and off the group of liquid crystal segments 
at equal intervals, the numerical aperture between the liquid crystal 
segments is modulated to change the diffraction angle of the laser beam. 
For example, by turning on the group of the liquid crystal segment with 
wide intervals, the diffraction angle is small. And, by turning on all of 
the liquid crystal segments at equal narrow intervals, the diffraction 
angle is large. 
FIG. 3(b) shows a second pattern of an example of a liquid crystal 
diffraction device to which the present invention can be applied. And the 
example of a liquid crystal diffraction device described in FIG. 3(b) is 
suitable for the second exemplified structure of a pick-up described in 
the FIG. 2(b). According to an operation signal from a central processing 
unit, a driver circuit 60 modulates each of gate circuits 61-1.about.2. 
Each of the electrodes 62 is fabricated to connect to one of the liquid 
crystal segment 6-31.about.2. When the outer liquid crystal segment 63-2 
is turned on to shut the outer spectrum of the laser beam, the laser beam 
is focused at far position because the numerical aperture is small. By 
turning on or off the liquid crystal segments selectively and changing the 
numerical aperture, the position of the focal point is moved. 
FIG. 4 shows a flowchart of an operation processing procedure in the 
central processing unit (CPU) to which an optical disk reading method of 
the present invention is applied. The CPU receives an operator's signal to 
access the Nth data layer of a multi-layered optical disk (Step 1: S1), 
wherein N is an integer greater than 1, and not greater than the total 
number of data layers M. Then the CPU refers to stored data in the CPU's 
memory for a diffraction angle to focus the laser beam onto the Nth data 
layer (S2). The CPU retrieves an operational coded signal, and transmits 
the signal to a liquid crystal diffraction device to modulate the liquid 
crystal diffraction device to deflect and focus the laser beam onto the 
Nth data layer (S3). After the liquid crystal diffraction device is 
modulated by the operational signal from the CPU, the CPU also operates a 
routing circuit to transmit a read-out signal from a detector which 
receives the laser beam reflected from the Nth data layer, to a selected 
decoding circuit in a data processor (S4). The routing circuit can connect 
and disconnect the read-out signal transmission path from the detector to 
the decoding circuit. At last, when a reproduction switch of a key 
operating unit is turned on by the operator (S5), the data reproduction is 
started (S6). 
Although the invention has been particularly shown and described, it is 
contemplated that various changes and modification may be made without 
departing from the scope of the invention as set forth in the following 
claims.