Tracking control apparatus for a multi-layer optical disc and method therefor

In a three-spot tracking control system for a multi-layer optical disc, stray light of the signal-reproducing main beam from a layer other than the layer being reproduced leaks into first and second photodetectors receiving the reflected beam of two auxiliary tracking controlling beams to render the tracking operation unstable. In order to overcome this drawback, a first photodetector detects one auxiliary reflected beam from one or the other of the information signal layers of a double-layer optical disc. The second photodetector detects the other reflected auxiliary beam from the information signal layer. A first upper envelope detector detects the upper envelope signal of the reflected auxiliary beam. A second upper envelope detector detects the upper envelope signal of the detection signal of the other reflected auxiliary beam. A first subtractor subtracts the detected output by the first upper envelope detector 6 from the detection signal of the first photodetector. A second subtractor subtracts the detected output by the second upper envelope detector from the detection signal of the second photodetector. A third subtractor performs subtraction between outputs of the first subtractor and the second subtractor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a tracking control apparatus for a multi-layer 
optical disc for controlling tracking of the multi-layer optical disc made 
up of plural information signal layers. More particularly, this invention 
relates to a tracking control apparatus for a multi-layer optical disc for 
controlling tracking of a main light beam according to a difference of the 
reflection of two auxiliary beams. 
2. Description of Related Art 
In conventional optical disc reproduction, a three-spot tracking system has 
been known in which a signal reproducing main beam and two auxiliary 
tracking-controlling beams, radiated on the disc with an radial offset of 
about one-half track with respect to the main beam, are used for 
controlling the tracking of the signal-reproducing main beam. The 
reflected beams of the two tracking-controlling auxiliary beams from the 
optical disc are detected by first and second photodetectors. 
Low-frequency components of the difference between signals obtained by the 
first and second photodetectors are used as a tracking error signal. 
In optical discs as storage means for computers or package media for music 
or video information, a recent tendency is toward high recording density. 
As one of the methods for achieving high recording density, a multi-layer 
optical disc, comprised of plural information signal layers as signal 
recording area, has been proposed. An example of such multi-layer optical 
disc is a double-layer optical disc comprised of two information signal 
recording layers as an information signal recording area. Referring to 
FIG. 1, a double-layer optical disc 26 includes a disc substrate 31 of a 
transparent synthetic resin material, such as polycarbonate, on one 
surface of which a first information signal layer 31a has been formed by a 
row of pits. A semi-transparent reflective film 32 of a thin dielectric 
film of, for example, silicon nitride SiN.sub.2, is formed on the first 
information signal layer 31a. The double-layer optical disc 26 also 
includes an intermediate layer 33 made of a transparent resin material, 
which is 40 .mu.m in thickness. A second information signal layer 33a is 
formed on the surface of the intermediate layer 32 opposite to its surface 
in contact with the semi-transparent reflective layer 32 by a row of pits. 
The double-layer optical disc 26 further includes a reflective layer 34 
of, for example, aluminum A1, formed on the second information signal 
layer 33a, and a protective or covered layer 35 deposited on the 
reflective film 34. The covered layer 35 may be a substrate as in a video 
disc, or a layer bonded to the substrate. 
For reading information signals from the double-layer optical disc 26, the 
focal position of the laser light radiated from an objective lens 25 of an 
optical pickup unit shown in FIG. 1 is varied in two stages. That is, for 
reading out information signals on the first information signal layer 31a, 
it suffices if a spot of a focal point by a laser light beam L.sub.1 is 
set on the row of pits recorded on the first information signal layer 31a. 
On the other hand, for reading out information signals on the second 
information signal layer 33a, it is sufficient if the spot of a focal 
point formed by the laser light L.sub.2 shown by a broken line in FIG. 1 
is set on the row of pits recorded on the second information signal layer 
33a. 
The optical disc may also comprise a first substrate bonded by a resin 
layer to a second substrate, wherein recorded surfaces may be provided on 
either opposed surface of each of the first and second substrates. 
Reproduction may then occur from each recording layer. 
The tracking control system employing the "so-called" three-beam method may 
also be used in reproducing the information signals from such 
double-layered optical disc 26. 
However, with the method employing a three-spot tracking control as 
described above, the light reflected from the layer other than the layer 
being reproduced, that is, the stray light, leaks into first and second 
photodetectors receiving the reflected light beams of the two 
tracking-controlling auxiliary beams. The result is that correct tracking 
error signals occasionally cannot be produced thus leading to an unstable 
tracking operation. 
For example, if stray light incident on the first photodetector is equal to 
that incident on the second photodetector, correct tracking error signals 
can be produced, thus assuring a stable tracking operation. However, if 
the amount of stray light leaking into one of the first and second 
photodetectors differs from that leaking into the other photodetector due 
to, for example, movement of stray light spots on first and second 
photodetector surfaces caused by transverse movements of the objective 
lens during tracking or due to mechanical position deviation of the 
photodetectors, an offset is produced in the tracking error signals, thus 
disabling correct tracking. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a tracking 
control apparatus for a multi-layer optical disc whereby the offset in the 
tracking error signals produced due to, for example, movement of stray 
light spots on first and second photodetector surfaces, caused by 
transverse movements of the objective lens during tracking or due to 
mechanical position deviation of the photodetectors, may be eliminated for 
enabling stable tracking. 
With the tracking control apparatus for a multi-layer optical disc 
according to the present invention, the above problem produced in tracking 
control of a multi-layer optical disc using the three-spot method may be 
solved by: subtracting the upper envelope signal of one of the reflected 
auxiliary beams detected by a first envelope detection means from a 
detection signal of one of the reflected auxiliary beams from the track of 
one of the information signal layers detected by the first photodetector 
means; subtracting the upper envelope signal of the other reflected 
auxiliary beam detected by a second upper envelope detection means from a 
detection signal of the other auxiliary reflected beam from the track of 
the one information signal layer detected by a second photodetector means 
by a second subtraction means; and by subtracting outputs of the first and 
second subtraction means by a third subtraction means. The result is that, 
in reproducing signals from a multi-layered optical disc, it becomes 
possible to eliminate offset in the tracking error signal produced by 
movement of the stray light spot on the photodetector surface caused by 
transverse movement of the objective lens during tracking or mechanical 
deviations of the photodetector positions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, a preferred embodiment of a tracking control 
apparatus for a multi-layer optical disc according to the present 
invention will be explained in detail. Although the invention is discussed 
in connection with a double-layer optical disc, it is equally applicable 
to pairs of recording layers respectively formed on each recording layer 
of each substrate of the optical disc. 
The present embodiment is directed to a tracking control apparatus for a 
double-layer optical disc comprised of first and second information signal 
layers layered together, as shown schematically in FIG. 1, as previously 
discussed. With the described tracking controlling apparatus, a 
signal-reproducing main beam and two tracking-controlling auxiliary beams 
are radiated on a track of an information signal layer of the 
double-layered optical disc. The tracking of a track on one of the 
information signal layers of the double-layered optical disc by the main 
beam is controlled responsive to the difference of the reflected light 
beams of the two auxiliary light beams from the information signal layer. 
With the present tracking control apparatus, a track of the other 
information signal recording layer of the double-layer optical disc is 
irradiated with the signal-reproducing main beam and two tracking 
controlling auxiliary beams for controlling the tracking by the main beam 
of a track of the other information signal layer of the double-layer 
optical disc responsive to the difference in the reflected auxiliary light 
beams from the information signal layer. 
Specifically, as shown in FIG. 2, the tracking controlling apparatus 1 for 
the double-layer optical disc 26 includes a first photodetector 2 for 
detecting one of two auxiliary light beams reflected from the information 
signal layer, and a second photodetector 3 for detecting the other 
reflected auxiliary light beam. The tracking controlling apparatus 1 also 
includes a first upper envelope detector 6 for detecting an upper envelope 
signal of the detection signals of the reflected auxiliary beam detected 
by the first photodetector 2 and a second upper envelope detector 7 for 
detecting an upper envelope signal of the detection signals of the other 
reflected auxiliary beam detected by the second photodetector 3. The 
tracking controlling apparatus also includes a first subtractor 8 for 
subtracting a detection output of the first upper envelope detector 6 from 
the detection signal of the first photodetector 2, a second subtractor 9 
for subtracting a detection output of the second upper envelope detector 7 
from the detection signal of the second photodetector 3, and a third 
subtractor 10 for performing subtraction between an output of the first 
subtractor 8 and an output of the second subtractor 9. 
The detection signal of the reflected auxiliary beam detected by the first 
photodetector 2 is converted by a current-to-voltage converting amplifier 
4 into a voltage signal which is amplified and then supplied to the first 
upper envelope detector 6 and to the first subtractor 8. The detection 
signal of the other reflected auxiliary beam, as detected by the second 
photodetector 3, is converted by a current-to-voltage converting amplifier 
5 into voltage signals which are then amplified and routed to the second 
upper envelope detector 7 and to the second subtractor 9. 
A subtraction output of the third subtractor 10 is bandwidth-limited by a 
low-pass filter (LPF) 11 before being outputted as a tracking error 
signal. 
The above-described various components of the tracking control apparatus 
for the double-layer optical disc are arranged in a circuit configuration 
shown in FIG. 3. That is, the current-to-voltage converting amplifier 4 is 
made up of a resistor R.sub.1 and an amplifier 12, while the 
current-to-voltage converting amplifier 5 is made up of a resistor R.sub.2 
and an amplifier 13. The first upper envelope detector 6 is made up of 
transistors Q.sub.1 and Q.sub.2 and a differential amplifier 14. The 
transistor Q.sub.1 has its collector grounded, its emitter connected to a 
resistor R.sub.3, and its base provided with an output signal of the 
current-to-voltage converting amplifier 4. The transistor Q.sub.2 has its 
base connected to an emitter of the transistor Q.sub.1, and its emitter 
connected to a parallel connection of a resistor R.sub.4 and a capacitor 
C.sub.1. The differential amplifier 14 has its positive input terminal 
connected to an emitter of the transistor Q.sub.1 and has its output fed 
back to its negative input terminal. The second upper envelope detector 7 
is made up of a transistor Q.sub.3, a transistor Q.sub.4 and a 
differential amplifier 15. The transistor Q.sub.3 has its collector 
grounded, its emitter connected to a resistor R.sub.5, and its base 
provided with an output signal of the current-to-voltage converting 
amplifier 5. The transistor Q.sub.4 has its base connected to an emitter 
of the transistor Q.sub.3 and its emitter connected to a parallel 
connection of a resistor R.sub.6 and a capacitor C.sub.2. The differential 
amplifier 15 has its positive input terminal connected to an emitter of 
the transistor Q.sub.4 and has its output fed back to its negative input 
terminal. 
The first subtractor 8 is comprised of a differential amplifier 16 having 
its positive input terminal connected via a resistor R.sub.9 to an output 
terminal of the current-to-voltage converting amplifier 4 and grounded via 
a resistor R.sub.1O. The differential amplifier 16 has its negative input 
terminal connected via a resistor R.sub.7 to an output terminal of the 
first upper envelope detector 6 and has its output fed back via a resistor 
R.sub.8 to its negative input terminal. The second subtractor 9 is 
comprised of a differential amplifier 16 having its positive input 
terminal connected via a resistor R.sub.13 to an output terminal of the 
current-to-voltage converting amplifier 5 and grounded via a resistor 
R.sub.14. The differential amplifier has its negative input terminal 
connected via a resistor R.sub.11 to an output terminal of the first upper 
envelope detector 6 and has its output fed back via a resistor R.sub.12 to 
its negative input terminal. The third subtractor 10 is comprised of a 
differential amplifier 18 having its positive input terminal connected via 
a resistor R.sub.17 to an output terminal of a second subtractor 9 and 
grounded via a resistor R.sub.18. The differential amplifier has its 
negative input terminal connected via a resistor R.sub.15 to an output 
terminal of the first subtractor 9 and has its output fed back via a 
resistor R.sub.16 to its negative input terminal. The LPF 11 is comprised 
of a resistor R.sub.19 and a capacitor C.sub.3. 
The tracking control apparatus 1 for the double-layer optical disc 26 is 
connected to an optical pickup unit 20 for the double-layer optical disc 
as shown in FIG. 4 for performing tracking control of the double-layer 
optical disc. Before providing an explanation of the operation of the 
tracking control apparatus for the double-layer optical disc 26, the 
optical pickup unit 20 for the double-layer optical disc 26 is first 
explained. 
The diffused laser light beam outgoing from a light source 21, such as a 
laser diode, is collimated by a collimating lens 22 and separated by a 
diffraction lattice 23 into 0-order light for the main beam and into .+-.1 
order light for the two auxiliary beams. The laser light separated into 
these three beams is passed through a beam splitter 24 and converged by an 
objective lens 25 so as to be radiated on a track of a first information 
signal layer 26a of the double-layer optical disc 26 as shown in FIGS. 4 
and 5. The main light spot formed by the 0-order light is termed G.sub.0, 
while the auxiliary light spots formed by the +1 order light and the 
auxiliary light spots formed by the -1 order light are termed G.sub.+ and 
G.sub.- respectively. 
The reflected laser beam from the first information signal layer 26a is 
passed through the objective lens 25 and enters a light condensing lens 27 
after separation and reflection by the beam splitter 24. The reflected 
laser beam condensed by the light condensing lens 27 is radiated via a 
cylindrical lens 28 onto a photodetector 29 provided with the first and 
second photodetectors 2 and 3. 
The photodetector 29 is configured as shown in FIG. 6 in which the first 
photodetector 2 and the second photodetector 3 are respectively arranged 
on opposite sides of a photodetector 30 for the reflected main beam. The 
first photodetector 2, the second photodetector 3 and the photodetector 30 
receive the reflected light spot g.sub.+ of the auxiliary light spot 
G.sub.+, the reflected light spot g of the auxiliary light spot G and the 
reflected light spot g.sub.0 of the main light spot G.sub.0, respectively. 
However, the reflected light of the signal-reproducing main beam from the 
layer of the double-layer optical disc 26 other than the layer being 
reproduced, that is, the stray light, leaks into the first and second 
photodetectors 2 and 3, respectively. 
If the stray light leaking into the first photodetector 2 and that leaking 
into the second photodetector 3 are equal to each other as shown in FIG. 
6, there is no problem raised in generating a tracking error signal. 
However, if the amount of the stray light leaking into the photoresistor 2 
differs from that leaking into the photodetector 3 due to movement of 
stray light spots on the photodetector surface caused by transverse 
movement of the objective lens during tracking or mechanical deviations in 
the photodetector positions, as shown in FIG. 7, there is produced an 
offset in the tracking error signal, such that correct tracking cannot be 
achieved. 
The tracking control apparatus 1 for the double-layer optical disc is 
configured as shown in FIG. 2, and operates in the manner now to be 
explained by referring to the signal waveforms shown in FIG. 8. Thus, it 
becomes possible to eliminate offset of the tracking error signal 
generated due to movement of the stray light spot on the photodetector 29 
caused by the transverse movement of the objective lens 25 during tracking 
or mechanical deviations in the positions of the photodetector 29 for 
assuring correct tracking. 
If the stray light is taken into account, the stray light leaking into the 
first and second photodetectors 2 and 3 operating as the photodetectors 
for the reflected beam for tracking control may be deemed as dc light not 
having signal components. If the as-detected signal light caused by the 
transverse movement of a low pre-set period of the objective lens, as one 
of the causes for offset, is considered for only the first photodetector 
2, the amount of the stray light entering the photodetector 29 is varied 
by transverse movement of the objective lens, such that a signal shown in 
FIG. 8C, that is a signal in the absence of the stray light as shown in 
FIG. 8A, dc-summed to the signal by the stray light, as shown in FIG. 8B, 
is obtained. Thus, by taking out the upper envelope detection signal shown 
in FIG. 8D from the signal shown in FIG. 8C, using the upper envelope 
detector 6, fluctuating components by the stray light can be extracted, 
whereas, by subtracting the signal shown in FIG. SD from the signal shown 
in FIG. 8C using the first subtractor 8, the signal freed of the 
fluctuations caused by the stray light, as shown in FIG. 8E, may be 
obtained. 
The same analysis holds for the output of the second photodetector 3. That 
is, a signal shown at FIG. 8D is obtained using the upper envelope 
detector 7, and a signal shown in FIG. 8E, freed of the fluctuations 
caused by the stray light by the second subtractor 9, is produced, and a 
difference signal is found by the third subtractor 10. The low-frequency 
signal of the difference output is used as a tracking error signal freed 
of the offset by the stray light, thus assuring a stable tracking 
operation. 
The tracking control apparatus for the multi-layer optical disc according 
to the present invention is not limited to the above-described embodiments 
and may be effectively employed for reproducing an optical disc having two 
or more layers. If the preset system is used, there may be obtained stable 
tracking characteristics not affected by movement of the stray light on 
the photodetector surface caused by mechanical deviation of the 
photodetector position.