Optical pickup for optical disk having multiple recording layers

An optical pickup for all optical disk having multiple recording layers is provided to exactly control a focal position according to the thickness of optical disk, by separately controlling focal positions on each recording layer, wherein a Fresnel lens for diffracting an optical beam generated from an optical source and an objective lens for focusing the diffracted beam are separately located, and focus driving coils are included for separately driving the Fresnel lens and objective lens by using a signal detected by an optical detector. As the interval between the Fresnel lens and objective lens is varied, the interval between focal points formed on each recording layer is also varied. Thus, the focal positions formed on each recording layer can be exactly controlled according to the thickness of optical disk. Therefore, information can be clearly reproduced without interference by simultaneously scanning each recording layer while exactly controlling the focal positions.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical pickup for an optical disk 
having multiple recording layers, and more particularly to, an optical 
pickup for an optical disk having multiple recording layers which are 
simultaneously scanned for recording or reading out information on or from 
the multiple recording layers of optical disk. 
The optical disk is widely known as an optical recording medium, which has 
multi-optical layers in which two or more recording layers on which 
information is recorded are overlapped. Also, an optical pickup from which 
the recorded information is read out by simultaneously scanning the 
multiple recording layers thereof is well known, as compared with the 
optical disk having such multiple recording layers. 
FIG. 1 shows a conventional optical pickup having dual recording layers. An 
optical beam generated from an optical source 1 is collimated by a 
collimating lens 2. The collimated beam is reflected by a beam splitter 3 
and transmitted to a Fresnel lens 4 as a diffraction element. Then, the 
transmitted light is incident to an objective lens 5. Thus, a plurality of 
beams diffracted by Fresnel lens 4 are incident to objective lens 5, so 
that beam spots are respectively formed on recording layers 7 and 8 of 
optical disk 6. 
The beam reflected from recording layers 7 and 8 of optical disk 6 passes 
through beam splitter 3 via objective lens 5 and Fresnel lens 4. The 
reflected beam having passed through beam splitter 3 is detected as an 
electrical signal by an optical detector 11 via a condensing lens 9 and a 
sensor lens 10. 
In the conventional optical pickup as described above, Fresnel lens 4 and 
objective lens 5 are jointly set in a body tube 12. Focus driving coils 13 
are attached to both sides of body tube 12 and permanent magnets 14 for 
generating magnetic flux are arranged on the exterior of body tube 12 near 
focus driving coil 13. Focus driving coils 13 vertically moves together 
with body tube 12 by the focus error signal extracted from the signal 
detected by optical detector 11. That is, objective lens 5 is driven 
together with Fresnel lens 4 so as to compensate a focusing error of 
objective lens 5 with respect to optical disk 6, which is caused due to 
the vertical vibration of the optical disk. 
In this structure where Fresnel lens 4 is driven together with objective 
lens 5, since two beam spots respectively formed on recording layers 7 and 
8 of optical disk 6 move in unison, there is a constant interval between 
the beam spots. The interval between recording layers 7 and 8, however, is 
varied due to variations in the thickness of the recording layers, caused 
by molding imperfections or deformation such as warpage. 
Thus, in the above-described conventional optical pickup, the focal 
positions of beam spots formed on the multiple recording layers cannot be 
exactly adjusted, so that the optical pickup cannot be used for an optical 
disk having the multiple recording layers. 
Also, according to the conventional optical pickup, since the two beam 
spots formed on the multiple recording layers of the optical disk 
originate from a single optical source, information cannot be 
independently recorded on two recording layers by an optical modulation 
method, so that the optical pickup can be applied only for reproduction 
and not for recording. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an optical pickup for 
an optical disk having multiple recording layers in which the focal points 
formed on the multiple recording layers can be separately adjusted. 
It is another object of the present invention to provide an optical pickup 
for an optical disk having multiple recording layers on which and from 
which information can be recorded and reproduced. 
To achieve the above object of the present invention, there is provided an 
optical pickup for an optical disk having multiple recording layers, 
comprising: an optical source for generating an optical beam; a 
diffraction optical element for diffracting the optical beam generated 
from the optical beam generator; an objective lens for focusing the beam 
diffracted by the diffraction optical element to form spots on each of the 
multiple recording layers; optical detecting means for detecting a 
predetermined electrical signal from the beam reflected from the optical 
disk; and driving means for independently driving the diffraction optical 
element and the objective lens by using the signals detected from the 
optical detecting means, to thereby exactly form focal points on each 
multiple recording layer. 
Also, the optical pickup according to the present invention comprises a 
detector for dividing beam reflected from each of multiple recording 
layers and detecting each signal from the divided reflected beam, so as to 
detect a signal for driving the objective lens and the diffraction optical 
element. 
To provide an optical pickup for an optical disk having multiple recording 
layers on or from each of which information is recorded or reproduced, 
there is provided an optical pickup for an optical disk having multiple 
recording layers, comprising: a plurality of optical sources for 
generating optical beams whose polarization directions are different from 
each other; a polarization diffraction optical element for 
polarization-diffracting the optical beam generated from the optical 
source; an objective lens for forming spots on each multiple recording 
layers by focusing the beam diffracted by the polarization diffraction 
optical element; a polarized beam splitter for polarization-dividing the 
beam reflected from the multiple recording layers according to the 
polarization direction; a plurality of optical detectors for detecting 
signals by receiving the reflected beam divided by the polarized beam 
splitter; and driving means for separately driving the polarization 
diffraction optical element and the objective lens by using the signals 
detected by a plurality of optical detectors.

DETAILED DESCRIPTION OF THE INVENTION 
According to the preferred embodiment shown in FIG. 2, the optical 
arrangement is the same as that of the conventional optical pickup shown 
in FIG. 1. However, the optical pickup of FIG. 2 further comprises a 
driving portion for driving a Fresnel lens 4 and an objective lens 5 along 
the optical axis, which is constructed differently from the conventional 
optical pickup of FIG. 1. That is, Fresnel lens 4 and objective lens 5 are 
separated from each other and focus driving coils 13A and 13B are wound 
around Fresnel lens 4 and objective lens 5, respectively. Also, permanent 
magnets 14A and 14B for generating a magnetic flux field in focus driving 
coils 13A and 13B are arranged in parallel with focus driving coils 13A 
and 13B, respectively. Here, permanent magnets 14A and 14B may be a single 
permanent magnet which will be described later. 
In FIG. 2, as described above, the optical beam generated from an optical 
source 1 is collimated by a collimating lens 2. The collimated optical 
beam is reflected by a beam splitter 3 and transmitted to a Fresnel lens 4 
as a diffraction element. Then, the transmitted light is incident to an 
objective lens 5. Objective lens 5 focuses the beam diffracted by Fresnel 
lens 4. Two beam spots selected from diffracted beams focused by objective 
lens 5 are formed on recording layers 7 and 8 of optical disk 6, 
respectively. 
The beams reflected from recording layers 7 and 8 of optical disk 6 pass 
through beam splitter 3 via objective lens 5 and Fresnel lens 4. The beam 
is detected as an electrical signal by optical detector 11 via a 
condensing lens 9 and a sensor lens 10. The signal detected by optical 
detector 11 is divided into signals corresponding to the data recorded on 
each of recording layers 7 and 8 and focus and track error information by 
a general circuit (not shown). Focus driving coil 13A for Fresnel lens 4 
and focus driving coil 13B for objective lens 5 are excited according to 
the focus error signal, to thereby be moved together with the lens 
corresponding to each. Also, a track driving coil (not shown) may be 
installed corresponding to each of Fresnel lens 4 and objective lens 5, 
such that the lenses are driven by the above track error signal for a 
tracking control. 
FIG. 3 shows a diffraction pattern of Fresnel lens 4. The diffraction 
pattern is composed of numerous concentric circles having a gradually 
widened spacing from the circumference down to an inner circle being a 
predetermined distance from the center. As shown in FIG. 4, the concentric 
circles have an alternately protruded and recessed cross-section. After 
collimated beam 15 is incident to Fresnel lens 4, the incident beam is 
diffracted into multiple beams, wherein the emitted beams include a 
zero-order beam 16A which is the incident beam passed through Fresnel lens 
without diffraction, i.e., transmittance, and symmetrical .+-. first-order 
beams 16B and 16C, which are diverged and converged at angles. 
Referring to FIG. 5, beams 16A, 16B and 16C diffracted by Fresnel lens 4 
form focal points 17A, 17B and 17C which are spaced on the same optical 
axis by objective lens 5. This plurality of focal points each of which may 
be formed on a recording layer of the optical disk by adjusting the 
diffraction rate of Fresnel lens 4 and the magnification power of 
objective lens 5. Here, since Fresnel lens 4 and objective lens 5 are 
separated from each other, the intervals among focal points 17A, 17B and 
17C formed on the optical axis can be varied when the interval between 
Fresnel lens 4 and objective lens 5 is varied by driving Fresnel lens 4 
and objective lens 5 using focus driving coils 13A and 13B and permanent 
magnets 14A and 14B. That is, the focal points can be exactly formed on 
each recording layer by properly driving each of focus driving coils 13A 
and 13B according to thickness of the optical disk shown in FIG. 2. 
FIG. 6 schematically shows the structure of the above-described driving 
portion according to a preferred embodiment of the present invention. 
Fresnel lens 4 and objective lens 5 are floated together with holders 18A 
and 18B for holding each lens while being separated from each other. Focus 
driving coils 13A and 13B are wound around holders 18A and 18B and a pair 
of permanent magnets 14' shared by focus driving coils 13A and 13B are 
arranged opposing each other. Permanent magnets 14' are supported by a 
yoke 19 on the inner sides of which the magnets are attached. A hole 20 in 
the center of yoke 19 is provided as a path through which the incident 
beam passes. 
FIG. 7 shows an optical pickup according to another preferred embodiment of 
the present invention. According to this preferred embodiment, the optical 
pickup comprises a polarization Fresnel lens 4', a polarization beam 
splitter 21 for polarization-dividing the beams reflected from multiple 
recording layers 7 and 8 of optical disk 6, two condensing lenses 9A and 
9B for detecting signals from each of two divided reflected beams and two 
optical detectors 11A and 11B. 
The beams of zero- and first-order 16A and 16B whose polarization 
directions are perpendicular with respect to each other are diffracted by 
polarization Fresnel lens 4'. Also, the polarizations of the beams 
reflected from each of recording layers 7 and 8 of optical disk 6 are 
perpendicular with respect to each other. The reflected beams are divided 
into two beams by polarization beam splitter 21: one being reflected from 
recording layer 7 and the other being reflected from recording layer 8. 
The two signals detected, without mutual interference, by optical 
detectors 11A and 11B include such information as the recorded data and 
focal positions. Thus, these two signals via a circuit means (not shown) 
separately drive focus driving coil 13A of polarization Fresnel lens 4' 
and focus driving coil 13B of objective lens 5, to thereby exactly control 
the focal positions on each recording layers 7 and 8 according to the 
thickness of optical disk 6. 
Next, FIG. 8 shows an optical pickup according to still another preferred 
embodiment of the present invention. This optical pickup is for both 
recording and reproducing such that information can be recorded on each of 
recording layers 7 and 8 and can be read out therefrom as well, by 
applying the optical pickup shown in FIG. 7. 
According to this preferred embodiment, an optical disk 6' having 
magneto-optical recording layers 7' and 8' is preferably used. A first 
optical source 1A generates P-polarized light. After the P-polarized light 
is collimated by a collimating lens 2A, the collimated beam is reflected 
by a beam splitter 22A and passes through polarized beam splitter 23. 
Then, the beam is focused on a first recording layer 7' of optical disk 6' 
as a spot, via a polarization Fresnel lens 4' and an objective lens 5. The 
beam reflected from first recording layer 7' passes through polarized beam 
splitter 23 and beam splitter 22A via objective lens 5 and polarization 
Fresnel lens 4' in sequence. Then, the beam passed through beam splitter 
22A is detected as an electrical signal by first optical detector 11A via 
condensing lens 9A and sensor lens 10A. On the other hand, a second 
optical source 1B generates S-polarized light. After the S-polarized light 
is collimated by a collimating lens 2B, the collimated lens passes through 
beam splitter 22B and is reflected by polarized beam splitter 23. Then, 
the reflected light is focused as a spot on a second recording layer 8' of 
optical disk 6' via Fresnel lens 4' and objective lens 5. The beam 
reflected from second recording layer 8' passes through objective lens 5 
and polarization Fresnel lens 4' in sequence. Then, the beam having passed 
through polarization Fresnel lens 4' is reflected by polarized beam 
Splitter 23 and beam splitter 22B in sequence and detected as an 
electrical signal by a second optical detector lib via condensing lens 9B 
and sensor lens 10B. Focus driving coil 13A of polarization Fresnel lens 
4' and focus driving coil 13B of objective lens 5 are driven by the 
signals of first and second optical detectors 11A and 11B, as described 
above. 
According to this optical pickup, since the optical disk uses different 
optical sources according to the recording layers, information can be 
separately recorded on each of recording layers. Also, the data recorded 
on each of recording layers can be read out by separately or 
simultaneously scanning each recording layer during reproduction. 
Especially, in recording and reading out information, since polarized 
light of opposite polarizations are used, information can be recorded and 
read out without interference, to thereby record and reproduce information 
without noise. 
As described above, in the construction of the optical pickup for an 
optical disk having multiple recording layers according to the present 
invention, there is provided an optical pickup having a driving portion 
for varying the interval between the diffraction optical element (Fresnel 
lens or polarization Fresnel lens) and the objective lens, wherein the 
diffraction optical element and the objective lens are spaced from each 
other, to thereby simultaneously scan each recording layer while forming 
focal points on each recording layer according to the thickness of the 
optical disk. As a result, when information of each recording layer is 
reproduced, the reproducing operation is stably performed without error 
and information is clearly reproduced without noise. 
Also, in the optical pickup according to the present invention, an optical 
system is independently constructed in each recording layer of an optical 
disk, together with the driving portion, so that information can be 
independently recorded on and read out from each recording layer. 
Therefore, there is provided an optical pickup for recording as well as 
reproducing.