U.S. patent application number 09/745939 was filed with the patent office on 2001-07-05 for optical wavefront modifier.
This patent application is currently assigned to PHILIPS CORPORATION. Invention is credited to Stallinga, Sjoerd, Vrehen, Joris Jan, Wals, Jeroen.
Application Number | 20010006429 09/745939 |
Document ID | / |
Family ID | 8241082 |
Filed Date | 2001-07-05 |
United States Patent
Application |
20010006429 |
Kind Code |
A1 |
Wals, Jeroen ; et
al. |
July 5, 2001 |
Optical wavefront modifier
Abstract
An optical wavefront modifier (27) is adapted for modifying a
wavefront of an optical beam passing through the modifier. The
modifier comprises a first and a second transparent electrode layer
(42, 43) and a flat medium (46) for modifying the wavefront in
dependence on electrical excitation of the medium and arranged
between the electrode layers. The electrode layers are adapted to
impress a first wavefront modification of a first order of a radius
in the cross-section of the beam in the plane of the medium and to
impress simultaneously a second wavefront modification of a second
order of the radius different from the first order.
Inventors: |
Wals, Jeroen; (Eindhoven,
NL) ; Vrehen, Joris Jan; (Eindhoven, NL) ;
Stallinga, Sjoerd; (Eindhoven, NL) |
Correspondence
Address: |
Jack E. Haken
c/o U.S. PHILIPS CORPORATION
Intellectual Property Department
580 White Plains Road
Tarrytown
NY
10591
US
|
Assignee: |
PHILIPS CORPORATION
|
Family ID: |
8241082 |
Appl. No.: |
09/745939 |
Filed: |
December 22, 2000 |
Current U.S.
Class: |
359/254 ;
359/245; G9B/7.065; G9B/7.102 |
Current CPC
Class: |
G11B 7/0956 20130101;
G02F 2203/18 20130101; G11B 7/13927 20130101; G11B 7/1369 20130101;
G02B 27/0068 20130101; G02B 27/0031 20130101; G02F 1/134309
20130101 |
Class at
Publication: |
359/254 ;
359/245 |
International
Class: |
G02F 001/03; G02F
001/07 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 1999 |
EP |
99204525.2 |
Claims
1. An optical wavefront modifier for modifying a wavefront of an
optical beam passing through the modifier, the modifier comprising
a first and a second transparent electrode layer and a medium for
modifying the wavefront in dependence on electrical excitation of
the medium and arranged between the electrode layers, the beam
having a cross-section in the plane of the medium, characterized in
that the electrode layers are adapted to impress a first wavefront
modification of a first order of a radius in the cross-section and
simultaneously a second wavefront modification of a second order of
the radius different from the first order.
2. Optical wavefront modifier according to claim 1, wherein the
first electrode layer comprises an electrode configuration for
effecting the first wavefront modification and the second electrode
layer comprises an electrode configuration for effecting the second
wavefront modification.
3. Optical wavefront modifier according to claim 1, wherein the
first electrode layer comprises an electrode configuration for
effecting both the first wavefront modification and the second
wavefront modification.
4. Optical wavefront modifier according to claim 1, wherein the
second order is one order lower than the first order.
5. Optical wavefront modifier according to claim 4, wherein the
first wavefront modification corresponds to coma and the second
aberration corresponds to astigmatism.
6. Optical wavefront modifier according to claim 4, wherein the
first wavefront modification corresponds to spherical aberration
and the second aberration corresponds to coma.
7. Optical wavefront modifier according to claim 1, wherein the
first wavefront modification and the second wavefront modification
are orthogonal over the cross-section of the beam.
8. A device for scanning an optical record carrier having an
information layer, comprising a radiation source for generating a
radiation beam, an objective system for converging the radiation
beam to a focus on the information layer, and a detection system
for intercepting radiation from the record carrier, characterized
in that an optical wavefront modifier according to any of the
preceding claims is arranged in the optical path between the
radiation source and the detection system.
9. Device according to claim 8, comprising an information
processing unit for error correction.
Description
[0001] The invention relates to an optical wavefront modifier for
modifying a wavefront of an optical beam passing through the
modifier, the modifier comprising a first and a second transparent
electrode layer and a medium for modifying the wavefront in
dependence on electrical excitation of the medium and arranged
between the electrode layers, the beam having a cross-section in
the plane of the medium. The invention also relates to a device for
scanning an optical record carrier having an information layer.
[0002] An optical wavefront modifier is used to change the shape of
the wavefront of a radiation beam by introducing path length
differences in dependence on the position in the cross-section of
the beam. It may be used to change properties of an optical beam
such as its vergence by introducing a focus curvature in the
wavefront of the beam or to change the direction of the beam by
introducing tilt. A wavefront modifier may also operate as a
wavefront compensator for compensating an undesired shape of the
wavefront of an optical beam, e.g. for removing spherical
aberration or coma from a wavefront.
[0003] European Patent Application No. 0 745 980 shows an optical
scanning device provided with a wavefront modifier used as a tilt
compensator. The wavefront compensator is an electrostriction
device arranged in the optical path between the radiation source
and the objective system. It comprises two electrode layers on each
side of an electrostrictive medium. One of the electrode layers
comprises three transparent electrodes, each of which covers part
of the cross-section of the beam in the plane of the medium. The
other electrode layer consists of single transparent electrode
covering the entire cross-section of the beam. The wavefront
modifier is used to introduce coma in the radiation beam in order
to compensate the coma caused by tilt of the record carrier being
scanned by the optical scanning device. It is a disadvantage of the
known wavefront modifier the aberration compensation does not
operate properly when the radiation beam is following a track on
the record carrier.
[0004] It is an object of the invention to provide a wavefront
modifier which provides a good aberration compensation independent
of the tracking of the focus.
[0005] This object is achieved if, according to the invention, the
electrode layers of the wavefront modifier are adapted to impress a
first wavefront modification of a first order of a radius in the
cross-section and simultaneously a second wavefront modification of
a second order of the radius different from the first order. The
invention is based on the insight that a first wavefront
modification, which is centred on the optical axis of the radiation
beam causes other wavefront modifications in the radiation beam
when the objective system is displaced from its centred position in
a transverse direction when following a track. The other
modifications have a radial order different from that of the first
modification. In general, the introduction of a first modification
in a displaced radiation beam requires the introduction of the
first modification and other modifications of different radial
order in the not-displaced radiation beam. The wavefront modifier
according to the invention introduces the first modification and at
least one other modification in the radiation beam. The wavefront
modifications are centred on the axis of the radiation beam, unless
otherwise stated.
[0006] The mathematical function describing spherical aberration
has a radial order of four, coma of three, astigmatism and defocus
of two and wavefront tilt of one.
[0007] The wavefront modifications may be introduced as a decentred
first modification or as a combination of a centred first
modification and a centred second modification. When introduced as
a first and a second modification, the first electrode layer
preferably comprises an electrode configuration for effecting the
first wavefront modification and the second electrode layer
comprises an electrode configuration for independently effecting
the second wavefront modification. Alternatively, the electrode
configuration of one electrode layer may be adapted to effect
independently both the first and second modification. The
independent control of the first and second modification allows a
compensation of a variable amount of displacement of the radiation
beam.
[0008] When the wavefront modification is introduced as a decentred
modification, the electrode configuration of one electrode layer
may be adapted to effect the modification. The amount of decentring
of the configuration is preferably substantially equal to the
displacement of the radiation beam, i.e. equal to the displacement
of the objective system. The electrode configuration may be
relatively simple. An off-centre modification may be described as a
linear combination of a first modification of the same type as the
off-centre modification but centred on the optical axis and a
centred second modification of a lower radial order than the first
modification. This can be explained as follows. The second
modification is the difference between the off-centre modification
and the same centred modification. For a small decentring, the
second modification is proportional to the derivative of the
modification in the radial direction of the decentring. In this
case the radial order of the second modification is at least one
order lower than that of the first modification.
[0009] In a special embodiment the wavefront modifier operates as
an aberration compensator, correcting an undesired aberration in a
radiation beam. When the modifier introduces a decentred
aberration, the amount of the aberration may be controlled by the
output signal of an aberration detector for measuring the
aberration in the radiation beam and the amount of decentring may
be controlled by the output of a position detector for measuring
the displacement of the radiation beam. Alternatively, the amount
of the decentred first aberration is controlled by a combination of
the output signals of the aberration detector and the position
detector, the magnitude of the decentring being fixed, and the sign
of the decentring is controlled by the sign of the output signal of
the position detector. When the modifier introduces a centred first
and second aberration, the amount of the first aberration may be
controlled by the output signal of the aberration detector and the
amount of the second aberration by the output signal of the
position detector.
[0010] The aberration detector may be a tilt detector for detecting
tilt of the record carrier, and the aberration compensator
introduces coma as a first aberration in the radiation beam, which
compensates the coma caused by the tilt. The astigmatism caused by
the off-centre objective system is preferably compensated by
astigmatism introduced as a second aberration by the aberration
compensator.
[0011] In a preferred embodiment, the wavefront modifier introduces
coma and astigmatism by means of two similar, transversely
displaced electrode configuration. Each of these structures
introduces coma in the radiation beam. Since the coma of each
configuration is off-centre, it can be described as a combination
of centred coma and astigmatism. If the objective system is
off-centre in one direction, one set of electrode structures will
be energised; when the objective system is offset in the opposite
direction, the other set of electrodes will be energised.
[0012] In a special embodiment the wavefront modifier comprises a
series of strip electrodes in a symmetrical arrangement. The strips
may have a curved shape. Any combination of aberrations may be
obtained by setting each of the strips at a specific voltage. In a
preferred embodiment the voltages are formed by a series
arrangement of resistors having taps the strip electrodes being
connected to the taps. The desired voltage distribution over the
strips can be achieved by setting the voltage at the end taps and
at least one intermediate tap. The electrode structure is
preferably etched in a transparent conductive layer. The same
conductive layer may be used to form the series arrangement of
resistors.
[0013] The first and second wavefront modifications are preferably
orthogonal over the cross-section of the radiation beam to ensure
that the first and second modification do not affect each other,
thereby improving the independence of the control of the
modifications.
[0014] A further aspect of the invention relates to a device for
scanning an optical record carrier having an information layer,
comprising a radiation source for generating a radiation beam, an
objective system for converging the radiation beam to a focus on
the information layer, and a detection system for intercepting
radiation from the record carrier, characterized in that an optical
wavefront modifier according to any of the preceding claims is
arranged in the optical path between the radiation source and the
detection system.
[0015] The objects, advantages and features of the invention will
be apparent from the following more particular description of
preferred embodiments of the invention, as illustrated in the
accompanying drawings, in which
[0016] FIG. 1 shows a scanning device according to the
invention;
[0017] FIG. 2 shows two laterally displaced comatic wavefront
distortions WD and the difference DIFF between them as a function
of radial position r in the radiation beam;
[0018] FIG. 3 shows a cross-section of an aberration compensator in
the form of a liquid crystal cell;
[0019] FIG. 4A shows an electrode configuration for introducing
decentred coma;
[0020] FIG. 4B shows two superposed electrode configurations for
introducing decentred coma;
[0021] FIG. 5 shows electrical connections between the electrode
configurations of FIG. 4A and a control circuit;
[0022] FIGS. 6A and B show two embodiments of a control circuit for
the electrode configurations of FIG. 4A;
[0023] FIG. 7 shows an electrode configuration for introducing
decentred coma;
[0024] FIG. 8A shows the value of the control voltages on the
electrodes in the configuration of FIG. 7;
[0025] FIG. 8B shows the dependence of the asymmetry factors
p.sub.+and p.sub.-on the displacement of the objective system;
[0026] FIGS. 9 and 10 show a series arrangement of resistors
connecting electrodes in an electrode configuration;
[0027] FIG. 11 shows an electrode configuration for introducing
centred astigmatism;
[0028] FIG. 12 shows a control circuit for an aberration
compensator having electrode configurations for both coma and
astigmatism; and
[0029] FIG. 13 shows an electrode configuration for introducing
spherical aberration.
[0030] FIG. 1 shows a device for scanning an optical record carrier
1. The record carrier comprises a transparent layer 2, on 1 side of
which information layer 3 is arranged. The side of the information
layer facing away from the transparent layer is protected from
environmental influences by a protection layer 4. The side of the
transparent layer facing the device is called the entrance face 5.
The transparent layer 2 acts as a substrate for the record carrier
by providing mechanical support for the information layer.
Alternatively, the transparent layer may have the sole function of
protecting the information layer, while the mechanical support is
provided by a layer on the other side of the information layer, for
instance by the protection layer 4 or by a further information
layer and a transparent layer connected to the information layer 3.
Information may be stored in the information layer 3 of the record
carrier in the form of optically detectable marks arranged in
substantially parallel, concentric or spiral tracks, not indicated
in the Figure. The marks may be in any optically readable form,
e.g. in the form of pits, or areas with a reflection coefficient or
a direction of magnetization different from their surroundings, or
a combination of these forms.
[0031] The scanning device comprises a radiation source 6, for
example a semiconductor laser, emitting a diverging radiation beam
7. A beam splitter 8, for example a semitransparent plate, reflects
the radiation beam towards a collimator lens 9, which converts the
diverging beam 7 into a collimated beam 10. The collimated beam 10
is incident on objective system 11. The objective system may
comprise one or more lenses and/or a grating. The objective system
11 has an optical axis 12. The objective system 11 changes the
collimated beam 10 to a converging beam 13, incident on the
entrance face 5 of the record carrier 1. The converging beam 13
forms a spot 14 on the information layer 3. Radiation reflected by
the information layer 3 forms a diverging beam 15, transformed into
a collimated beam 16 by the objective system 11 and subsequently
into a converging beam 17 by the collimator lens 9. The beam
splitter 8 separates the forward and reflected beams by
transmitting at least part of the converging beam 17 towards a
detection system 18. The detection system captures the radiation
and converts it into electrical output signals 19. A signal
processor 20 converts these output signals to various other
signals. One of the signals is an information signal 21, the value
of which represents information read from the information layer 3.
The information signal is processed by an information processing
unit for error correction 86. Other signals from the signal
processor 20 are the focus error signal and radial error signal 22.
The focus error signal represents the axial difference in height
between the spot 14 and the information layer 3. The radial error
signal represents the distance in the plane of the information
layer 3 between the spot 14 and the centre of a track in the
information layer to be followed by the spot. The focus error
signal and the radial error signal are fed into a servo circuit 23,
which converts these signals to servo control signals 24 for
controlling a focus actuator and a radial actuator respectively.
The actuators are not shown in the Figure. The focus actuator
controls the position of the objective system 11 in the focus
direction 25, thereby controlling the actual position of the spot
14 such that it coincides substantially with the plane of the
information layer 3. The radial actuator controls the position of
the objective lens 11 in a radial direction 26, thereby controlling
the radial position of the spot 14 such that it coincides
substantially with the central line of track to be followed in the
information layer 3. The tracks in the Figure run in a direction
perpendicular to the plane of the Figure.
[0032] The scanning device of FIG. 1 has a relatively large
tolerance range for tilt of the optical record carrier 1. It
thereto determines the aberration caused by the tilted record
carrier in the converging beam 13, and compensates the aberration
by introducing a wavefront distortion in the collimated beam 10.
The wavefront distortion is introduced by a wavefront modifier
operating as an aberration compensator 27 arranged in the
collimated beam 10. A control circuit 28 controls the wavefront
distortion via control signals 29. The value of the aberration to
be compensated is determined by an aberration detector, which, in
this embodiment, is a tilt detector 30. The tilt detector emits a
radiation beam 31 towards the optical record carrier 1 and detects
the angle of the beam reflected by the record carrier. The position
of the spot of the reflected beam is a measure for the angle and,
hence, for the tilt of the record carrier. The measured tilt is
directly proportional to the coma in the converging beam 13. Hence,
the tilt signal 32, i.e. the output signal of the tilt detector 30,
can be used directly as input for the control circuit 28, thereby
controlling the amount of coma introduced by the aberration
compensator 27.
[0033] The tilt detector 30 may be of any type. The tilt signal may
also be derived from a combination of detector output signals 19.
In that case the tilt detector forms part of the control circuit
28.
[0034] The wavefront distortion introduced by the aberration
compensator 27 will only compensate the aberration introduced by
the tilted record carrier if the introduced aberration is correctly
centred with respect to the objective system 11. The compensation
is not correct anymore, if the introduced aberration is centred on
the axis of collimated beam 10 and the objective system is
displaced in a radial direction 26 because of radial tracking. The
effect of this displacement is shown in FIG. 2, giving wavefronts
in that radial cross-section of the radiation beam in which the
objective system 11 has its radial displacement d. The displacement
d is normalised on the radius of the entrance pupil of the
objective system. Drawn curve 37 represents a comatic wavefront
distortion WD, centred on the optical axis of the radiation beam 10
at r=0 and introduced by the aberration compensator 27. The dashed
line 38 represents the comatic wavefront distortion to be
compensated and caused by the tilted optical record carrier 1, and
displaced from the optical axis by a distance d due to a
displacement of the objective system 11. It is clear from the
Figure that, when the displacement d is zero, the introduced
aberration 37 will perfectly cancel the aberration 38, thereby
providing a spot 14 on the information layer 3 of the record
carrier 1 of high quality. When the displacement d is not equal to
zero, the wavefronts 37 and 38 will not cancel each other, thus
causing an imperfect compensation. The resulting wavefront error
DIFF is the difference between curves 37 and 38, shown in FIG. 2 as
line 39. For small displacements d, the difference 39 is
proportional to the derivative of line 37 with respect to the
co-ordinate in the direction of the displacement. The resulting
wavefront error is one radial order lower than the wavefront WD,
and in this case is astigmatism, the value of which is proportional
to the displacement d and the amount of coma to be compensated.
This astigmatism must also be compensated by the aberration
compensator 27. A more detailed analysis of the wavefront errors
shows, that a decentred comatic wavefront not only introduces
astigmatism but also a small amount of wavefront tilt and defocus.
The wavefront tilt and defocus will be corrected automatically by
the radial and focus servo respectively.
[0035] The measurement of the position of the objective system 11
in the radial direction 26, required as input for the aberration
compensation, is performed by a position detector 33 as shown in
FIG. 1. A position signal 34 generated by the position detector is
used as input for the control circuit 28. The position of the
objective system 11 may be measured using any known position
measuring method. An optical method is preferred, because it does
not affect the mechanical properties of the objective system. The
position may also be derived from the detector output signals 19,
as is known inter alia from U.S. Pat. No. 5,173,598 (PHN 13695). In
that case the position detector forms a part of the signal
processor 20.
[0036] FIG. 3 shows an embodiment of the aberration compensator, in
the form of a liquid crystal cell. The cell comprises two plane
parallel transparent plates 40 en 41, made of for instance glass.
On the inner sides of the transparent plates transparent electrode
layers 42 and 43 are arranged. The inner sides of the electrode
layers are covered with alignment layers 44 and 45 respectively. A
nematic liquid crystal material 46 is arranged between the two
alignment layers. The liquid crystal material may be replaced by a
ferro-electric medium, when higher switching speeds are required.
The electrode layer comprises transparent conductors, made of for
instance indium tin oxide. The refractive index of the liquid
crystal material is controlled by the voltage difference between
the electrode layers 42 and 43. Since the refractive index
determines the optical path length through the liquid crystal layer
46, a temporal and/or spatial variation of the voltage difference
can be used to change the wavefront of a radiation beam passing
through the aberration compensator. Although the Figure shows a
medium in the form of a flat liquid crystal layer, the medium may
be curved. The thickness of the medium may vary as a function of
position in the cross-section of the radiation beam, thereby
reducing the requirements imposed on the control voltages.
[0037] FIG. 4A shows the electrode structures in electrode layer 42
and 43. These electrode structures are adapted to introduce
off-centre coma in the radiation beam. The electrode structures
comprise various electrodes in the form of electrically conducting
transparent regions separated by small non-conducting intermediate
regions, not shown in the Figure. The electrode layer 42 comprises
electrodes 51 to 55. Similarly, the electrode layer 43 comprises
electrodes 56 to 60. The Figure shows a plan view of the electrode
layers 42 and 43. The intersection of the optical axis 35 of
collimated beam 10 with the electrode layers is indicated by the
cross 50. The electrode structure is adapted to introduce a comatic
wave front aberration in the radiation beam passing through the
liquid crystal cell in the form of the Zernike polynomial
(3r.sup.3-2r) cos .theta., where r-.theta. are the polar
co-ordinates in the cross-section of the radiation beam. The angle
.theta. is zero along the horizontal direction in the Figure, from
the cross 50 towards electrode 54. The width and position of
electrode 54 is indicated by the dot and dash line 36 in FIG. 2.
The width is chosen such that the electrode 54 covers those regions
of the aberration compensator where the value of the Zernike
polynomial (3r.sup.3-2r) cos .theta. is larger than a predetermined
value `a`. In practice, `a` has a value between 0.1 and 0.35, and
preferably has a value approximately equal to 0.25. The same
applies to the other electrodes. The electrode structure in layer
42 is offset in the left direction with respect to the centre 50.
The electrode structure in the electrode layer 43 is offset to the
right with respect to the centre 50. The amount of offset is
determined by the maximum displacement of the objective system 11.
FIG. 4B shows a plan view of both electrode layers 42 and 43
superposed. The offset of the electrode structures is in the radial
direction 26 as indicated in FIG. 1.
[0038] FIG. 5 shows a cross-section of the electrode layers 42 and
43, where the conductive regions are shown separated from one
another. The electrode 51 and 55 are electrically connected,
likewise electrodes 53 and 54, 56 and 60, 58 and 59 are pairwise
electrically connected. These electrical connections may be made
outside the electrode layers or in the electrode layers by means of
small transparent conductive strips. Although the three regions 52
in FIG. 5 are connected in the layer as indicated by FIG. 4A, FIG.
5 also shows an external connection of the three regions for
clarity's sake only. The cross-section of FIG. 5 is along line A-A
shown in FIG. 4B. The control circuit 28 provides the control
signals 29, six of which are indicated in FIG. 5 by V.sub.1 to
V.sub.6. Control signal V.sub.1 is applied to electrode 52, V.sub.2
to electrodes 51 and 55 and V.sub.3 to electrodes 54 and 53.
Likewise control signal V.sub.4 is connected to electrodes 57,
control signal V.sub.5 to electrodes 56 and 60 and V.sub.6 to
electrodes 59 and 58.
[0039] The aberration compensator 27 may be controlled by applying
various DC voltages to its electrodes. However, it is preferred to
use AC voltages for the control in view of the stable operaition of
the liquid crystal. FIG. 6A shows an embodiment of the control
circuit 28, providing the required AC control voltages. The tilt
signal 32 is used as input for a voltage-to-voltage converter 61,
which provides at its output a first control signal 62, having
value .DELTA.V, dependent on the tilt signal 32. The first control
signal 62 is connected to an adder 63. A voltage source 63 provides
a reference voltage V.sub.0 to the adder 63. The adder has three
output signals D.sub.1, D.sub.2 and D.sub.3, the values of which
are V.sub.0, V.sub.0+.DELTA.V, V.sub.0-.DELTA.V, respectively. The
three output signals D.sub.1-D.sub.3 are used as input for a
multiplier 64. A square-wave generator 64' provides a square wave
signal, having a fixed amplitude and a predetermined frequency,
preferably lying in the range between 1 and 10 kHz. This square
wave signal is used as input for the multiplier 64. The multiplier
provides three AC control signals, A.sub.1, A.sub.2 and A.sub.3, as
output signals. Each of the three output signals has a square-wave
form and a zero average value. The peak-peak amplitude of signal
A.sub.1 is equal to V.sub.0, that of signal A.sub.2 is equal to
V.sub.0+.DELTA.V, that of signal A.sub.3 is equal to
V.sub.0-.DELTA.V. The sign and magnitude of the control signals
A.sub.1-A.sub.3 are such, that, when applied to the aberration
compensator 27, the correct amount of coma is introduced in the
collimated beam 10 to compensate the coma caused by the amount of
tilt of the optical record carrier 1 as represented by the tilt
signal 32. Thereto, the value of .DELTA.V is proportional to the
value of the tilt signal. The control signals A.sub.1, A.sub.2 and
A.sub.3 are connected to six change-over switches 65, which have
the signal V.sub.1 to V.sub.6 as output signals. The six output
signals V.sub.1 to V.sub.6 can be switched between the control
signal A.sub.1, A.sub.2 or A.sub.3 and ground. The switches 65 are
controlled by a switch control circuit 66, which has the position
signal 34 as input. When the position signal is positive, the six
switches 65 are in the positions as shown in FIG. 6A. When the
position signal is negative, the six switches are in their other
positions. Hence, in the drawn position of the switches, the
electrodes 56-60 are all connected to the ground, and varying
voltages are applied to the electrodes 51-55, such that a comatic
wavefront aberration is introduced which is offset to the left hand
side in FIG. 5 with respect to the optical axis 35. When the sign
of the position signal 34 reverses, the electrodes 51-55 are
connected to the ground, and the varying voltages are applied to
the electrodes 56-60. The resulting comatic wavefront aberration is
off centre to the right hand side with respect to axis 35. The
value of the predetermined voltage V.sub.0 depends on the
properties of the aberration compensator 27, in particular the
liquid crystal material, and is chosen such that the response of
the compensator is proportional to .DELTA.V.
[0040] FIG. 6B shows an alternative embodiment of the control
circuit 28. The tilt signal 32 is used as input for a
voltage-to-voltage converter 161, which provides at its output a
first control signal having a value 1/2.DELTA.V, proportional to
the amount of tilt. The tilt signal 32 and the position signal 34
are used as input for a multiplier 160, which provides at its
output a second control signal having a value of
1/2(x/x.sub.0).DELTA.V, where x is proportional to the displacement
of the objective system and x.sub.0 is the maximum displacement of
the objective system. The first and second control signal are fed
into an adder 162, forming two output signals
.DELTA.V.sub.1=1/2.DELTA.V-1/2(x/x.- sub.0).DELTA.V and
.DELTA.V.sub.2=1/2.DELTA.V+1/2(x/x.sub.0).DELTA.V. A voltage source
165 provides a reference voltage V.sub.0 to an adder 163. The adder
also has the signals .DELTA.V.sub.1 and .DELTA.V.sub.2 as inputs
and forms five signals having the values V.sub.0,
V.sub.0-.DELTA.V.sub.1, V.sub.0+.DELTA.V.sub.1,
V.sub.0-.DELTA.V.sub.2 and V.sub.0+.DELTA.V.sub.2. A square-wave
generator 166 provides a square wave signal, having a fixed
amplitude and a predetermined frequency, preferably lying in the
range between 1 and 10 kHz. The square wave signal and the five
signals are used as input for a multiplier 164. The multiplier
provides six AC control signals V.sub.1 to V.sub.6, which are
connected to the aberration compensator 27. Each of the AC control
signals has a square-wave form and a zero average value. The
peak-peak amplitude of the signals V.sub.1 to V.sub.6 is V.sub.0,
V.sub.0-.DELTA.V.sub.1, V.sub.0+.DELTA.V.sub.1, V.sub.0,
V.sub.0-.DELTA.V.sub.2 and V.sub.0+.DELTA.V.sub.2, respectively.
The signals V.sub.1, V.sub.2 and V.sub.3 have the same phase;
likewise V.sub.4, V.sub.5 and V.sub.6. The two groups of signals
may have the same phase or may be mutually 180.degree. out of
phase. The amplitudes of V.sub.0, .DELTA.V.sub.1 and .DELTA.V.sub.2
depend on the properties or the aberration compensator and the
phase between the groups of signals, and are chosen such that the
response of the compensator is proportional to .DELTA.V.sub.1 and
.DELTA.V.sub.2.
[0041] A proper balancing of aberrations in this embodiment of the
aberration compensator requires that the displacement between the
two electrode structures in the electrode layers 42 and 43 is equal
to approximately half the maximum peak-peak displacement of the
objective system 11. If the maximum peak-peak displacement of the
objective system is e.g. from -400 to +400 .mu.m, the displacement
between the electrode structures is preferably 400 .mu.m.
[0042] The match between the wavefront aberration introduced by the
aberration compensator 27 and the Zernike polynomial for coma may
be improved by increasing the number of electrodes in the electrode
layers 42 and 43. FIG. 7 shows an electrode configuration that can
be used in the aberration compensator 27. The electrodes form a
series of small strips with a small spacing, causing a smooth
transition of the refractive index of the liquid crystal material
under one electrode to the refractive index of the liquid crystal
material under the neighbouring electrode. The reduction of the
phase changes between electrodes reduces the higher order
aberrations, even when the objective system 11 is positioned
off-centre. The particular width of the electrodes of the
embodiment, decreasing with increasing radius, as shown in FIG. 7
allows the electrodes to be controlled with a voltage that
increases linearly with the strip of the electrode. If the 2N+1
strips are numbered consecutively with an index running as -N,
-N+1, . . . 0, 1, . . . N, then the strip with index j covers that
area in the (x,y) plane that comply with 1 2 j - 1 2 N + 1 < W
31 ( x , y ) < 2 j + 1 2 N + 1
[0043] W.sub.31(x,y)=(x.sup.2+y.sup.2)x is the Seidel polynomial
for coma, and x,y are normalised co-ordinates in the cross-section
of the radiation beam in the plane of the aberration compensator,
where x is in the direction of displacement of the objective
system. This electrode structure introduces a comatic wavefront
aberration in the beam passing through the aberration compensator.
The aberration is not of the Zernike type but of the Seidel type,
which has the advantage of a simpler layout of the electrodes, each
electrode having a connection outside the cross-section of the
beam, and a simple scheme for the control voltages. The tilt and
defocus, which are inherently introduced into the radiation beam 10
when using Seidel aberrations, will be compensated automatically by
the focus and radial tracking servo of the device.
[0044] The electrode configuration 67 shown in FIG. 7 may be used
in both electrode layers 42 and 43, and displaced with respect to
one another as indicated in FIG. 4B. The control of the voltages of
the electrodes in the two electrode layers can be carried out by a
control circuit similar to the one shown in FIGS. 6A and B. In an
alternative embodiment, the electrode configuration 67 is arranged
in electrode layer 42 and centred on the optical axis 35. The
electrode layer 43 comprises a single electrode covering the entire
cross-section of the radiation beam 10 and set at a fixed
potential. When controlled by a voltage that increases linearly
from one electrode to the next, the electrode configuration will
give rise to centred comatic wavefront aberration. The astigmatism,
required when the objective system 11 is off-centre, can be
introduced by an asymmetric control of the electrodes as indicated
in FIG. 8. FIG. 8 shows the voltage as a function of the electrode
number, where electrode number zero is the central electrode of the
electrode configuration, which is set at a voltage V.sub.0. The
drawn line 68 indicates the linearly increasing voltage for the
generation of centred coma. The dashed line 69 indicates the
voltages for simultaneously generating coma and astigmatism.
[0045] The electrode configuration 67 as shown in FIG. 7 requires a
relatively large number of voltages to be generated by the control
circuit 28. The number of voltages to be generated by the control
circuit can be reduced, if the electrode configuration is provided
with a series arrangement of resistors that forms the required
voltages. FIG. 7 shows a series arrangement made up of resistors
70, the arrangement being provided with a central terminal 71 and
two end-terminals 72 and 73. The three terminals 71, 72 and 73
allow both a control by a linear voltage indicated by drawn line 68
and by a voltage indicated by dashed line 69 in FIG. 8A. A more
accurate control can be obtained if the number of terminals is
increased. The voltages applied to end terminals 72 and 73 may be
chosen asymmetrical with respect to the voltage V.sub.0 on the
central terminal 71. The voltage Vj on strip j can then be written
as 2 V j = V 0 - p j N V ,
[0046] where .DELTA.V is equal to (V.sub.+N-V.sub.-N)/2N if there
is no displacement of the objective system. The asymmetry factor
p.sub.+is used for j.gtoreq.0 and p.sub.-for j<o for one sign of
the tilt signal; p.sub.-is used for j.gtoreq.0 and p.sub.+for
j<0 for the other sign of the tilt signal. The values of the
factors depend on the displacement d as indicated in FIG. 8B, where
the drawn line represents the values of p.sub.+and the dashed line
those of p.sub.-.
[0047] FIG. 9 shows an electrode configuration wherein the series
arrangement of resistors is integrated in the conductive layer of
the electrode layer. The embodiment has five electrodes 76-80
separated by small non-conductive strips. The three terminals 81,
82 and 83 are connected by four resistors 84, formed by strips of
conductive layer, connected in series between the terminals. A high
resistance can be obtained by decreasing the width of the strips
that make up the resistors 84. Five taps 85 connect the resistors
with the electrodes.
[0048] FIG. 10 shows an alternative embodiment of the electrode
configuration of the aberration compensator 27. The configuration
comprises a structure 88 for generation of coma, similar to the
structure shown in FIG. 7. The individual strips of the structure
88 are connected by taps in the form of strips 89 to a resistor
maze 90. The maze forms resistors of equal value between subsequent
taps 89. The control voltages are applied to the configuration
through terminals 91, 92 and 93. The extent of the maze may be
reduced by arranging these strips of the maze in a zigzag
structure. Alternatively, the strips of the maze may be arranged
around the structure 88.
[0049] The resistor of the elements in the maze must be
sufficiently large to ensure a tolerable low level of dissipations
and sufficiently small to ensure an RC-time of the cell that is
much smaller than the period of the AC-voltage.
[0050] The coma caused by tilt of the record carrier 1 may also be
compensated by an aberration compensator that introduces centred
coma and centred astigmatism. Thereto, the aberration compensator
27 is provided with electrode layer 42 having an electrode
configuration for introducing centred coma, and electrode layer 43
having an electrode configuration for introducing centred
astigmatism. The electrode configuration for centred coma is
similar to the electrode configuration 42 shown in FIG. 4A but
centred on the intersection 50 of optical axis 35. The centred coma
may also be introduced by the electrode configuration 67 shown in
FIG. 7.
[0051] FIG. 11 shows an electrode configuration 95 in electrode
layer 43 for introducing centred astigmatism. The electrode pattern
is centred on the optical axis 35. A circle 96 in the electrode
configuration indicates the cross-section of the radiation beam in
the plane of the configuration. The electrodes in both electrode
layers may be confined to the area within the beam cross-section
96, or may extend outside the beam cross-section. The configuration
95 is adapted to introduce astigmatism in the Zernike form, which
can be described as Z.sub.22=x.sup.2-y.sup.2. The normalised
co-ordinates x,y are indicated in the Figure. This Zernike form for
astigmatism is particularly suitable for an aberration compensator
which also introduces coma in the Seidel form. In its simplest
form, the electrode configuration comprises a central electrode 97
and four side electrodes 98-101. The position of the border between
the electrodes and the control voltages is determined as follows.
The points in the configuration with Z.sub.22(x,y)>a are set at
a voltage V.sub.10=V.sub.0-.DELTA.V. The points in the
configuration with -a<Z.sub.22(x,y)<a are set at a voltage
V.sub.11=V.sub.0'. The points in the configuration with
Z.sub.22(x,y)<-a are set at a voltage
V.sub.12=V.sub.0'+.DELTA.V. The voltage .DELTA.V is proportional to
the amount of astigmatism to be introduced. The value of the
parameter a is preferably in the range from 0.10 to 0.60, and, more
preferably, substantially equal to 0.25. The electrode
configuration shown in FIG. 11 is based on a=0.25.
[0052] FIG. 12 shows a control circuit for the electrical control
of an aberration compensator having both the electrode
configuration 67 for introducing coma in the radiation beam passing
through the compensator and the electrode configuration 95 for
introducing astigmatism in the beam. The control circuit can also
be used if the electrode configurations have both a Zernike layout
or both a Seidel layout. The control of the coma configuration 67
is similar to the control shown in FIGS. 6A and B. Hence, voltage
converter 105, first control signal 106, adder 107 and voltage
source 108 are similar to the corresponding elements 61 to 64' in
FIGS. 6A and B. The DC output signals D.sub.4, D.sub.5 and D.sub.6
of adder 107 correspond to the output signals D.sub.1, D.sub.2 and
D.sub.3, respectively as shown in FIG. 6A. The control of the
astigmatism configuration 95 uses the tilt signal 32 and the
position 34 as input signals. A multiplier 109 forms the product of
the two signals. The product is a measure for the astigmatism
introduced into the radiation beam by the combination of a centred
comatic aberration introduced by the wavefront compensator 27 and a
displaced objective system 11. The product is output as a second
control signal 110 and used as input for an adder 111. A voltage
source 112 supplies a voltage V.sub.0' to the adder. The adder has
three DC output signals D.sub.7, D.sub.8 and D.sub.9, having the
values V.sub.0'+.DELTA.V, V.sub.0', V.sub.0'-.DELTA.V respectively,
where .DELTA.V is the value of the second control signal 110. The
DC output signals D.sub.4 to D.sub.9 are connected to a multiplier
113, which forms output signals V.sub.7 to V.sub.12. A square-wave
generator 114, similar to the square-wave generator 64' in FIG. 6A,
supplies a square wave signal to the multiplier 113. The multiplier
113 multiplies each of the six input signals D.sub.4 to D.sub.9
with the square-wave signal, resulting in six square wave output
signals V.sub.7 to V.sub.12, respectively, having the wave form of
the output of the square-wave generator 114 and an amplitude
corresponding to the signals D.sub.4 to D.sub.9. The output signals
V.sub.7, V.sub.8 and V.sub.9 are similar to the output signals
A.sub.1, A.sub.2 and A.sub.3, respectively, in FIG. 6A, and are
connected to the terminals 71, 72 and 73, respectively, of the
electrode configuration 67 shown in FIG. 7. The output voltage
V.sub.10 is connected to side electrodes 98 and 100 of electrode
configuration 95 shown FIG. 11. The output voltage V.sub.11 is
connected to the central electrode 97, and the output voltage
V.sub.12 is connected to the side electrodes 99 and 101.
[0053] The aberration compensator 27 in the above described
embodiments compensates coma caused by tilt of the record carrier
1, taking into account the position of the objective system 11. The
position of the objective system can also be taken into account for
compensators that introduce aberrations other than coma, for
instance spherical aberration, caused for instance by variations in
the thickness of the transparent layer 2 of the record carrier one.
When an optical beam in which centred spherical aberration has been
introduced, passes through a displaced objective system, the beam
after passage through the objective system will suffer from coma
which is linear in the displacement and astigmatism which is
quadratic in the displacement of the objective system. The
compensation of the spherical aberration can be corrected for the
displacement of the objective system in a way similar to the
correction in the above described embodiments of the aberration
compensator. A first adapted embodiment of the aberration
compensator comprises a first electrode layer having an electrode
configuration for generating spherical aberration as shown in FIG.
13, the centre of which is displaced with respect to the
intersection of the optical axis 35 with the electrode layer, and a
second electrode layer having a similar electrode configuration for
generating spherical aberration, but displaced in a direction
opposite to the configuration in the first electrode layer. A
second embodiment of the aberration compensator comprises a first
electrode layer having an electrode configuration for generating a
centred spherical aberration, and a second electrode layer having
an electrode configuration for generating centred coma. The two
electrode configurations may also be combined into a single
electrode layer. A third embodiment of the aberration compensator
comprises three electrode layers and two liquid crystal layers
between them. One layer is provided with an electrode configuration
for generating centred spherical aberration. A second layer is
provided with a configuration for generating centred coma and a
third layer with a configuration for generating centred
astigmatism. The aberration compensator is controlled by the
position signal 34 representing the position of the objective
system and a signal representing the amount spherical aberration in
the radiation beam returning from the record carrier. A sensor for
measuring the spherical aberration in the radiation beam is
described in the European Application having filing number
98204477.8 (PHN 17.266). The amount of spherical aberration may be
predetermined and appropriate for the thickness of the transparent
layer.
[0054] FIG. 13 shows a electrode configuration 116 for generating
spherical aberration. The Zernike representation of the aberration
is Z.sub.40=6(x.sup.2+y.sup.2) (x.sup.2+y.sup.2-1)+1. The borders
between the electrodes and the voltages applied to them can be
derived as follows. The points in the configuration with
Z.sub.40(x,y)>a, i.e. the central area 117 and the ring 121, are
set at a voltage V.sub.0-.DELTA.V. The points in the configuration
complying with -a<Z.sub.40(x,y)<a, i.e. the rings 118 and
120, are set at a voltage V.sub.0. The points in the pupil with
Z.sub.40(x,y)<-a, i.e. the ring 119, are set at a voltage
V.sub.0+.DELTA.V. The parameter `a` is preferably in the range from
0.20 to 0.70. The electrode configuration shown in FIG. 13 is for
a={square root}{square root over (3)}/4=0.433. This value of a
gives equal surface areas for the electrodes to which a voltage
V.sub.0-.DELTA.V is applied and those to which V.sub.0+.DELTA.V is
applied. The electrode configuration for generating spherical
aberration may be simplified by forming three concentric rings and
applying different voltages to them.
[0055] The electrode configurations for generating spherical
aberration, coma and/or astigmatism may be combined into a single
electrode configuration by a suitable division of the electrode
layer into separate electrodes and a corresponding adaptation of
the control circuit. The aberration compensator may comprise one
electrode layer for introducing two aberrations, e.g. coma and
astigmatism, and one electrode layer for introducing another
aberration, e.g. spherical aberration.
[0056] The above embodiments of the wavefront modifier operate as
aberration compensators. It will be clear, that the wavefront
modifier can also be used to introduce a desired wavefront shape in
a radiation beam without correcting an undesired wavefront shape.
As an example, the modifier may introduce defocus as a first
wavefront modification of the radiation beam and tilt as a second
wavefront modification, correcting for a displacement of the
radiation beam. Such a modifier may be used in a focus servo loop
together with an objective lens being capable of axial movement,
the modifier and the objective lens controlling the axial position
of the focus spot in two different frequency ranges.
* * * * *