Proceedings of the 6th International Workshop on Adaptive Optics for Industry and Medicine. Ed. 1
J.C. Dainty. Galway, June 2007.
CONVERGING AND DIVERGING LIQUID CRYSTAL LENSES
A. K. KIRBY, P. J. W. HANDS, G. D. LOVE
Durham University, Dept. of Physics,
Durham, DH1 3LE, UK
We report on recent work on the application of liquid crystals to variable lenses. We
demonstrate the construction and operation of a novel and simple form of modal LC lens
which offers both converging and diverging modes of operation, as well as tip/tilt and
astigmatism.
1. Introduction
Considerable work has been carried out in the field of electronically variable
lenses (see, e.g. [1-3] and these proceedings). We have reported previously on
the design and production of modal liquid crystal (LC) lenses [4-5] and their use
as adaptive focus elements. We have recently developed a new LC device
which overcomes some of the limitations of the modal LC lens;
•
•
The fabrication of the modal LC lens is complicated by the requirement
for a high-resistance electrode, which is normally achieved by
depositing an extremely thin layer of Indium Tin Oxide (ITO) onto a
glass substrate. Producing the required thickness with good uniformity
is problematic, and the deposition process typically has a low yield.
The new LC device has a simple construction and requires only
medium-resistance ITO coated glass substrates, which are rather easier
to manufacture.
A normal modal LC lens can be driven to provide a positive
(converging) lens of variable power. The new device can also be
driven to provide a lens which can be varied from positive to negative
optical powers. The effective optical throw of the lens, for a given cell
thickness, is doubled compared to the modal lens. Correspondingly the
relaxation time, for a given lens throw, is reduced by a factor of
approximately 4.
2Proceedings of the 6th International Workshop on Adaptive Optics for Industry and Medicine. Ed.
J.C. Dainty. Galway, June 2007.
•
The optical power of the device along x- and y-axes can be controlled
independently, allowing tip, tilt and limited astigmatic correction.
2. Construction and operation
The basic principle of operation of the device is illustrated in figures 1 and 2.
One substrate of the device is grounded (lower electrode, as shown), and timevarying voltages (V1, V2) are applied to the opposite sides of the other (upper)
electrode. If the V1 & V2 are out of phase ( φ =1800) then the RMS of the field
between the upper and lower electrodes takes on the form illustrated by the
dashed line in figure 2. The solid line indicates the equivalent optical phase
LC
Glass substrate
V2
V1
Electrodes
shift.
Figure 1. 1-D schematic and 3D illustration of LC device
16
.
14
Vrms/HeNe waves
12
10
8
6
4
2
0
-5
-4
-3
-2
-1
0
1
2
x(mm)
Figure 2. Phase (solid) and voltage (dashed) profiles for ΔφV1-V2 =1800
3
4
5
Proceedings of the 6th International Workshop on Adaptive Optics for Industry and Medicine. Ed. 3
J.C. Dainty. Galway, June 2007.
By altering the relative phase and voltages of V1 and V2 ( φ) and by the addition
of a bias-voltage term the shape and scale of the voltage profile, and hence the
phase profile, can be adjusted – particularly the ‘plateau’ in the centre of the
phase profile can be eliminated.
All liquid crystals exhibit an inversion of the dielectric anisotropy at some
driving field frequency. For most materials this ‘crossover frequency’ is
impractically high to be of use – typically several MHz, at which frequency
dielectric losses in the material cause heating and usually melting, taking the
material out of the liquid crystal phase. There do exist some materials,
collectively known as dual-frequency liquid crystals, which have a low
crossover frequency, typically a few kHz. Using on of these materials (Niopik
LC1001), we can produce a negative (diverging lens).
The effect of driving with a field frequency above it’s crossover frequency is
that the LC molecules will tend to realign normal to the applied field, i.e. in the
plane of the cell, whereas in the more conventional case of driving with a field
frequency below the crossover frequency, the molecules tend to align with the
applied field. In other words, driving with a low frequency causes the cell to
switch ‘on’ and driving with a high frequency causes the cell to switch ‘off’.
This mechanism is normally used to improve switching times of cells; however
it can be used to produce a negative-lens form. If both high and low frequency
fields are applied simultaneously, the LC molecules respond essentially to the
difference in the torques and settle at an equilibrium position.
If the cell is biased using a low-frequency drive voltage with no inter-electrode
phase difference ( φ =0), and a high-frequency drive voltage with φ=1800 are
applied simultaneously, then the voltage difference profile, VLF-VHF, and the
corresponding optical phase profile, take on the form shown in figure 3.
4Proceedings of the 6th International Workshop on Adaptive Optics for Industry and Medicine. Ed.
J.C. Dainty. Galway, June 2007.
12
3.5
3
10
Vrms
2
6
1.5
4
HeNe waves
2.5
8
Vlf-Vhf
Optical
1
2
0.5
0
0
0
2
4
x (mm)
6
8
10
Figure 3. Production of negative (diverging) lens, using dual-frequency LC materials. Solid line
represents the difference between the low frequency bias voltage and the high-frequency structure
voltage. The dashed trace represents the corresponding optical phase profile.
3. Results
Figures 4-6 show interferograms and unwrapped phase profiles for converging
lens operation with varying drive voltages.
5
4
HeNe waves
3
2
X-profile
Y-profile
1
0
0
2
4
6
8
10
-1
-2
x(mm)
Figure 4. Interferogram and unwrapped x- and y- phase profiles for Vrms(x,y)=5.65V,
Vrms(bias)=2.12V, φ =1800
Proceedings of the 6th International Workshop on Adaptive Optics for Industry and Medicine. Ed. 5
J.C. Dainty. Galway, June 2007.
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7
6
HeNe waves
5
4
X profile
Y profile
3
2
1
0
0
2
4
6
8
10
-1
-2
x(mm)
Figure 5. Interferogram and unwrapped x- and y- phase profiles for Vrms(x,y)=5.65V,
Vrms(bias)=2.82V, φ =1800
16
14
12
HeNe waves
10
8
X-profile
6
Y-profile
4
2
0
0
2
4
6
8
10
-2
-4
x(mm)
Figure 6. Interferogram and unwrapped x- and y- phase profiles for Vrms(x,y)=4.60V,
Vrms(bias)=3.54V, φ =1800
Both the positive and negative (converging and diverging) lens operation of the
device and independent control of x- and y- axes is demonstrated in Figure 7,
which shows the production of astigmatism.
6Proceedings of the 6th International Workshop on Adaptive Optics for Industry and Medicine. Ed.
J.C. Dainty. Galway, June 2007.
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12
10
HeNe waves
8
X-profile
6
Y-profile
4
2
0
0
2
4
6
8
10
-2
x(mm)
Figure 7. Interferogram and unwrapped x- and y- phase profile using drive voltages of
VxLF=5.65 rms, V xLF_bias=2.12Vrms, φx =φy =1800, VyLF=6.7 Vrms, V yHF=2.35 Vrms
Acknowledgments
The original idea of this method of addressing LCs was proposed by Dr.
Alexander Naumov. His help and enthusiasm for LC adaptive optics is
gratefully acknowledged. This work was funded by the EPSRC and the EU
Eurocores SONS programme.
References
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