2009 Issue 1 - Raytheon

E
Electric Field Oscillation
of Incident Laser Beam
Liquid crystals are useful in laser-beam
steering because they can produce an electronically
controlled optical phase change of
up to 2 π. This change is caused solely by
the action of the LC material rather than by
differing lens thicknesses. Figure 2 shows
the extreme orientations of the molecules:
with maximum voltage applied across the
device (red) and with no voltage (blue).
An incident laser beam impinging upon the
front surface of a powered OPA device
(Figure 1) encounters spatially distributed LC
molecules in various degrees of rotation.
The laser beam’s electric field oscillates
along the long axis of the molecules having
no voltage, but along the short axis of the
fully rotated molecules (seen in Figure 2).
This difference results in essentially two different
materials having different refractive
indices: the extraordinary (nE ) and ordinary
(nO ) indices, respectively. The difference
between these refractive indices is defined
as birefringence — a temperature-dependent
property existing only in the liquid crys-
k
ne
Striped
Electrodes
Transparent
Substrates
no
Common
Electrode
Figure 2. An electric field incident upon the relaxed LC (blue) passes through a material with
the extraordinary refractive index, but an electric field incident upon the reoriented LC (red)
passes through a material with the ordinary refractive index.
tal phase — and this is the second defining
molecular characteristic of LC. Increasing
birefringence improves beam steering by
allowing the necessary phase change to
be accumulated over a shorter propagation
distance through the cell, thereby allowing
the device’s thickness to be reduced. In the
UCF research, birefringence is improved by
elongating the π-electron conjugation of
the molecule.
The last defining characteristic of LC
material is its molecular-restoring forces.
Applying voltage potential causes LC molecules
to rotate, but, just as importantly,
removing the voltage allows the molecular
restoring-forces to return LC molecules to
their resting orientation. For the operational
mode used by OPAs, the elastic splay constant
and rotational viscosity are the primary
determining forces to be considered. The
elastic splay constant is a measure of the
ease with which a material can be made to
move (as the LC molecule rotates and reorients).
The rotational viscosity value is a
EO/LASERS
measure of the material’s resistance to
movement. These can be grouped into the
temperature-dependent visco-elastic coefficient.
The lower the coefficient, the more
easily the LC material can move, which
increases switching speed.
The materials designed by the UCF team
minimize the visco-elastic coefficient by limiting
the molecular weight and cross-sectional
area of the LC molecules. Combining
the three defining characteristics, LC molecules
for beam steering and adaptive optics
can be compared by using a figure of merit
(FoM), which is defined as birefringence
squared divided by the visco-elastic coefficient
and plotted against temperature over
the range in which the molecule exists as a
liquid crystal. The FoM is proportional to the
switching speed of a device that is onewavelength
thick.
The liquid crystal material produced by the
UCF research has an optimal FoM of 45,
versus a 3.9 FoM for the commercial liquid
crystal mixture E7. The improved material
has enabled Raytheon’s OPA group to
steadily reduce the switching time and
improve steering efficiency over the
past six years, and to capture new business
in high-energy-directed weapons and
laser communications.
Amanda Parish
amanda_j_parish@raytheon.com
1 A radian is a measure of angular orientation.
Two π radians = 360 degrees.
RAYTHEON TECHNOLOGY TODAY 2009 ISSUE 1 37