Electrically Controlled Liquid Crystal Devices for Beam Steering

1
Electrically Controlled Liquid Crystal Devices
for Beam Steering

• C. Titus
• D. Voloschenko
• S. Shiyanovskii
• P.J. Bos
• O.D. Lavrentovich
• Liquid Crystal Institute Chemical Physics
  Program
• Kent State University, Kent, OH 44242
• BMDO/AFOSR grant F49620-96-1-0449
• April 99

2
Content

• Cholesteric diffraction gratings
• Beam-deflecting switches

3
Part 1. Cholesteric Gratings

• 1. Optics of Cholesterics
• 2. Problem of Alignment
• 3. Switchable grating performance at constant
  periodocity
• 4. Problem of Field-controlled Periodicity
• and Computer Simulations
• 5. Continuous Beam Steering

4
Optics of cholesterics
Ch periodical modulation of dielectric
susceptibility
h
h
E
E
No diffraction for Eh
Diffraction for E h
5
Cholesteric Cells as Diffraction Gratings
L
?
thin cell, Raman-Nath grating
thick cell, Bragg grating
6
Problem of Alignment
uniform stripe texture that does not depend on
the varying pitch of the cholesteric
Alignment needed
7
Idea of uniform alignment simultaneous action
of the electric field and uniaxial surface
anchoring
planar state, no field
modulated state, medium field in-plane
uniformity is fixed by surface alignment
homeotropic state, strong field
8
Experimental realization Field-switchable
cholesteric structures (periodicity does not
change with the field)
U0 V, planar texture
U4 V, modulated structure (6 micron
period) produces diffraction, next slide
U6 V, homeotropic structure
9
Computer Simulations to Optimize the Cholesteric
GratingsBasic formulas
10
Computer Simulations 3D view
11
Computer Simulations, Vertical cross-section
12
Electric-Field Induced Instability and Creation
of Stripes (computer simulations, please click on
the picture)
13
Electrically-Controlled Periodicity of
Cholesteric Grating
20 mm
U1.1 V
U1.75 V
14
Click in the picture for Movie
15
Continuous Beam Steering in Raman-Nath regime
V
L
?
P 1.7 µm L 2.5 µm
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Electrically-switchable gratings
Diffraction pattern on a screen
Screen
Cholesteric cell
Incident beam
U4 V Field On
U0 V Field Off
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Beam Steering in Bragg regime
P 0.7 µm L 2.3 µm
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SUMMARY-1
1.
Simultaneous dielectric reorientation and
unidirectional anchoring produce
electrically-controlled diffraction gratings
2.
Cholesteric gratings are capable of
both
Bragg and Raman-
Nath diffraction
,
depending on pitch and cell's thickness
3.
Field-controlled periodicity allows for
continuous beam steering
4.
Basic parameters

diffraction angles 10-50 degrees

steering angles up to 20 degrees

response times 20 ms
for some transitions, 0.5 sec

diffraction efficiency 15 -30
(
non-optimized cells)

controlling voltages less than 5 V
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Part 2 Prismatic Beam Steering Device

• Based on Birefringent Prisms
• Easily extendible to a large number of beam
  positions.
• Very simple, low cost construction
• High throughput
• Clear paths to fast response.

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Birefringent Prisms

• A birefringent prism refracts light of orthogonal
  polarizations to different angles.
• The angles are just given by Snells law of
  refraction.
• By changing the polarization of the input light,
  the beam exiting the prism can be deflected by
  the difference of qe and qo (independent of
  wavelength)

qe
qo
Snells Law no sinqp sin qe ne sinqp sin qo
qp
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A Birefrinent/Isotropic Bi-Prism

• Consider that have isotopic prism with index of
  refraction equal to no of birefringent prism.
• One polarization is now undeflected.
• Other is deflected by
• d sin-1(no sin(sin-1((ne/no) sina))-a (just
  from Snells law again) where a is the prism
  angle

(for no1.5 ne1.7)
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Basic Switching Unit

• Is combination of a Bi-prism and a electrically
  controllable polarization rotator. (simple TN
  cell)
• TN cell in non-rotating state causes no light
  deflection
• (Vnr applied)
• TN cell in rotating state causes light deflection
  
• (Vr applied)

Vr
23
The Beam Steering Device

• Stack with N basic units that have a values in a
  binary sequence.
• Has 2N beam steering positions.
• For example
• 3 of 16 positions

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The 4 Bit Demo Device

• The prisms
• Made using wedge shaped liquid crystal cells.
• Cells are filled with SmA material. ( to
  minimize effect of thermal director fluctuations
  found in nematics)
• The polarization state rotators
• Made using Twisted Nematic (TN) cells
• Same technology (and cost) as watch displays.

Click in picture to start and stop Movie
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The Data

• LSB deflection is 0.002 rads ( beam divergence is
  0.001 rads)
• Low nearly constant insertion loss for all
  positions (no AR considerations have been
  implemented)

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Remaining Issues

• Faster Switching Speed
• Increased manufacturability
• Optimized Design for equally spaced output
  positions
• Larger number of steering angles
• Larger Deflection angles

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Faster Switching Speed

• Current time is about 100 ms. ( in the visible
  times at 1.5 microns could be 10X these values)
• TN cells optimized for speed will yield approx.
  10ms switching.
• Very easy
• Tunable Birefringence devices will yield 3ms
  switching at room temp, and 1ms at 50 degrees C.
• Easy
• Ferroelectric materials will allow switching in
  the micro-second region.
• Lots of work has been done in this area, and
  processes are well documented.

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Manufacturability

• TN cells are similar to watch displays.
• Basic manufacturing processes are well
  understood.
• More care is needed however to minimize
  scattering.
• Birefringent Prisms
• Could use inorganic based prisms, but are
  expensive
• SmA filled cells are inexpensive and easy to
  build, but currently built one at a time.
• Polymerized liquid crystal films could yield a
  high volume process.

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Equally Spaced Beam Positions

• Spacing of beam positions is currently only
  approximately uniform.
• We have looked at this in detail and designed a
  system that allows very equally spaced positions.

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Larger Deflection Angles

• With well defined beam positions, a passive
  micro-prism array can be used to achieve any
  deflection angle.

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Larger Number of Positions

• Number of positions 2N
• as long as smallest a yields deflection of
  greater than a beam diameter
• and largest value of a is not unreasonably
  large ( 20 degrees?)
• The above 2nd issue can be made less restricting
  by doubling prisms
• If single unit limits are reached, multiple units
  can be considered
• number of positions is limitless.

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Optical Calculations of Diffractive Systems

• FDTD method is being developed.
• Direct solver of Maxwells eqns.
• For a LC over a single electrode,
• the far field diffraction pattern has been
  calculated.
• Next is multiple electrodes to calculate
  diffraction of optical phased arrays.

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Summary Part 2

• Simple, low cost beam steering device based on
  birefringent prisms.
• A 4-bit beam steering device has been
  demonstrated to show high efficiency.
• Paths toward improvements are clear.
• The Finite Difference Time Domain method of
  calculating optics of diffractive systems.
• Far field diffraction of from a single electrode
  LC device has been calculated
• Multi-electrode systems are next.