Liquid Crystal Variable Retarders Twisted

Liquid Crystal Variable Retarders Twisted
Polarizers
CUSTOM LIQU ID CRYSTAL CAPABILITIES
Liquid Crystal Variable Retarders
Retarders
A basic building block of Meadowlark Optics’ line of
liquid crystal products is the Liquid Crystal Variable
Retarder (LCVR). A single one of these devices can
replace an entire series of standard crystalline retarders.
They are electronically adjustable from nearly 0 waves
(or beyond with an optional compensator) to over half
wave in the order of 10 milliseconds. An advanced use of
LCVRs is described in the application note “Stokes
Polarimetry Using Liquid Crystal Retarders”, which is
available on our Web site at www.meadowlark.com.
While we typically list our standard products as the
Liquid Crystal Variable Retarder, Attenuator and
Polarization Rotator, we also have the ability to utilize
Liquid Crystals in other ways that are extremely useful.
The Twisted Nematic Liquid Crystal Device (TN
Device) provides our customers with potential for custom
applications where a standard LCVR might not be
appropriate. The Liquid Crystal Circular Polarizer
(LCCP) is one way to achieve isolation with a liquid
crystal cell. At Meadowlark Optics we never cease
working on polarization solutions for our customers. We
hope the information below will provide our customers
with new ideas that will challenge us to create new,
exciting solutions for polarization control.
0.1
Transmission
0.8
0.04
0.4
0.02
0
2
14
Spatial Light
Modulators
4
6
8
10
12
Cell Thickness (arb. units)
0
0
2
4
6
8
10
12
14
U
U=2d(/)
Fig. 4-1 Transmission by a twisted-nematic cell between
parallel polarizers as a function of thickness
and/or wavelength. Inset shows cell thickness change only.
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Page 32
Custom
•
Mounts
If the twist is gentle when compared to the wavelength
of the light, the polarization will simply follow the twist
of the liquid-crystal molecules. Such a cell is said to be
operating in the “Mauguin limit” and its rotation is quite
achromatic. The polarization rotation angle is equal to
the twist angle for all wavelengths, which are short
enough for the twist to be viewed as sufficiently gentle.
When this is not the case, the cell will no longer act as a
pure rotator. The result of inputting linearly polarized
light is no longer an output of rotated linearly polarized
light, but rotated elliptically polarized light. However, for
certain discrete wavelengths, depending on the
birefringence of the liquid crystal and the thickness of
the cell, the pure rotation characteristic is retained. This
is illustrated in figure 4-1, which shows the transmission
Polarimeters
does not affect the light. At low voltage, however, the
twist does affect the light, causing rotation of the
polarization.
A twisted-nematic liquid-crystal cell is constructed in the
same manner as a standard LCVR except the alignment
of the liquid-crystal molecules is twisted 90°. As in an
LCVR, high voltage (~10V) aligns the molecules with
the field and removes the birefringence and therefore
Fax (303) 833-4335
0.06
0.6
0
One is often only interested in the ‘s’ and ‘p’ polarization
of an optical system, or, in the case of a digital optical
switch, only two states are frequently required. If you
desire to switch the polarization state between only two
angles, for example 0 and 90°, a twisted-nematic device
is an excellent solution. A big advantage of the twisted
nematic device over an LCVR is the simplicity of the
driving scheme. High voltage (above ~10V) gives 0
rotation and low voltage (below ~1V) gives 90° rotation,
so you need not concern yourself with exact voltages or
tight tolerances. Also, the field of view is wide when
compared to an LCVR, because the cell is being used in
a situation where the optical axis of the liquid crystal
molecules is not at an arbitrary angle to the light but is
either parallel or perpendicular to it.
•
0.08
0.2
Twisted-Nematic Liquid Crystal Cell
Te l ( 3 0 3 ) 8 3 3 - 4 3 3 3
Liquid
Crystals
1
Polarizers
CUSTOM LIQU ID CRYSTAL CAPABILITIES
105
Contrast Ratio
Retarders
104
1000
100
10
1
1510
1520
1530
1540
1550
1560
1570
1580
1590
Wavelength (nm)
Liquid
Crystals
Normally White Contrast
Normally Black Contrast
Fig.4-2 Contrast of a Twisted-Nematic Liquid-Crystal Cell
(normalized to 1), of a 90° twisted nematic cell between
parallel polarizers to be a function of the variable,
where d is the thickness of the cell, is the birefringence, and is the wavelength. Where the curve first
goes to zero is termed the “first minimum” and this
position is typically used. The next highest transmission
minimum is called the “second minimum” and so on. In
this plot, moving along the horizontal axis can be viewed
as increasing thickness or decreasing wavelength.
One might ask, given the achromaticity of thicker cells
“why use the first minimum?” The simple answer is
speed. The switching speed of an LC is a strong function
of the cell thickness; generally, speed drops quadratically
with the thickness. Thus, while a cell operating at a
particular wavelength in the first minimum condition
might switch in 10 to 50msec, one designed to operate
achromatically (for example to transmit <1% between
parallel polarizers) over the entire visible range can take
several seconds to switch.
Figure 4-2 shows the high contrasts of several thousands
to one, which can be achieved in practice with these
type cells. The curve termed “normally black contrast”
was taken between parallel polarizers where low voltage
gives a dark state and high voltage yields a bright state.
The curve termed “normally white contrast” was taken
between perpendicular polarizers where the dark state
occurs at high voltage.
A liquid-crystal circular polarizer (LCCP) is built much
as our standard liquid crystal products but utilizes a
cholesteric rather than nematic liquid crystal. These LC
molecules are aligned to form a helix whose axis is
perpendicular to the optical surfaces. Interference effects
cause light with the same wavelength (in the medium)
and handedness as the LC molecule’s helix to be
reflected. Thus, at their design wavelength, these liquidcrystal cells reflect one circular component of
polarization, for example right-handed circularlypolarized light, and transmit the other circular
component. An unpolarized or linearly-polarized beam
will be divided into two circularly-polarized beams.
Because circularly-polarized light changes handedness
upon specular reflection, these polarizers perform well as
optical isolators. Figure 4-3 shows the performance of a
typical device being used as an isolator. Because neither
polarization is absorbed, LCCPs can also be used as
polarizing mirrors, or in certain situations, as a laser-line
polarizing beamsplitter. Compared to a typical circular
polarizer, composed of a dichroic polarizer and a quarterwave retarder, an LCCP has significantly better
transmission performance. With an LCCP, 95% or more
of properly polarized light will be transmitted, compared
to 80% or less for a dichroic circular polarizer. Because
they are transparent at wavelengths away from their
center wavelength, they will easily transmit a probe beam
or can be stacked to extend their range.
Custom
Mounts
Polarimeters
Spatial Light
Modulators
U=2d(/),
Liquid Crystal Circular Polarizers
Page 33
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Retarders
When using a liquid-crystal circular polarizer, certain
properties should be kept in mind. The center
wavelength is a function of temperature. For example,
the center wavelength of a device working at 850 nm
will drop by roughly 0.5 nm/°C. The operating
wavelength is also a weak function of angle. Being an
interference phenomena, the center wavelength, c,
follows the typical Bragg formula,
Polarizers
CUSTOM LIQU ID CRYSTAL CAPABILITIES
c=ocos(),
where o is the center wavelength at normal incidence
and is the angle of incidence relative to the normal.
Clearly, one must also always remember that both a
transmitted and reflected beam exists. Of more subtle
concern is the fact that, unlike other reflecting surfaces,
the handedness of circularly polarized light does not
change upon reflection from an LCCP as it does with an
ordinary specularly reflecting surface. Therefore, light
passing through an LCCP that becomes right-handed
circularly polarized will become left-handed upon
reflection and will therefore be rejected when it again
reaches the LCCP. This is the basis of their use as
isolators. Because it is still left-handed circularly polarized,
upon subsequent reflection from an ordinary surface, the
rejected beam will be able to pass through the LCCP. The
simplest way to protect from these “secondary” reflections
is to slightly tip the LCCP so that reflected light is sent
out of the optical path into a beam block.
100
60
Liquid
Crystals
% Transmission
80
40
20
0
0.9
0.95
1
1.05
1.1
Relative Wavelength (␭Ⲑ␭c)
Fig. 4-3 Isolation performance of a LCCP
Polarimeters
Mounts
The performance of these types of liquid crystal devices is
established primarily by two factors. The first is related to
the “secondary” reflection noted above. Because of this
effect, the polarization purity will be degraded by any
reflections from the outer glass surface of the cell. Thus a
high quality anti-reflective coating is quite important for
high isolation. Secondly, there are typically a few
nanometers of residual retardance in these cells, which
will lower both the circularity and the isolation. This
effect becomes most significant at short wavelengths
dropping isolations of roughly 99.9% in the near IR to
only about 99% towards the blue. Better isolations are
possible but these tend to induce a small amount of
scattering.
Spatial Light
Modulators
only half integer numbers of pitches within the LC helix.
Thus 49.5, 50 or 50.5 helical pitches may fit between
glass substrates but not 49.2 or 50.1, etc. Slight
irregularities in the thickness cause the LC’s helical pitch
to minutely contract or expand until the next allowed
number of pitches becomes energetically favorable. At
this point, there is an abrupt change in pitch to the
opposite extreme. The center wavelength follows the
pitch. The two curves in figure 4-3 show the change in
optical performance as one crosses or proceeds between
one of these pitch disclinations. Again, while they do
not significantly affect the performance at the central
wavelength, they do cause a narrowing of the useful
bandpass.
Pitch dislocations are essentially a cosmetic defect,
which does not affect the performance of an LCCP at its
design wavelength. The surface alignment layer allows
Custom
Te l ( 3 0 3 ) 8 3 3 - 4 3 3 3
•
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Page 34
light modulator section for details and specifications on
these innovative products.
Liquid
Crystals
Spatial Light
Modulators
Polarimeters
Mounts
Custom
The long axis of the liquid crystal molecules defines the
extraordinary (or slow) index. With no voltage present,
the molecules lie parallel to the windows and maximum
retardance is obtained. When voltage is applied across the
liquid crystal layer, the molecules tip parallel to the applied
electric field. As voltage increases, the effective
birefringence decreases, causing a reduction in retardance.
Custom retardances can be achieved by using high
birefringent materials and/or increased liquid crystal layer
thickness. Birefringence of liquid crystal materials
decreases at longer wavelengths, requiring proper
evaluation and design for optimum performance.
Meadowlark Optics’ Liquid Crystal Variable Retarders
are used throughout the visible and near infrared region.
Our liquid crystal retarders are sensitive to temperature
and wavelength changes, and can be calibrated to
provide high precision tunable retarders, insensitive to
temperature or wavelength change.
Liquid crystal retarders offer outstanding performance
over large incidence angles. Material type, cavity
thickness, and especially operating voltage play a large
role in determining the acceptable input angle.
Phase control or modulation is possible for light linearly
polarized parallel to the slow axis. Electrical control of
the effective extraordinary index allows precision tuning
of an optical phase delay in the propagating beam.
Variable attenuators with no mechanical rotation are
configured by placing a Liquid Crystal Variable Retarder
between crossed polarizers. Full 180° linear polarization
rotation can easily be achieved by combining the Liquid
Crystal Variable Retarder with a fixed quarter waveplate.
Liquid crystal spatial light modulators consist of
individually controllable pixels. These devices are used
in a variety of intensity and/or phase control applications
where spatial variation is required. Refer to the spatial
Page 35
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•
A
Meadowlark Optics’ award-winning Liquid
Crystal Variable Retarders provide precise solidstate retardance tunability. These true zero-order
IN
devices are precision engineered, offering
NIN
excellent performance in the visible to near
infrared wavelength ranges. When combined
with other optical components, our Liquid
Crystal Variable Retarders produce electrically controllable
attenuation, linear polarization rotation, or phase
modulation.
WAR
D
Meadowlark Optics’ liquid crystal retarders are constructed
using precision polished, optically flat fused silica windows
spaced a few microns apart. The cavity is filled with
nematic liquid crystal material and sealed. This assembly
ensures excellent transmitted wavefront quality and low
beam deviation required for many demanding applications.
Liquid Crystal Variable Retarders
W
Retarders
L
iquid Crystal Variable Retarders are solid state, realtime, continuously tunable waveplates. Nematic liquid
crystals are birefringent materials whose effective
birefringence can be changed by varying an applied voltage.
G
Polarizers
POLARIZATION CONTROL WITH LIQU ID CRYSTALS
Voltage
(volts)
Retardance
V2
= /2
2<V<4
/4 < < /2
V4
= /4
4<V<7
0 < < /4
V7
=0
Output
Fig.4-4 Output polarization forms for different
retardance values of a compensated variable retarder
with horizontal linearly polarized input
Continuous tuning of retarders over a broad wavelength
range is required for many applications. This added
versatility makes real-time polarization conversion
possible with a single Liquid Crystal Variable Retarder
and electronic controller. Figure 4-4 shows a variety of
output polarization forms achieved with a single device.
Pure phase modulation is accomplished by aligning the
optic axis of the liquid crystal retarder parallel to a
linearly polarized input beam.
A Liquid Crystal Variable Retarder is the fundamental
component used in the following devices and systems.
• variable attenuators
• polarization rotators
• variable beamsplitters
• optical compensators
• spatial light modulators • polarimeters
• non-mechanical shutters • tunable filters
• beam steerers
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•
e-mail: [email protected]
Polarizers
LIQU ID CRYSTAL VARIABLE RETARDERS
T
Retarders
hese products all use nematic liquid crystal materials
to electrically control polarization. Meadowlark
Optics’ standard liquid crystal products provide tunable
retardation by changing the effective birefringence of the
material with applied voltage, thus altering the
transmitted light to some elliptical polarization form.
Percent Transmission
100
80
400
800
1200
;;
;;
1600
Wavelength (nm)
Fig. 4-5 Typical LC Transmission
Anisotropic nematic liquid crystal molecules form
uniaxial birefringent layers in the liquid crystal cell. An
essential feature of nematic material is that, on average,
molecules are aligned with their long axes parallel, but
with their centers randomly distributed as shown in
figure 4-6(a). With no voltage applied, the liquid crystal
molecules lie parallel to the glass substrates and
maximum retardation is achieved.
When voltage is applied, liquid crystal molecules begin to
tip perpendicular to the fused silica windows as shown in
figure 4-6(b). As voltage increases, molecules tip further
causing a reduction in the effective birefringence and
hence, retardance. Molecules at the surface, however, are
unable to rotate freely because they are pinned at the
alignment layer. This surface pinning causes a residual
retardance of ~30 nm even at high voltage (20 volts).
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Spacer
LC Molecules
(a) Maximum Retardance (V = 0)
LC Molecules
tipped with
applied voltage
(b) Minimum Retardance (V >> 0)
Fig. 4-6 Liquid Crystal Variable Retarder construction showing
molecular alignment (a) without and (b) with applied voltage
(drawing not to scale)
•
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Custom
•
ITO
Alignment Layer
Mounts
Te l ( 3 0 3 ) 8 3 3 - 4 3 3 3
;;
;;
Fused Silica
Polarimeters
As with any birefringent material, retardance is
dependent upon thickness and birefringence. Liquid
crystal material birefringence depends on operating
wavelength, drive voltage, and temperature. The overall
retardance of a liquid crystal cell decreases with
increasing temperature (approximately -0.4% per C).
90
Spatial Light
Modulators
We achieve zero (or any custom) retardance with a
subtractive fixed polymer retarder, called a compensator,
attached to the liquid crystal cell. Negative retardance
values are sometimes preferred, for example, when
converting between right- and left-circularly polarized
states. Figure 4-7 illustrates retardance as a function of
voltage for a typical Liquid Crystal Variable Retarder
with and without an attached compensator. Placing a
compensated Liquid Crystal Variable Retarder between
two high extinction polarizers creates an excellent
optical attenuator, with convenient electronic control.
Liquid
Crystals
Our precision Liquid Crystal Variable Retarders require
unique fabrication and assembly steps. We construct these
retarders using optically flat fused silica windows coated
with our transparent conductive indium tin oxide (ITO).
Our ITO coating is specially designed for maximum
transmission from 450-1800 nm. A thin dielectric layer is
applied over the ITO and gently rubbed, to provide for
liquid crystal molecular alignment. Two windows are then
carefully aligned and spaced a few microns apart. The
cavity is filled with birefringent nematic liquid crystal
material. Electrical contacts are attached and the device is
environmentally sealed. We carefully place the Liquid
Crystal Variable Retarder in an anodized aluminum
housing such that the fast and slow axes are both at 45°
relative to a convenient mounting hole.
Page 36
Polarizers
LIQU ID CRYSTAL VARIABLE RETARDERS
(a)
Retarders
Retardance (waves)
0.75
momentarily removed to allow the liquid crystal
molecules to undergo natural relaxation. Response time
achieved with the transient nematic effect is also shown
in figure 4-8. Our programmable D3040 Controller
described on page 45 can provide the necessary TNE
voltage profiles.
0.50
0.25
0
5
10
15
20
15
20
Our standard Liquid Crystal Variable Retarders provide a
minimum retardance range of ~30 nm to at least halfwave at the specified wavelength. With an attached
compensator, retardance is guaranteed from zero to at
least half-wave at the specified wavelength. Custom
retardance ranges (up to a few waves) and custom
compensators are available. Contact our Sales
Department to discuss your requirements.
Voltage (volts rms)
(b)
Liquid
Crystals
Retardance (waves)
0.75
0.50
0.25
0.00
0
5
10
Voltage (volts rms)
Polarimeters
Mounts
Custom
AP
ICATIO
PL
Response Time
Liquid Crystal Variable Retarder response time
depends on several parameters, including layer
NO E
thickness, viscosity, temperature, variations in
T
drive voltage, and surface treatment. Liquid
crystal response time is proportional to the square of the
layer thickness and therefore, the square of the total
retardance.
Liquid crystal devices should be electrically driven with
an AC waveform with no DC component to prevent
N
Spatial Light
Modulators
Fig. 4-7 Liquid Crystal Variable Retarder performance at 632.8 nm,
21 °C (a) without compensator, and (b) with compensator
Each Liquid Crystal Variable Retarder is supplied with
retardance versus voltage performance data for your
specified wavelength. A coaxial cable with BNC
connector is provided for easy attachment to an
electronic controller.
Response time also depends upon direction of the
retardance change. If the retardance increases, response
time is determined solely by mechanical relaxation of the
molecules. If retardance decreases in value, response time is
much faster due to the increased electric field across the
liquid crystal layer. Typical response time for our standard
visible Liquid Crystal Variable Retarder is shown in figure
4-8. It takes about 5 ms to switch from one-half to zero
waves (low to high voltage) and about 20 ms to switch
from zero to one-half wave (high to low voltage).
Response time improves by using custom materials with
high birefringence and a thinner liquid crystal layer. At
higher temperature, material viscosity decreases, also
contributing to a faster response.
Another technique involves the Transient Nematic
Effect (TNE) to improve response times. With this drive
method, a high voltage spike is applied to accelerate the
molecular alignment parallel to the applied field. Voltage
is then reduced to achieve the desired retardance. When
switching from low to high retardance all voltage is
Page 37
Te l ( 3 0 3 ) 8 3 3 - 4 3 3 3
•
(a) Applied voltage to
LC Variable
Retarder
90%
4 ms
10%
22 ms
0%
t1
t2
(b) Typical temporal
response for halfwave operation
at 632.8 nm
(21C)
t3 t4
90%
(c) Improved temporal
response using
Transient Nematic
Effect
10%
0%
Fig. 4-8 Temporal response of LC Variable Retarder
“The temporal response of a liquid-crystal
device seems very complicated. Where can I
find some clarification?”
PROBLEM
SOLUTION See our application note on temporal
response of liquid crystal devices at www.meadowlark.com.
Fax (303) 833-4335
•
e-mail: [email protected]
ionic buildup which can damage the liquid crystal layer.
S PECIFICATIONS
Retarder Material:
Nematic liquid crystal
Substrate Material:
Optical quality synthetic
fused silica
Wavelength Range:
450-1800 nm (specify)
Retardance Range:
Without compensator:
With compensator:
A temperature sensing and control option can be added
to our Liquid Crystal Variable Retarders for accurate
controlling of the operating temperature. The sensor is
attached directly to a fused silica substrate outside the
Retardance Uniformity:
1.230
0.375
0.275
8-32 TAP
All dimensions in inches
Model LVR-100
Fig. 4-9 Model LVR-100 dimensions
/4
Surface Quality:
40-20 scratch and dig
Beam Deviation:
2 arc min
Reflectance (per surface):
0.5%
Diameter Tolerance:
±0.005 in.
Temperature Range:
see page 46
Spatial Light
Modulators
F
2% rms variation
over clear aperture
Transmitted Wavefront
Distortion (at 632.8 nm):
Recommended Safe
Operating Limit:
D
30 nm to /2 wave
0 to /2 wave
custom ranges are available
Liquid
Crystals
Meadowlark
Optics
Retarders
We require a 2 kHz square wave of adjustable amplitude
for controlling our Liquid Crystal Variable Retarders
(LCVR). Either our Model B1020 or D3040 Controller
described on pages 44-45, will ensure this requirement
is met.
1.000
500 W/cm2 CW
300 mJ/cm2 10 ns, visible
S
C.A.
SMB
connector
Polarizers
LIQU ID CRYSTAL VARIABLE RETARDERS
All dimensions in inches
O RDERING I NFORMATION
t
0.500
8-32 TAP
Fig. 4-10 Models LVR-200 and LVR-300 dimensions
PROBLEM
SOLUTION
“I need to measure the polarization state of
light. Can I use the retardance tunability
of your LCVR to do this?”
Absolutely! See our application note
on Stokes polarimetry at
www.meadowlark.com.
■
■
0.70
0.75
LVR - 200
3.00
1.60
1.00
LVR - 300
With Attached Compensator (0 to /2 wave)
1.00
0.37
1.23
LRC - 100
2.00
0.70
0.75
LRC - 200
3.00
1.60
1.00
LRC - 300
•
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Page 38
Custom
Fax (303) 833-4335
2.00
* A temperature sensing and control option for series ‘200’ and ‘300’ is available.
Please be sure to append “-TSC” to the part number when ordering. For custom
retardance ranges or special compensator values, please call for a free quotation.
Please contact our sales department to obtain a price list for
our standard components.
•
LVR - 100
Temperature Sensing and Control Option*
We offer standard liquid crystal variable retarders to cover
4 regions of the spectrum:
VIS: 400 - 700nm
IR 1: 650 - 950nm
IR 2: 900 - 1250nm
IR 3: 1200 - 1700nm
Please specify wavelength region when placing your order.
Performance from 450 to 1800 nm
Computer control capability
Temperature control options
Unequaled measurement accuracy
Te l ( 3 0 3 ) 8 3 3 - 4 3 3 3
Part
Number
Mounts
■
Thickness
t (in.)
Without Compensator (30 nm to /2 wave)
1.00
0.37
1.23
K EY B ENEFITS
■
Clear
Aperture (in.)
Polarimeters
Diameter
D (in.)
Maximum transmission is dependent upon properties of
the Liquid Crystal Variable Retarder as well as the
polarizers used in your system. An unpolarized light
source is used for illumination.
Linear
Polarized
Output
f
Mounts
Polarimeters
Entrance
Polarizer
45˚
s
Compensated
Liquid Crystal
Variable
Retarder
Exit
Polarizer
60
40
20
0
2
6
4
8
10
Voltage (volts rms)
Fig. 4-11 Standard Liquid Crystal Variable Attenuator
design uses crossed linear polarizers.
With crossed polarizers, light transmission is maximized
by applying the correct voltage to achieve half-wave
retardance from the liquid crystal cell as shown in figure
4-11. Half-wave operation rotates the incoming
polarization direction by 90°, so that light is passed by
the second polarizer. Minimum transmission is obtained
with the retarder operating at zero (or a whole number
of) waves.
Transmission decreases as the applied AC voltage
amplitude increases (half- to zero-waves retardance).
The relationship between transmittance T and
retardance (in degrees) for crossed polarizer
configuration is given by:
T() = 1/2 [1 - cos()] Tmax
Custom
80
Fig.4-12 Normalized transmittance of Liquid Crystal Variable Attenuator
with crossed linear polarizers at a single wavelength
40
Percent Transmittance
Unpolarized
Input
100
(normalized to Tmax)
eadowlark Optics’ Liquid Crystal Variable
Attenuator offers real-time, continuous control
of light intensity. Our attenuator consists of a Liquid
Crystal Variable Retarder (with attached compensator)
operating between crossed linear polarizers.
Contrast ratio is defined as the maximum transmission
(obtained with the liquid crystal cell at half-wave
operation) divided by the minimum transmission
(obtained with the liquid crystal cell at zero waves).
Values exceeding 1000:1 (see figure 4-14) can be
obtained for a single wavelength by optimizing the
applied voltage levels for minimum and maximum
transmission. We guarantee a minimum contrast ratio of
500:1 at your specified wavelength.
Percent Transmittance
M
Spatial Light
Modulators
Liquid
Crystals
Retarders
Polarizers
LIQU ID CRYSTAL VARIABLE ATTENUATOR
30
20
10
400
500
600
700
800
Wavelength (nm)
Fig. 4-13 Transmittance as a function of wavelength for
Liquid Crystal Variable Attenuator, optimized for 550 nm,
with polarizers and unpolarized input
where Tmax is the maximum transmittance when
retardance is exactly one-half wave (or 180).
Page 39
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•
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A
■
■
Continuous control of light intensity
Computer control capability
High contrast ratio
S PECIFICATIONS
Nematic liquid crystal with
Birefringent polymer
Polarizer Material:
Dichroic polymer
Substrate Material:
Optical quality synthetic
fused silica
Custom devices for near infrared applications, utilizing
appropriate dichroic polarizers, can also be manufactured.
Please see the section on Polarizers for a selection of
available polarizers.
Model B1020 and D3040 controllers listed on pages 4445 offer the precision necessary to obtain accurate and
repeatable intensity control for your application.
Contrast Ratio:
≥ 500:1 at single wavelength,
1mm beam
Transmittance:
30% with unpolarized input
4
Transmitted Wavefront
Distortion (at 632.8 nm):
/4 (each component)
3
Surface Quality:
≤ 40-20 scratch and dig
Beam Deviation:
≤ 2 arc min
Reflectance (per surface):
0.5% at normal incidence
Diameter Tolerance:
±0.005 in.
Temperature Range:
10 °C to 50 °C
2
1
400
500
600
700
800
Wavelength (nm)
Fig. 4-14 Typical Contrast Ratio of a
Liquid Crystal Variable Attenuator optimized at 550 nm
1 W/cm2 CW
(with dichroic polarizers)
Polarimeters
Recommended Safe
Operating Limit:
Log (Contrast Ratio)
450-700 and 900-1550 nm
Spatial Light
Modulators
Wavelength Region:
Liquid
Crystals
Retarder Material:
Retarders
Liquid Crystal Variable Attenuator can be
configured with high efficiency calcite or
beamsplitting polarizers to maximize light transmittance
and increase damage threshold. With a linearly polarized
input beam and a calcite polarizer, transmittance values
exceed 90% at most wavelengths. Very high contrast
ratios, in excess of 5000:1, can be achieved with custom
double attenuators. In this design, two Liquid Crystal
Variable Retarders are combined with three polarizers.
K EY B ENEFITS
■
Polarizers
LIQU ID CRYSTAL VARIABLE ATTENUATOR
O RDERING I NFORMATION
Clear
Aperture (in.)
Part
Number
1.00
0.37
LVA - 100 - 2.00
0.70
LVA - 200 - 3.00
1.60
LVA - 300 - Mounts
Diameter
D (in.)
Please specify your operating wavelength in nanometers when
ordering.
Custom
Custom sizes of our Liquid Crystal Variable Attenuators are available. Call for a quote.
Please contact our sales department to obtain a price list for our standard components.
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Page 40
A quarter-wave retarder converts elliptical polarization
formed by the Liquid Crystal Variable Retarder to
linear polarization. The rotation angle is equal to onehalf the retardance change from the Liquid Crystal
Variable Retarder.
Response time depends upon the desired amount of
rotation. Small rotations have longer response times.
Polarization purity is defined as the ratio of the rotated
linear component to the orthogonal component. A
selected rotation is very sensitive to applied voltage and
operating temperature. On average, polarization purity,
or contrast ratio is better than 150:1.
O
ur Liquid Crystal Polarization Rotator continuously
rotates the polarized direction of a monochromatic,
linearly polarized input beam. Our design consists of a
Liquid Crystal Variable Retarder combined with a zeroorder polymer quarter-wave retarder. The fast axis of one
retarder is oriented at 45 to the slow axis of the second.
Linearly polarized input must be parallel to the quarterwave retarder slow axis. Polarization rotation is achieved
by electrically controlling the retardance of the Liquid
Crystal Variable Retarder, eliminating any mechanical
motion.
We provide test data including the required voltages
corresponding to polarization orientations from
approximately -40° to approximately 140° rotation in
10° increments. These measurements are taken at room
temperature for your specified wavelength.
Standard Liquid Crystal Polarization Rotators are
supplied without an input polarizer. Input polarization
direction must be precisely aligned for optimum
performance. Please call if you require an input polarizer.
Input
Polarization
Polarimeters
Spatial Light
Modulators
Liquid
Crystals
Retarders
Polarizers
LIQU ID CRYSTAL POLARIZATION ROTATOR
s
f
Compensated
LC Variable
Retarder
Mounts
Rotated
Output
Polarization
s
f
4
Retarder
Custom
Fig.4-15 Operation of Liquid Crystal Polarization Rotator showing complete rotation of a linearly polarized input beam
Page 41
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K EY B ENEFITS
■
■
■
■
O RDERING I NFORMATION
Continuous rotation of linearly polarized light
Computer control capability
High polarization purity
180 degree polarization rotation
High power capability
Nematic liquid crystal with
Birefringent polymer
Part
Number
1.00
0.37
1.23
LPR - 100 - 2.00
0.70
0.79
LPR - 200 - 3.00
1.60
1.00
LPR - 300 - Custom sizes of our Liquid Crystal Polarization Rotators are
available. Call for a quote.
Optical quality synthetic
fused silica
Polarization Rotation:
0-180
Polarization Purity:
150:1 average
Transmittance:
92% with polarized input
Transmitted Wavefront
Distortion (at 632.8 nm):
/4
Surface Quality:
40-20 scratch and dig
Beam Deviation:
2 arc min
Reflectance (per surface):
0.5% at normal incidence
Diameter Tolerance:
±0.005 in.
Temperature Range:
10 °C to 50 °C
Polarimeters
450-1800 nm (specify)
Please contact our sales department to obtain a price list for
our standard components.
Spatial Light
Modulators
Wavelength:
Recommended Safe
Operating Limit:
Thickness
t (in.)
Liquid
Crystals
Substrate Material:
Clear
Aperture (in.)
Please specify your operating wavelength in nanometers
when ordering.
S PECIFICATIONS
Retarder Material:
Diameter
D (in.)
Retarders
■
Polarizers
LIQU ID CRYSTAL POLARIZATION ROTATOR
500 W/cm2 CW
300 mJ/cm2 10 ns, visible
Mounts
Custom
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Page 42
Polarizers
Retarders
BASIC LIQU ID CRYSTAL CONTROLLER
O
ur basic controller, Model B1020 is specially
designed to integrate with any single Meadowlark
Optics liquid crystal device described in this section.
Manual adjustment of the voltage amplitude controls
liquid crystal retardance. Figure 4-7 on page 38
illustrates the relationship between voltage and
retardance.
Independent voltage settings allow easy and repeatable
selection of two retardance values. Often, it is desirable
to modulate between the two states. For example,
switching between 1/4 wave and 3/4 wave retardance
changes linearly polarized light to either right or left
circular. A manual toggle allows easy switching
between states.
Our Model B1020 comes equipped with its own internal
modulation control. The dial adjusts regular switching
frequency between the two voltage settings. An external
input allows liquid crystal retardance modulation to run
synchronously with other equipment.
Each Meadowlark Optics’ Liquid Crystal Variable
Retarder is supplied with a plot of retardance versus
voltage. Using a true RMS voltmeter with your Model
B1020 Controller and the retardance plot ensures
accurate retardance to voltage correlation. An optional
Voltmeter Adapter Cable simultaneously connects a
liquid crystal device and digital voltmeter to the Model
B1020 Controller.
NOTE: Previous generations of
Meadowlark LC Controllers used BNC to
SMB cables. Adapters and replacement
cables are available. Please contact
Meadowlark for more information.
Custom
Mounts
Polarimeters
Spatial Light
Modulators
Liquid
Crystals
Fig. 4-16 Front panel layout for Model B1020 Controller
Page 43
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OUTPUT
O
N
EXTERNAL
MODULATION
O
F
F
Fig. 4-17 Rear panel layout for Model B1020 Controller
S PECIFICATIONS
Output Voltage:
2 kHz AC square wave
adjustable 0-25 V rms
50% duty cycle
minimal DC bias
Voltage Resolution:
3 mV
Internal Modulation:
0.5-150 Hz
External Modulation:
TTL compatible input
12 V maximum
DC-1 kHz
Power Requirements:
115/230 VAC
50/60 Hz
5W
Dimensions (L x W x H):
7.0 x 5.0 x 3.1 in.
Weight:
2.0 lbs
O RDERING I NFORMATION
Part
Number
Item
Basic Liquid Crystal Controller
B1020
Voltmeter Adapter Cable
B1020-VAC
BNC to SMB adapter
BNC-SMB
Please contact our sales department to obtain a price list for our
standard components.
Fax (303) 833-4335
•
e-mail: [email protected]
Polarizers
D3040 QUAD CELL LIQU ID CRYSTAL DIGITAL INTERFACE
T
Retarders
he D3040 Quad Cell Liquid Crystal Interface is
designed for ease of use and high precision control of
up to four Meadowlark nematic liquid crystal devices at
one time.
The D3040 controller is available in either Basic or
Advanced Package options.
The D3040 comes standard with CellDRIVE 3000 Basic
software to allow for independent control of the
amplitude of the 2-kHz square wave drive for four
separate nematic liquid crystal cells. The Advanced
Package includes all the functionality of the Basic
Package plus the added features of the CellDRIVE 3000
Advanced software and capability for temperature
monitoring and control on one channel. The CellDRIVE
3000 Advanced software allows for modulation of the
amplitude of the 2-kHz square wave drive using specific
functions incusing sinusoidal, square, triangle, sawtooth,
and transient nematic effect waveforms.
D3040 Features
Liquid
Crystals
• USB or RS-232 PC to Controller Unit Interface
• Includes National Instruments LabView™ Virtual
Instrument drivers to interface with custom software
• Independent control of voltage levels on 4 channels
to 1 mV resolution
• Compact and simple to use
• Microsoft® HyperTerminal configuration file included
Spatial Light
Modulators
Polarimeters
CellDRIVE 3000 Basic provides timeinvariant amplitude control of Liquid
Crystal Variable Retarders connected
to the D3040.
Mounts
The D3040 Advanced Package includes the functionality of the Basic
Package plus temperature control on one channel and CellDRIVE 3000
Advanced software. CellDRIVE 3000 Advanced software is designed to
meet the requirements of most LCVR applications. It provides the
capability to select and configure a variety of waveforms, including
transient nematic effect waveforms, for each output channel.
Custom
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Polarizers
D3040 QUAD CELL LIQU ID CRYSTAL DIGITAL INTERFACE
Spatial Light
Modulators
Liquid
Crystals
Retarders
Basic package includes:
S PECIFICATIONS
• D3040 Controller Unit
• Power supply and power cable
• USB and RS232 cables
• CellDRIVE 3000 Basic software
• National Instruments™ LabVIEW™
virtual instruments driver
• D3040 User’s Manual
Fundamental
Drive Waveform:
2-kHz square wave
Modulation Amplitude:
0 to 10 Vrms
Modulation Resolution:
1 mV (0.155 mV using Lab
VIEW subroutines)
DC Offset:
< 5 mV
Advanced package includes:
LC cell to Controller
Connection:
PC-Controller
Communications Interface: USB or RS232
• D3040 Controller Unit
• Power supply and power cable
• USB and RS232 cables
• LC-Controller interface cable
• Temperature control cable
• CellDRIVE 3000 Advanced software
• National Instruments™ LabVIEW™
virtual instruments driver
• Temperature monitoring and control
• D3040 User’s Manual
with TSC option may not be compatible with the TSC
option in the D3040.
NOTE: Previous generations of Meadowlark LC
Controllers used BNC to SMB cables. Adapters and
replacement cables are available. Please contact
Meadowlark for more information.
Mounts
Polarimeters
Power Requirements:
100-240 VAC, 47-63 Hz, 500 mA
Safety and Environmental
Certification:
CE compliant
External Dimensions:
9.5" x 6.25" x 1.5"
Weight:
<2 lbs.
Advanced Package only
Modulation Waveforms:
NOTE: Previous generations of Meadowlark LC devices
SMA-SMB connectors,
2m length
Temperature Control:
sinusoidal
triangle
square
sawtooth
transient nematic effect
Closed-looped feedback controlled
active heating/passive cooling
± 1°C of setpoint (nominal)
Minimum System Requirements:
• Windows® 98/ME/2000/XP
• PC with Pentium II class processor
• 32 MB RAM
• 6 MB hard drive space
• CD ROM drive
• USB or RS232 COM Port
• Use of LabVIEW™ Instrument Library requires
LabVIEW™ version 6.1 or newer full development
system
O RDERING I NFORMATION
D3040-BASIC
Advanced
D3040-ADV
Extra SMA-SMB Cables
SMA-SMB
Custom
Basic
Page 45
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