Multi-Wave Liquid Crystal Variable Retarders / Wave Plates

Overview
Specs
Performance
Switching Time
Applications
LC Controller
Custom Capabilities
Patterned Retarders
Feedback

Operating Principle

High Retardance

Low Retardance

In their nematic phase, liquid crystal molecules have an ordered orientation, which together with the stretched shape of the molecules creates an optical anisotropy. When an electric field is applied, the molecules align to the field and the level of birefringence is controlled by the tilting of the LC molecules.

Selection Guide for LC Retarders
Type	Clear Aperture
Half Wave	Ø10 mm or Ø20 mm
Half Wave, Thermally Stabilized	Ø10 mm
Full Wave	Ø10 mm or Ø20 mm
Full Wave, Thermally Stabilized	Ø20 mm
Multi-Wave	Ø10 mm
Multi-Wave, Integrated Controller	Ø10 mm
Custom LC Retarders

Features

Variable Wave Plate to Continuously Control the Polarization State of Light
Retardance Range
- LCC1115-A: ~60 nm to >6λ
- LCC1115-B and LCC1115-C: ~400 nm to >6λ
Clear Aperture: Ø10 mm
Three Antireflection Coatings Available
- LCC1115-A: 350 - 700 nm
- LCC1115-B: 650 - 1050 nm
- LCC1115-C: 1050 - 1700 nm
Surface Quality: 40-20 Scratch-Dig
Retardance Uniformity: <λ/50 Over the Entire Clear Aperture

Thorlabs' Multi-Wave Liquid Crystal Variable Retarders (LCVR) use nematic liquid crystal cells to function as variable wave plates. The absence of moving parts provides quick switching times on the order of milliseconds (see the Switching Time tab for details). Our multi-wave retarders are AR coated for 350 - 700 nm, 650 - 1050 nm, or 1050 - 1700 nm and can reach up to >6λ retardance (see the Performance tab for transmission and retardance data). An LC cell spacer that is 30 µm thick in the LCC1115-A or 50 µm thick in the LCC1115-B and LCC1115-C aids each retarder in achieving its maximum retardance value. The retarders have Ø10 mm clear apertures with a 1" outer diameters, making them compatible with any of our Ø1" optics mounts for 8 mm thick optics. An engraved white line on the front of each housing marks the slow axis.

Performance
These liquid crystal variable retarders provide excellent uniformity, low optical losses, and low wavefront distortion. They also provide quick switching times, a broad operating temperature range, and a broad wavelength range. Please see the Specs, Performance, and Switching Time tabs for complete details.

Operation
A Liquid Crystal Variable Retarder consists of a transparent cell filled with a solution of Liquid Crystal (LC) molecules and functions as a variable wave plate. The orientation of the LC molecules is determined by the alignment layer in the absence of an applied voltage. The alignment layer is composed of an organic polyimide (PI) coating whose molecules are aligned in the rubbing direction during manufacturing. Due to the birefringence of the LC material, this LC retarder acts as an optically anisotropic wave plate, with its slow axis, marked on the mechanical housing, parallel to the surface of the retarder. Two parallel inner faces of the cell wall are coated with a transparent conductive film so that a voltage can be applied across the cell. When an AC voltage is applied, the LC molecules will reorient from their default alignment according to the applied Vrms.

Controller
The LCC25 controller, sold below, provides active DC offset compensation, while applying an AC voltage (0 to 25 V_rms). The DC offset compensation automatically zeros the DC bias across the LC device in order to counteract the buildup of charges. It is fully compatible with all of the liquid crystal retarders sold by Thorlabs.

Item #	LCC1115-A	LCC1115-B	LCC1115-C
Wavelength Range	350 - 700 nm	650 - 1050 nm	1050 - 1700 nm
Liquid Crystal Material	Nematic Liquid Crystal
Retardance Range	~60 nm to >6λ	~400 nm to >6λ
Clear Aperture	Ø10 mm
Surface Quality	40-20 Scratch-Dig
Switching Speed (Rise / Fall, Typical)^a	423 ms / 70 ms (1 λ to 2 λ)	820 ms / 190 ms (1 λ to 3 λ)	720 ms / 187.5 ms (1 λ to 2 λ)
AR Coating	R_avg < 0.5% at all Air-to-Glass Surfaces for Specified Wavelength Range
Wavefront Distortion	≤λ/4 (@ 635 nm)
Retardance Uniformity	<λ/50 Over the Entire Clear Aperture
Housing Outer Diameter	1"
Storage Temperature	-30 to 70 °C
Operation Temperature	-20 to 45 °C
Compatible Mounts	RSP1 (RSP1/M), CRM1 (CRM1/M), CRM1P (CRM1P/M), KM100

Specified at 25 °C. Switching speed is highly dependent on several factors, including voltage change and cell temperature. See the Switching Time tab for more details.

Variable Full-Wave Retarder Performance Graphs
Item #	Wavelength Range	Retardance @ 25 °C	Retardance vs. Temperature	Transmission
LCC1115-A	350 - 700 nm		(635 nm)
LCC1115-B	650 - 1050 nm		(780 nm)
LCC1115-C	1050 - 1700 nm		(1550 nm)

LC Retarder Performance

In their nematic phase, liquid crystals molecules have an ordered orientation, which together with the stretched shape of the molecules, creates an optical anisotropy. When an electric field is applied, the molecules align to the field and the level of effective retardance is controlled by the tilting of the LC molecules. To minimize effects due to ions in the material, an LC device must be driven using an alternating voltage. Our LCC25 controller is designed to minimize the DC bias in the driving signal in the operating range of 0 V to 25 V.

Due to changes in the molecular polarizability, the LC material exhibits higher chromatic dispersion at short wavelengths and comparably small chromatic dispersion at long wavelengths. To account for this, we provide the retardance data at one or two select wavelengths within the product's wavelength range in the table to the right.

Additionally, the LC retardation also depends on the temperature of the device. As temperature increases, the retardation decreases with it. However, as seen in the Switching Time tab, the switching speed of the LC improves at higher temperatures. Generally, the LC's refractive indices (both ordinary and extraordinary) change more drastically as temperature nears the LC's clearing temperature. As such, we choose to use materials with a high clearing temperature to minimize the temperature dependence when used at room temperature.

Click Here to Download Retardance Data
Click Here to Download Transmission Data

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Graph shows variation in retardance over a period of 154 weeks.

Long-Term Stability

Our liquid crystal retarders exhibit consistent performance over time. The graph to the right shows the retardance vs. voltage for one previous-generation LCC1112-A three-quarter wave retarder, driven by our LCC25 liquid crystal controller over 154 weeks. The retardance was tested once per week and varied only slightly over the testing period. For the complete set of data from testing each week, please click below to download the full data file.

The graph below to the left shows that the retardance varies only slightly at a constant voltage, while the graph below to the right shows that the voltage varies only slightly at a constant retardance. Similar consistency in performance can also be expected for our other models of retarders. To maximize the long-term stability of our retarders, we recommend always using the LCC25 controller. It is specifically designed to reduce the DC voltage offset, thus minimizing charge buildup and maximizing stability.

Click Here to Download Long-Term Performance Data

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Liquid Crystal Retarder Sample Switching Time

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Switching Time Decreases with Smaller Retardance Changes

LC Retarders Switching Time

Liquid crystal retarders feature a short switching time compared to mechanical variable wave plates due to the lack of moving parts. The switching time of a liquid crystal retarder depends on several variables, some of which are controlled in the manufacturing process, and some by the user.

In general liquid crystal retarders will always switch faster when changing from a high to a low birefringence value. Additionally, the higher the operating temperature is, the faster the retarder will switch from one state to another due to the decreased viscosity at the higher temperature.

For any given retarder, the switching speed will always be faster at higher voltages. The graph to the right depicts examples of switching between different voltages. If trying to achieve faster switching speed, we recommend using the retarder together with a fixed waveplate, to use the variable retarder at a higher voltage.

In addition, the material's viscosity and hence the switching speed also depend on temperature of the LC material. As can be seen below, the switching speed can increase by as much as two times by heating the LC retarder. Our standard LC retarders are designed to work at temperatures of up to 45 °C, where they can still maintain the specified retardation. If additional speed is required, the retarders can work at temperatures up to 70 °C, but the maximum retardation value will be lower.

The switching speed also is directly proportional to the thickness of the LC retarder, the rotational viscosity of the LC material, and the dielectric anisotropy of the LC material. However, since each of those variables affects other operating parameters as well, our LC retarders are designed to optimize overall performance, with a special emphasis on switching time. We also offer OEM and custom LC retarders optimized for other parameters, as well as faster liquid crystal retarders. Contact techsupport@thorlabs.com for details.

Sample Switching Times at Various Temperatures

Switching times were tested by measuring the rise time from V₁ to V₂ and the fall time from V₂ to V₁ with the liquid crystal retarder being held at the specified temperature. V₁ was fixed in all of the tests and corresponds to the control voltage when the LC retarder is at 1 λ retardation. V₂ is the voltage at the target retardation value. Please note that switching times at lower voltages (for instance, if V₁=5 V) are longer than the switching times specified below.

LCC1115-A

Temperature	V₁	V₂	Rise Time (ms)	Fall Time (ms)
From 1 λ to 2 λ
25 °C	3.1	1.92	423	70
45.1 °C	3.1	1.92	150	20
59.9 °C	3.1	1.92	140	20
70.2 °C	3.1	1.92	100	20
From 1 λ to 4 λ
25 °C	3.1	1.34	1580	100
45.1 °C	3.1	1.34	480	50
59.9 °C	3.1	1.34	340	30
70.2 °C	3.1	1.34	260	27
From 1 λ to 8 λ
25 °C	3.1	1.04	1953	140
45.1 °C	3.1	1.04	940	58
59.9 °C	3.1	1.04	630	40
70.2 °C	3.1	1.04	435	35

LCC1115-B

Temperature	V₁	V₂	Rise Time (ms)	Fall Time (ms)
From 1 λ to 3 λ
25 °C	10	3.82	820	190
45.1 °C	10	3.82	350	110
59.9 °C	10	3.82	280	110
70.2 °C	10	3.82	145	60
From 1 λ to 7 λ
25 °C	10	2.3	1020	235
45.1 °C	10	2.3	740	140
59.9 °C	10	2.3	637.5	100
70.2 °C	10	2.3	537.5	80
From 1 λ to 14 λ
25 °C	10	1.34	3200	360
45.1 °C	10	1.34	2530	180
59.9 °C	10	1.34	2100	120
70.2 °C	10	1.34	1460	100

LCC1115-C

Temperature	V₁	V₂	Rise Time (ms)	Fall Time (ms)
From 1 λ to 2 λ
25 °C	4.94	2.96	720	187.5
45.1 °C	4.94	2.96	282.5	86.5
59.9 °C	4.94	2.96	192.5	75
70.2 °C	4.94	2.96	97.5	62.5
From 1 λ to 4 λ
25 °C	4.94	1.98	2850	230
45.1 °C	4.94	1.98	1405	135
59.9 °C	4.94	1.98	722.5	100
70.2 °C	4.94	1.98	477.5	99.5
From 1 λ to 6 λ
25 °C	1.36	1.185	4700	390
45.1 °C	1.36	1.185	1835	200
59.9 °C	1.36	1.185	1185	130
70.2 °C	1.36	1.185	1550	105

Liquid Crystal Retarder Schematic

Drawing indicates the slow and fast axes

Alignment

In order to precisely align the axis of the liquid crystal cell, mount the retarder in an appropriate rotation mount (e.g. the RSP1(/M) or the CRM1P(/M) for our Ø10 mm clear aperture retarders and RSP2(/M) or the LCRM2(/M) for our Ø20 mm clear aperture retarders). Then set up a detector or power meter to monitor the transmission of a beam through a pair of crossed linear polarizers. Next place the LC retarder between two crossed polarizers with the slow axis aligned with the transmission axis of the first polarizer. Then slowly rotate it until the transmitted intensity is minimized. In this configuration, the LC retarder is ready for phase modulation applications.

To operate as a light intensity modulator or shutter, again find the minimum transmitted intensity as prescribed above. Once the minimum is found, rotate the retarder by ±45°. This will maximize the transmitted intensity through the crossed polarizers for most LC retarders (e.g., zero-order quarter- or half-wave plates). However, this rule of thumb does not rigidly hold for multi-wave phase retarders using broadband sources due to the wavelength dependency of the retardation.

The slow (extraordinary) axis of the liquid crystal retarder corresponds to the orientation of the long axis of the liquid crystal molecules when no voltage is being applied. Applying a voltage will cause the orientation direction of the liquid crystal molecules to rotate out of the plane of the drawing, changing the retardation. Thorlabs LC retarders are nematic liquid crystal devices, which must be driven with an AC voltage in order to prevent the accumulation of ions and free charges, which degrades performance and can cause the device to burn out.

Applications

Polarization Control with a Liquid Crystal Variable Retarder
The LCVR can be effectively used as a variable zero-order wave plate over a broad spectrum of wavelengths. The optical axis of the LCVR is defined as the major axis of the liquid crystal molecules when no voltage is being applied to the cell, which are all aligned due to the LC alignment layer. When using the LCVR to control the polarization of a beam, the linearly polarized input beam should be aligned so that its polarization axis is oriented at an angle of 45° with respect to the optical axis of the LCVR in order to maximize the dynamic range of the optic. The schematic below shows how the output state of polarization will change as retardance is decreased (RMS voltage increased).

Polarization Control

Pure Phase Retarder with Liquid Crystal Variable Retarder
In order to only effect the phase of the incident beam, the linearly polarized input beam must have its polarization axis aligned with the optical axis of the liquid crystal retarder. As V_rms is increased, the phase offset in the beam is decreased. Pure phase retarders are often used in interferometers to alter the optical path length of one arm of the interferometer with respect to the other. With an LCVR, this can be done actively.

LCC25 Specifications
Electrical Specs
Adjustable Output Voltage	± 25 V
External Input Voltage	0 to 5 VDC Square Wave
Voltage Resolution	1.0 mV
Adjustable Internal Modulation Frequency	0.5 to 150 Hz @ 50% Duty Cycle
Switching Frequency	2,000 ± 5 Hz, 50% Duty Cycle
Slew Rate	10 V/µs
DC offset	±10 mV
Max Output Current	15 mA
AC Power	85 – 264 VAC, 47 – 63 Hz, 25 VA
Fuse Rating	125 mA, 5 x 20 mm SLO-BLO
Warm Up Time	30 Minutes
Physical Specs

External Input Connector	BNC
External Input Enable	Front Panel: INT/EXT enable Key
External Input Indicator	Green LED
Output Connector	BNC
Output Enable	Front Panel: OUTPUT ENABLE Key
Output Indicator	Green LED
Rotary Knob	Digital Encoder
Display	LCD 16 x 2
Power Switch	Rocker Switch
USB interface	USB Standard B Plug
Dimensions	5.75″ x 3″ x 12.2″ 146 mm x 78 mm x 309 mm
Weight	3.6 lbs
Operating Temperature Range	10 to 40 °C
Maximum Relative Humidity	85%
Other	Tilting Rubber-Padded Feet

The LCC25 liquid crystal variable retarder and polarization rotator controller produces a 2000 Hz square wave output with an amplitude that can be varied from 0 to 25 V_rms. The output amplitude can be set via the front panel controls, the USB interface, and the external input. Both the front panel and USB interface allow the user to select two voltage levels, Voltage 1 and Voltage 2. When the LCC25 is operated in the constant voltage mode, the output of the controller will be a 2000 Hz square wave with an amplitude equal to either of the two set voltage levels (Figure A). If the LCC25 controller is operating in the modulation mode, the output 2000 Hz square wave will be modulated in amplitude between the two voltage settings with a modulation frequency that can be set by the user to be between 0.5 and 150 Hz (Figure B).

The modulated mode can be used to measure the response time of the LC retarder.

External or remote control of the LCC25 is possible using the external input or the USB interface. The external input accepts a 0 to 5 VDC TTL signal that modulates the 0 to 25 V_rms output of the LCC25 between the two set voltages. The USB interface can be used to send line commands to the controller so that the LCC25 can be used in automated lab sequences.

In order to prevent the separation and build up of charges in the liquid crystal layer, the LCC25 will automatically detect and correct any DC offset in real time to within ±10 mV.

Figure A. A plot of the output voltage of the LCC25 Liquid Crystal Controller when it is being operated in the constant voltage mode.

Figure B. A plot of the output voltage of the LCC25 Liquid Crystal Controller when the output voltage is being modulated between the two set voltages.

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Screen shot of the GUI interface in Modulation Mode.

Software for the LCC25 Controller

Software

Version 3.2.0

GUI Interface for controlling the Liquid Crystal Retarder Controller via a PC. To download, Click the button below.

GUI Interface
The GUI interface included with the software provides access to all of the settings of the liquid crystal retarder controller. For example, the user can select one of two user-defined voltages or a modulation mode that oscillates between these two voltages at a user-defined frequency. As shown in the above screen shot, the applied voltage is shown in a plot with respect to time. Both the output and external input can be turned on and off via the GUI. In addition, advanced features allow the user to define a custom waveform by specifying the starting voltage, ending voltage, the voltage step size, and the dwell time. The waveform may be previewed on the screen before it is output to the retarder, and it may be saved so that the LCC25 can be restarted quickly in the future. The GUI is available as a stand-alone or LabVIEW based version for flexibility in implementation.

Custom Software Development
Users may also use the provided C/C++ and LabVIEW software development kits for implementing the liquid crystal retarder controller with other instruments. Sample C++ code and LabVIEW programs help to illustrate how the C commands and LabVIEW VIs can be utilized. Full documentation on the available commands is provided with the software.

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Liquid Crystal Cell Seal Application

Thorlabs' Custom Liquid Crystal Capabilities

Thorlabs offers a large variety of liquid crystal retarders from stock, including 1/2-, 3/4-, full-wave, and multi-wave models with a Ø10 mm or Ø20 mm clear aperture as well as 1/2-wave temperature-controlled models. However, we also offer OEM and custom retarders. The retardance range, coating, rubbing angle, temperature stabilization, and size can be customized to meet many unique optical designs. We also offer other custom liquid crystal devices, such as empty LC cells, polarizaton rotators, and noise eaters. For more information about ordering a custom liquid crystal device, please contact Thorlabs' technical support.

Our engineers work directly with our customers to discuss the specifications and other design aspects of a custom liquid crystal retarder. They will analyze both the design and feasibility to ensure the custom products are manufactured to high-quality standards and in a timely manner.

Polyimide (PI) Coating and Rubbing - Custom Alignment Angle
In their nematic phase, liquid crystal molecules naturally align to an average orientation, which together with their stretched shape, creates an optical anisotropy, or direction-dependent optical effect. The orientation of the LC molecules in an LC cell, in the absence of an applied voltage, is determined by the alignment layer, created by the polyimide (PI) coating and rubbing angle. Rubbing creates grooves, which the liquid crystal molecules will align to. Users can choose any initial orientation of LC molecules by specifying the rubbing angle.

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Custom Liquid Crystal Cell Without Case

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Liquid Crystal Cell Filling in a Vacuum Chamber

Custom Cell Spacing
The wall spacing inside of the liquid crystal cell, which determines the thickness of LC material, can be customized during the manufacturing process. The retardance range of an LC cell is dependent on the LC material thickness:

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Custom Liquid Crystal Cell Test Setup

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Custom Liquid Crystal Cell Test Result

Here, δ is the retardance in waves, d is the thickness of the LC material, λ_ν is the wavelength of light, and Δn is the birefringence of the LC material used. Thus, for a given wavelength, the retardance is determined by the wall spacing inside the LC cell (i.e., the thickness of LC layer). We offer standard retardance ranges of λ/2 to 30 nm, 3λ/4 to 30 nm, and λ to 30 nm, but higher retardance ranges may also be ordered.

Custom Liquid Crystal Material
Customers can also provide their own liquid crystal material, and Thorlabs will use it to fill the liquid crystal cell. Since different liquid crystal materials have different birefringence values, varying the material enables a different retardance range.

Temperature Control/Switching Time
A temperature sensor can also be integrated into the LC variable retarder. Using a temperature controller and a heater, the temperature of the retarder can be actively stabilized to within ±0.1 °C. The viscosity of the liquid crystal material is lowered at higher temperatures, allowing the retarder to switch from one state to another due to the decreased viscosity. An active temperature control system can be used to heat the retarder, allowing it to operate at higher switching speeds.

Assembly / Housing
If desired, we can manufacture custom liquid crystal retarders without housings.

Testing
Each LC retarder is tested for birefringence, uniformity, and fast axis angle, using the measurement setup shown in the photo to the left. The equipment measures the 2-dimensional birefringence distribution using wave plates and a CCD camera. The image to the right shows a sample test result of a liquid crystal retarder, showing excellent uniformity.

For More Information
Contact Thorlabs' technical support for more information about our custom liquid crystal device options or to place an order.

Custom Capability	Custom Specification
Patterned Retarder Size	Ø100 µm to Ø2"
Patterned Retarder Shape	Any
Microretarder Size	≥Ø30 µm
Microretarder Shape	Round or Square
Retardance Range @ 632.8 nm	50 to 550 nm
Substrate	N-BK7, UV Fused Silica, or Other Glass
Substrate Size	Ø5 mm to Ø2"
AR Coating	-A: 350 - 700 nm -B: 650 - 1050 nm -C: 1050 - 1700 nm

Liquid Crystal Retarder Smaple Switching Time

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Figure 1: Patterned Retarder with Random Distribution

Features

Build a Custom Microretarder
Customize Size, Shape, and Substrate Material
Retardance Range: 50 - 550 nm
Fast Axis Resolution: <1º
Retardance Fluctuations Under 30 nm

Applications

3D Displays
Polarization Imaging
Diffractive Optical Applications: Polarization Gratings, Polarimetry, and Beam Steering

Thorlabs offers customizable patterned retarders, available in any pattern size from Ø100 µm to Ø2" and any substrate size from Ø5 mm to Ø2". These custom retarders are composed of an array of microretarders, each of which has a fast axis aligned to a different angle than its neighbor. The size and shape of the microretarders are also customizable. They can be as small as 30 µm and in shapes including circles and squares. This control over size and shape of the individual microretarders allows us to construct a large array of various patterned retarders to meet nearly any experimental or device need.

These patterned retarders are constructed from our liquid crystals and liquid crystal polymers. Using photo alignment technology, we can secure the fast axis of each microretarder to any angle within a resolution of <1°. Figures 1 - 3 show examples of our patterned retarders. The figures represent measured results of the patterned retarder captured on an imaging polarimeter and demonstrate that the fast axis orientation of any one individual microretarder can be controlled deterministically and separately from its neighbors.

The manufacturing process for our patterned retarders is controlled completely in house. It begins by preparing the substrate, which is typically N-BK7 or UV fused silica (although other glass substrates may be compatible as well). The substrate is then coated with a layer of photoalignment material and placed in our patterned retarder system where sections are exposed to linearly polarized light to set the fast axis of a microretarder. The area of the exposed sections depends on the desired size of the microretarder; the fast axis can be set between 0° and 180° with a resolution <1°. Once set, the liquid crystal cell is constructed by coating the device with a liquid crystal polymer and curing it with UV light.

Thorlabs' LCP depolarizers provide one example of these patterned retarders. In principle, a truly randomized pattern may be used as a depolarizer, since it scrambles the input polarization spatially. However, such a pattern will also introduce a large amount of diffraction. For our depolarizers, we designed a linearly ramping fast axis angle and retardance that can depolarize both broadband and monochromatic beams down to diameters of 0.5 mm without introducing additional diffraction. For more details, see the webpage for our LCP depolarizers.

By supplying Thorlabs with a drawing of the desired patterned retarder or an excel file of the fast axis distribution, we can construct almost any patterned retarder. For more information on creating a patterned retarder, please contact Tech Support.