Features

• Variable Wave Plate to Actively Control the Polarization State of Light
• Retardance Range: ~30 nm to λ/2
• Clear Aperture: Ø10 mm or Ø20 mm
• Temperature-Stabilized Option for Ø10 mm Clear Aperture Retarder
• Sufrace Quality: 20-10 Scratch-Dig
• Retardance Uniformity: <λ/50 Over the Entire Clear Aperture

Thorlabs' Half-Wave Liquid Crystal Variable Retarders (LCVR) use a nematic liquid crystal cell to function as a variable wave plate. The absence of moving parts provides quick switching times on the order of milliseconds (see the Switching Time tab for details). AR coatings are available for three wavelength ranges: 350 - 700 nm, 650 - 1050 nm, or 1050 - 1620 nm (see LC Retarders tab for transmission and retardance data). Thorlabs' offers two options for clear aperture size, Ø10 mm and Ø20 mm. Our Ø10 mm LCC1111 retarders feature a 1" outer diameter, making them compatible with any of our Ø1" optics mounts for 8 mm thick optics. The Ø20 mm retarders have a 2" outer diameter, providing compatibility with any of our Ø2" optics mounts for 13 mm thick optics.

These liquid crystal variable retarders provide excellent uniformity, low optical losses and low wavefront distortion. Our retarders also provide quick switching time, a broad operating temperature range, and a broad wavelength range. Please see the Specs and LC Retarders tabs for complete details. We also offer temperature-stabilized half-wave retarders for more consistent performance.

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. Two parallel faces of the cell wall are coated with a transparent conductive film so that a voltage can be applied across the cell. The orientation of the LC molecules is determined by the alignment layer in the absence of an applied voltage. When an AC voltage is applied, the molecules will change from their default orientation based on the applied rms value of the voltage. Hence, the phase offset in a linearly polarized beam of light can be actively controlled by varying the applied voltage.

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.

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 birefringence 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 in the driving signal for an operating range of 0 V to 25 V. To accomplish this the LCC25 controller automatically zeros the DC bias across the LC device in order to counteract the buildup of charges. The LC material also exhibits some chromatic dispersion due to changes in the molecular polarizability. To account for this, we provide the retardation data below for two wavelengths in each wavelength range.

Additionally, the LC retardation also depends on the temperature of the device. As temperature increases the material density decreases and 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.

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Click Here to Download Transmission Data

LCC1111-A & LCC1221-A (350 - 700 nm)

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LCC1111-B & LCC1221-B (650 - 1050 nm)

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LCC1111-C & LCC1221-C (1050 - 1620 nm)

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The fall time decreases with larger voltage changes. Similarly, the rise time also decreases for larger voltage 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 low to high 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 faster switching speeds are desired, we recommend using the retarder together with a fixed wave plate so that the retarder can be used at a larger 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 up to 50 °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 is also 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 custom and OEM LC retarders optimized for other parameters, as well as faster liquid crystal retarders. See the Custom Capabilities tab above, or 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 is fixed at 10 V for all the tests, and V₂ is the voltage at which the retardation is the maximum specified value for the retarder (1/2 λ). Please note that switching times at lower voltages (for instance, if V₁=5 V) are longer than the switching times specified below.

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 or the CRM1P for our Ø10 mm clear aperture retarders and RSP2 or the LCRM2 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 and then rotate it until the transmitted intensity is minimized.

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. Thorlabs LC retarders are nematic liquid crystal devices, which must be driven with an AC voltage in order to prevent the separation and build up of charge, which 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).

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.

The LCC25 liquid crystal variable retarder 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 ouptput 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 2.0.0

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-, and full-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, 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.