Full-Wave Liquid Crystal Retarder with Temperature Control

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.

Features

• Variable Wave Plate to Actively Control the Polarization State of Light
• Retardance Range: ~30 nm to λ
• Ø20 mm Clear Aperture
• Surface Quality: 40-20 Scratch-Dig
• Retardance Uniformity: <λ/50 Over the Entire Clear Aperture

Thorlabs' Thermally Stabilized Full-Wave Liquid Crystal Variable Retarder (LCVR) uses 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). This liquid crystal retarder features an integrated heater which will hold the temperature of the retarder constant. Temperature stabilization provides constant retardance even if the ambient temperature changes and also allows for faster switching times. Each retarder includes a Y-cable which allows it to be connected with one of our LC Controllers and our TC300B Heater and TEC Temperature Controller (all available separately below).

The LCC1223T-A is AR coated for visible light from 350 to 700 nm (see the Performance tab for transmission and retardance data). This retarder features a Ø20 mm clear aperture and has an outer diameter of 2", making it compatible with any of our Ø2" optic mounts for 12 mm thick optics (such as Item # LCRM2A or Item # RSP2).

Performance
This liquid crystal variable retarder provides excellent uniformity, low optical losses, and low wavefront distortion. It also offers a quick switching time, 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 V_rms.

Controllers
The LCC25 and KLC101 liquid crystal controllers provide active DC offset compensation while applying an AC voltage (0 to 25 V_rms). The DC offset compensation automatically zeroes the DC bias across the LC device in order to counteract the buildup of charge. The TC300B Heater and TEC Temperature Controller provides precise PID regulated control of temperature.

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 and KLC101 controllers, sold below, are 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 two select wavelengths within the product's wavelength range in the graph below.

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 Performance Data

LCC1223T-A (350 - 700 nm)

Click to Enlarge

Click to Enlarge

Click to Enlarge

Click to Enlarge
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 our LCC25 or KLC101 controllers. They are designed to reduce the DC voltage offset, thus minimizing charge buildup and maximizing stability.

Click Here to Download Long-Term Performance Data

Click to Enlarge

Click to Enlarge

Click to Enlarge

LC Retarders Switching Time

Liquid crystal (LC) 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 of which are controlled 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. The LCC1223T-A retarder is designed to work at temperatures up to 45 °C.

For any given retarder, the switching speed will always be faster at higher 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.

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 Tech Support 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 (λ). Please note that switching times at lower voltages (for instance, if V₁=5 V) are longer than the switching times specified below.

Alignment

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 to the right, 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.

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 CRM1PT(/M) for our Ø10 mm clear aperture retarders and RSP2(/M) or the LCRM2A(/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.

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.

Damage Threshold Data for Thorlabs' Liquid Crystal Variable Retarder

The specifications to the right are measured data for Thorlabs' Thermally Stabilized Liquid Crystal Variable Retarder.

Laser Induced Damage Threshold Tutorial

The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.

Testing Method

Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.

First, a low-power/energy beam is directed to the optic under test. The optic is exposed in 10 locations to this laser beam for 30 seconds (CW) or for a number of pulses (pulse repetition frequency specified). After exposure, the optic is examined by a microscope (~100X magnification) for any visible damage. The number of locations that are damaged at a particular power/energy level is recorded. Next, the power/energy is either increased or decreased and the optic is exposed at 10 new locations. This process is repeated until damage is observed. The damage threshold is then assigned to be the highest power/energy that the optic can withstand without causing damage. A histogram such as that below represents the testing of one BB1-E02 mirror.

The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm² (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.

Example Test Data
Fluence	# of Tested Locations	Locations with Damage	Locations Without Damage
1.50 J/cm2	10	0	10
1.75 J/cm2	10	0	10
2.00 J/cm2	10	0	10
2.25 J/cm2	10	1	9
3.00 J/cm2	10	1	9
5.00 J/cm2	10	9	1

According to the test, the damage threshold of the mirror was 2.00 J/cm² (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.

Continuous Wave and Long-Pulse Lasers

When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) [1]. Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions.

When pulse lengths are between 1 ns and 1 µs, laser-induced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.

LIDT in linear power density vs. pulse length and spot size. For long pulses to CW, linear power density becomes a constant with spot size. This graph was obtained from [1].

Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.

In order to use the specified CW damage threshold of an optic, it is necessary to know the following:

1. Wavelength of your laser
2. Beam diameter of your beam (1/e²)
3. Approximate intensity profile of your beam (e.g., Gaussian)
4. Linear power density of your beam (total power divided by 1/e² beam diameter)

Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below. 

The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).

Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):

While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application. 

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.

Pulsed Lasers

As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.

Pulses shorter than 10^-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism [2]. In contrast, pulses between 10^-7 s and 10^-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.

Pulse Duration	t < 10-9 s	10-9 < t < 10-7 s	10-7 < t < 10-4 s	t > 10-4 s
Damage Mechanism	Avalanche Ionization	Dielectric Breakdown	Dielectric Breakdown or Thermal	Thermal
Relevant Damage Specification	No Comparison (See Above)	Pulsed	Pulsed and CW	CW

When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:

LIDT in energy density vs. pulse length and spot size. For short pulses, energy density becomes a constant with spot size. This graph was obtained from [1].

1. Wavelength of your laser
2. Energy density of your beam (total energy divided by 1/e² area)
3. Pulse length of your laser
4. Pulse repetition frequency (prf) of your laser
5. Beam diameter of your laser (1/e² )
6. Approximate intensity profile of your beam (e.g., Gaussian)

The energy density of your beam should be calculated in terms of J/cm². The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e² beam.

Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately [3]. A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm² at 1064 nm scales to 0.7 J/cm² at 532 nm):

You now have a wavelength-adjusted energy density, which you will use in the following step.

Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT [4]. For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.

The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:

Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10^-9 s and 10^-7 s. For pulses between 10^-7 s and 10^-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.

---

[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).

Click to Enlarge
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 and full-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, polarization 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.

Click to Enlarge
Custom Liquid Crystal Cell Without Case

Click to Enlarge
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:

Click to Enlarge
Custom Liquid Crystal Cell Test Setup

Click to Enlarge
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.