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Multi-Wave Liquid Crystal Variable Retarder with Integrated Controller
LCC2415-VIS Variable Retarder Connected to an RSP1 Rotation Mount with an SM1T2 Lens Tube Coupler
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.
Designed for the visible wavelength range (400 - 700 nm), this Multi-Wave Liquid Crystal Variable Retarder offers an absolute retardance that can be continuously varied from 0 to 4000 nm, corresponding to a maximum possible retardance of 5.71λ at 700 nm and 10λ at 400 nm. The retardance magnitude is controlled by applying a voltage to an LC cell, shown schematically in the Operating Principle box to the right. Because this wave plate contains no moving parts, it is able to achieve switching times on the order of hundreds of milliseconds, as shown in the Performance tab.
The included Windows® software package allows the user to set the retardance (in units of waves, degrees, or nm) or the voltage across the liquid crystal cell (0 V to 10 V, with 0.2 mV resolution). For retardance sequences, the user needs to supply an external trigger signal (5 V TTL) through the female SMC connector on the housing (see photo below) to advance from step to step; the step will occur on the falling edge of the TTL signal. An SMC cable, sold separately, is required to trigger a sequence.
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USB and SMC Connectors on the Side of LCC2415-VIS Retarder
This variable wave plate has a clear aperture of Ø10 mm, and it is contained in a compact 60.0 mm x 60.0 mm x 18.0 mm (2.36" x 2.36" x 0.71") housing for simple integration into an optical setup. The housing incorporates both the liquid crystal (LC) cell and the controller, so no other external boxes need to be placed onto the tabletop. For mounting, the housing contains internal SM1 (1.035"-40) threads on both sides for our Ø1" lens tubes, four 4-40 taps on both sides for our 30 mm cage systems, and two 8-32 (M4) taps for Ø1/2" posts.
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Mechanical Drawing of LCC2415-VIS(/M)
These plots were obtained using a typical retarder unit; the actual performance will vary from unit to unit. All units are factory calibrated before shipment so that they will provide retardance of 0 nm to 4000 nm (±1%) for input wavelengths in the 400 - 700 nm range when operated from the included software. The unit-specific calibration data is available upon request.
All performance data was taken at normal incidence at an ambient temperature of 25 °C.
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Click for Raw Data at 5 nm Increments in the 400 - 700 nm Range
This plot shows the drive voltage, VRMS, that the software applies to the liquid crystal cell to achieve the desired retardance magnitude.
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Click for Raw Data
The shaded region in this plot indicates the LCC2415-VIS's design wavelength range.
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Graph shows variation in retardance over a period of 154 weeks.
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.
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 LCC2415-VIS retarder is designed to work at temperatures up to 70 °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 email@example.com for details.
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This plot gives switching times of our LCC2415-VIS retarder at 25 °C for V1 = 3.1 VRMS and V2 = 1.34 VRMS. At an input wavelength of 400 nm, these drive voltages produce a retardance of 505 nm and 2835 nm, respectively. The rise time is greater than the fall time, meaning that a retardance increase takes longer than the equivalent retardance decrease.
Software for the LCC2415-VIS Multi-Wave Variable Retarder with Integrated Controller
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Main Window of LCC2415-VIS User Interface
The GUI also allows easy entry of user-defined retardance sequences, as shown in the screenshot to the right. A sequence consists of a number of steps, each of which has its own retardation value. In addition to manual entry, the software allows sequences to be defined by a starting value, an ending value, and the step size between start and end. Sequences can also be imported from and exported to CSV format.
Once the sequence is entered, the user needs to supply an external trigger signal (5 V TTL) through the female SMC connector on the housing to advance from step to step; the step will occur on the falling edge of the TTL signal. SMC cables are sold separately.
Custom Software Development
The liquid crystal (LC) cells used in Thorlabs' liquid crystal retarders consist of a transparent cell filled with a solution of liquid crystal molecules. Two parallel faces of the cell wall are coated with a transparent conductive film so that a voltage can be applied across the cell.
As shown in the pictures to the right, in the absence of an applied voltage, the orientation of the LC molecules is determined by the alignment layer that is created by the organic polyimide coating and rubbing angle. The molecules' orientation, together with the stretched shape of the molecules, creates the birefringence necessary for retardance. The retardance, δ, in units of wavenumbers, is given by the following formula:
Here, d is the effective path length through the liquid crystal solution, λ is the wavelength, and Δn is the difference in the refractive index between the parallel and perpendicular polarization directions.
When an AC voltage is applied across the cell, the molecules will change from their default orientation and align to the applied electric field. The level of birefringence is determined by the tilting of the LC molecules and controlled by the VRMS magnitude. Hence, the phase of a linearly polarized beam of light can be actively modulated by varying the applied voltage.
It is important to note that the LC molecules adjacent to the alignment layer (shown in green in the pictures above) do not respond to the applied field, preventing the retardance imparted by the LC cell from being truly zero. Hence, we have included an extra, fixed-retardance wave plate in the construction of the LCC2415-VIS, which compensates for the retardation imparted by the non-moving LC molecules.
Damage Threshold Data
The specifications to the right are measured data for Thorlabs' LCC2415-VIS Liquid Crystal 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.
Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.
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.
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) . 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.
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:
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.
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 . 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.
When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:
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 . 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 . 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.
 R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
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Liquid Crystal Cell Seal Application
Thorlabs' Custom Liquid Crystal Capabilities
Thorlabs offers a large variety of liquid crystal (LC) retarders from stock, including half-, three-quarter-, and full-wave models with a Ø10 mm or Ø20 mm clear aperture as well as half-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
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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
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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, λν 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 the 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
Temperature Control / Switching Time
Assembly / Housing
For More Information
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Figure 1: Patterned Retarder with Random Distribution
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.
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Figure 2: Patterned Retarder with a Spiral Distribution
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Figure 3: Patterned Retarder with a Pictoral Distribution