Meadowlark Optics

Resources

Articles and Application Notes

POLARIZATION COMPONENTS

A new technology for performing high-precision Stokes polarimetry is presented. One traditional Stokes polarimetry configuration relies on mechanical devices such as rapidly rotating waveplates that are undesirable in vibration-sensitive optics experiments. Another traditional technique requires division of a light signal into four components that are measured individually; this technique is limited to applications in which signal levels are sufficient that intensity reduction does not diminish the signal-to-noise ratio.

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The immediate purpose of a polarimeter such as the LCPM-3000 liquid crystal polarimeter from Meadowlark Optics is to measure the vector components that combine to describe the polarization state of light. A polarimeter can also be used as a precision diagnostic tool; not only is it useful for characterizing light signals and sources, it is also effective at precisely characterizing optical components through their effect on the polarization state of light.

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This application note briefly describes polarized light, retardation and a few of the tools used to manipulate the polarization state of light. Also included are descriptions of basic component combinations that provide common light manipulation tools such as optical isolators, light attenuators, polarization rotators and variable beam splitters.

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Many are enabled by new materials including polymers and liquid crystals. We survey here these and other relatively new devices and components available commercially that open new possibilities for astronomers.

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 Beam splitting polarizer cubes consisting of two right angle prisms cemented together after one hypotenuse is coated have become important optical components in many optical systems. Usually the coating stack is of the MacNeille design. We present and compare an alternative coating structure consisting of a very fine wire grid structure on the cube hypotenuse that has performance advantages of improved polarization purity over an extended range of wavelengths and angles. Modern lithography permits wire spacings and dimensions that are small enough for good polarizer performance at visible wavelengths as well as near infrared wavelengths.

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Measured extinction ratio on high quality linear polarizers depends on test system geometry. The measurement becomes especially challenging for polarizers with extinction ratio expected to exceed 106. We describe methods capable of measurements of high purity polarizers at and above 106 extinction ratio. We discuss the geometrical factors affecting the measured results that may be pertinent in determining what performance is achievable in a users system.

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 Light is a transverse electromagnetic wave. Every light wave has a direction of propagation with electric and magnetic fields that are perpendicular to the direction of propagation of the wave.

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For many applications involving polarized light, it is important that the azimuthal angle of a polarization optic (i.e. polarizer, retarder, etc…) be accurately aligned to a physical datum or to an eigenaxis of another polarization optic. A simpleopto-mechanical tool for azimuthal alignment can be used to perform accurate alignments and consists of two "rotatable" mounts. One mount holds a polarizer, while the other holds a half-wave retarder. The method of swings is used to aid in the azimuthal alignment of the polarization optic and is illustrated using the Poincaré Sphere. Additionally, imperfections in polarization optics are discussed.

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The ability to accurately rotate the polarization of incident light while minimizing any losses in polarization purity has applications in optical switching, polarimetry, and microscopy. Polarization rotators utilizing tunable birefringent plates, such as liquid crystal (LC) devices, have the advantage of non-mechanically tuning the devices’ retardance. However, these devices properly work with incident light within a very specific wavelength range. Ferroelectric liquid crystal (FLC) devices can switch between two orthogonal states of linear polarization, and other response times much faster than their nematic liquid crystal cell counterparts. An achromatic polarization rotator can be constructed with an FLC cell between two half-wave plates that have been constructed to produce a half-wave retardance at a certain design wavelength. This results in a device that others fast response times and high polarization purity over a broader wavelength range.

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The Fabry-Perot is an optically resonant cavity formed by two partically reflecting mirrors that ideally are non-absorbing and therefore transmit the light tht is not reflected.

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While the liquid crystal industry is primarily driven by the display industry, increasingly important applications in science and engineering have emerged such as beam steering, wavefront modulation and polarization switching and control. We will discuss some of the differences in construction techniques needed to produce a precision optical device rather than a flat panel display along with development work being carried out at Meadowlark Optics in some of the above areas. These include polarization switches capable of greater than 5000:1 contrast and high efficiency beam steering for precision interferometer gauges.

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Since the human eye is insensitive to polarization, there is a large amount of information in many situations, which is not readily utilized. Measuring the polarization state of light is useful in many research fields including biology, chemistry, astronomy and remote sensing. The first portion of the paper discusses the simple application of accurately measuring the retardance value and fast axis position of an unknown waveplate. We will mention some of the many polarimetry applications especially in the context of non-mechanical, liquid crystal based polarimeter experimental technique. Some of these examples are from biology showing tissue birefringence changes, astronomy for solar imaging, polarimetric visualization and landmine detection.

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Meadowlark Optics first introduced nematic liquid crystal (LC) devices for precision optical applications more than 25 years ago. These devices are variable retarders, sometimes called LCVRs. They provide electrical control of retardance at low voltages, usually 20 volts (2 kHz square wave AC) or less. Figure 1 shows an example of the relationship of retardation to voltage for one of these devices. In many applications they have supplanted their much higher voltage cousin, the Pockels cell.

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Beamsteering using liquid crystals can be achieved with refractive or diffractive implementations. The common thread in these various structures is that the liquid crystal is employed as an optical phase modulator.

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Novel tunable polarization interference filters (PIF) employing active liquid crystal devices are presented, and the principles of operation are described.

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Novel tunable polarization interference filters Cholesteric liquid crystals were the first liquid-crystalline phase observed.

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Meadowlark Optics manufactures four types of non-mechanical, vibration free, shutters. Three of these require polarized light input, preferably linearly polarized. They are used in conjunction with an exit polarizer. The extinction ratio between the open and closed states depends strongly on the polarization purity of the input beam or input polarizer and on the quality of the exit polarizer.

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Liquid crystal tunable filters are gaining wide acceptance in such diverse areas as optical fiber communications, astronomy, remote sensing, pollution monitoring, color generation for display and medical diagnostics.

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This article highlights Meadowlark Optics Liquid Crystal Variable Retarders’s role in Lidar measurements of aerosol layers asscoiated with the 2015 Calbuco eruption.

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This article highlights Meadowlark Optics Ferroelectric LC Rotator’s role in creation of a highly versatile stellar polarimeter developed at the University of New South Wales(UNSW).

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Liquid crystal (LC) technology, a critical component in a diverse range of optics for visible wavelengths, has recently been adapted into devices for the mid-wave infrared (MWIR). Optics designs, including variable retarders, attenuators, linear polarization rotators, and tunable filters, have been modified for optimal performance over the range of 3.6 to 5.7 microns.

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During the last two decades, Liquid Crystal Variable Retarder (LCVR) technology has matured and advanced as reliable and well-understood technology for ground applications to the point of being recently integrated in space-based optical instruments for the first time. LCVR cells use nematic liquid crystals to electronically tune the birefringence of the device in order to control the polarization of the transmitted light.

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Meadowlark Optics’ Liquid Crystal Tunable Filters facilitate spectral analysis of not only  a collimated beam, but more importantly of an entire object. The Tunable Filter passes  only a narrow bandwidth of light while blocking all others with the spectral range. The  pass band can be shifted to a new color in the blink of an eye. This combination of pass  band and speed is equivalent to literally thousands of dichroic or interference filters on  hundreds of filter wheels. This enables the user to acquire images at thousands of  different wavelengths in a short amount of time. Applications of Liquid Crystal Tunable  Filters include fluorescence microscopy, absorption microscopy, Raman microscopy, and  solar astronomy.

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Analysis of polarized light is an important diagnostic tool in many applications including  remote sensing, solar astronomy, atomic and molecular spectroscopy and material analysis.  For example, solar magnetic field measurements can be obtained by polarization analysis of  certain components of the solar spectrum. Surface roughness and anisotropy of materials can  be determined by investigating the depolarization of light upon reflection. 

 

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Introducing Meadowlark Optics’ fastest family of liquid  crystal devices. Using a proprietary method of construction switching speeds are dramatically increased  over the standard liquid crystal family. These cells also have a symmetric switching speed so sluggish relaxation rates are no longer the limiting factor for time response. (In normal Nematic devices, relaxation rate is quadratically dependent on thickness. e.g. twice the cell thickness leads  to four times the switching time.)  Still an analog device, these cells can be used as variable attenuators, as polarization rotators and as variable retarders. Thin cells with less than a wave of retardance and up to 50,000 nm of  retardance are possible. The cells can also be compensated to provide a zero retardance point. Meadowlark is please to discuss custom requirements.

 

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This application note briefly describes polarized light, retardation and a few of the tools used to manipulate the polarization state of light. Also included are descriptions of basic component combinations that provide common light manipulation tools such as optical isolators, light attenuators, polarization rotators and variable beam splitters.

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Many are enabled by new materials including polymers and liquid crystals. We survey here these and other relatively new devices and components available commercially that open new possibilities for astronomers.

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Retarders or waveplates are tools for polarization modification in bulk optical systems. These devices usually have a strong wavelength dependence in their performance, making them suitable for use over a wavelength band on the order of a few percent of the center wavelength for which they are made.

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Precision control of polarization is increasingly important in disciplines such as solar polarimetry, optical communications, biomedical imaging, military target identification, chemical analysis, and wavelength filtering.

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Retarders or waveplates are useful devices for modifying the polarization of light. Until recently these have always been free standing optical elements, usually made using solid uniaxial crystals such as quartz, calcite or magnesium fluoride. Meadowlark Optics has specialized in application of new materials for polarization modification.

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Error sources that afflict true zero order, compound zero order and multi-order linear retarders include variations in temperature, angle of incidence, wavelength, and the presence of multiple reflections. This application note contains technical information designed to aid the user in producing the highest quality results and in making the best purchasing choice among retarder types for the intended application.

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Error sources that afflict true zero order, compound zero order and multi-order linear retarders include  variations in temperature, angle of incidence, wavelength, and the presence of multiple reflections. This  application note contains technical information designed to aid the user in producing the highest quality  results and in making the best purchasing choice among retarder types for the intended application.

 

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SPATIAL LIGHT MODULATORS

Adaptive optics was first utilizedto correct for aberrations that are introduced whenimaging through atmospheric turbulence.In monochromatic imaging systemsor laser communication systemswavefront correctionis most easily accomplished by adding a liquid crystal spatial light modulator to the imaging system.

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In order to understand biological functionsat a system level it is necessary to image interconnectivity of processes in real time. This is particularly relevant in neuroscience, where the BRAIN initiative is funding research to understand how the brain functions, and how that function is altered by disease.

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Despite extensive research, brain function and neurological diseases are poorly understood. Complexities arise from the quantity of neurons in the brain and from the densely interconnected networks of intermixed cell types.There is a need for methods that noninvasively probe the underlying micro-circuitry in the brain with single-cell resolution.

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Fiber optic communications have long been of interest, but more recently the technology is being coupled with SLMs for imaging deep in tissue where scattering would otherwise prevent optical techniques from being practical. Much research is focused on imaging through multi-mode fibers due to the small form factor that minimizes damage to surrounding tissue.

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With the recent award of the Nobel Prize to Betzig and Moerner there has been a significant increase in awareness of point spread function (PSF) engineering. In this case the SLM is placed in the emission arm of a microscope. Moerner demonstrated use of PSF engineering with a Meadowlark OpticsSLM for super-resolution imaging and 3D localization of fluorescence emitters.

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Much research is aimed at improving imaging in a variety of conditions such as: correcting for spherical aberrationsin 3D imaging far from the ideal focal plane, correcting for motion related artifacts, and developing new imaging modalities.In each case the SLM is used as a powerful tool to supplement and enhance capabilities of traditional microscopes.

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Holographic optical tweezing uses tightly focused laser beams to manipulate the 3D position of objects within a field of view. This can be used for research in fundamental physics, biological studies, and cold atom trapping. The SLM is usedto modulate the phase of an incident laser beam to create a 3D volume of focal points.

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 By modulating the phase and/or amplitude of spectral component of broadband femtosecond lasers it is possible to generate arbitrarily shaped ultrafast optical waveforms. Applications for this technology include optical communications, biomedical optical imaging, high power laser amplifiers, quantum control, and laser-electron beam interactions.

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 Xi, S., Wang, X., Song, L., Zhu, Z., Zhu, B., Huang, S., ... & Wang, H. (2017). Experimental study on optical image encryption with asymmetric double random phase and computer-generated hologram. Optics express, 25(7), 8212-8222.

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 Liu, S. L., Zhou, Z. Y., Liu, S. K., Li, Y. H., Li, Y., Xu, Z. H., ... & Shi, B. S. (2018). Coherent manipulation of a three-dimensional maximally entangled state. arXiv preprint arXiv:1807.07257.

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Starshynov, I., Ghafur, O., Fitches, J., & Faccio, D. (2019). Coherent control of light for non-line-of-sight imaging. arXiv preprint arXiv:1908.04094.

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McManamon, P. F., Bos, P. J., Escuti, M. J., Heikenfeld, J., Serati, S., Xie, H., & Watson, E. A. (2009). A review of phased array steering for narrow-band electrooptical systems.Proceedings of the IEEE,97(6), 1078-1096.

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Knarr, S. H. (2019). Compressive characterization of high-dimensional classical and quantum light sources. University of Rochester.

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 Eilers, H., Anderson, B. R., & Gunawidjaja, R. (2018, February). Authentication via wavefront-shaped optical responses. In Photonic Instrumentation Engineering V(Vol. 10539, p. 1053916). International Society for Optics and Photonics.

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Muralidharan, A., Uzcategui, A. C., McLeod, R. R., & Bryant, S. J. (2019). Stereolithographic 3D Printing for Deterministic Control over Integration in Dual-Material Composites. Advanced Materials Technologies, 1900592.

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Dayton, D., Spencer, M., Hassall, A., & Rhoadarmer, T. (2018, September). Distributed-volume optical disturbance generation in a scaled-laboratory environment using nematic liquid-crystal phase modulators. In Unconventional and Indirect Imaging, Image Reconstruction, and Wavefront Sensing 2018(Vol. 10772, p. 107720H). International Society for Optics and Photonics.

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Stockley, J. E., Sharp, G. D., Serati, S. A., & Johnson, K. M. (1995). Analog optical phase modulator based on chiral smectic and polymer cholesteric liquid crystals.Optics letters,20(23), 2441-2443.

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 Ewing, T. K., & Folks, W. R. (2005, May). Liquid crystal on silicon infrared scene projectors. InTechnologies for Synthetic Environments: Hardware-in-the-Loop Testing X(Vol. 5785, pp. 36-45). International Society for Optics and Photonics.

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Despite extensive research, brain function and neurological diseases are poorly understood. Complexities arise from the quantity of neurons in the brain and from the densely interconnected networks of intermixed cell types. Tools neuroscientists have traditionally relied upon include the patch clamp, which probes electrical activity of a single neuron, and fMRI, which images activity in volumes containing millions of neurons.

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Wavefront shaping devices such as deformable mirrors, liquid crystal spatial light modulators (SLMs), and active lenses are of considerable interest in microscopy for aberration correction, volumetric imaging, and programmable excitation. Liquid crystal SLMs are high resolution phase modulators capable of creating complex phase profiles to reshape or
redirect light within a three-dimensional (3D) volume. Recent advances in Meadowlark Optics (MLO) SLMs reduce losses by increasing fill factor from 83.4% to 96%,and improving resolution from 512 x 512 pixels to 1920 x 1152 pixels while maintaining a liquid crystal response time of 300 Hz at 1064 nm. This paper summarizes new SLM capabilities, and benefits for microscopy.

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VIDEOS

POLARIZERS - Wire Grid

POLARIZERS - Linear Summary

POLARIZERS - Linear Overview

POLARIZERS - Beam Splitting

POLARIZERS - Dichroic

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Non-Mechanical Polarization Rotation