Articles/Application Notes

Retarders or waveplates are tools for polarization modi?cation 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.

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 speci?c wavelength range. Ferroelectric liquid crystal (FLC) devices can switch between two orthogonal states of linear polarization, and o?er 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 o?ers fast response times and high polarization purity over a broader wavelength range.

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.

` 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.

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.

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.

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.

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.

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.

This application note describes how two Liquid-Crystal Variable Retarders (LCVRs) can be used to build a Stokes Polarimeter to accurately characterize the polarization state of a beam of light.

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.

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.

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.

This application note details factors that effect the response time for Nematic Liquid Crystal optics. The response time of a liquid-crystal variable retarder depends on several factors, including the LC layer thickness, viscosity, temperature and surface treatment as well as the driving waveform. The response time is also sensitive to the direction of the retardance change as well as the absolute value of the LC retardance.

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.

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.

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.

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.