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. Display and tunable laser applications are examples that can require consistent polarization modification over a much broader wavelength range. We discuss new methods and designs for dramatically increasing range of performance and review older methods as well. We show examples of achievable performance using modern polymer and liquid crystal materials.
We present here methodology and instrumentation for the precise measurement of retardance and optic axis
orientation of retarder assemblies for the Daniel K. Inouye Solar Telescope. This solar telescope will perform
broadband polarimetry of the sun. Each Meadowlark assembly is made up of three compound zero order retarders that
must have a retardance variation of less than 6.33 nanometers across the greater than 110 millimeter clear aperture.
The retardation of each component was measured using a combination of spectral transmission scans and ellipsometry,
with test wavelengths of less than a 0.45 nanometer bandwidths and yielding a standard deviation in measurements of
less than 0.001 waves.
A technique for the measurement of the near zero window (Infrasil® and CaF2) retardance is shown, in addition to
retardance measurements of the component waveplates. An average retardance of 0.63 nm for CaF2 and 0.28 nm for
Infrasil® was found. Finally, a technique for determining the optic axis tilt of each crystal waveplate using laser
ellipsometry is discussed.
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.
We constructed these designs using material selected for optimal optical behavior in this wavelength range. Description and characterization of these chosen component materials is included along with the performance of each device. We present design challenges, along with future plans and possibilities for MWlR LC technology.
Reactive mesogen retarders or waveplates provide new opportunities for flexibility in the design of optical
systems requiring polarization control. These true zero order retarders are less than 10 microns thick and
can be used either as a free standing film or they can be coated as a film on other optical elements such as
lenses or mirrors. The slow axis direction can be patterned to form small, even microscopic, discrete
retarders or continuously varied to make radial or axial polarizers. We describe in detail the properties of
these new optical retarders.
The "Association de Satellites Pour l'Imagerie et l'Interférométrie de la Couronne Solaire", ASPIICS, is a solar
coronagraph to be flown on the PROBA 3 Technology mission of the European Space Agency. ASPIICS heralds the
next generation of coronagraphs for solar research, exploiting formation flying to gain access to the inner corona under
eclipse-like conditions in space. The science goal is high spatial resolution imaging and two-dimensional
spectrophotometry of the Fe XIV, 530.3 nm, emission line. This work describes a liquid crystal Lyot tunable-filter and
polarimeter (LCTP) that can implement this goal. The LCTP is a bandpass filter with a full width at half maximum of
0.15 nm at a wavelength of 530.3 nm. The center wavelength of the bandpass is tunable in 0.01 nm steps from 528.64
nm to 533.38 nm. It is a four stage Lyot filter with all four stages wide-fielded. The free spectral range between
neighboring transmission bands of the filter is 2.7 nm. The wavelength tuning is non-mechanical using nematic liquid
crystal variable retarders (LCVR's). A separate LCVR of the Senarmont design, in tandem with the filter, is used for the
polarimetric measurements. A prototype of the LCTP has been built and its measured performances are presented here.
Meadowlark Optics has successfully built and demonstrated a liquid crystal based tunable filter with novel FWHM
tunability. This allows separate control over both the location of a narrow spectral bandpass and the width of the
bandpass function. This non-mechanical, imaging filter thus enables random access of the visible to near IR spectrum
and also controlling the specificity of the transmitted light. We will discuss both the relative trade-offs in this filter
design space and present data from functional units.
Beamsplitting 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.
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.
Meadowlark Optics has developed a variety of spatial light modulators (SLMs) for nondisplay applications. They operate by electrical control of the birefringence of nematic liquid crystals to achieve spatial and temporal modulation of phase and amplitude of light. They modulate phase by electrically varying the effective extraordinary index, n, of a uniaxial layer of nematic liquid crystal. This varies the optical thickness of the layer for light that is linearly polarized parallel to the optic axis of the layer. The SLM's can modulate amplitude for light that is linearly polarized at 45° to this direction if they are followed by an orthogonal or parallel linear polarizer.
We report progress to develop a miniature reflective display with a silicon backplane that can be used to project full color video images. This display has sufficient speed for sequential color at full video rates. It can tolerate the high light flux levels required for projecting bright images. The display has more than 640 X 480 pixels on a 20 micron pitch. Each pixel is an 18 micron square mirror. The mirror surface can be either overcoated aluminum or a high efficiency dielectric mirror.