We demonstrate a novel class of elements called Far-Field Geometric Phase Holograms (FGPH) capable of producing far-field output images free of chromatic distortion for a broad range of input wavelengths. The FGPH utilizes the geometric phase which applies the same phase profile to any incident wave regardless of wavelength. Thus, the fidelity of an image produced by an FGPH is the same for all wavelengths. However, being a diffractive element, the FGPH is still dispersive in that the size of a generated image depends on the replay wavelength according to the diffraction equation. In this paper, we give theory for the ideal FGPH element, describing its replay characteristics and unique polarization properties. We experimentally realize an FGPH element using photo-aligned liquid crystals patterned with a direct-write system. We characterize the fabricated element and show the theory to be valid. Generally, this new class of polarization sensitive elements can produce broadband undistorted images with high diffraction efficiency.
We recently reported on a mathematical formalism for analyzing the result of a direct-write scanning system applied to photoaligned liquid crystal films. We use that formalism to study the direct-write recording of polarization gratings (PGs). First, we evaluate three scan paths in simulation and experiment, describe their tradeoffs and practical constraints, and identify the most favorable. Second, we explore the parameter space of direct-write PGs in simulation, which includes four dimensions in general: grating period, line spacing, beam size, and spatially averaged fluence. Using this analysis, we predict that a certain portion of the parameter space should be optimal, leading to high diffraction efficiency and well-aligned PGs. Finally, we experimentally fabricate and characterize nine PGs with scan parameters within and around this optimal parameter space and conclude that the prediction is validated. This work is the first in-depth study of direct-write PGs; it identifies many challenges and solutions, and shows, for the first time, direct-write recorded PGs with quality equivalent to those recorded via holography. In particular, we demonstrate a PG (20 μm period) with first-order diffraction efficiency 99.5%, 0.2% haze, and polarization contrast of 2000.
The Apodizing Phase Plate (APP) is a phase-only pupil-plane coronagraph that suppresses starlight in a D-shaped region from 2 to 7 λ D around a target star. Its performance is insensitive to residual tip-tilt variations from the AO system and telescope structure. Using liquid crystal technology we develop a novel and improved version of the APP: the broadband vector Apodizing Phase Plate (vAPP). The vAPP prototype consists of an achromatic half-wave retarder pattern with a varying fast axis encoding phase structure down to 25 microns. The fast axis encodes the required phase pattern through the vector phase, while multiple twisting liquid crystal layers produce a nearly constant half-wave retardance over a broad bandwidth. Since pupil phase patterns are commonly designed to be antisymmetric, two complementary PSFs are produced with dark holes on opposite sides.
We summarize results of the characterization of our latest vAPP prototype in terms of pupil phase reconstruction and PSF contrast performance. The liquid crystal patterning technique allows us to manufacture more extreme phase patterns than was possible before. We consider phase-only patterns that yield higher contrasts and better inner working angles than previous APPs, and patterns that produce dark regions 360 degrees around the PSF core. The possibility of including a phase ramp into the coronagraph is demonstrated, which simplifies the vAPP into a single optic. This additional phase ramp removes the need for a quarter-wave plate and a Wollaston prism, and enables the simplified implementation of a vAPP in a filter wheel at a pupil-plane location. Since the phase ramp is analogous to a polarization grating, it generates a (polarized) spectrum of a planet inside the dark hole, and thus allows for instantaneous characterization of the planet.
High speed spectral imaging is useful for a variety of tasks spanning industrial monitoring, target detection, and chemical
identification. To better meet these needs, compact hyperspectral imaging instrumentation, capable of high spectral
resolution and real-time data acquisition and processing, are required. In this paper, we describe the first snapshot imaging
spatial heterodyne Fourier transform spectrometer based on birefringent crystals and polarization gratings. This includes
details about its architecture, as well as our preliminary proof of concept. Finally, we discuss details related to the
calibration of the sensor, including our preliminary investigations into high speed data reconstruction and calibration using
neural networks. With such an approach, it may be feasible to reconstruct and calibrate an entire interferogram cube in
one step with minimal Fast Fourier Transform (FFT) processing.
Spatial heterodyne interferometry (SHI) is a technique based on Fourier transform spectroscopy. As such, many of the benefits, such as high spectral resolving power, can be realized. Furthermore, unlike a Fourier transform spectrometer, an SHI is able to minimize the number of required samples for a given resolving power and spectral range. The calibration and detailed modeling of a polarization spatial heterodyne interferometer (PSHI) are detailed. Unlike our original first-order ray tracing model, the new model is based on the Jones matrix formalism. Using this improved model, we explore the nonideal aspects of the PSHI, including interference effects caused by retardance errors in the polarization grating and quarter wave plate. To minimize the influence of these errors, a calibration procedure is described based on a linear operator theory. Finally, the Jones matrix model and calibration procedure are validated through a series of simulations and experiments.
Spatial heterodyne interferometry (SHI) is a spectral measurement technique based on Fourier Transform Spectroscopy (FTS). One main benefit of an FTS lies in its higher spectral resolving power over direct measurement (dispersive) systems; however, accessing this higher resolving power can result in longer measurement times without heterodyning techniques. In this paper, the calibration and modeling of a polarization SHI is detailed, based on the Jones matrix formalism. With this, we explore non-ideal aspects of the polarization grating, such as zero-order light leakage. This light leakage causes crosstalk that can introduce errors in the spectral calibration. To minimize error, a calibration procedure is introduced based on a linear operator theory. Finally, the Jones matrix model and calibration procedure are validated through a series of experiments.
The apodizing phase plate (APP) is a solid-state pupil optic that clears out a D-shaped area next to the core
of the ensuing PSF. To make the APP more efficient for high-contrast imaging, its bandwidth should be as
large as possible, and the location of the D-shaped area should be easily swapped to the other side of the PSF.
We present the design of a broadband APP that yields two PSFs that have the opposite sides cleared out.
Both properties are enabled by a half-wave liquid crystal layer, for which the local fast axis orientation over
the pupil is forced to follow the required phase structure. For each of the two circular polarization states, the
required phase apodization is thus obtained, and, moreover, the PSFs after a quarter-wave plate and a polarizing
beam-splitter are complementary due to the antisymmetric nature of the phase apodization. The device can be
achromatized in the same way as half-wave plates of the Pancharatnam type or by layering self-aligning twisted
liquid crystals to form a monolithic film called a multi-twist retarder. As the VAPP introduces a known phase
diversity between the two PSFs, they may be used directly for wavefront sensing. By applying an additional
quarter-wave plate in front, the device also acts as a regular polarizing beam-splitter, which therefore furnishes
high-contrast polarimetric imaging. If the PSF core is not saturated, the polarimetric dual-beam correction can
also be applied to polarized circumstellar structure. The prototype results show the viability of the vector-APP
concept.
Over the last few years, Boulder Nonlinear Systems (BNS) and North Carolina State University (NCSU) have developed
a new beam steering technique that uses a stack of thin liquid crystal polarization gratings (LCPGs) to efficiently and
non-mechanically steer a beam over a large field-of-regard (FOR) in discrete steps. This technology has been
successfully transferred to BNS through an exclusive license agreement, and a facility has been completed to enable
commercial production of these devices. This paper describes the capabilities enabled by both the LCPGs and the
successful transfer of this technology.
Over the last several years, we have pioneered liquid crystal polarization gratings (PGs), in both switchable and
polymer versions. We have also introduced their use in many applications, including mechanical/non-mechanical
laser beam steering and polarization imaging/sensing. Until now, conventional holographic congurations were
used to create PGs where the diameter of the active area was limited to 1-2 inches. In this paper, we discuss a
new holography setup to fabricate large area PGs using spherical waves as the diverging coherent beams. Various
design parameters of this setup are examined for impact on the quality of the recorded PG profile. Using this
setup, we demonstrate a large area polymer PG with approximately 66 inch square area, and present detailed
characterization.
We have developed a 3D Finite Difference Time Domain (FDTD) algorithm to model obliquely incident waves
through arbitrary birefringent and dichroic media with transverse periodic boundaries. Beginning with arbitrary
conductivity and permittivity tensors, we employed the split-field method (SFM) to enable broadband sources
with oblique incidence. We terminate our boundaries with a uniaxial perfectly matched layer (UPML) in one
dimension and periodic boundaries in the other two dimensions. The algorithm is validated via several case
studies: a polarizer pair, a twisted nematic liquid crystal, and an array of conducting particles. Using this
approach, we simulate for the first time polarization gratings with light obliquely incident in directions orthogonal
to the grating vector (i.e., at oblique angles outside the normal diffraction plane).
Polarization gratings (PGs) as polarization sensitive diffractive optical elements work in broadband (UV to Mid-
IR) with nearly 100% diffraction efficiency. We have introduced and utilized the PGs in different types of beam
steering modules presented in our previous papers. Here, we describe and demonstrate a nonmechanical beam
steering device based on passive gratings, liquid crystal (LC) polymer PGs. The device covers a large-angle
Field-Of-Regard (FOR) with high efficiency, and is based on a stack of alternating LC half-wave plates and
LC polymer PGs. The half-wave plates are switchable and are used to select the handedness of the circularly
polarized input beam. The polymer PGs diffract the input beam to either of the first diffraction orders based
on the circular handedness of the beam previously selected. When compared with conventional beam steering
methods based on active gratings (ternary and quasi-ternary designs), this technique is experimentally able to
steer an equivalent number of angles with similar efficiency, but fewer LC cells, and hence, fewer transparent
electrodes and lower absorption. We successfully demonstrate the ability to steer 80° FOR with roughly 2.6°
resolution at 1064 nm wavelength.
We introduce and demonstrate a quasi-ternary nonmechanical beam steering design based on Polarization Gratings
(PGs). That uses a single wave plate and N PGs to generate 2(N+1)-1 steering angles. When compared to
conventional binary (2N) or ternary (3N) liquid crystal PG steering designs, this technique uses fewer elements
arranged in a simpler configuration to obtain the same number of steering angles. This advantageous property
can be achieved by selecting proper diffraction angles and alignment of the PGs. Due to fewer elements per
stage, losses due to electrode absorption and Fresnel reflections are reduced, thereby increasing the overall steering
efficiency. Using this approach, we demonstrate a four-stage (N = 4) quasi-ternary beam steering device
that achieves 52° Field Of Regard (FOR) with 1.7° resolution (31 steering angles) at 1550 nm wavelength.
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