Stephen C. McClain

First results from a dual photoelastic-modulator-based polarimetric camera

We report on the construction and calibration of a dual photoelastic-modulator (PEM)-based polarimetric camera operating at 660?nm. This camera is our first prototype for a multispectral system being developed for airborne and spaceborne remote sensing of atmospheric aerosols. The camera includes a dual-PEM assembly integrated into a three-element, low-polarization reflective telescope and provides both intensity and polarization imaging. A miniaturized focal-plane assembly consisting of spectral filters and patterned wire-grid polarizers provides wavelength and polarimetric selection. A custom push-broom detector array with specialized signal acquisition, readout, and processing electronics captures the radiometric and polarimetric information. Focal-plane polarizers at orientations of 0 degrees and -45 degrees yield the normalized Stokes parameters q=Q/I and u=U/I respectively, which are then coregistered to obtain degree of linear polarization (DOLP) and angle of linear polarization. Laboratory test data, calibration results, and outdoor imagery acquired with the camera are presented. The results show that, over a wide range of DOLP, our challenging objective of uncertainty within +/-0.005 has been achieved.

Polarization ray tracing in anisotropic optically active media. II. Theory and physics

Refraction, reflection, and amplitude relations are derived that apply to polarization ray tracing in anisotropic, optically active media such as quartz. The constitutive relations for quartz are discussed. The refractive indices and polarization states associated with the two modes of propagation are derived as a function of wave direction. A procedure for refracting at any uniaxial or optically active interface is derived that computes both the ray direction and the wave direction. A method for computing the optical path length is given, and Fresnel transmission and reflection equations are derived from boundary conditions on the electromagnetic fields. These ray-tracing formulas apply to uniaxial, optically active media and therefore encompass uniaxial, non-optically active materials and isotropic, optically active materials.

Polarization ray tracing in anisotropic optically active media. I: Algorithms

Procedures for performing polarization ray tracing through birefringent media are presented in a form compatible with the standard methods of geometrical ray tracing. The birefringent materials treated include the following: anisotropic optically active materials such as quartz, non-optically active uniaxial materials such as calcite, and isotropic optically active materials such as mercury sulfide and organic liquids. Refraction and reflection algorithms are presented that compute both ray directions and wave directions. Methods for computing polarization modes, refractive indices, optical path lengths, and Fresnel transmission and reflection coefficients are also specified. A numerical example of these algorithms is given for analyzing the field of view of a quartz rotator.

Polarization ray tracing in anisotropic optically active media

Procedures for performing polarization ray tracing through birefringent media are presented in a form compatible with the standard methods of geometric ray tracing. The birefringent materials treated include the following: anisotropic optically active materials such as quartz, non-optically active uniaxial materials such as calcite, and isotropic optically active materials such as mercury sulfide or organic liquids. Refraction and reflection algorithms are presented which compute both ray directions and wave directions. Methods for computing polarization modes, refractive indices, optical path lengths, and Fresnel transmission and reflection coefficients are also specified.

Depolarization in liquid-crystal televisions

The depolarization of a TVT-6000 liquid-crystal television has been measured to vary between 2% and 9% as a function of bias voltage, angle of incidence, and incident polarization state.

Recent results of the second generation of vector vortex coronagraphs on the high-contrast imaging testbed at JPL

The Vector Vortex Coronagraph (VVC) is an attractive internal coronagraph solution to image and characterize exoplanets. It provides four key pillars on which efficient high contrast imaging instruments can be built for ground- and space-based telescopes: small inner working angle, high throughput, clear off-axis discovery space, and simple layout. We present the status of the VVC technology development supported by NASA. We will review recent results of the optical tests of the second-generation topological charge 4 VVC on the actively corrected High Contrast Imaging Testbed (HCIT) at the Jet Propulsion Laboratory (JPL). New VVC contrast records have been established.

Taking the vector vortex coronagraph to the next level for ground- and space-based exoplanet imaging instruments: review of technology developments in the USA, Japan, and Europe

The Vector Vortex Coronagraph (VVC) is one of the most attractive new-generation coronagraphs for ground- and space-based exoplanet imaging/characterization instruments, as recently demonstrated on sky at Palomar and in the laboratory at JPL, and Hokkaido University. Manufacturing technologies for devices covering wavelength ranges from the optical to the mid-infrared, have been maturing quickly. We will review the current status of technology developments supported by NASA in the USA (Jet Propulsion Laboratory-California Institute of Technology, University of Arizona, JDSU and BEAMCo), Europe (University of Li`ege, Observatoire de Paris- Meudon, University of Uppsala) and Japan (Hokkaido University, and Photonics Lattice Inc.), using liquid crystal polymers, subwavelength gratings, and photonics crystals, respectively. We will then browse concrete perspectives for the use of the VVC on upcoming ground-based facilities with or without (extreme) adaptive optics, extremely large ground-based telescopes, and space-based internal coronagraphs.

Aberrations of a horizontal–vertical depolarizer

We use ray-trace equations for uniaxial birefringent materials to derive third-order estimates for aberrations that are produced in imaging through uniaxial plates and horizontal-vertical (HV) depolarizers. An HV depolarizer is a spatial pseudodepolarizer; it converts a uniform input polarization state into a continuum of spatially varying polarization states in an output beam. An HV depolarizer consists of two birefringent wedges whose crystal axes are crossed at 90 degrees . The interface between the wedges is inclined, which leads to a spatially varying retardance that provides the spatial pseudodepolarization. In HV depolarizers, spherical aberration, astigmatism, and image doubling are the principal aberrations for on-axis objects. Only spherical aberration occurs in isotropic plates, while the presence of birefringent wedges introduces astigmatism and image doubling. It is shown that image separation is proportional tothe magnitude of the retardance variation. Image separation is independent of the thickness, wedge angle, and refractive indices that are used to achieve this variation. A computer program is used to perform an exact birefringent ray trace and produces spot diagrams that confirm the aberration estimates.

Achromatic athermalized retarder fabrication

A method for fabricating an achromatic, athermalized quarter-wave retarder is presented that involves monitoring retardance during polishing. A design specified by thicknesses alone is unlikely to meet specification due to uncertainties in birefringence. This method facilitates successful fabrication to a retardance specification despite these uncertainties. A retarder made from sapphire, MgF(2), and quartz was designed, fabricated, and its performance validated for the 0.470 to 0.865 μm wavelength region. Its specifications are as follows: at wavebands centered at 0.470, 0.660, and 0.865 μm, the band-averaged retardance should be 90°±10° for all fields and retardance should change less than 0.1° for a 1° change in temperature. Retarder fabrication accommodated birefringence and thickness uncertainties via the following steps. The first plate was polished to a target thickness. The retardance spectrum of the first plate was then measured and used to determine a retardance target for the second plate. The retardance spectrum of the combined first and second plates was then used to specify a retardance target for the third plate. The retardance spectrum of the three plates in combination was then used to determine when the final thickness of the third plate was reached.

Depolarization measurements of an integrating sphere

Mueller-matrix polarimetry performed in the visible and near IR indicates that an integrating sphere acts as an ideal depolarizer to the 0.5% accuracy of the polarimeter. The integrating sphere emits unpolarized light regardless of the incident polarization state.