Low Polarization Optical System Design

1
Low Polarization Optical System Design
Anna-Britt Mahler, Paula Smith, Neil Beaudry,
Greg Smith, Russell Chipman University of
Arizona College of Optical Sciences

Summary
Example The Multiangle SpectroPolarimetric
Imager (MSPI)
Polarization measurement
The Multiangle SpectroPolarimetric Imager (MSPI)
is a 3-mirror off-axis telescope designed to
study atmospheric aerosols from space. To
accomplish this, the MSPI camera must measure
degree of linear polarization to and accuracy of
0.5, which requires that it have diattenuation
of less than 0.1.
Polarization-sensitive systems require low
instrumental polarization. For such systems, the
optical system designer must consider surface
geometries, optical element materials and surface
coatings that will minimize polarization
aberrations, perform end-to-end polarization
aberration modeling, and measurements of element
and system polarization properties.
After obtaining coating samples from the vendor,
coatings were measured using the Polarization
Laboratory Mueller Matrix Imaging Polarimeter
(MMIP). The MMIP measures absolute reflectance,
and Mueller matrix images fully characterize the
polarization of the sample.
Mueller Matrix Imaging Polarimeter
What is meant by low polarization?
Low polarization indicates low diattenuation and
retardance. Diattenuation is polarization-depende
nt transmission or reflection, retardance is the
difference in phase shift for different
polarizations.
Measurements did not match the models well, so
thicknesses and refractive indices of the coating
layers were optimized to match the model to the
measured data.
End-to-end system polarization modeling
Special mirror coatings were designed to minimize
MSPI camera polarization by diattenuation
compensation of the three mirrors. This concept
is shown to the right, where the diattenuation
due to coatings 1, 2, and 3 are cascaded into a
combined diattenuation (purple).
Systems sensitive to instrumental polarization
include interferometric systems, polarimeters and
ellipsometers, systems with large angles of
incidence, broad spectral band systems, remote
sensing radiometers and spectrometers, liquid
crystal displays and projectors, monochromators
and fold mirrors.
Optimization resulted in new refractive index
values, and the measured data now matches the
modeled data well
These mirror coatings were then applied to the
mirror surfaces in Code V and diattenuation pupil
maps were generated by raytracing. Pupil maps
like these can give a comprehensive look at the
polarization aberrations present in the system.
Coatings are now being optimized for low
diattenuation and high reflectance using the new
refractive index values.
Polarization is introduced by surface geometry
and material properties such as birefringence.
Low polarization design requires modeling of
polarization introduced by optical elements and
end-to-end polarization aberration analysis of
the optical system. Measurements of coating or
dielectric samples during the design phase allow
models to better simulate system performance.
Acknowledgments
This research has been supported by NASA Jet
Propulsion Laboratories and the University of
Arizona College of Optical Sciences.
Length of tick mark indicates magnitude of
diattenuation, direction indicates dominant
polarization direction
On-axis field has max diattenuation of 0.19
Off-axis field has max diattenuation of 0.33