Enhanced ORCA and CLARREO Depolarizers Using AR Microstructures

Next generation Earth Science Satellites ORCA and CLARREO are designed to measure our planet's ocean and climate health. Using hyper-spectral imaging at wavelengths ranging from the UV through NIR, these instruments will record the levels of the earth's temperature rise over the course of a decade. To make such detailed measurements, polarization effects at various wavelengths due to multiple factors must be eliminated using an optical device known as a "de-polarizer". For the CLARREO de-polarizer, four quartz windows are needed to randomize the polarization state of the observed reflected light spectrum. Multiple reflections from 4 surfaces produce losses up to 14% of the incident light, a level high enough to produce "ghost" effects superimposed on the desired earth images resulting in reduced image contrast and greater measurement error. An anti-reflection (AR) treatment is needed that can withstand the radiation and temperature effects caused by the mission environment while reducing reflection losses to levels of fractions of one percent. A new type of AR treatment, being developed for many military and commercial applications, is based on surface relief microstructures fabricated directly in a window, optic, or sensor material. AR microstructures (ARMs) can suppress internal reflections to levels unattainable by conventional thin-film AR coating technology. To extend the performance benefits of ARMs to hyper-spectral imaging systems, it is proposed that the fabrication processes developed for fused silica, glass, silicon, and many other optical materials be adapted for use with the quartz and magnesium fluoride depolarizers planned for the ORCA and CLARREO missions. In addition, an investigation of innovative surface microstructure technology is proposed for the fabrication of a new type of non-scattering, micro-textured depolarizer with inherent AR properties that can be applied to multiple optical elements within a spectrometer system.

Durable, high performance AR microstructures will replace thin-film AR coatings used in applications ranging from automotive, computer, and portable device displays, to window glass, eyeglasses, sunglasses, camera lenses, optical telecommunication fibers, laser systems, and astronomical instruments. An important near term commercial application involves the integration of AR microstructures directly into the surfaces of imaging sensors that detect light over a wide color spectrum ranging from the ultraviolet through the far infrared. AR microstructures are beginning to have a great impact on reducing energy consumption for new solid-state lighting products based on light emitting diode technology, and on energy efficiency where the wide bandwidths and acceptance angles of AR microstructures allow collection and conversion of more photons by conventional solar panels.

The primary near-term NASA missions that can benefit from the new microstructure AR and depolarizer treatments are the hyperspectral imaging instruments of ORCA and CLARREO. Other NASA missions in operation or being developed include ACCLAIM, a sub-orbital platform employing a hyperspectral imager for cloud and atmospheric aerosol monitoring, GLORY, a total solar irradiance measurement system, the Orbiting Carbon Observatory, a dedicated global atmospheric CO2 measurement system, and AQUARIUS, a global sea surface salinity measurement system. Several mid-term NASA missions that may also experience a direct or indirect benefit from the new microstructure technology include HyspIRI, a hyperspectral instrument for agricultural land surface health monitoring, GEO-CAPE, an atmospheric air quality and ocean health monitoring spectrometer, ASCENDS, a multi-frequency laser system for day and night monitoring of CO2 levels in the atmosphere, and ACE, a backscatter lidar and polarimeter system for aerosol and cloud profiling and open ocean monitoring.

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