The accuracy of satellite remote sensing of trace gases by imaging spectrometers depends highly on the uniformity of the instrument spectral response function. Studies have shown that scene inhomogeneity across the spectrometer’s slit width (spectral direction) can cause errors in the measured spectral radiances, leading to trace gas retrieval inaccuracy. One mitigation approach recent imaging spectrometers use is a slit homogenizer to redistribute scene radiance within the slit. This on-board hardware device functions like a slab waveguide, with rays making multiple bounces between narrowly spaced, highly reflective, plane-parallel mirrors. This presents a challenge as any difference in s- and p-polarization reflectance for the mirror surfaces tends to multiply with each bounce, producing a net linear polarization sensitivity (LPS) in the system’s throughput which also results in retrieval error. Our solution is a slit homogenizer design that mitigates for LPS by employing total internal reflection (TIR) and a birefringent internal medium. TIR ensures high and equal reflectance while a birefringent material such as sapphire, with an appropriately oriented optic axis, provides high-order retardance between bounces. This introduces polarization scrambling in the manner of a Lyot-depolarizer. We provide a basic analysis of the device’s geometrical optics, detailing the crystalline optic axis orientation for the device cut from a sapphire boule and readily-available R-cut material. Preliminary lab testing was performed on three mirror-pair and two sapphire plate homogenizers at multiple visible wavelengths. The results show that our sapphire plate slit homogenizers decrease LPS by at least an order of magnitude compared to the mirror-based ones.
The monitoring of Earth’s atmosphere requires routine measurements of many gasses and aerosols. The most common technique to perform this task is hyperspectral imaging (HSI). However, with the push to integrate HSI sensing capabilities on small platforms, e.g. cubesats and UAVs, the development of smaller, cheaper, higher performing, and low power HSI systems is necessary. Current HSI systems are composed of a large and complex assortment of lenses, filters and cameras that are large, heavy, expensive, and intolerant to physical shocks—all things that make them challenging for use in space-based sensing and imaging applications. The metamaterial filter described in this work eliminates the need for many of the previously necessary optics because it can spectrally filter light independent of the lights angle of incidence—this allows for a focused beam of light to be filtered by the metamaterial. This is in distinct contrast to grating-based HSI systems where the spectrometer requires collimated light. Additionally, the metamaterial filter is designed to filter light only at the desired spectral bands; this is a great benefit for small-platform systems because of the substantially reduced data rate and required computational resources.
The health of Earth’s atmosphere and its ecosystems are of vital importance to humanity. To assess the current state of the atmosphere and its rate of degradation, the monitoring of atmospheric gasses and particulates is necessary. The development of next-generation Low size, weight, and power (SWaP) sensors and instruments which are required for this task is a high priority for NASA’s Earth Science Technology Office (ESTO). The primary tool to monitor atmospheric gasses is hyperspectral imaging (HSI). Current HSI systems are composed of a large and complex assortment of lenses, filters and cameras that are large, heavy, expensive, and intolerant to physical shocks—all things that make them challenging for use in space-based sensing and imaging applications. As an alternative, a Low SWaP sensor is made possible by integrating a compact HSI sensor onto a CubeSat or SmallSat platform, which is much cheaper to deploy vs. a conventional satellite. To facilitate this, metamaterials are employed at the detector level to reduce the optical components required for HSI, while still providing comparable performance. The metamaterial studied here replaces a conventional grating disperser in a HSI system, by being compatible with a focused beam (fast optics) while spectrally filtering a particular spectral channel.
We have developed a low SWaP-C enabling Metamaterial Spectrometer (MMS) device for hyperspectral imaging in the MWIR. Our chip-based MMS device couples a Distributed Bragg Stack filter with a sub-wavelength dielectric resonator metasurface. The former gives the device a narrow passband, while the latter can be pixelated into an arbitrary number of parallel spectral channels, each with an independently engineerable center wavelength and bandwidth to create a hyperspectral or multispectral filter. The all-dielectric structure provides low optical loss vs. metallic plasmonic resonators. The metasurface resonators are engineered to accept light across a wide angle-of-incidence cone while being integrated directly into existing focal plane array (FPA) detectors. A wide acceptance cone of light eliminates the need for collimating optics, thereby reducing the SWaP requirements of the MMS relative to competing technologies. The MMS can be fabricated on a wafer scale using standard nanofabrication techniques, which are cost-effective for highvolume manufacturing. Although our initial prototype has been implemented in the MWIR, the generalized MMS structure can be implemented in other infrared spectral ranges by via appropriate choices of materials and rescaling of dimensions. Potential commercial applications of the hyperspectral MMS include environmental monitoring, medical diagnostics, antiterrorism, forensics, and food safety.
The Ball Fiber Optical Comb Demo is a lab-based system which is used to develop space applications for optical frequency combs. These developments utilize the broadband optical coherence of the frequency comb to expand the capabilities of ground test and orbital systems used for optical wave-front measurement, control of adaptive optics, precision ranging, and reference frequency stabilization. The work expands upon a NIST-developed all-fiber frequency comb that exhibits high stability in a compact, enclosed package.
Previously demonstrated applications for frequency combs include: Spectroscopy, distance and velocity measurement, frequency conversion, and timing transfer. Results from the Ball system show the characterization and performance of a frequency comb system with a technological path-to-space. Demonstrations in high precision metrology and long distance ranging are also presented for application in adaptive and multi-body optical systems.
The NASA ESTO-funded Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) development project demonstrates a reconfigurable multi-order airborne spectrometer and tests the performance of spectra separation and filtering on the sensor spectral measurements and subsequent trace gas and aerosol retrievals. The activities support mission risk reduction for the UV-Visible air quality measurements from geostationary orbit for the TEMPO and GEMS missions1 . The project helps advance the retrieval algorithm readiness through retrieval performance tests using scene data taken with varying sensor parameters. We report initial results of the project.
We have designed and built a testbed demonstrating an angle sensor that measures the relative angular position between
two free-flying spacecraft when used in conjunction with a distance-metrology system. In flight, one spacecraft would
carry an LED beacon while the other would carry the sensor system. Our fixed, staring testbed sensor demonstrated a 10
degree capture range with 0.1 arcsec resolution over the inner 1 arcmin of field, and an update rate of over 100 Hz. The
testbed showed this performance for simulated spacecraft separations of 100 to 1000 meters.
The Terrestrial Planet Finder Interferometer (TPF-I) is a future NASA mission for mid-infrared astronomy in space, using formation flying to position the telescopes. A unique and significant challenge for TPF-I is control of stray light from thermally emitting objects near the starlight beam paths, such as sunshades and other warm parts of the neighboring spacecraft. A proposed strategy for stray light control in these missions is simple geometric shading of the beam-transport optics from the emitting objects, but this intrinsically limits the maximum inter-spacecraft separation. We present a preliminary study of diffractive beam propagation to set lower limits on the baffle diameters. This and other geometric constraints then lead to specific estimates of the maximum inter-spacecraft separation.
KEYWORDS: Mirrors, Space operations, Solar radiation models, Sensors, Systems modeling, Wavefronts, Optical alignment, Performance modeling, Imaging systems, Control systems
The proposed Micro-Arcsecond X-ray Imaging Mission (MAXIM) uses an array of spacecraft containing grazing incidence optics to create and acquire an image on a distant detector spacecraft. Among the technical challenges facing the mission, maintaining an acceptably small wavefront error in the optical system is addressed in this paper. Starting with a performance model for the observatory and both analytically- and raytrace-based optical sensitivities to misalignment and figure error, an error budget is constructed that includes the effects of the individual optical surfaces, the alignment of the optical elements within the 4-mirror periscope sub-assemblies, and the relative alignment of the many periscopes that make up the MAXIM optical imaging system. At this stage of conceptual development, the allocations to different sub-systems that affect wavefront error is based on the philosophy of "spreading the pain" associated with performance requirements of the contributing elements. The performance model and error budget become tools with which to explore different architectures and requirements allocations as the mission concept develops.
The Ozone Mapping and Profiler Suite (OMPS) for the United States National Polar-orbiting Operational Environmental Satellite System (NPOESS) consists of a two sensor suite and Level 1 and 2 data
processing algorithms to produce calibrated radiance data and ozone total column and profile values. We describe the profiling system design that matches the limb-observing space sensor performance to
measurement requirements of the retrieval algorithm and uses algorithm techniques to achieve the data quality needed for limb-scatter-based ozone profiling.
The Ozone Mapping and Profiler Suite (OMPS) is being developed for the United States National Polar-orbiting Operational Environmental Satellite System (NPOESS). We describe the optical design and predict the performance of the OMPS earth limb-imaging spectrometer. Limb-scattered solar radiation is measured at selected ultraviolet (UV), visible, and near infrared (NIR) wavelengths to determine ozone profile concentrations for the altitude range of 8 to 60 km. The sensor consists of a telescope with three separate crosstrack fields of view of the limb, a prism spectrometer covering 290 to 1050 nm, and a solar-diffuser calibration mechanism. The sensor provides 3 km vertical resolution profiles of atmospheric radiance with channel spectral resolutions (full-width at half-maximum, FWHM) ranging from 2.7 nm in the UV to 35 nm in the NIR and handles the demanding spectral and spatial dynamic range of the limb-scattered solar radiation with the required sensitivity for ozone retrievals.
We describe the features of the optical system for Terrestrial Planet Finder, a space-based, cryogenic interferometer for direct detection of Earth-type planets around nearby stars. Destructive interference in a stellar interferometer suppresses stellar glare by a factor of several thousand or more, and phase chopping distinguishes planet light from symmetric backgrounds. The mid-IR is favorable for detecting planetary emission relative to that from the star, and this spectral region also offers important molecular signatures indicative of key atmospheric gases.
Several proposed spacecraft missions require positional knowledge of their optical elements to very high precision. This knowledge can be provided by a metrology system based on a laser interferometer incorporating the spacecraft optics. We present results from fabrication and testing of a lab-based frequency-modulated (FM) Michelson interferometer intended to maintain length stability to a few picometers. The instrument can be used to make precise relative distance measurements or it can be used to characterize orientation and polarization effects of system components commonly used in metrology gauges. External frequency modulation of a frequency-stabilized laser source and phase-sensitive detection are used to detect changes in the arm length difference of the interferometer. Arm length adjustments are made via a closed loop feedback system. A second system having a shared beampath with the primary system monitors the performance of the primary system. Preliminary data, operating in an ambient lab environment, demonstrate control to roughly 20 picometers rms for measurement times around 100 seconds.
Several proposed space-based interferometry missions require positional knowledge of their optical elements to very high precision. To achieve the desired stellar position measurement precision, the internal optical path difference of the stellar interferometer must be measured to within 10 picometers. This knowledge can be provided by a metrology system based on a laser interferometer incorporating the spacecraft optics. We present results from fabrication and testing of a lab-based frequency-modulated (FM) Michelson interferometer intended to maintain length stability to a few picometers. The instrument can be used to make precise relative distance measurements or it can be used to characterize orientation and polarization effects of system components commonly used in metrology gauges. External frequency modulation of a frequency- stabilized laser source and phase-sensitive detection are used to detect changes in the arm length difference of the interferometer. Arm length adjustments are made via a closed loop feedback system. A second system having a shared beampath with the primary system monitors the performance of the primary system. Preliminary data, operating in an ambient lab environment, demonstrate control to roughly 6 picometers rms for measurement times around 10 seconds.
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