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NASA NTRS2005

A GCM Recent History of Northern Martian Polar Layered Deposits: Contribution from Past Equatorial Ice Reservoirs

Polar layered deposits are exposed in the walls of the troughs cutting the north polar cap of Mars. They consist of alternating ice and dust layers or layers of an ice-dust mixture with varying proportions and are found throughout the cap. Layers thickness ranges from meters to several tens of meters with an approximately 30 meter dominant wavelength. Although their formation processes is not known, they are presumed to reflect changes in ice and dust stability over orbital and axial variations. Intensive 3-D LMD GCM simulations of the martian water cycle have been thus performed to determine the annual rates of exchange of surface ice between the northern cap and tropical areas for a wide range of obliquity and orbital parameters values.These rates have been employed to reconstruct an history of the northern cap and test simple models of dust-ice layers formation over the last 10 Ma orbital variations. We use the 3-D water cycle model simulated by the 3-D LMD GCM with an intermediate grid resolution (7.5 longitude x 5.625 latitude) and 25 vertical levels. The dust opacity is constant and set to 0,15. No exchange of ice with regolith is allowed. The evolution of the northern cap over obliquity and orbital changes (eccentricity, Longitude of perihelion) has been recently described with this model. High summer insolation favors transfer of ice from the northern pole to the Tharsis and Olympus Montes, while at low obliquity, unstable equatorial ice is redeposited in high-latitude and polar areas of both hemisphere. The disappearance of the equatorial ice reservoir leads to a poleward recession of icy high latitude reservoirs, providing an additional source for the cap accumulation during each obliquity or orbital cycle. Furthering the efforts, a quantitative evolution of ice reservoirs is here investigated for various astronomical conditions.

Levrard, B.↗

NASA NTRS2010

Polycyclic Aromatic Hydrocarbons

Carbonaceous materials play an important role in space. Polycyclic Aromatic Hydrocarbons (PAHs) are a ubiquitous component of the carbonaceous materials. PAHs are the best-known candidates to account for the IR emission bands. They are also thought to be among the carriers of the diffuse interstellar absorption bands (DIBs). PAH ionization states reflect the ionization balance of the medium while PAH size, composition, and structure reflect the energetic and chemical history of the medium. A major challenge is to reproduce in the laboratory the physical conditions that exist in the emission and absorption interstellar zones. The harsh physical conditions of the ISM -low temperature, collisionless, strong UV radiation fields- are simulated in the laboratory by associating a molecular beam with an ionizing discharge to generate a cold plasma expansion. PAH ions and radicals are formed from the neutral precursors in an isolated environment at low temperature and probed with high-sensitivity cavity ringdown spectroscopy in the NUV-NIR range. Carbon nanoparticles are also formed during the short residence time of the precursors in the plasma and are characterized with time-offlight mass spectrometry. These experiments provide unique information on the spectra of large carbonaceous molecules and ions in the gas phase that can now be directly compared to interstellar and circumstellar observations (IR emission bands, DIBs, extinction curve). These findings also hold great potential for understanding the formation process of interstellar carbonaceous grains. We will review recent progress in the experimental and theoretical studies of PAHs, compare the laboratory data with astronomical observations and discuss the global implications.

Salama, Farid↗

NASA NTRS2017

Recent Progress in Laboratory Astrophysics and Astrochemistry Achieved with the COSmIC Facility

We describe the characteristics and the capabilities of the laboratory facility, COSmIC, that was developed at NASA Ames to generate, process and analyze interstellar, circumstellar and planetary analogs in the laboratory. COSmIC stands for "Cosmic Simulation Chamber" and is dedicated to the study of neutral and ionized molecules and nanoparticles under the low temperature and high vacuum conditions that are required to simulate various space environments such as diffuse interstellar clouds, circumstellar outflows and planetary atmospheres. COSmIC integrates a variety of state-of-the-art instruments that allow recreating simulated space conditions to generate, process and monitor cosmic analogs in the laboratory. The COSmIC experimental setup is composed of a Pulsed Discharge Nozzle (PDN) expansion, that generates a plasma in the stream of a free supersonic jet expansion, coupled to high-sensitivity, complementary in situ diagnostics: cavity ring down spectroscopy (CRDS) and laser induced fluorescence (LIF) systems for photonic detection, and Reflectron Time-Of-Flight Mass Spectrometer (ReTOF-MS) for mass detection. Recent results obtained using COSmIC will be highlighted. In particular, the progress that has been achieved in the domain of the diffuse interstellar bands (DIBs) and in monitoring, in the laboratory, the formation of circumstellar dust grains and planetary atmosphere aerosols from their gas-phase molecular precursors. Plans for future laboratory experiments on interstellar and planetary molecules and grains will also be addressed, as well as the implications of the studies underway for astronomical observations and past and future space mission data analysis.

Salama, Farid↗

NASA NTRS1975

A computer-aided telescope pointing system utilizing a video star tracker

The Video Inertial Pointing (VIP) System developed to satisfy the acquisition and pointing requirements of astronomical telescopes is described. A unique feature of the system is the use of a single sensor to provide information for the generation of three axis pointing error signals and for a cathode ray tube (CRT) display of the star field. The pointing error signals are used to update the telescope's gyro stabilization and the CRT display is used by an operator to facilitate target acquisition and to aid in manual positioning of the telescope optical axis. A model of the system using a low light level vidicon built and flown on a balloon-borne infrared telescope is briefly described from a state of the art charge coupled device (CCD) sensor. The advanced system hardware is described and an analysis of the multi-star tracking and three axis error signal generation, along with an analysis and design of the gyro update filter, are presented. Results of a hybrid simulation are described in which the advanced VIP system hardware is driven by a digital simulation of the star field/CCD sensor and an analog simulation of the telescope and gyro stabilization dynamics.

Murphy, J. P.↗

NASA NTRS2017

Techniques for High Contrast Imaging in Multi-Star Systems II: Multi-Star Wavefront Control

Direct imaging of exoplanets represents a challenge for astronomical instrumentation due to the high-contrast ratio and small angular separation between the host star and the faint planet. Multi-star systems pose additional challenges for coronagraphic instruments because of the diffraction and aberration leakage introduced by the additional stars, and as a result are not planned to be on direct imaging target lists. Multi-star wavefront control (MSWC) is a technique that uses a coronagraphic instrument's deformable mirror (DM) to create high-contrast regions in the focal plane in the presence of multiple stars. Our previous paper introduced the Super-Nyquist Wavefront Control (SNWC) technique that uses a diffraction grating to enable the DM to generate high-contrast regions beyond the nominal controllable region. These two techniques can be combined to generate high-contrast regions for multi-star systems at any angular separations. As a case study, a high-contrast wavefront control (WC) simulation that applies these techniques shows that the habitable region of the Alpha Centauri system can be imaged reaching 8 times 10(exp -9) mean contrast in 10 percent broadband light in one-sided dark holes from 1.6-5.5 lambda (wavelength) divided by D (distance).

Sirbu, D.↗

NASA NTRS2001

Effects of Manufacturing and Deployment on Thin Films for the NGTS Sunshade

The Next Generation Space Telescope (NGST) is being developed as an advanced astronomical observatory. The NGST proposes to utilize several thin film membrane layers to create a shield for protection of the telescope from solar thermal energy and stray light, The shield will take the form of a polygon, approximately 15 x 30 m, with individual membrane layers positioned so that they do not come in contact with one another. The membrane shield will be deployed and supported by a series of booms, which will be packed into a small volume for launch. Finally, the shield will be deployed on orbit. Several film materials are being considered for the membrane shield, including CPI, Kapton E, Kapton RN, and Upilex. Each of these polyimide materials was tested to determine their durability over the 10-year mission. New facets of materials testing have been introduced in this study to develop performance data with greater realism to actual use, particularly that of degradation from packing, launch and deployment processing. Materials were exposed to handling that simulated the life of the materials from manufacture through deployment with standardized fixtures and then exposed to a simulated, L2, 10-year radiation environment. Mechanical and thermal radiative properties were measured before and after each phase of testing. This paper summarizes the program and test results.

Wooldridge, Eve M.↗

NASA NTRS1988

Planetary astronomy: Rings, satellites, and asteroids

Studies of planetary rings focus on the dynamical processes that govern astronomically observable ring properties and structure. These investigations thus help reveal properties of the rings as well as probe the gravity fields of the planets. Satellite studies involve interpretation of orbital motion to extract information regarding the gravity fields of the outer planets and the physical properties of the satellites themselves. Asteroid lightcurve work is designed to investigate the large-scale shapes of the asteroids, as well as to reveal anomalous features such as major topography, possible satellites, or albedo variations. Work on the nature of viscous transport in planetary rings, emphasizing the role of individual particles' physical properties, has yielded a method for estimating both angular momentum and mass transport given an optical-thickness gradient. This result offers the prospect of ringlet instability, which may explain the square-profile ringlets in Saturn's C Ring. Thermal and reflected lightcurves of 532 Herculina have been interpreted to show that albedo variations cannot be the primary cause of variations. A lightcurve simulation has been developed to model complex asteroidal figures. Bamberga was observed during the December occultation as part of the joint LPL-Lowell program.

Greenberg, Richard↗

NASA NTRS2010

Characterization of Orbital Debris Photometric Properties Derived from Laboratory-Based Measurements

To better characterize and model optical data acquired from ground-based telescopes, the Optical Measurements Center (OMC) at NASA/JSC attempts to emulate illumination conditions seen in space using equipment and techniques that parallel telescopic observations and source-target-sensor orientations. The OMC uses a 75 Watt Xenon arc lamp as a solar simulator, an SBIG CCD camera with standard Johnson/Bessel filters, and a robotic arm to simulate an object's position and rotation. The laboratory uses known shapes, materials suspected to be consistent with the orbital debris population, and three phase angles to best match the lighting conditions of the telescope based data. The fourteen objects studied in the laboratory are fragments or materials acquired through ground-tests of scaled-model satellites/rocket bodies as well as material samples in more/less "flight-ready" condition. All fragments were measured at 10 increments in a full 360 rotation at 6 , 36 , and 60 phase angles. This paper will investigate published color photometric data for a series of orbital debris targets and compare it to the empirical photometric measurements generated in the OMC. Using the data acquired over specific rotational angles through different filters (B, V, R, I), a color index is acquired (B-R, R-I). Using these values and their associated lightcurves, this laboratory data is compared to observational data obtained on the 1 m telescope of the Astronomical Institute of the University of Bern (AUIB), the 0.9 m operated by the Small- and Medium-Aperture Research Telescope System (SMARTS) Consortium and the Curtis-Schmidt 0.6 m Michigan Orbital Debris Space Debris Telescope both located at Cerro Tololo Inter-American Observatory (CTIO). An empirical based optical characterization model will be presented to provide preliminary correlations between laboratory based and telescope-based data in the context of classification of GEO debris objects.

Cowardin, H.↗

NASA NTRSDate not supplied

On the New Optical Constants Database (OCdb) and its Importance for the Interpretation of Observational Data

The Optical Constants database(ocdb.smce.nasa.gov) came online in February 2023 and provides complex refractive indices of laboratory-generated organic refractory materials and ices relevant to (exo) planetary and astrophysical environments.The goal of the OCdb is to centralize published optical constants data to facilitate both their access by the scientific community and their use to analyze observational data returned by space missions and ground-based observatories. Computational tools are also under development to facilitate scientific use of the available OCdb optical constants data sets. Investigators generating laboratory optical constants are therefore encouraged to contribute their data to OCdb in order to increase the availability of their data and to enhance the scientific effectiveness of the database. Optical constants are critical input parameters in models (e.g.,radiative transfer, atmospheric, and reflectance spectral models)that are used to simulate the absorption, reflection, and scattering of light due to solid materials present in planetary and astrophysical environments (planets, their satellites, exoplanets, asteroids, comets, protoplanetary disks, etc.), and are key to the compositional interpretation of observations. We will first present the infrastructure of the OCdb and show how to use and contribute to it. We will introduce the two large NASA projects, namely, the Laboratory Astrophysics Directed Work Package and the NASA Center for Optical Constants, that have been instrumental in (i) developing the OCdb, (ii) generating planetary-and astrophysics-relevant ices and organic refractory materials from gas and ice irradiation in the laboratory, and (iii) determining their optical constants for inclusion in OCdb. We will also present two studies that are making use of these optical constants to interpret observations of Titan’s atmosphere and Pluto’s surface. These studies show the importance of measuring optical constants of laboratory-generated materials, and their impact on the models used to analyze and interpret astronomical observations. These studies also demonstrate the essential importance of such a database and the need for optical constants of a broad range of materials and wavelengths to enable the scientific community and to maximize the scientific return from space missions (e.g., Cassini, New Horizons, SOFIA, JWST).

The Optical Constants database↗

NASA NTRS2020

Beam Propagation Through Atmospheric Turbulence Using an Altitude-Dependent Structure Profile with Non-Uniformly Distributed Phase Screens

Modeling the effects of atmospheric turbulence on optical beam propagation is a key element in the design and analysis of free-space optical communication systems. Numerical wave optics simulations provide a particularly useful technique for understanding the degradation of the optical field in the receiver plane when the analytical theory is insufficient for characterizing the atmospheric channel. Motivated by such an application, we use a split-step method modeling the turbulence along the propagation path as a series of thin random phase screens with modified von Karman refractive index statistics using the Hufnagel-Valley turbulence profile to determine the effective structure constant for each screen. In this work, we employ a space-to-ground case study to examine the irradiance and phase statistics for both uniformly and non-uniformly spaced screens along the propagation path and compare to analytical results. We find that better agreement with the analytical theory is obtained using a non-uniform spacing with the effective structure constant for each screen chosen to minimize its contribution to the scintillation in the receiver plane. We evaluate this method as a flexible alternative to other standard layered models used in astronomical imaging applications.

Chahine, Yousef K.↗

NASA NTRS1995

High-Density Amorphous Ice, the Frost on Interstellar Grains

Most water ice in the universe is in a form which does not occur naturally on Earth and of which only minimal amounts have been made in the laboratory. We have encountered this 'high-density amorphous ice' in electron diffraction experiments of low-temperature (T less than 30 K) vapor-deposited water and have subsequently modeled its structure using molecular dynamics simulations. The characteristic feature of high-density amorphous ice is the presence of 'interstitial' oxygen pair distances between 3 and 4 A. However, we find that the structure is best described as a collapsed lattice of the more familiar low-density amorphous form. These distortions are frozen in at temperatures below 38 K because, we propose, it requires the breaking of one hydrogen bond, on average, per molecule to relieve the strain and to restructure the lattice to that of low-density amorphous ice. Several features of astrophysical ice analogs studied in laboratory experiments are readily explained by the structural transition from high-density amorphous ice into low-density amorphous ice. Changes in the shape of the 3.07 gm water band, trapping efficiency of CO, CO loss, changes in the CO band structure, and the recombination of radicals induced by low-temperature UV photolysis all covary with structural changes that occur in the ice during this amorphous to amorphous transition. While the 3.07 micrometers ice band in various astronomical environments can be modeled with spectra of simple mixtures of amorphous and crystalline forms, the contribution of the high-density amorphous form nearly always dominates.

Jenniskens, P.↗

NASA NTRS2011

Integrated Lens Antennas for Multi-Pixel Receivers

Future astrophysics and planetary experiments are expected to require large focal plane arrays with thousands of detectors. Feedhorns have excellent performance, but their mass, size, fabrication challenges, and expense become prohibitive for very large focal plane arrays. Most planar antenna designs produce broad beam patterns, and therefore require additional elements for efficient coupling to the telescope optics, such as substrate lenses or micromachined horns. An antenna array with integrated silicon microlenses that can be fabricated photolithographically effectively addresses these issues. This approach eliminates manual assembly of arrays of lenses and reduces assembly errors and tolerances. Moreover, an antenna array without metallic horns will reduce mass of any planetary instrument significantly. The design has a monolithic array of lens-coupled, leaky-wave antennas operating in the millimeter- and submillimeter-wave frequencies. Electromagnetic simulations show that the electromagnetic fields in such lens-coupled antennas are mostly confined in approximately 12 15 . This means that one needs to design a small-angle sector lens that is much easier to fabricate using standard lithographic techniques, instead of a full hyper-hemispherical lens. Moreover, this small-angle sector lens can be easily integrated with the antennas in an array for multi-pixel imager and receiver implementation. The leaky antenna is designed using double-slot irises and fed with TE10 waveguide mode. The lens implementation starts with a silicon substrate. Photoresist with appropriate thickness (optimized for the lens size) is spun on the substrate and then reflowed to get the desired lens structure. An antenna array integrated with individual lenses for higher directivity and excellent beam profile will go a long way in realizing multi-pixel arrays and imagers. This technology will enable a new generation of compact, low-mass, and highly efficient antenna arrays for use in multi-pixel receivers and imagers for future planetary and astronomical instruments. These antenna arrays can also be used in radars and imagers for contraband detection at stand-off distances. This will be enabling technology for future balloon-borne, smaller explorer class mission (SMEX), and other missions, and for a wide range of proposed planetary sounders and radars for planetary bodies.

Lee, Choonsup↗

NASA NTRS2006

Simulating Responses of Gravitational-Wave Instrumentation

Synthetic LISA is a computer program for simulating the responses of the instrumentation of the NASA/ESA Laser Interferometer Space Antenna (LISA) mission, the purpose of which is to detect and study gravitational waves. Synthetic LISA generates synthetic time series of the LISA fundamental noises, as filtered through all the time-delay-interferometry (TDI) observables. (TDI is a method of canceling phase noise in temporally varying unequal-arm interferometers.) Synthetic LISA provides a streamlined module to compute the TDI responses to gravitational waves, according to a full model of TDI (including the motion of the LISA array and the temporal and directional dependence of the arm lengths). Synthetic LISA is written in the C++ programming language as a modular package that accommodates the addition of code for specific gravitational wave sources or for new noise models. In addition, time series for waves and noises can be easily loaded from disk storage or electronic memory. The package includes a Python-language interface for easy, interactive steering and scripting. Through Python, Synthetic LISA can read and write data files in Flexible Image Transport System (FITS), which is a commonly used astronomical data format.

Armstrong, John↗

NASA NTRS2004

Proceedings from the 2nd International Symposium on Formation Flying Missions and Technologies

Topics discussed include: The Stellar Imager (SI) "Vision Mission"; First Formation Flying Demonstration Mission Including on Flight Nulling; Formation Flying X-ray Telescope in L2 Orbit; SPECS: The Kilometer-baseline Far-IR Interferometer in NASA's Space Science Roadmap Presentation; A Tight Formation for Along-track SAR Interferometry; Realization of the Solar Power Satellite using the Formation Flying Solar Reflector; SIMBOL-X : Formation Flying for High-Energy Astrophysics; High Precision Optical Metrology for DARWIN; Close Formation Flight of Micro-Satellites for SAR Interferometry; Station-Keeping Requirements for Astronomical Imaging with Constellations of Free-Flying Collectors; Closed-Loop Control of Formation Flying Satellites; Formation Control for the MAXIM Mission; Precision Formation Keeping at L2 Using the Autonomous Formation Flying Sensor; Robust Control of Multiple Spacecraft Formation Flying; Virtual Rigid Body (VRB) Satellite Formation Control: Stable Mode-Switching and Cross-Coupling; Electromagnetic Formation Flight (EMFF) System Design, Mission Capabilities, and Testbed Development; Navigation Algorithms for Formation Flying Missions; Use of Formation Flying Small Satellites Incorporating OISL's in a Tandem Cluster Mission; Semimajor Axis Estimation Strategies; Relative Attitude Determination of Earth Orbiting Formations Using GPS Receivers; Analysis of Formation Flying in Eccentric Orbits Using Linearized Equations of Relative Motion; Conservative Analytical Collision Probabilities for Orbital Formation Flying; Equations of Motion and Stability of Two Spacecraft in Formation at the Earth/Moon Triangular Libration Points; Formations Near the Libration Points: Design Strategies Using Natural and Non-Natural Ares; An Overview of the Formation and Attitude Control System for the Terrestrial Planet Finder Formation Flying Interferometer; GVE-Based Dynamics and Control for Formation Flying Spacecraft; GNC System Design for a New Concept of X-Ray Distributed Telescope; GNC System for the Deployment and Fine Control of the DARWIN Free-Flying Interferometer; Formation Algorithm and Simulation Testbed; and PLATFORM: A Formation Flying, RvD and Robotic Validation Test-bench.

Source record↗

NASA NTRS2001

Thermal Performance of the Hubble Space Telescope (HST) Solar Array-3 During the Disturbance Verification Test (DVT)

The Hubble Space Telescope (HST) is one of NASA's most productive astronomical observatories. Launched in 1990, the HST continues to gather scientific data to help scientists around the world discover amazing wonders of the universe. To maintain HST in the fore front of scientific discoveries, NASA has routinely conducted servicing missions to refurbish older equipment as well as to replace existing scientific instruments with better, more powerful instruments. In early 2002, NASA will conduct its fourth servicing mission to the HST. This servicing mission is named Servicing Mission 3B (SM3B). During SM3B, one of the major refurbishment efforts will be to install new rigid-panel solar arrays as a replacement for the existing flexible-foil solar arrays. This is necessary in order to increase electrical power availability for the new scientific instruments. Prior to installing the new solar arrays on HST, the HST project must be certain that the new solar arrays will not cause any performance degradations to the observatory. One of the major concerns is any disturbance that can cause pointing Loss of Lock (LOL) for the telescope. While in orbit, the solar-array temperature transitions quickly from sun to shadow. The resulting thermal expansion and contraction can cause a "mechanical disturbance" which may result in LOL. To better characterize this behavior, a test was conducted at the European Space Research and Technology Centre (ESTEC) in the Large Space Simulator (LSS) thermal-vacuum chamber. In this test, the Sun simulator was used to simulate on-orbit effects on the solar arrays. This paper summarizes the thermal performance of the Solar Array-3 (SA3) during the Disturbance Verification Test (DVT). The test was conducted between 26 October 2000 and 30 October 2000. Included in this paper are: (1) brief description of the SA3's components and its thermal design; (2) a summary of the on-orbit temperature predictions; (3) pretest thermal preparations; (4) a description of the chamber and thermal monitoring sensors; and (6) presentation of test thermal data results versus flight predictions.

Nguyen, Daniel H.↗

NASA NTRS2005

Surface charging and x-ray emission from insulator surfaces induced by collisions with highly charged ions : relevance to cometary and planetary sp

Characteristic X-ray emission lines are detected from simulants of comet surfaces as they undergo collisions with highly charged ions (HCIs). The HCI projectiles are O+2-O+7. Ion energies are varied in the range (2-7)q keV, where q is the ion charge state. The targets are the insulator minerals olivine, augite, and quartz. It is found that the emission of characteristic K-L, K-M X-rays appears to proceed during positive charging of the surface by the HCI beam. When one uses low-energy, flood-gun electrons to neutralize the surface charge, the X-ray emission is eliminated or greatly reduced, depending on the flood-gun current. Acceleration of background electrons onto the charged surface results in excitation of elemental transitions, including the K-L2 and K-L3 target X-ray emission lines of Mg and Si located spectroscopically at 1253.6 and 1739.4 eV, respectively. Also observed are emission lines from O, Na, Ca, Al, and Fe atoms in the target and charge-exchange lines via surface extraction of electrons by the O+q electric field. Good agreement is found in the ratio of the measured X-ray yields for Mg and Si relative to the ratio of their electron-impact K-shell ionization cross sections. The present study may serve as a guide to astronomers as to specific observing X-ray energies indicative of solar/stellar wind or magnetospheric ion interactions with a comet, planetary surface, or circumstellar dust.

comets↗

NASA NTRS2017

Narrowband Rejection of Reaction Wheel and Environmental Disturbances for the WFIRST OMC Testbed

The Wide Field Infrared Survey Telescope (WFIRST) is a NASA observatory with two scientific instruments. The first is the Wide Field Instrument (WFI) designed to perform wide field imaging and surveys of the near infrared (NIR) sky. The second is a coronagraph (CGI) that will enable astronomers to detect and measure properties of planets in other solar systems. The coronagraph requires 0.5 milli-arcsecond (mas) RMS pointing per axis of the line of sight (LOS) to achieve a contrast of 1x10-9. This paper discusses the approach used for achieving this level of pointing performance on the occulting mask coronagraph (OMC) testbed at JPL. This testbed uses a low-order wavefront sensing (LOWFS) camera and fast steering mirror (FSM) to suppress injected LOS jitter and environmental LOS jitter. The injected jitter includes representative broadband spacecraft attitude control system (ACS) LOS motion and tonal LOS jitter caused by the reaction wheel assemblies (RWA). The environmental jitter includes thermal LOS variations and harmonics of the line noise. The LOWFS camera uses high flux from the obscured science target to achieve high rate measurements of the LOS. These measurements are augmented with fictitious RWA tachometer information and an estimate of the line noise frequency. A LOS servo using a combination of feedback and feedforward control is used in the testbed to compensate for all of the disturbance sources and to mitigate jitter caused by in-band camera noise. The feedforward uses a novel robust least mean squares (RLMS) filter algorithm to reject the RWA and line frequency tones. High fidelity models of the sensors, disturbances and actuator are presented in this paper. These models were developed as part of a simulation of the LOS control system. Performance results from the hardware testbed at the Jet Propulsion Laboratory (JPL) are discussed in this paper.

Shields, Joel↗

NASA NTRS2008

WGM Resonators for Terahertz-to-Optical Frequency Conversion

Progress has been made toward solving some practical problems in the implementation of terahertz-to-optical frequency converters utilizing whispering-gallery-mode (WGM) resonators. Such frequency converters are expected to be essential parts of non-cryogenic terahertz- radiation receivers that are, variously, under development or contemplated for a variety of applications in airborne and spaceborne instrumentation for astronomical and military uses. In most respects, the basic principles of terahertz-to-optical frequency conversion in WGM resonators are the same as those of microwave (sub-terahertz)-to-optical frequency conversion in WGM resonators, various aspects of which were discussed in the three preceeding articles. To recapitulate: In a receiver following this approach, a preamplified incoming microwave signal (in the present case, a terahertz signal) is up-converted to an optical signal by a technique that exploits the nonlinearity of the electromagnetic response of a whispering-gallery-mode (WGM) resonator made of LiNbO3 or another suitable electro-optical material. Upconversion takes place by three-wave mixing in the resonator. To ensure the required interaction among the optical and terahertz signals, the WGM resonator must be designed and fabricated to function as an electro-optical modulator while simultaneously exhibiting (1) resonance at the required microwave and optical operating frequencies and (2) phase matching among the microwave and optical signals circulating in the resonator. Downstream of the WGM resonator, the up-converted signal is processed photonically by use of a tunable optical filter or local oscillator and is then detected. The practical problems addressed in the present development effort are the following: Satisfaction of the optical and terahertz resonance-frequency requirement is a straightforward matter, inasmuch as the optical and terahertz spectra can be measured. However, satisfaction of the phase-matching requirement is more difficult. The approach followed in the present development is to perform computer simulations of the microwave and optical signals circulating in the resonator to test for phase matching. To enable excitation of the terahertz WGM resonator mode, it is also necessary to ensure phase matching between that mode and the incoming terahertz radiation. In the present development, the incoming signal is coupled into the WGM resonator via a tapered waveguide in the form of a fused silica rod. The phase-matching requirement is satisfied at one point along the taper; the rod is positioned with this point in proximity to the WGM resonator. To maximize the conversion efficiency, it is necessary to maximize the spatial overlap among the terahertz and optical modes in the WGM resonator. In the absence of a special design effort to address this issue, there would be little such overlap because, as a consequence of a large difference between wavelengths, the optical and terahertz modes would be concentrated at different depths from the rim of a WGM resonator. In the present development, overlap is ensured by constructing the WGM resonator as a ring (see figure) so thin that the optical and terahertz modes are effectively forced to overlap.

Strekalov,Dmitry↗