Laser spectral analysis systems are increasingly being considered for in situ analysis of the atomic and molecular composition of selected rock and soil samples on other planets . Both Laser Induced Breakdown Spectroscopy (LIBS) and Raman spectroscopy are used to identify the constituents of soil and rock samples <i>in situ</i>. LIBS instruments use a high peak-power laser to ablate a minute area of the surface of a sample. The resulting plasma is observed with an optical head, which collects the emitted light for analysis by one or more spectrometers. By identifying the ion emission lines observed in the plasma, the constituent elements and their abundance can be deduced. In Raman spectroscopy, laser photons incident on the sample surface are scattered and experience a Raman shift, exchanging small amounts of energy with the molecules scattering the light. By observing the spectrum of the scattered light, it is possible to determine the molecular composition of the sample.<p> </p>For both types of instruments, there are advantages to physically separating the light collecting optics from the spectroscopy optics. The light collection system will often have articulating or rotating elements to facilitate the interrogation of multiple samples with minimum expenditure of energy and motion. As such, the optical head is often placed on a boom or an appendage allowing it to be pointed in different directions or easily positioned in different locations. By contrast, the spectrometry portion of the instrument is often well-served by placing it in a more static location. The detectors often operate more consistently in a thermally-controlled environment. Placing them deep within the spacecraft structure also provides some shielding from ionizing radiation, extending the instrument’s useful life. Finally, the spectrometry portion of the instrument often contains significant mass, such that keeping it off of the moving portion of the platform, allowing that portion to be significantly smaller, less massive and less robust.<p> </p>Large core multi-mode optical fibers are often used to accommodate the optical connection of the two separated portions of such instrumentation. In some cases, significant throughput efficiency improvement can be realized by judiciously orienting the strands of multi-fiber cable, close-bunching them to accommodate a tight focus of the optical system on the optical side of the connection, and splaying them out linearly along a spectrometer slit on the other end.<p> </p>For such instrumentation to work effectively in identifying elements and molecules, and especially to produce accurate quantitative results, the spectral throughput of the optical fiber connection must be consistent over varying temperatures, over the range of motion of the optical head (and it’s implied optical cable stresses), and over angle-aperture invariant of the total system. While the first two of these conditions have been demonstrated, spectral observations of the latter present a cause for concern, and may have an impact on future design of fiber-connected LIBS and Raman spectroscopy instruments. In short, we have observed that the shape of the spectral efficiency curve of a large multi-mode core optical fiber changes as a function of input angle.
The Multispectral Thermal Imager (MTI) was designed as an imaging radiometer with absolute calibration requirements established by Department of Energy (DOE) mission goals. Particular emphasis was given to water surface temperature retrieval using two mid wave and three long wave infrared spectral bands, the fundamental requirement was a surface temperature determination of 1K at the 68% confidence level. For the ten solar reflective bands a one-sigma radiometric performance goal of 3% was established. In order to address these technical challenges a calibration facility was constructed containing newly designed sources that were calibrated at NIST. Additionally, the design of the payload and its onboard calibration system supported post launch maintenance and update of the ground calibration. The on-orbit calibration philosophy also included vicarious techniques using ocean buoys, playas and other instrumented sites; these became increasingly important subsequent to an electrical failure which disabled the onboard calibration system. This paper offers various relevant lessons learned in the eight-year process of reducing to practice the calibration capability required by the scientific mission. The discussion presented will include observations pertinent to operational and procedural issues as well as hardware experiences; the validity of some of the initial assumptions will also be explored.
Accurate coregistration of images from the Multispectral Thermal Imager (MTI) is needed to properly align bands for spectral analysis and physical retrievals, such as water surface temperature, land-cover classification, or small target identification. After accounting for spacecraft motion, optical distortion, and geometrical perspective, the irregularly-spaced pixels in the images must be resampled to a common grid. What constitutes an optimal resampling depends, to some extent, on the needs of the user. A good resampling trades off radiometric fidelity, contrast preservation for small objects, and cartographic accuracy -- and achieves this compromise without unreasonable computational effort. The standard MTI coregistration product originally used a weighted-area approach to achieve this irregular resampling, which generally over-smoothes the imagery and reduces the contrast of small objects. Recently, other resampling methods have been implemented to improve the final coregistered image. These methods include nearest-neighbor resampling and a tunable, distance-weighted resampling. We will discuss the pros and cons of various resampling methods applied to MTI images, and show results of comparing the contrast of small objects before and after resampling.
The Robotic Lunar Observatory (ROLO) project has developed a spectral irradiance model of the Moon that accounts for variations with lunar phase through the bright half of a month, lunar librations, and the location of an Earth-orbiting spacecraft. The methodology of comparing spacecraft observations of the Moon with this model has
been developed to a set of standardized procedures so that comparisons can be readily made. In the cases where observations extend over several years (e.g., SeaWiFS), instrument response degradation has been determined with precision of about 0.1% per
year. Because of the strong dependence of lunar irradiance on geometric angles, observations by two spacecraft cannot be directly compared unless acquired at the same time and location. Rather, the lunar irradiance based on each spacecraft instrument calibration can be compared with the lunar irradiance model. Even single observations by an instrument allow inter-comparison of its radiometric scale with
other instruments participating in the lunar calibration program. Observations by SeaWiFS, ALI, Hyperion and MTI are compared here.
One element of a multi-year calibration program between the National Institute of Standards and Technology (NIST) and the National Aeronautical and Space Administration (NASA) Earth Observing System (EOS) Project Science Office has been the development and deployment of a portable transfer radiometer for verifying the thermal-infrared scales being used for flight-instrument pre-launch calibration. This instrument, the Thermal-infrared Transfer Radiometer (TXR), has been built and the first deployment test was completed successfully, as has been reported previously.1 The 5 µm channel, based on a photovoltaic Indium Antimonide (InSb) detector, so far has demonstrated a pre-deployment to post-deployment uncorrected repeatability of better than 30 mK to 60 mK, which is sufficient to enable intercomparisons at useful uncertainty levels for the EOS program. However, the 10 µm channel, based on a photovoltaic Mercury Cadmium Telluride (MCT) detector, shows uncorrected repeatability levels of about 0.5 K, the response changes being induced by cryocycling. This paper describes the technique that has been developed for correcting these changes. A portable black body check-source travels with the TXR that is used to verify the repeatability during the deployment trip. The check-source, in combination with the stability of the 5 µm channel, is used to restore a higher accuracy scale to the 10 µm channel than would otherwise be possible. This application is analogous to the use of an on-orbit calibration source to check for and correct for launch-induced or degradation-induced flight instrument detector response changes.
The Multispectral Thermal Imager (MTI) is a satellite-based imaging system that provides images in fifteen spectral bands covering large portions of the spectrum from 0.45 through 10.7 microns. This article describes the current MTI radiometric image calibration, and will provide contrast with pre-launch plans discussed in an earlier article. The MTI system is intended to provide data with state-of-the-art radiometric calibration. The on-orbit calibration relies on the pre-launch ground calibration and is maintained by vicarious calibration campaigns. System drifts before and between the vicarious calibration campaigns are monitored by several on-board sources that serve as transfer sources in the calibration of external images. The steps used to transfer calibrations to image products, additional radiometric data quality estimates performed as part of this transfer, and the data products associated with this transfer will all be examined.
The Multispectral Thermal Imager (MTI) is a research and development project sponsored by the Department of Energy and executed by Sandia and Los Alamos National Laboratories and the Savannah River Technology Center. Other participants include the U.S. Air Force, universities, and many industrial partners. The MTI mission is to demonstrate the efficacy of highly accurate multispectral imaging for passive characterization of industrial facilities and related environmental impacts from space. MTI provides simultaneous data for atmospheric characterization at high spatial resolution. Additionally, MTI has applications to environmental monitoring and other civilian applications. The mission is based in end-to-end modeling of targets, signatures, atmospheric effects, the space sensor, and analysis techniques to form a balanced, self-consistent mission. The MTI satellite nears completion, and is scheduled for launch in late 1999. This paper describes the MTI mission, development of desired system attributes, some trade studies, schedule, and overall plans for data acquisition and analysis. This effort drives the sophisticated payload and advanced calibration systems, which are the overall subject of the first session at this conference, as well as the data processing and some of the analysis tools that will be described in the second segment.
The Multispectral Thermal Imager Optical Assembly (OA) has been fabricated, assembled, successfully performance tested, and integrated into the flight payload structure with the flight Focal Plane Assembly (FPA) integrated and aligned to it. This represents a major milestone achieved towards completion of this earth observing E-O imaging sensor that is to be operated in low earth orbit. The OA consists of an off- axis three mirror anastigmatic (TMA) telescope with a 36 cm unobscured clear aperture, a wide-field-of-view (WFOV) of 1.82 degrees along the direction of spacecraft motion and 1.38 degree across the direction of spacecraft motion. It also contains a comprehensive on-board radiometric calibration system. The OA is part of a multispectral pushbroom imaging sensor which employs a single mechanically cooled focal plane with 15 spectral bands covering a wavelength range from 0.45 to 10.7 micrometer. The OA achieves near diffraction-limited performance from visible to the long-wave infrared (LWIR) wavelengths. The two major design drivers for the OA are 80% enpixeled energy in the visible bands and radiometric stability. Enpixeled energy in the visible bands also drove the alignment of the FPA detectors to the OA image plane to a requirement of less than plus or minus 20 micrometer over the entire visible detector field of view (FOV). Radiometric stability requirements mandated a cold Lyot stop for stray light rejection and thermal background reduction. The Lyot stop is part of the FPA assembly and acts as the aperture stop for the imaging system. The alignment of the Lyot stop to the OA drove the centering and to some extent the tilt alignment requirements of the FPA to the OA.
The Multi-spectral Thermal Imager (MTI) will be a satellite- based imaging system that will provide images in fifteen spectral bands covering large portions of the spectrum from 0.45 through 10.7 microns. An important goal of the mission is to provide data with state-of-the-art radiometric calibration. The on-orbit calibration will rely on the pre-launch ground calibration and will be maintained by vicarious calibration campaigns. System drifts before and between the vicarious calibration campaigns will be monitored by several on-board sources that serve as transfer sources in the calibration of external images. These sources can be divided into two groups: a set of sources at an internal aperture, primarily intended to monitor short term drifts in the detectors and associated electronics; and two sources at the external aperture, intended to monitor longer term drifts in the optical train before the internal aperture. The steps needed to transfer calibrations to image products, additional radiometric data quality estimates performed as part of this transfer, and the data products associated with this transfer will all be examined.
MTI is a comprehensive research and development project that includes up-front modeling and analysis, satellite system design, fabrication, assembly and testing, on-orbit operations, and experimentation and data analysis. The satellite is designed to collect radiometrically calibrated, medium resolution imagery in 15 spectral bands ranging from 0.45 to 10.70 micrometer. The payload portion of the satellite includes the imaging system components, associated electronics boxes, and payload support structure. The imaging system includes a three-mirror anastigmatic off-axis telescope, a single cryogenically cooled focal plane assembly, a mechanical cooler, and an onboard calibration system. Payload electronic subsystems include image digitizers, real-time image compressors, a solid state recorder, calibration source drivers, and cooler temperature and vibration controllers. The payload support structure mechanically integrates all payload components and provides a simple four point interface to the spacecraft bus. All payload components have been fabricated and tested, and integrated.
Los Alamos National Laboratories has completed the design, manufacture and calibration of a vacuum-compatible, tungsten lamp, integrating sphere. The light source has been calibrated at the National Institute of Standards and Technology and is intended for use as a calibration standard for remote sensing instrumentation. Calibration 2(sigma) uncertainty varied with wavelength from 1.21% at 400 nm and 0.73% at 900 nm, to 3.95% at 2400 nm. The inner radius of the Spectralon-coated sphere is 21.2 cm with a 7.4 cm square exit aperture. A small satellite sphere is attached to the main sphere and its output coupled through a stepper motor driven aperture. The variable aperture allows a constant radiance without effecting the color temperature output from the main sphere. The sphere's output is transmitted into a vacuum test environment through a fused silica window that is an integral part of the outer housing of the vacuum shell assembly. The atmosphere within this outer housing is composed of 240 degree(s)K nitrogen gas, provided by a custom LN<SUB>2</SUB> vaporizer unit. Use of the nitrogen gas maintains the internal temperature of the sphere at a nominal 300 degree(s)K +/- 10 degree(s). The calibrated spectral range of the source is 0.4 micrometers through 2.4 micrometers . Three, color temperature matched, 20 W bulbs together with a 10 W bulb are within the main integrating sphere. Two 20 W bulbs, also color temperature matched, reside in the satellite integrating sphere. A silicon and a germanium broadband detector are situated within the inner surface of the main sphere. Their purpose is for the measurement of the internal broadband irradiance. A fiber-optic-coupled spectrometer measures the internal color temperature that is maintained by current control on the lamps. Each lamp is independently operated allowing for radiances with common color temperatures ranging from near 0.026 W/cm<SUP>2</SUP>/sr to about 0.1 W/cm<SUP>2</SUP>/sr at a wavelength of 0.9 micrometers (the location of the peak spectral radiance).
Many remote sensing applications rely on imaging spectrometry. Here we use imaging spectrometry for thermal and multispectral signatures measured from a satellite platform enhanced with a combination of accurate calibrations and on-board data for correcting atmospheric distortions. Our approach is supported by physics-based end- to-end modeling and analysis, which permits a cost-effective balance between various hardware and software aspects.
The Optical Assembly (OA) for the Multispectral Thermal Imger (MTI) program has been fabricated, assembled, and successfully tested for its performance. It represents a major milestone achieved towards completion of this earth observing EO imaging sensor that is to be operated in low earth orbit. Along with its wide field of view, 1.82 degrees along-track and 1.38 degrees cross-track, and comprehensive on-board calibration system, the pushbroom imaging sensor employs a single mechanically cooled focal plane with 15 spectral bands covering a wavelength range from 0.45 to 10.7 micrometers . The OA has an off-axis three-mirror anastigmatic telescope with a 36-cm unobscured clear aperture. The two key performance criteria, 80 percent enpixeled energy in the visible and radiometric stability of 1 percent 1 (sigma) in the visible/near-IR and short wavelength IR, of 1.45 percent 1 (sigma) in the medium wavelength IR, and of 0.53 percent 1 (sigma) long wavelength IR, as well as its low weight and volume constraint drive the overall design configuration of the OA and fabrication requirements.
A radiometric calibration station (RCS) is being assembled at the Los Alamos National Laboratory (LANL) which will allow for calibration of sensors with detector arrays having spectral capability from about 0.4-15 micrometers. The configuration of the LANL RCS is shown. Two blackbody sources have been designed to cover the spectral range from about 3-15 micrometers, operating at temperatures ranging from about 180-350 K within a vacuum environment. The sources are designed to present a uniform spectral radiance over a large area to the sensor unit under test. THe thermal uniformity requirement of the blackbody cavities has been one of the key factors of the design, requiring less than 50 mK variation over the entire blackbody surface to attain effective emissivity values of about 0.999. Once the two units are built and verified to the level of about 100 mK at LANL, they will be sent to the National Institute of Standards and Technology (NIST), where at least a factor of two improvements will be calibrated into the blackbody control system. The physical size of these assemblies will require modifications of the existing NIST Low Background Infrared (LBIR) Facility. LANL has constructed a bolt-on addition to the LBIR facility that will allow calibration of our large aperture sources. Methodology for attaining the two blackbody sources at calibration levels of performance equivalent to present state of the art will be explained in the paper.
Los Alamos National Laboratories has begun construction of a visible/infrared radiometric calibration station that will allow for absolute calibration of optical and IR remote sensing instruments with clear apertures less than 16 inches in diameter in a vacuum environment. The calibration station broadband sources will be calibrated at the National Institute of Standards and Technology (NIST) and allow for traceable absolute radiometric calibration to within +/- 3% in the visible and near IR (0.4-2.5 micrometers ), and less than +/- 1% in the infrared, up to 12 micrometers . Capabilities for placing diffraction limited images of for sensor full-field flooding will exist. The facility will also include the calibration of polarization and spectra effects, spatial resolution, field of view performance, and wavefront characterization. The configuration of the vacuum calibration station consists of an off-axis 21 inch, f/3.2, parabolic collimator with a scanning fold flat in collimated space. The sources are placed, via mechanisms to be described, at the focal plane of the off-axis parabola. Vacuum system pressure will be in the 10<SUP>-6</SUP> Torr range. The broadband white-light source is a custom design by LANL with guidance from Labsphere Inc. The continuous operating radiance of the integrating sphere will be from 0.0-0.006 W/cm<SUP>2</SUP>/Sr/micrometers (upper level quoted for approximately 500 nm wavelength). The blackbody source is also custom designed at LANL with guidance from NIST. The blackbody temperature will be controllable between 250-350 degree(s)K. Both of the above sources have 4.1 inch apertures with estimated radiometric instability at less than 1%. The designs of each of these units will be described. The monochromator and interferometer light sources are outside the vacuum, but all optical relay and beam shaping optics are enclosed within the vacuum calibration station. These sources are to be described, as well as the methodology for alignment and characterization.
Several optomechanical tasks for the Los Alamos National Laboratory's (LANL) Free- Electron Laser (FEL) were set by the envisioned project goals as early as 1988. Unfortunately, the FEL project has been set aside due to funding constraints. The tasks reported on here required extensive modeling for final adaptability into the FEL environment. The systems to be described are best identified as (1) a Brewster attenuation device, (2) an optical mode relay lens system, (3) a spectral harmonics band-filtering system, (4) a 25-nm micropulse spectrometer system, (5) a 12.5-nm micropulse spectrometer system, (6) a 0.6-nm micropulse spectrometer system, and (7) a reflective mode profile rotator. The Brewster attenuation device was successfully used inside the FEL resonator. The optical mode relay lens system, spectral harmonics band filtering system, and reflective mode profile rotator were completed but never used. The 25-nm micropulse spectrometer was optically and mechanically completed, but the detector electronics were never finished. The 12.5- and 0.6-nm micropulse spectrometers were never assembled, due to hardware that was common to the 25-nm system.
Two versions of the 'Phase-Step Mirror' (PSM), a novel optical component that prevents the formation of sidebands in a Free-Electron Laser (FEL) were tested on the Los Alamos National Laboratory (LANL) APEX FEL. Sideband suppression and frequency control with high extraction efficiency and single line, transform limited operation were demonstrated. The results of our LANL experiments and computer simulations showed that for very high gain applications, the first-order sideband is completely suppressed, but the laser gain is so strong that on about pass 300 the sideband at the second-order or next free spectral range of the PSM appears. This second-order sideband may be suppressed by designing a PSM with grooves having two alternating depths, one chosen to suppress the first-order sideband, and the other, the second-order sideband.
The optical cavity of the Boeing free-electron laser (FEL) was reconfigured as a semiconfocal ring resonator with two glancing incidence hyperboloid-paraboloid telescopes. The challenge for this experiment was the complexity of the ring resonator compared to the simplicity of a concentric cavity. The ring resonator's nonspherical mirror surfaces, its multiple elements, and the size of the components contributed to the problems of keeping the optical mode of the resonator matched to the electron beam in the wiggler. Several new optical diagnostics were developed to determine when the optical mode in the FEL was spatially and temporally matched to the electron beam through the wiggler. These included measurements of the focus position and Rayleigh range of the ring resonator optics to determine the spatial match of the optical mode through the wiggler, and a measurement of the position of the optical axis for multiple passes around the ring resonatorto determine the stability of the resonator alignment. This paper also describes the optical measurements that were necessary to achieve reliable lasing. The techniques for measuring ring resonator Rayleigh range and focus position, multiple pass alignment, cavity length, optical energy per micropulse, peak power, optical extraction, small signal gain, ringdown loss, lasing wavelength, electron bunch pulse width, and energy slew are discussed.
The optical cavity of the Boeing visible free electron laser was reconfigured from a concentric cavity to a glancing incidence ring resonator in late 1989 and was operated until December 1990. the crucial requirement for the optical cavity of an FEL is to provide an optical mode which is spatially and temporally matched to the electron beam as it moves through the wiggler. Several new optical diagnostics were developed to determine when the above requirement was satisfied. This paper will discuss those diagnostics which achieved and maintained the alignment of the ring resonator within tolerance to lase and measured the quality of lasing. The new diagnostics included measurements of the focus position and Rayleigh range of the ring resonator optics to determine the spatial match of the optical mode through the wiggler, and a measurement of the position of the optical axis for multiple passes around the ring resonator to determine the stability of the resonator alignment. Accelerator performance was determined by measuring the electron beam pulse width and charge, which indicated electron beam brightness, and by measuring the width of the spontaneous emission spectrum, which gave an indication of the alignment between the electron beam and the optical axis. Temporal overlap of electron and optical pulses was assured by measuring the optical cavity length. In addition, several other diagnostics which indicated FEL performance will be described: optical energy per micropulse, small signal gain, ringdown loss, laser pulse width, laser wavelength, and time resolved spectroscopy.
The 10-pm Los Alamos free-electron laser (FEL) facility is being upgraded. The conventional electron gun and bunchers have been replaced with a much more compact 6-MeV photoinjector accelerator. By adding existing parts from previous experiments, the primary beam energy will be doubled to 40 MeV. With the existing 1-mS wiggler (Xv, 2.7 cm) and resonator, the facility can produce photons with wavelengths from 3 to 10 im when lasing on the fundamental mode and produce photons in the visible spectrum with short period wigglers or harmonic operation. After installation of a 150° bend, a second wiggler will be added as an amplifier. The installation of laser transport tubes between the accelerator vault and an upstairs laboratory will provide experimenters with a radiation free environment for experiments. At the time of writing (Jan. 1990), the injector plus one additional tank has been installed and tested with beam to an energy of 17 MeV.