David G. MacDonnell

Elevation information in tail (EIT) technique for lidar altimetry.

A technique we refer to as Elevation Information in Tail (EIT) has been developed to provide improved lidar altimetry from CALIPSO lidar data. The EIT technique is demonstrated using CALIPSO data and is applicable to other similar lidar systems with low-pass filters. The technique relies on an observed relation between the shape of the surface return signals (peak shape) and the detector photo-multiplier tube transient response (transient response tail). Application of the EIT to CALIPSO data resulted in an order of magnitude or better improvement in the CALIPSO land surface 30-meter elevation measurements. The results of EIT compared very well with the National Elevation Database (NED) high resolution elevation maps, and with the elevation measurements from the Shuttle Radar Topography Mission (SRTM).

Fully transparent photon sieve

Regular photon sieve (PS) may only have up to ~25% transmission of light. The low transmission limits its applications in many fields such as satellite remote sensing when the reflected light incident on the PS is relatively weak. Binary PS was developed to overcome the low transmission problem of PS. However, binary PS which involves using different optical materials/thicknesses in different zones of the PS at a nanometer or micron scale, is not easy to manufacture. Therefore, in this study, we developed a fully transparent PS concept. We can use laser photolithography to simply make holes on a sheet of fully transparent material. With specifically designed optical thickness and PS-patterned pinholes, the transparent sheet can effectively focus light to its focal point. This concept is validated both by the finite-difference time domain (FDTD) modeling and by laboratory prototypes in this study.

Fabrication of photon sieves by laser ablation and optical properties

In this work, we demonstrate the feasibility and performance of photon sieve diffractive optical elements fabricated via a direct laser ablation process. Pulses of 50 ns width and wavelength 1064 nm from an ytterbium fiber laser were focused to a spot diameter of approximately 35 µm. Using a galvanometric scan head writing at 100 mm/s, a 30.22 mm2 photon sieve operating at 633 nm wavelength with a focal length of 400 mm was fabricated. The optical performance of the sieve was characterized and is in strong agreement with numerical simulations, producing a focal spot size full-width at half-maximum (FWHM) of 45.12 ± 0.74 µm with a photon sieve minimum pinhole diameter of 62.2 µm. The total time to write the photon sieve pattern was 28 seconds as compared to many hours using photolithography methods. We also present, for the first time to our knowledge in the literature, thorough characterization of the influence of angle of incidence, temperature, and illumination wavelength on photon sieve performance. Thus, this work demonstrates the potential for a high speed, low cost fabrication method of photon sieves that is highly customizable and capable of producing sieves with low or high numerical apertures.

CLARREO Reflected Solar Spectrometer: Restrictions for Instrument Sensitivity to Polarization

The foundation for future space mission Climate Absolute Radiance and Refractivity Observatory (CLARREO) is the ability to produce climate change benchmark records and provide on-orbit calibration standard through the highly accurate and Système Internationale-traceable observations. The accuracy of CLARREO measurements is set to 0.3% (k=2) for spectrally resolved reflectance. The instrument sensitivity to polarization and polarization of reflected light at the top of atmosphere are the sources for systematic uncertainty. In this paper, we estimate radiometric errors due to polarization effects for CLARREO benchmark and reference intercalibration observations. Data from the Polarization and Anisotropy of Reflectance for Atmospheric Sciences coupled with Observations from Lidar (PARASOL) instrument, a spaceborne polarimeter, have been used in combination with the orbital modeling of Earths sampling. For the CLARREO benchmark data, we used simulated annual nadir sampling for the polar orbit with 90° inclination, and for the intercalibration with cross-track sensors on the JPSS, such as CERES and VIIRS, we simulated on-orbit matched data sampling. Selected PARASOL data over one full solar year provided polarization parameters in visible (VIS) spectral range. For estimating polarization in near infrared (NIR) spectral range, we used a radiative transfer model. Our results show that to limit error contribution due to polarization to half of the allowed total, the sensitivity to polarization of CLARREO reflected solar instrument should not exceed 0.5% (k=2) in spectral range from VIS to NIR.

Lidars utilizing vortex laser beams

We are investigating the potential of the “vortex” laser beam to provide additional information of natural scenes from aircraft and space-based lidars. This type of beam has a spatial wavefront with a helical twist that creates an optical singularity on axis, and carries orbital angular momentum. We will report on preliminary results for differences in Rayleigh-Mie scattering, and scattering from rough surfaces, and plans for future studies.

Flying in a spacecraft constellation: a coordination puzzle.

CNES has been operating the PARASOL spacecraft as a member of the Afternoon (A-Train) Satellite constellation for nearly 5 years and the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) for more than 3 years. Whereas PARASOL is a French spacecraft operated by CNES, CALIPSO is a French platform with a National Aeronautics and Space Administration (NASA) payload (Lidar) and jointly operated by CNES and NASA. In this frame, a number of challenges and puzzling situations have been experienced in the A-train and in the CNESNASA cooperation.