The Active Sensing of CO<sub>2</sub> Emissions over Nights, Days, and Seasons (ASCENDS) CarbonHawk Experiment Simulator (ACES) is a NASA Langley Research Center instrument funded by NASA’s Science Mission Directorate that seeks to advance technologies critical to measuring atmospheric column carbon dioxide (CO<sub>2</sub>) mixing ratios in support of the NASA ASCENDS mission. The ACES instrument, an Intensity-Modulated Continuous-Wave (IM-CW) lidar, was designed for high-altitude aircraft operations and can be directly applied to space instrumentation to meet the ASCENDS mission requirements. Airborne flight campaigns have been used to demonstrate ACES’ advanced technologies critical for a spaceborne instrument with lower platform consumption of size, mass, and power, and with improved performance. ACES recently flew on the NASA DC-8 aircraft during the 2017 NASA ASCENDS/Arctic-Boreal Vulnerability Experiment (ABoVE) airborne measurement campaign to test ASCENDS-related technologies in the challenging Arctic environment. Data were collected over a wide variety of surface reflectivities, terrain, and atmospheric conditions during the campaign’s eight research flights. ACES also flew during the 2017 and 2018 Atmospheric Carbon and Transport – America (ACT-America) Earth Venture Suborbital - 2 (EVS-2) campaigns along with the primary ACT-America CO<sub>2</sub> lidar, Harris Corporation’s Multi-Frequency Fiber Laser Lidar (MFLL). Regional CO<sub>2</sub> distributions of the lower atmosphere were observed from the C-130 aircraft during the ACT-America campaigns in support of ACT-America’s science objectives. The airborne lidars provide unique remote data that complement data from more traditional in situ sensors. This presentation shows the applications of CO<sub>2</sub> lidars in meeting these science needs from airborne platforms and an eventual spacecraft.
Global atmospheric carbon dioxide (CO<sub>2</sub>) measurements for the NASA Active Sensing of CO<sub>2</sub> Emissions over Nights, Days, and Seasons (ASCENDS) space mission are critical for improving our understanding of global CO<sub>2</sub> sources and sinks. Advanced Intensity- Modulated Continuous-Wave (IM-CW) lidar techniques are investigated as a means of facilitating CO<sub>2</sub> measurements from space to meet the ASCENDS measurement requirements. In recent numerical, laboratory and flight experiments we have successfully used the Binary Phase Shift Keying (BPSK) modulation technique to uniquely discriminate surface lidar returns from intermediate aerosol and cloud contamination. We demonstrate the utility of BPSK to eliminate sidelobes in the range profile as a means of making Integrated Path Differential Absorption (IPDA) column CO<sub>2</sub> measurements in the presence of optically thin clouds, thereby eliminating the need to correct for sidelobe bias errors caused by the clouds. Furthermore, high accuracy and precision ranging to the surface as well as to the top of intermediate cloud layers, which is a requirement for the inversion of column CO<sub>2</sub> number density measurements to column CO<sub>2</sub> mixing ratios, has been demonstrated using new hyperfine interpolation techniques that takes advantage of the periodicity of the modulation waveforms. This approach works well for both BPSK and linear swept-frequency modulation techniques. The BPSK technique under investigation has excellent auto-correlation properties while possessing a finite bandwidth. A comparison of BPSK and linear swept-frequency is also discussed in this paper. These results are extended to include Richardson-Lucy deconvolution techniques to extend the resolution of the lidar beyond that implied by limit of the bandwidth of the modulation, where it is shown useful for making tree canopy measurements.
Operating high power laser diode arrays in long pulse regime of about 1 msec, which is required for pumping 2-micron
thulium and holmium-based lasers, greatly limits their useful lifetime. This paper describes performance of laser diode
arrays operating in long pulse mode and presents experimental data on the active region temperature and pulse-to-pulse
thermal cycling that are the primary cause of their premature failure and rapid degradation. This paper will then offer a
viable approach for determining the optimum design and operational parameters leading to the maximum attainable
Future robotic and crewed lunar missions will require safe and precision soft-landing at scientifically interesting sites
near hazardous terrain features such as craters and rocks or near pre-deployed assets. Presently, NASA is studying the
ability of various 3-dimensional imaging sensors particularly lidar/ladar techniques in meeting its lunar landing needs.
For this reason, a Sensor Test Range facility has been developed at NASA Langley Research Center for calibration and
characterization of potential 3-D imaging sensors. This paper describes the Sensor Test Range facility and its application
in characterizing a 3-D imaging ladar. The results of the ladar measurement are reported and compared with simulated
image frames generated by a ladar model that was also developed as part of this effort. In addition to allowing for
characterization and evaluation of different ladar systems, the ladar measurements at the Sensor Test Range will support
further advancement of ladar systems and development of more efficient and accurate image reconstruction algorithms.
Most Lidar applications rely on moderate to high power solid state lasers to generate the required transmitted pulses. However, the reliability of solid state lasers, which can operate autonomously over long periods, is constrained by their laser diode pump arrays. Thermal cycling of the active regions is considered the primary reason for rapid degradation of the quasi-CW high power laser diode arrays, and the excessive temperature rise is the leading suspect in premature failure. The thermal issues of laser diode arrays are even more drastic for 2-micron solid state lasers which require considerably longer pump pulses compared to the more commonly used pump arrays for 1-micron lasers. This paper describes several advanced packaging techniques being employed for more efficient heat removal from the active regions of the laser diode bars. Experimental results for several high power laser diode array devices will be reported and their performance when operated at long pulsewidths of about 1msec will be described.
Space-based laser and lidar instruments play an important role in NASA's plans for meeting its objectives in both Earth Science and Space Exploration areas. Almost all the lidar instrument concepts being considered by NASA scientist utilize moderate to high power diode-pumped solid state lasers as their transmitter source. Perhaps the most critical component of any solid state laser system is its pump laser diode array which essentially dictates instrument efficiency, reliability and lifetime. For this reason, premature failures and rapid degradation of high power laser diode arrays that have been experienced by laser system designers are of major concern to NASA. This work addresses these reliability and lifetime issues by attempting to eliminate the causes of failures and developing methods for screening laser diode arrays and qualifying them for operation in space.
2-micron solid-state lasers operating at moderate to high pulse energies require high power quasi-CW laser diode arrays (LDAs) operating at a nominal wavelength of 792 nm with pulse durations of at least one millisecond. This long pulse duration is one of the main causes of limited lifetimes for these arrays. Such relatively long pulse durations cause the laser diode active region to experience high peak temperatures and drastic thermal cycling. This extreme localized heating and thermal cycling of the active regions are considered the primary contributing factors for both gradual and catastrophic degradation of LDAs. This paper describes the thermal characteristics of various LDA packages, providing valuable insight for improving their heat dissipation and increasing their lifetime. The experiment includes both direct measurement of thermal radiation of the LDA facet using a high resolution IR camera and indirect measurement of LDA active region temperature by monitoring the wavelength shift of the near-IR light. The result of thermal measurements on different quasi-CW LDA packages and architectures is reported.
A compact high pulse energy Ti:Sapphire laser with its third harmonic has been developed for airborne ozone differential absorption lidar to study the distribution and concentration of ozone throughout the troposphere. The Ti:Sapphire laser, pumped by a commercial frequency-doubled Nd:YAG laser with a pulse repetition frequency of 20 Hz is seeded by a single mode 900 nm diode laser. More than 130 mJ/pulse was achieved at the fundamental wavelength of 900 nm. Two nonlinear, Lithium Triborate crystals were used for the Third Harmonic Generation resulting in output pulse energy of more than 39mJ at 300nm, which is used as the off-line wavelength of an airborne ozone DIAL system. The energy conversion efficiency from 900 nm to 300 nm was 30 percent as compared to the theoretical value of 36 percent.