Polymer composites are one of the most attractive near-term means to exploit the unique properties of carbon nanotubes and graphene. This is particularly true for composites aimed at electronics and photonics, where a number of promising applications have already been demonstrated. One such example is nanotube-based saturable absorbers. These can be used as all-optical switches, optical amplifier noise suppressors, or mode-lockers to generate ultrashort laser pulses. Here, we review various aspects of fabrication, characterization, device implementation and operation of nanotube-polymer composites to be used in photonic applications. We also summarize recent results on graphene-based saturable absorbers for ultrafast lasers.

Nanotube–Polymer Composites for Ultrafast Photonics

CO2, CH4, and N2O are recognised as the most important greenhouse gases, the concentrations of which increase rapidly through human activities. Space-borne integrated path differential absorption lidar allows global observations at day and night over land and water surfaces in all climates. In this study we investigate potential sources of measurement errors and compare them with the scientific requirements. Our simulations reveal that moderate-size instruments in terms of telescope aperture (0.5–1.5 m) and laser average power (0.4–4 W) potentially have a low random error of the greenhouse gas column which is 0.2% for CO2 and 0.4% for CH4 for soundings at 1.6 μm, 0.4% for CO2 at 2.1 μm, 0.6% for CH4 at 2.3 μm, and 0.3% for N2O at 3.9 μm. Coherent detection instruments are generally limited by speckle noise, while direct detection instruments suffer from high detector noise using current technology. The wavelength selection in the vicinity of the absorption line is critical as it controls the height region of highest sensitivity, the temperature cross-sensitivity, and the demands on frequency stability. For CO2, an error budget of 0.08% is derived from our analysis of the sources of systematic errors. Among them, the frequency stability of ± 0.3 MHz for the laser transmitter and spectral purity of 99.9% in conjunction with a narrow-band spectral filter of 1 GHz (FWHM) are identified to be challenging instrument requirements for a direct detection CO2 system operating at 1.6 μm.

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Space-borne remote sensing of CO2, CH4, and N2O by integrated path differential absorption lidar: a sensitivity analysis

Q-switched output of 1.1 J/pulse at a 2.053 microm wavelength has been achieved in a diode-pumped Ho: Tm: LuLF laser with a side-pumped rod configuration in a master-oscillator-power-amplifier (MOPA) architecture. This is the first time to our knowledge that a 2 microm laser has broken the joule per pulse barrier for Q-switched operation. The total system efficiency reaches 5% and 6.2% for single- and double-pulse operation, respectively. The system produces an excellent 1.4 times transform-limited beam quality.

1 J/pulse Q-switched 2 µm solid-state laser

We have developed a lidar technique for measuring the tropospheric C02 concentrations as a candidate for NASA's planned ASCENDS mission. Our technique uses two pulsed laser transmitters allowing simultaneous measurement of a C02 absorption line in the 1570 nm band, 02 extinction in the Oxygen A-band and surface height and backscatter. The lidar measures the energy and time of flight of the laser echoes reflected from the atmosphere and surface. The lasers are stepped in wavelength across the C02 line and an 02 line region during the measurement. The receiver uses a telescope and photon counting detectors, and measures the background light and energies of the laser echoes from the surface along with scattering from any aerosols in the path. The gas extinction and column densities for the C02 and 02 gases are estimated from the ratio of the on- and off- line signals via the DIAL technique. Time gating is used to isolate the laser echo signals from the surface, and to reject laser photons scattered in the atmosphere. We have developed an airborne lidar to demonstrate the C02 measurement from the NASA Glenn Lear 25 aircraft. The airborne lidar steps the pulsed laser's wavelength across a selected C02 line with 20 steps per scan. The line scan rate is 450 Hz and laser pulse widths are I usec. The time resolved laser backscatter is collected by a 20 cm telescope, detected by a photomultiplier and is recorded by a photon counting system. We made initial airborne measurements on flights during October and December 2008. Laser backscatter and absorption measurements were made over a variety of land and water surfaces and through thin and broken clouds. Atmospheric C02 column measurements using the 1571.4, 1572.02 and 1572.33 nm C02 lines. Two flights were made above the DOE SGP ARM site at altitudes from 3-8 km. These nights were coordinated with DOE investigators who Hew an in-situ C02 sensor on a Cessna aircraft under the path. The increasing C02 line absorptions with altitudes were evident and comparison with in-situ measurements showed agreements to 6 ppm. This spring we improved the aircraft's nadir window. During July and August 2009 we made 9 additional 2 hour long flights and measured the atmospheric C02 absorption and line shapes using the 1572.33 nm C02 line. Measurements were made at stepped altitudes from 3-13 km over a variety of surface types in Nebraska, Illinois, the SGP ARM site, and near and over the Chesapeake Bay in North Carolina and Virginia. Strong laser signals and clear line shapes were observed at all altitudes, and some measurements were made through thin clouds. The flights over the ARM site were underflown with in-situ measurements made from the DOE Cessna. The Oklahoma and east coast t1ights were coordinated with a LaRC/ITT C02 lidar on the LaRC UC-12 aircraft, a LaRC in~situ C02 sensor, and the Oklahoma flights also included a JPL C02 lidar on a Twin Otter aircraft. Ed Browell and Gary Spiers led the LaRC and JPL teams. More details of the t1ights, measurements and analysis will be described in the presentation.

Pulsed Airborne Lidar Measurements of Atmospheric CO2 Column Absorption and Line Shapes from 3-13 km Altitudes

We report initial measurements of atmospheric CO2 column density using a pulsed airborne lidar operating at 1572 nm. It uses a lidar measurement technique being developed at NASA Goddard Space Flight Center as a candidate for the CO2 measurement in the Active Sensing of CO2 Emissions over Nights, Days and Seasons (ASCENDS) space mission. The pulsed multiple-wavelength lidar approach offers several new capabilities with respect to passive spectrometer and other lidar techniques for high-precision CO2 column density measurements. We developed an airborne lidar using a fibre laser transmitter and photon counting detector, and conducted initial measurements of the CO2 column absorption during flights over Oklahoma in December 2008. The results show clear CO2 line shape and absorption signals. These follow the expected changes with aircraft altitude from 1.5 to 7.1 km, and are in good agreement with column number density estimates calculated from nearly coincident airborne in-situ measurements.

https://www.tandfonline.com/doi/pdf/10.1111/j.1600-0889.2010.00502.x

Pulsed airborne lidar measurements of atmospheric CO 2 column absorption

A differential absorption lidar has been built to measure CO2 concentration in the atmosphere The transmitter is a pulsed single-frequency Ho:Tm:YLF laser at a 205-μm wavelength A coherent heterodyne receiver was used to achieve sensitive detection, with the additional capability for wind profiling by a Doppler technique Signal processing includes an algorithm for power measurement of a heterodyne signal Results show a precision of the CO2 concentration measurement of 1%–2% 1σ standard deviation over column lengths ranging from 12 to 28 km by an average of 1000 pulse pairs A preliminary assessment of instrument sensitivity was made with an 8-h-long measurement set, along with correlative measurements with an in situ sensor, to determine that a CO2 trend could be detected

Coherent differential absorption lidar measurements of CO2.

A data acquisition and signal processing system is being developed for a 2-micron airborne wind profiling coherent Doppler lidar system. This lidar, called the Doppler Aerosol Wind Lidar (DAWN), is based on a Ho:Tm:LuLiF laser transmitter and 15-cm diameter telescope. It is being packaged for flights onboard the NASA DC-8, with the first flights in the summer of 2010 in support of the NASA Genesis and Rapid Intensification Processes (GRIP) campaign for the study of hurricanes. The data acquisition and processing system is housed in a compact PCI chassis and consists of four components such as a digitizer, a digital signal processing (DSP) module, a video controller, and a serial port controller. The data acquisition and processing software (DAPS) is also being developed to control the system including real-time data analysis and display. The system detects an external 10 Hz trigger pulse and initiates the data acquisition and processing process, and displays selected wind profile parameters such as Doppler shift, power distribution, wind directions and velocities. Doppler shift created by aircraft motion is measured by an inertial navigation/GPS sensor and fed to the signal processing system for real-time removal of aircraft effects from wind measurements. A general overview of the system and the DAPS as well as the coherent Doppler lidar system is presented in this paper.

https://ntrs.nasa.gov/api/citations/20100025487/downloads/20100025487.pdf

Data Acquisition and Processing System for Airborne Wind Profiling with a Pulsed, 2-Micron, Coherent-Detection, Doppler Lidar System

Future robotic missions to Mars and, eventually, human missions to Mars will require landing massive spacecraft with “pin point” accuracy, e.g., the planned Mars Sample Return (MSR) mission will require “pin point” landing accuracy to rendezvous with the previously cached Mars samples to be returned to Earth and the first human mission to Mars, with payloads estimated to be in excess of 40 metric tons, must land very close to the cargo spacecraft that precede it on the journey to Mars. Hence, “pin point” entry, descent and landing (EDL) has become a major technological driver in future massive robotic and human mission to Mars [1]. To achieve “pin point” EDL on Mars, we must predict the atmospheric density, atmospheric winds and atmospheric dust level to an accuracy previously unobtainable. To develop an accurate and precise predictive model of the atmosphere of Mars, we propose a Marsorbiting LIDAR system to measure/monitor the density, winds and dust in the atmosphere of Mars over two Mars years. The LIDAR measurements will be used to develop an accurate model of the atmosphere of Mars to be used for “pin point” EDL for future Mars missions. Introduction: Planetary winds have implications to understanding planetary weather and conducting planetary exploration. In the case of Mars, the interplay between winds, dust storms and the radiative feedback is critical to the prediction of operational meteorology but remains poorly understood. Two major components of the Martian weather are the episodically strong winds and the dust storms. Both have implications to understanding Martian weather and to the prediction of hazardous conditions for landing and exploration on the surface of Mars. The feedback between winds (driven by thermal gradients), dust storms (sustained suspension of high optical depth dust clouds) and the consequential changes in the thermal gradients, is recognized as fundamental to the Martian weather. In addition to the weather/climate issues, the knowledge of the 3-dimensional wind field would be beneficial and could be critical to both the design and execution of future robotic and human expedition missions. As an example, the wind field (and wind shear throughout the atmosphere) is a crucial factor in the entry, descent, and landing of instrumented craft and subsequent airborne and surface exploration. Approach: The lack of measurements of Martian atmospheric density in the 30-80 km range, dust storm formation and movements, and horizontal wind patterns in the 0-20 km range pose significant risks to aerocapture, and entry, descent, and landing (EDL) of future robotic and human Mars missions. Systematic measurement of the Mars atmospheric density and winds will be required over several Mars years, supplemented with day-of-entry operational measurements. To date, there have been 6 successful U.S. robotic landings on Mars (the two Viking landers, Mars Pathfinder, MER: Spirit and Opportunity, and Mars Phoenix). Atmospheric density and wind reconstruction has been performed for each of these entries. At present, all of NASA’s Mars atmospheric density and wind models have these 6 entries (at widely scattered positions and seasons) as their basis, supplemented by coarse orbital measurements of atmospheric opacity and temperature. This lack of data leads to a large uncertainty in prediction of the Martian atmospheric density and winds in the altitude regime where deceleration of landers will occur. This uncertainty will have a dramatically large impact on mass, cost and risk. The proposed 2-m Doppler/DIAL lidar system will provide the critically needed density and winds data to reduce the risks of future Mars landing missions. Research programs were initiated at NASA Langley Research Center during 2005, jointly funded by NASA Science Mission Directorate and Exploration Systems Mission Directorate, to first model the lidar’s performance and then build a breadboard lidar system demonstrating a measurement capability for wind, CO2 concentration, and aerosols suited to meteorological and climatological application for Mars. This 2-m coherent DIAL system can simultaneously measure wind by a Doppler technique and CO2 concentration by a differential absorption technique. Since the source of the backscatter is atmospheric aerosols, aerosol/dust profiling is inherently included. The first step in development was to model performance of the lidar in the Martian atmosphere. Simpson Weather Associates (SWA) and NASA/LaRC conducted a study to compare several lidar concepts that could provide critical observations of winds, aerosols and, perhaps, gases from orbit around Mars. During the two year study we defined the target atmosphere and observational requirements with explicit reference to lidar technologies; modified an existing Doppler Lidar Simulation Model for application to Mars missions; generated the lidar system performance requirements; and identified the technology “tall poles” such as power demands, weight and space hardening. During this study, the issue of atmospheric density and its role in managing entry, descent and landing became a pursuit to model the application of 2-m coherent DIAL to providing density profiles during the entry phase. In this presentation we review briefly the approach and findings of this study. Mars Lidar Simulation Model (MLSM): Prior to establishing lidar technology requirements for a Mars mission, an existing Doppler wind Lidar Simulation Model (DLSM) was modified to work with Mars atmospheres (Figure 1). Figure 1: The main page for the Mars Lidar Simulation Model based upon the established Doppler wind Lidar Simulation Model used in many space based instrument studies. Given that the initial choice of lidar receiver/detection technology was 2-m coherent, the aerosol backscatter and CO2 radiative properties at that wavelength had to be specified. Data from the TES and climate data sets from the European Mars GCM (Forget LMD; Lewis AOPP (Oxford)) were used to generate reference profiles for use in the MLSM (Mars Lidar Simulation Model). The reference wind profile used to bound the study in terms of wind speeds and vertical distribution of winds is shown in Figure 2. In terms of aerosol backscatter, we made the following assumptions: work to column optical depths at 9-m Use published size distributions and inferred dust physical properties to convert to 2-m properties Assume constant mixing ratio 0-40 km. Assume constant mixing ratio for CO2 The MLSM optical properties code was run for several size distribution parameters until observed optical depths were computed. Once the backscatter coefficients and vertical profiles were established, several weather situations were chosen from the Mars GCM (LMD) which provided 3D fields of temperature, pressure and density during various dust load scenarios. Figure 2: Reference wind profile A critical feature calculated by the MLSM for predicting lidar performance is the dust backscatter throughout the atmosphere. Qualitatively, the backscatter is expected to be quite large—Mars is known to be a rather dusty place—compared to the calculations lidar researchers are accustomed to in designing for the Earth atmosphere. An interesting note is in the nomenclature of studying Mars and Earth: on Earth the term used for backscattering particles is "aerosol," whereas for Mars the term is "dust." An example calculation of the backscatter coefficient for Mars (and Earth for comparison) is shown in Fig. 3. Figure 3: Atmospheric dust backscatter on Mars (red curves) and Earth (blue curves). This calculation demonstrates the very high level of backscatter that would be encountered in Mars— several orders of magnitude greater than found on Earth. Such a high level of backscatter is good for lidar operation in that very strong signal levels can be encountered with a modest lidar design. Another aspect of the Martian atmosphere illuminated by this modeling effort is the effect of the low atmospheric pressure of Mars on the shape of the CO2 absorption lines. The absorption lines, which are the basis for the DIAL measurements, are rather strong and narrow compared to the case for Earth—Figure 4 quantifies this comparison. Control of the laser spectrum becomes critical for the Mars application. 0 100 200 300 400 500 600

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M. J. Kavaya

Papers

Coherent differential absorption lidar measurements of CO2.

Data Acquisition and Processing System for Airborne Wind Profiling with a Pulsed, 2-Micron, Coherent-Detection, Doppler Lidar System

A Mars-Orbiting 2-micron LIDAR System to Monitor the Density, Winds and Dust of the Atmosphere of Mars