Atmospheric water vapor and carbon dioxide are important greenhouse gases that significantly contribute to the global radiation budget on Earth. A 2-micron triple-pulse, Integrated Path Differential Absorption (IPDA) lidar instrument for ground and airborne atmospheric carbon dioxide and water vapor concentration measurements using direct detection was developed at NASA Langley Research Center. This active remote sensing instrument provides an alternate approach with significant advantages for measuring atmospheric concentrations of the gases. A high energy pulsed laser transmitter approach coupled with sensitive receiver detection provides a high-precision measurement capability by having a high signal-to-noise ratio. This paper presents the concept, development, integration and testing of the 2-micron triple-pulse IPDA. The integration includes the various IPDA transmitter, receiver and data acquisition subsystems and components. Ground and airborne testing indicated successful operation of the IPDA lidar.
A ground-based 2-micron Differential Absorption Lidar (DIAL) CO2 profiling system for atmospheric boundary layer studies and validation of space-based CO2 sensors is being developed and tested at NASA Langley Research Center as part of the NASA Instrument Incubator Program. To capture the variability of CO2 in the lower troposphere a precision of 1-2 ppm of CO2 ( <0.5%) with 0.5 to 1 km vertical resolution from near surface to free troposphere (4-5 km) is one of the goals of this program. In addition, a 1% (3 ppm) absolute accuracy with a 1 km resolution over 0.5 km to free troposphere (4-5 km) is also a goal of the program. This DIAL system leverages 2-micron laser technology developed under NASA’s Laser Risk Reduction Program (LRRP) and other NASA programs to develop new solid-state laser technology that provides high pulse energy, tunable, wavelength-stabilized, and double-pulsed lasers that are operable over pre-selected temperature insensitive strong CO2 absorption lines suitable for profiling of lower tropospheric CO2. It also incorporates new high quantum efficiency, high gain, and relatively low noise phototransistors, and a new receiver/signal processor system to achieve high precision DIAL measurements. This presentation describes the capabilities of this system for atmospheric CO2 and aerosol profiling. Examples of atmospheric measurements in the lidar and DIAL mode will be presented.
The 2-micron wavelength region is suitable for atmospheric carbon dioxide (CO2) measurements due to the existence of distinct absorption features for the gas at this wavelength region [1]. For more than 20 years, researchers at NASA Langley Research Center (LaRC) have developed several high-energy and high repetition rate 2-micron pulsed lasers [2]. Currently, LaRC team is engaged in designing, developing and demonstrating a triple-pulsed 2-micron direct detection Integrated Path Differential Absorption (IPDA) lidar to measure the weighted-average column dry-air mixing ratios of carbon dioxide (XCO2) and water vapor (XH2O) from an airborne platform [1, 3-5]. This novel technique allows measurement of the two most dominant greenhouse gases, simultaneously and independently, using a single instrument. This paper will provide status and details of the development of this airborne 2-micron triple-pulse IPDA lidar. The presented work will focus on the advancement of critical IPDA lidar components. Updates on the state-of-the-art triple-pulse laser transmitter will be presented including the status of seed laser locking, wavelength control, receiver and detector upgrades, laser packaging and lidar integration. Future plans for IPDA lidar ground integration, testing and flight validation will also be discussed. This work enables new Earth observation measurements, while reducing risk, cost, size, volume, mass and development time of required instruments.
An airborne 2-μm triple-pulse integrated path differential absorption (IPDA) lidar is currently under development at NASA Langley Research Center (LaRC). This lidar targets both atmospheric carbon dioxide (CO2) and water vapor (H2O) column measurements, simultaneously. Advancements in the development of this IPDA lidar are presented in this paper. Updates on advanced two-micron triple-pulse high-energy laser transmitter will be given including packaging and lidar integration status. In addition, receiver development updates will also be presented. This includes a state-of-the-art detection system integrated at NASA Goddard Space Flight Center. This detection system is based on a newly developed HgCdTe (MCT) electron-initiated avalanche photodiode (e-APD) array. Future plan for IPDA lidar system for ground integration, testing and flight validation will be discussed.
Methane is a potent greenhouse gas and on a per molecule basis has a warming influence 72 times that of carbon dioxide over a 20 year horizon. Therefore, it is important to look at near term radiative effects due to methane to develop mitigation strategies to counteract global warming trends via ground and airborne based measurements systems. These systems require the development of a time-resolved DIAL capability using a narrow-line laser source allowing observation of atmospheric methane on local, regional and global scales. In this work, a demonstrated and efficient nonlinear conversion scheme meeting the performance requirements of a deployable methane DIAL system is presented. By combining a single frequency 1064 nm pump source and a seeded KTP OPO more than 5 mJ of 1.6 μm pulse energy is generated with conversion efficiencies in excess of 20%. Even without active cavity control instrument limited linewidths (50 pm) were achieved with an estimated spectral purity of ~95%. Tunable operation over 400 pm (limited by the tuning range of the seed laser) was also demonstrated. This source demonstrated the critical needs for a methane DIAL system motivating additional development of the technology.
A compact remote Raman spectroscopy system was developed at NASA Langley Research center and was
previously demonstrated for its ability to identify chemical composition of various rocks and minerals. In
this study, the Raman sensor was utilized to perform time-resolved Raman studies of various samples such
as minerals and rocks, Azalea leaves, and a few fossil samples. The Raman sensor utilizes a pulsed 532 nm
Nd:YAG laser as excitation source, a 4-inch telescope to collect the Raman-scattered signal from a sample
several meters away, a spectrograph equipped with a holographic grating, and a gated intensified CCD
(ICCD) camera system. Time resolved Raman measurements were carried out by varying the gate delay
with fixed short gate width of the ICCD camera, allowing measurement of both Raman signals and
fluorescence signals. Rocks and mineral samples were characterized, including marble, which contains
CaCO3. Analysis of the results reveals the short (~10-13 s) lifetime of the Raman process and shows that the
Raman spectra of some mineral samples contain fluorescence emission due to organic impurities. Also
analyzed were a green (pristine) and a yellow (decayed) sample of Gardenia leaves. It was observed that
the fluorescence signals from the green and yellow leaf samples showed stronger signals compared to the
Raman lines. It was also observed that the fluorescence of the green leaf was more intense and had a
shorter lifetime than that of the yellow leaf. For the fossil samples, Raman shifted lines could not be
observed due to the presence of very strong short-lived fluorescence.
NASA Langley Research Center has been developing 2-micron lidar technologies over a decade for wind measurements,
utilizing coherent Doppler wind lidar technique and carbon dioxide measurements, utilizing Differential Absorption
Lidar (DIAL) technique. Significant advancements have been made towards developing state-of-the-art technologies
towards laser transmitters, detectors, and receiver systems. These efforts have led to the development of solid-state lasers
with high pulse energy, tunablility, wavelength-stability, and double-pulsed operation. This paper will present a review
of these technological developments along with examples of high resolution wind and high precision CO2 measurements
in the atmosphere. Plans for the development of compact high power lasers for applications in airborne and future space
platforms for wind and regional to global scale measurement of atmospheric CO2 will also be discussed.
A compact remote Raman sensor system was developed at NASA Langley Research Center. This sensor is an
improvement over the previously reported system, which consisted of a 532 nm pulsed laser, a 4-inch telescope, a
spectrograph, and an intensified CCD camera. One of the attractive features of the previous system was its portability,
thereby making it suitable for applications such as planetary surface explorations, homeland security and defense
applications where a compact portable instrument is important. The new system was made more compact by replacing
bulky components with smaller and lighter components. The new compact system uses a smaller spectrograph
measuring 9 x 4 x 4 in. and a smaller intensified CCD camera measuring 5 in. long and 2 in. in diameter. The previous
system was used to obtain the Raman spectra of several materials that are important to defense and security applications.
Furthermore, the new compact Raman sensor system is used to obtain the Raman spectra of a diverse set of materials to
demonstrate the sensor system's potential use in the identification of unknown materials.
An InGaAsSb/AlGaAsSb phototransistor has been validated for lidar atmospheric remote sensing. The validation was performed using the Raman-shifted eye-safe aerosol lidar (REAL) at the National Center for Atmospheric Research. Although the device is optimized for detection around the 2-µm wavelength, the validation was performed at 1.543 µm, where mature commercial detectors are available. Simultaneous measurement of the atmospheric backscatter signals using the custom-built phototransistor and commercial InGaAs avalanche photodiode indicated good agreement between both devices. The validation included detecting 11-km-range hard targets, 5-km atmospheric structure consisting of cirrus clouds, and a near-field boundary layer. Far-field low intensity and spatially narrow atmospheric features were also detectable with the new phototransistor. Preliminary results related to systematic effects are discussed in the first attempt of incorporating a phototransistor in a lidar system.
Different methods of energy estimation for a differential absorption lidar (DIAL) system at NASA Langley Research
Center in Virginia are investigated in this paper. The system is a 2- &mgr;m wavelength coherent Doppler lidar called
VALIDAR that has been traditionally used for measuring wind. Recent advances in laser wavelength control have
allowed the new use of this lidar for measuring atmospheric CO2 concentration by a DIAL technique. In order to realize
accurate DIAL measurements, optimal signal processing techniques are required to represent the energy of the
heterodyned backscatter signals. The noise energy was estimated by minimizing the mean square error in its estimate
and was used to normalize its adverse influence on accurate estimation of the concentration of CO2 in the atmosphere.
The impact of different methods on the statistics of CO2 concentration measurements is compared.
Recent and future explorations of Mars and lunar surfaces through rovers and landers have spawned great interest in
developing an instrument that can perform in-situ analysis of minerals on planetary surfaces. Several research groups
have anticipated that for such analysis, Raman spectroscopy is the best suited technique because it can unambiguously
provide the composition and structure of a material. A remote pulsed Raman spectroscopy system for analyzing
minerals was demonstrated at NASA Langley Research Center in collaboration with the University of Hawaii. This
system utilizes a 532 nm pulsed laser as an excitation wavelength, and a telescope with a 4-inch aperture for collecting
backscattered radiation. A spectrograph equipped with a super notch filter for attenuating Rayleigh scattering is used to
analyze the scattered signal. To form the Raman spectrum, the spectrograph utilizes a holographic transmission grating
that simultaneously disperses two spectral tracks on the detector for increased spectral range. The spectrum is recorded
on an intensified charge-coupled device (ICCD) camera system, which provides high gain to allow detection of
inherently weak Stokes lines. To evaluate the performance of the system, Raman standards such as calcite and
naphthalene are analyzed. Several sets of rock and mineral samples obtained from Ward's Natural Science are tested
using the Raman spectroscopy system. In addition, Raman spectra of combustible substances such acetone and isopropanol are also obtained.
High quality infrared (IR) quantum detectors are important for several applications, such as atmospheric remote sensing, chemical detection and absorption spectroscopy. Although several IR detectors are commercially available, with different materials and structures, they provide limited performance regarding the signal-to-noise ratio and the corresponding minimum detectable signal. InGaAsSb/AlGaAsSb heterojunction based phototransistors show strong potential for developing IR sensors with improved performance.
In this paper, the performance of a novel n-p-n InGaAsSb/AlGaAsSb heterojunction phototransistor is presented. This performance study is based on experimental characterization of the device dark current, noise and spectral response. Detectivity of 1.7x109 cmHz 1/2/W at 2-μm was obtained at 100°C temperature and 2 V bias voltage. This corresponds to a responsivity of 94.7 A/W and an internal gain of 156 with about 38% quantum efficiency. Reducing the temperature to -30°C allows to increase the bias to 3V and enhance the detectivity to 8.7x1010 cmHz1/2/W at the same wavelength, which corresponds to a responsivity of 386.5 A/W and an internal gain of 288.2 with about 83% quantum efficiency. The device impulse response and linearity, including the corresponding dynamic range, also are presented. Impulse response analysis indicated a settling time of about 1.1 μs at 2V and 100°C, while linearity measurements indicated a constant responsivity in the radiation intensity range of 1.6x10-7 W/cm2 and 31.6 mW/cm2.
For exploration of planetary surfaces, detection of water and ice is of great interest in supporting existence of life on other planets. Therefore, a remote Raman spectroscopy system was demonstrated at NASA Langley Research Center in collaboration with the University of Hawaii for detecting ice-water and hydrous minerals on planetary surfaces. In this study, a 532 nm pulsed laser is utilized as an excitation source to allow detection in high background radiation conditions. The Raman scattered signal is collected by a 4-inch telescope positioned in front of a spectrograph. The Raman spectrum is analyzed using a spectrograph equipped with a holographic super notch filter to eliminate Rayleigh scattering, and a holographic transmission grating that simultaneously disperses two spectral tracks onto the detector for higher spectral range. To view the spectrum, the spectrograph is coupled to an intensified charge-coupled device (ICCD), which allows detection of very weak Stokes line. The ICCD is operated in gated mode to further suppress effects from background radiation and long-lived fluorescence. The sample is placed at 5.6 m from the telescope, and the laser is mounted on the telescope in a coaxial geometry to achieve maximum performance. The system was calibrated using the spectral lines of a Neon lamp source. To evaluate the system, Raman standard samples such as calcite, naphthalene, acetone, and isopropyl alcohol were analyzed. The Raman evaluation technique was used to analyze water, ice and other hydrous minerals and results from these species are presented.
Profiling of atmospheric carbon dioxide (CO2) is important for understanding the natural carbon cycle on Earth and its influence on global warming and climate change. Differential absorption lidar is a powerful remote sensing technique used for profiling and monitoring atmospheric constituents. Recently there has been an interest to apply this technique, at the 2 μm wavelength, for investigating atmospheric CO2. This drives the need for high quality detectors at this wavelength. Although 2 μm detectors are commercially available, the quest for a better detector is still on. The detector performance, regarding quantum efficiency, gain and associated noise, affects the DIAL signal-to-noise ratio and background signal, thereby influencing the instrument sensitivity and dynamic range. Detectors based on the III-V based compound materials shows a strong potential for such application.
In this paper the detector requirements for a long range CO2 DIAL profiles will be discussed. These requirements were compared to newly developed III-V compound infrared detectors. The performance of ternary InGaSb pn junction devices will be presented using different substrates, as well as quaternary InGaAsSb npn structure. The performance study was based on experimental characterization of the devices dark current, spectral response, gain and noise. The final results are compared to the current state-of-the-art InGaAs technology. Npn phototransistor structure showed the best performance, regarding the internal gain and therefore the device signal-to-noise ratio. 2-μm detectivity as high as 3.9x1011 cmHz1/2/W was obtained at a temperature of -20°C and 4 V bias voltage. This corresponds to a responsivity of 2650 A/W with about 60% quantum efficiency.
Tropospheric chemistry is considered to be the next frontier of atmospheric chemistry, and understanding and predicting the global influence of natural and human-induced effects on tropospheric chemistry will be the next challenge for atmospheric research over the foreseeable future. A geostationary Earth orbit (GEO) vantage point provides an ideal location for measuring spatially and temporally resolved distributions of trace gas species. One powerful technique for making this measurement is LIght Detection And Ranging (lidar) using solid-state lasers. Presently, NASA has a notional plan for using lidars for tropospheric chemistry measurements, but from low Earth orbit (LEO). While permitting high spatially resolved measurements, LEO measurements, however, lack the temporal resolution required to monitor important atmospheric processes and transport. A GEO instrument will require a more energetic and efficient lidar system in order to permit accurate measurements. In this study, we investigated the capability of a lidar for tropospheric profiling of chemical species and we develop a roadmap for the requisite technologies.
A combined inelastic (Raman) and elastic (Mie-Rayleigh) scattering and Laser-Induced Fluorescence (LIF) active remote sensing (RLIF) system is proposed as a mast-mounted instrument for the Mars Science Laboratory (MSL). This remote RLIF system will be capable of reconnaissance and identification of mineral, organic, and biogenic materials as well as conducting atmospheric studies of Mars. This system is based on the prototypes developed with partial support from NASA at the University of Hawaii. The proposed RLIF system will perform active optical imaging and spectroscopy out to 100 m on the surface features. In the elastic backscattering mode, the range of RLIF can be extended to >5-km because the cross section of Mie-Rayleigh scattering is several orders of magnitude higher than that of Raman cross-sections of molecular species. Results obtained with the University of Hawaii’s portable remote Raman and LIF system and the portable Mie-Rayleigh prototype lidar are presented. With the remote Raman system, measurements of mineral calcite (CaCO3), liquid hydrocarbons and solid naphthalene polycrystals have been verified to 100 m range. The LIF sensor will provide near real time in situ remote data that will complement analytical laboratory and contact suite instrumentation on the Mars rover.
Two-micron detectors are critical for atmospheric CO2 profiling using the lidar technique. InGaAs and HgCdTe detectors are commercially available for this wavelength but they lack sufficient gain, which limits their detectivity. The characterization results of a novel AlGaAsSb/InGaAsSb phototransistor for 2-µm application are reported. The device was developed by AstroPower, Inc. for NASA Langley Research Center. Spectral response measurements showed the highest responsivity in a 1.9- to 2.1-µm region with a maximum value of 2650 A/W at 2 µm. A 2-µm detectivity of 3.9×1011 cm Hz1/2/W was obtained, which corresponds to noise equivalent power of 4.6×10–14 W/Hz1/2.
Richard Ferrare, Edward Browell, Syed Ismail, Susan Kooi, Vince Brackett, Marian Clayton, Anthony Notari, Carolyn Butler, John Barrick, Glenn Diskin, Barry Lesht, Frank Schmidlin, Dave Turner, David Whiteman, Larry Miloshevich
Measurements of water vapor profiles over the Southern Great Plains acquired by two different lidars are presented. NASA's airborne DIAL Lidar Atmospheric Sensing Experiment (LASE) system measured water vapor, aerosol, and cloud profiles during the ARM/FIRE Water Vapor Experiment (AFWEX) in November-December 2000 and during the International H2O Project (IHOP) in May-June 2002. LASE measurements acquired during AFWEX are used to characterize upper troposphere water vapor measured by ground-based Raman lidars, radiosondes, and in situ aircraft sensors. LASE measurements acquired during IHOP are being used to better understand the influence water vapor variability on the initiation of deep convection and to improve the quantification and prediction of precipitation associated with these storms. The automated Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) Raman Lidar (CARL) has been routinely measuring profiles of water vapor mixing ratio, relative humidity, aerosol extinction, aerosol backscattering, and aerosol and cloud depolarization during both daytime and nighttime operations. Aerosol and water vapor profiles acquired since March 1998 are used to investigate the seasonal variability of the vertical distributions of water vapor and aerosols.
Knowledge of the spatial and temporal distribution of atmospheric carbon dioxide (CO2) is important for understanding the carbon natural cycle, predicting its future levels and its impact on global warming and climate changes. Laser technology has advanced considerably during the past few years in the 2-micron region where strong optimum lines are available for measuring CO2 using the Differential Absorption Lidar (DIAL) technique. Although several types of detectors might be suitable for this particular wavelength, an ideal device would have high gain, low noise and narrow spectral response peaking around the wavelength of interest. This increases the signal-to-noise ratio and minimizes the background signal, thereby increasing the instrument sensitivity and dynamic range. In this paper the detector requirements for a long range CO2 DIAL measurement will be presented. The requirements were compared to commercially available and newly developed infrared (IR) detectors. The IR detectors considered for this study consist of the well developed InGaAs and HgCdTe p-n junction photodiodes, beside the newly developed and proposed InGaAsSb and InGaSb detectors. All of the detectors were characterized and their performances were compared with the CO2 DIAL detector requirements. The characterization experiments included spectral response, dark current and noise measurements. CO2 DIAL measurements using InGaAs detectors were attempted and indicated the need for better detector performance. While InGaAs detectors showed the closest performance to the instrument requirements, InGaSb detectors indicated a promising solution.
This paper is presented to give a general description of the ORACLE project and of the technology development results obtained to date. ORACLE is a feasibility study of a fully automated differential absorption lidar for global measurements of tropospheric and stratospheric ozone and aerosols with high vertical and horizontal resolution. The proposed program includes both novel technology demonstrations and obtaining scientific data from spacecraft. These data are needed to address key issues in atmospheric research including the depletion of stratospheric ozone, global warming, atmospheric transport and dynamics, tropospheric ozone budgets, atmospheric chemistry, and the atmospheric impact of hazards. Only a space-based lidar system can provide the required spatial resolution for ozone and aerosols in both the stratosphere and the troposphere on a global scale at all required altitudes. To deliver these data, the most novel technologies such as all-solid-state lasers, photon-counting detectors and ultra-lightweight deployable telescopes must be employed in the mission payload.
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