Ground- and air-based lidar systems have been providing volumetric information about air quality, the ozone, global warming, and wind and weather patterns for years. But the real goal is to make it work from outer space.
From its early days as a simple technique for studying clouds to its current role in measuring air pollution, tracking wind and weather patterns, and monitoring changes in the ozone, light detection and ranging (lidar) has played a key role in the study of the Earth's atmosphere for more than 30 years. Operating both from the ground and from the air, lidar makes it possible to rapidly obtain three-dimensional profiles of atmospheric properties such as temperature and wind velocity and atmospheric constituents such as water, oxygen, and aerosols.
The result has been a growing body of knowledge about Earth's environment, atmospheric processes, and man's effect on both. Now researchers are hoping to pull the microscope back even further and take lidar's unique capabilities into space to gain a better understanding of global wind patterns, global warming, air pollution, and ozone depletion. In fact, the last decade has seen a surge of consideration of lidar for space-based environmental monitoring missions, due in part to advances in solid-state lasers, crystal growth, and diode lasers for pumping.
"Lidar can be used for anything you want to do in the atmosphere," said Upendra Singh, technologist, Systems Engineering Competency, at NASA Langley Research Center (Hampton, VA). "And to go into space, it has a lot of advantages because the atmosphere is inverted. When you launch something from the ground, the particles in the lower atmosphere are quite numerous, which makes it difficult for a laser beam to penetrate it and get to the upper atmosphere. But from space, you have very few particles, which means you get very strong signal returns. The dynamic range in the upper atmosphere is much enhanced from space."
A typical lidar system comprises a laser transmitter, receiver telescope, photodetectors, and range-resolving detection electronics, and works by sending out a laser pulse (usually from a frequency-shifted Nd:YAG laser) and measuring the absorption or emission in the reflected signal (see Fig. 1). The laser transmits a short pulse of light in a specific direction or in multiple directions. The light interacts with molecules in the air and the molecules send a small fraction of the light back to the telescope where it is measured by photodetectors. The amount of time it takes the light to return to the telescope indicates altitude, the wavelength shift of the light identifies the type of molecules that scattered the light, and the intensity of the light represents the concentration of molecules.
"Lidar gives you a direct measurement of atmospheric scattering and absorption," said Ed Browell, head of the lidar applications group at NASA Langley. "It is a pulsed system like radar, but uses lasers and tunable laser wavelengths to determine where a process is happening and measure it over time. You can tell where the scattering takes place, see how much absorption is occurring, and then determine how much of a gas is present. You not only know where you are in altitude and where you are making the measurement, you know the direction of the laser energy as it travels through the atmosphere."
Velocity and volume
Different types of lidar measure different atmospheric properties and constituents such as aerosol particles, ice crystals, water vapor, or trace gases. Direct-detection and coherent Doppler lidar determine the velocity of a target by measuring the Doppler shift in the return beam, making it useful for wind studies and weather forecasting. Michigan Aerospace (MAC; Ann Arbor, MI) is commercializing Doppler lidar systems for wind measurement based on technology originally developed at the University of Michigan. Dubbed direct-detection fringe imaging lidar, the MAC system uses both 532- and 355-nm wavelengths in conjunction with photon recycling.
The MAC system has been used in ground-based experiments in New Hampshire and Hawaii to measure wind velocity from altitudes of 0.5 to 20 km and will make its first airborne trip aboard a high-altitude balloon in 2005. Built by MAC, the University of New Hampshire, and Raytheon for the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Air Force, the project is intended to demonstrate direct-detection Doppler lidar from a downward-looking, space-like environment.
"Meteorologists use numerical models to forecast weather where the primary data is wind," said Carl Nardell, president of MAC. "But those models are only as good as the data you put in them. The 'holy grail' is to be able to use space-based lidar for wind studies."
Coherent Technologies (CTI; Louisville, CO) has built its business on developing coherent laser radar for military and remote sensing applications. For the last three years CTI has augmented this business with a product that provides coherent Doppler measurements of wind velocity, direction, and related applications such as airport wind hazard, wind shear, and turbulence detection. The company's WindTracer system is a pulsed Doppler lidar that uses an IR laser (2 µm) to bounce off aerosols (dust particles). The light reflected back to the system measures the wind and identifies the strength and location of wind gusts, microbursts, and vortices. The WindTracer has been installed at airports from Colorado to Hong Kong for wind shear and turbulence surveillance and is also used for meteorological applications.
"We have focused on coherent Doppler lidar because we saw a market out there for measuring radial winds in a volumetric area, out to 10 km," said Hal Bagley, vice president of CLR Photonics, a division of CTI. "You want to be able to detect the velocity of the wind and the direction it is blowing and map it on the computer screen to allow the user to 'see' the winds. Doppler lidar measures the frequency shift of the reflected light, which is proportional to wind velocity. We use coherent Doppler, versus direct-detection Doppler, because the increased sensitivity allows us to operate in very clear air conditions and still measure what the winds are doing."
Raman lidar detects particular atmospheric components such as water vapor by measuring the wavelength-shifted return from selected molecules. Researchers at Los Alamos National Laboratory (LANL; Los Alamos, NM) have developed a mobile Raman lidar system that can produce 2-D maps of evapotransportation (the discharge of water to the atmosphere by plants). The LANL system has a spatial resolution of about 1.5 m over a distance of 700 m. It uses an excimer laser with 400 mJ/pulse of energy at 200 Hz and wavelengths of 248 and 351 nm, respectively, for daytime and nighttime operations. The receiver uses a 24-in. telescope and scanning optics to allow three-dimensional volumetric imaging of water-vapor mixing ratio. The receiving detector system provides simultaneous measurement of the elastic back-scattered UV radiation from aerosols and clouds, plus nitrogen and water Raman signatures. Working with the U.S. Bureau of Reclamation, the LANL lidar group is using a mobile ground-based version of its Raman system to measure how much water is being used by vegetation on the Rio Grande River (see Fig. 2). Future efforts include upgrading the system to measure CO2 in the troposphere.
"While differential absorption lidar can look at specific species, it has very crude spatial recognition in the first few hundred meters of the atmosphere, which is where the bulk of the water vapor coming off the Earth resides," said Daniel Cooper, project leader, Experimental Atmospheric and Climate Physics at LANL. "Our system looks at Raman scattering in the UV, focusing on specific molecules as part of the spatial process."
Differential absorption lidar (DIAL) relies upon multiple-wavelength absorption to measure chemical concentrations such as ozone, water vapor, and pollutants. In water-vapor DIAL, for example, laser pulses are transmitted at two wavelengths, one on a water-vapor absorption line and another off-line. If the two wavelengths are close together, then for both wavelengths the scattering by molecules and particles is essentially equal. The difference in the returns between the two wavelengths is then due entirely to absorption by water-vapor molecules; the ratio of the backscatter at the two wavelengths as a function of range can be used to calculate the water-vapor concentration profile.
Over the last 30 years, lidar studies at NASA Langley have focused primarily on ultraviolet DIAL measurements of aerosols and water vapor in the stratosphere (up to 50 km above Earth's surface). In field experiments ranging from ozone studies in the Arctic to biomass burning in the southern hemisphere, intercontinental transport of air pollution across the Pacific and the North Atlantic, and storm tracking in the Midwest and the Atlantic, NASA Langley uses two types of DIAL systems. One comprises a pair of frequency-doubled Nd:YAG and Nd:YAG-pumped dye lasers that generate UV and IR wavelengths around 1064, 620, 600, 310, 300, and 289 nm. The IR and visible wavelengths measure aerosols and clouds, while the UV wavelengths measure ozone.
The NASA LASE (Lidar Atmospheric Sensing Experiment) system, used for water-vapor measurements in the upper troposphere, comprises a double-pulsed Ti:sapphire laser that operates in the 815-nm absorption band of water vapor and is pumped by a frequency-doubled flashlamp-pumped Nd:YAG laser. In field experiments, the NASA lidar systems typically fly on a DC-8 over specified regions for eight hours a day over six weeks (see Fig. 3). NASA's ultimate goal, though, is to take lidar into space to gain an even better understanding of atmospheric processes and changes.
"We focus on DIAL because it is the main technique that will go into space," Browell said. "We can make measurements from the ground and air over tens of kilometers and from space over hundreds of kilometers through the atmosphere."
Space-based lidar
Scientists have long known that space-based lidar would provide optimum coverage of global wind and weather patterns and atmospheric changes with high resolution, but the development of space-based lidar has proven difficult. About half of NASA's space lidar missions have encountered some sort of difficulty; for example, the Lidar In-space Tchnology Experiment (LITE) on the space shuttle was successful despite pulse energy degradation in both of the system's lasers. Similarly, the Geoscience Laser Altimeter System (GLAS), was launched in January 2003 to make continuous measurements of Earth's atmosphere from space. However, just two months into the mission, the first of the system's three lasers failed; upon investigation, the problem has been attributed to issues with the diode pumping arrays.
Technology challenges such as these have prompted NASA to establish the Laser Risk Reduction Program (LRRP), with the goal of bringing together the agency's various lidar projects and creating a more collaborative—and successful—approach to space-based lidar. Under the direction of Singh and colleagues from NASA Langley and NASA Goddard, the LRRP is charged with ensuring that lasers developed for space-based remote sensing are adequately qualified for long-term science measurements in outer space. Specific technology-development efforts initiated by the LRRP include 1-µm solid-state pulsed lasers for altimetry, 2-µm solid-state pulsed lasers for tropospheric wind profiling, pump laser diodes for both laser types, and wavelength converson technology. In addition, the LRRP hopes to create a consortium to take these technologies into the commercial realm.
"The government has the skills and can do the innovation, but we should not be in the business of building space-based hardware," Singh said. "Once we develop the knowledge base, we should transfer it to industry to further develop and produce the technology."
Several commercial efforts are already under way to improve the lasers and receptors used in space-based lidar. ITT Industries Advanced Engineering & Sciences (Reston, VA) is developing optical parametric oscillators (OPOs) and optical parametric amplifiers (OPAs) to improve the tunability of solid-state lasers in the UV (see Fig. 4). The technology is initially being developed for NASA Goddard for atmospheric studies of the ozone on high-altitude aircraft and unmanned aerial vehicles in the stratosphere.
"One of the major challenges for having a useful, reliable lidar system is having a good laser transmitter," said Scott Higdon, principal scientist with ITT Advanced Engineering and Sciences. "So we put a lot of emphasis on developing laser-transmitter technology that allows us to get to a number of regions of the wavelength spectrum."
ITT starts with fairly mature solid-state laser technology—the Nd:YAG—then converts single-frequency output to other wavelengths using the OPOs and OPAs. By combining harmonic conversion of the Nd:YAG laser with the OPO process, ITT says it is able to build a laser with higher repetition rate and lower pulse energy but potentially much higher power than what is possible with macropulsed systems—that is, 10 to 100 mJ at repetition rates up to 50 Hz.
"The real challenge here is you need UV output from 290 to 320 nm—290 to 300 nm for toposphere, 320 to 325 nm for stratosophere," Higdon said. "But it is very challenging to get high-average-power output at those wavelengths; if you need to be at other wavelengths beyond doubled, tripled, and quadrupled, this means more complexity, lower average power, less reliability, and more expense." ITT is also working on a 1-kHz Nd:YAG laser and a Nd:YAG transmitter combined with an OPA to do CO2 measurements in the IR (around 1.6 µm). Other projects include using OPA conversion to move into the mid-IR (around 3.4 µm) for methane detection.
"When you have a lot smaller payload space available you need smaller lidar systems, so our thrust is to make a compact and reliable source," Higdon said. "Working at 1 kHz allows this because it gets away from optical damage problems and really high-power pulsed-energy Nd:YAG lasers."