A brief overview is made of recent progress and current status of lasers for atmospheric lidarapplications. Emphasis is placed on recent developments in high-power or high PRF, fixed wavelength lasers. Improvements in solid-state (Ho, Nd, Tm), diode-pumping, CO2 and Dyelasers are covered, along with advances in new solid-state laser materials, non-linear opticalmaterials for OPO and frequency conversion, Raman shifting techniques, and recent advancesin better modeling of energy transfer dynamics in multi-level sensitized solid-state lasers.
The development of the DIAL technique has been limited by the availability of suitable tunablelaser sources. This review describes the four types of tunable laser that are most widelyused in DIAL applications. These are dye lasers, tunable solid state lasers, line tunable carbond ioxide lasers and optical parametric oscillators.
The Network for the Detection of Stratospheric Change is an international cooperation providinga set of high-quality, remote-sensing instruments at observing stations around the globe. A brief description of the NDSC and its goals is presented. Lidar has been selected as the NDSC instrument for measurements of stratospheric profiles of ozone, temperature and aerosol. The Jet Propulsion Laboratory has developed and implemented two stratospheric lidarsystems for NDSC. These are located at Table Mountain, California, and at Mauna Loa, Hawaii. These systems, which utilize differential absorption lidar, Rayleigh lidar, Raman lidarand backscatter lidar, to measure ozone, temperature and aerosol profiles in the stratosphereare briefly described. Examples of results obtained for both long-term and individual profilesare presented.
Measurements of ozone in the troposphere have become increasingly important during thepast decade with reports of ozone increases on global scales that not only threaten the vitalityof the plant and the animal kingdom, but can also, since ozone is a greenhouse gas, contributeto global warming. The lidar application that is described here is specifically designed toaccommodate the needs of the scientific community to accurately measure tropospheric ozoneover a period of many years as one indicator of global change, and is intended as a prototypefor a network of tropospheric ozone lidars.
Water vapor is one of the most important constituents in the Earth's atmosphere. It fuelsstorm development and is the most infra-red active at mospheric molecule. It plays a majorrole in dynamics and radiative transfer. The Raman lidar development at NASA Goddard isable to measure water vapor profiles, at night, from near the surface to ranges of 9-10 kmalong any elevation angle from horizon to horizon in a single plane. The lidar has been used ina number of field studies since it's first deployment in the fall of 1991. Comparisons of lidar derivedmoisture profiles with radio-sonde measurements have shown good agreement for warmmoist conditions. Consistent disagreement has been noted for sondes using carbon hygristorswhen the relative humidity is below about 20%
New technology for compact, eye safe lidar is surveyed. Advances with solid state lasers anddetectors permit efficient lidar designs that meet requirements for full time atmospheric monitoring. Micro pulse lidar is an approach that employs kHz pulse rate Nd : YLF lasers and microJoule pulse energies at visible wavelengths. Eye safety is an essential factor and is obtained bybeam expansion. High performance is realized by solid state Geiger mode avalanche photodiodephoton counting signal acquisition. The capability to monitor all significant cloud andaerosol scattering by a compact instrument has been demonstrated. Pseudorandom modulation CW lidar is under development. CW diode lasers are used directly as transmitters with theadvantage of wavelength tunability for water vapor and other differential absorption measurements. Background photon noise and dynamic range are problems for CW lidar. Advancedconventional pulsed diode laser lidar and near infrared lidar at fundamentally eye safewavelengths are in development for some applications.
A good knowledge of the temperature field and its variability in the middle atmosphere isneeded to understand the causes of its natural variability and to estimate the possible impact ofanthropogenic perturbations. Rayleigh-Raman lidars provide a powerful tool to reach thisgoal. Rayleigh backscattering of a laser beam by atmospheric molecules allows to measure densityand temperature profiles above 30 km. The network of Rayleigh lidars has shown thepotential of this method and has been used during the past 15 years to obtain new results concerningthe structure and the climatology of the middle atmosphere and to validate satellitemeasurements. Recent improvements in lidar techniques allow to extend downwards thetemperature profiles into the lower stratosphere and the upper troposphere using vibrationaland rotational Raman scattering by the main gases of the atmosphere, O2 and N2.
Pulsed solid-state coherent laser radar (lidar) systems are rapidly evolving as useful sensorsfor a variety of ground-based and airborne measurement applications. This paper describessuch measurement applications and sample demonstration measurement results associatedwith wake vortex detection and tracking, windshear and gust front detection, and wind fieldmonitoring in support of Space Shuttle operations.
Meteoric ablation in the upper atmosphere is the source of a dense layer of atomic Na between 80-105 km with maximum densities averaging 4×103 cm-3. The Na layer is a sensitivetracer of the wind, thermal, and density structure of this region and can be probed with exceptionalaccuracy and resolution using resonance fluorescence lidar techniques. Modern narrowbandtunable lasers are now being used in Na lidar systems to measure routinely winds andtemperatures with accuracies approaching ±1 m/s and ±1 K. We describe the Na Wind/Temperature lidar technique and present examples of observations made during the recent ALOHA-93 Campaign in Hawaii.
Since the early 1980's, airborne lidar systems have been used for making remote measurementsof ozone, water vapor, and aerosols in studies of many important atmospheric processes. Recent ozone and aerosol investigations have been conducted in the troposphere over thetropical Atlantic and western Pacific and in the Arctic stratosphere, and water vapor, aerosols, and clouds in the troposphere were measured over land and water during daytime and nighttimeconditions. Advanced airborne systems are under development to demonstrate autonomousoperation of these lidar systems and to expand their measurement capabilities. In thenear future, lidar systems will be used in space to investigate a wide variety of global atmosphericprocesses. This paper describes the NASA Langley Research Center's airborne differentialabsorption lidar systems, discusses measurements with these systems in recent atmosphericinvestigations, and describes the development of advanced airborne and spacebornelidar systems.
The application of a mobile DIAL (differential absorption lidar) remote sensing system inmeasurements of gas fluxes is discussed. UV-DIAL has been employed in the monitoring ofsulphur dioxide and mercury emissions both from natural and anthropogenic sources. The concentration distributions downwind from the sources, deduced from vertical lidar scans ortraverses under the plumes, were combined with wind data to determine the atmospheric fluxes of these gases. Measurements of the total flux of sulphur dioxide from the Italian volcanoes Etna, Stromboli and Vulcano were performed from an oceanographic research ship makingtraverses under the volcanic plumes with the lidar system sounding vertically. Some resultsfrom lidar measurements on atmospheric atomic mercury in Italian geothermal fields are alsopresented. Finally, some examples of remote monitoring of industrial emissions are given. Themobile DIAL system has been used for measurements on several Swedish industrial plantswithin the framework of a control programme commissioned by the Swedish Environmental Protection Agency.
With the invention of the laser, lidar systems were developed to probe the atmosphere for avariety of purposes, including the opening of a new frontier of cloud physics research. Theirhigh range resolution and increased sensitivity to small hydrometeors were suited for determiningthe properties of some overlooked types of clouds (e. g., cirrus), which over timeassumed increased stature for our understanding of climate. Although one-channel lidars candefine the physical boundaries of clouds, the most widely exploited technique has been polarizationdiversity, which relies on fundamental principles to discriminate between water and icephase clouds. Our understanding of the properties of all types of clouds has been enhanced bypolarization lidar field studies, often using a synergistic multiple remote sensorapproach. Theincorporation of polarization diversity into lidar systems based on spectroscopic approachesalso promises to increase their utility and accuracy. We graphically illustrate here the currentscope of lidar research pertaining to clouds.
Three techniques are reviewed which are presently applied to retrieve optical and microphysicalparameters of the stratospheric aerosol from lidar backscatter signals. Measurement examplesare presented to demonstrate the potential of the different methods and to documentthe stratospheric perturbation caused by the violent eruptions of Mount Pinatubo in June 1991. Lidar observations of the optical depth and microphysical properties such as the particlesurface-area concentration are needed to support model calculations performed to quantifythe influence of the volcanic aerosol on the global climate and the stratospheric ozone chemistry. The measurement examples show that the optical depth of the Pinatubo aerosol layer wasof the order of 0.1 during the first two years after the eruption. The surface-area concentrationexceeded 10 μm2 cm-3 which is regarded to be a threshold for a significant depletion of ozonein the lower stratosphere during the winter seasons 1992 and 1993.
Since the explosive volcanic eruption of Mt. Pinatubo on June 15, 1991, extensive lidarobservations have been made in the world. In this paper we review mainly lidar observationalresults obtained by the Effects of the Pinatubo eruption on Climate (EPIC) project from June1991 to April 1994. The first increase of the aerosols from the Pinatubo eruption was observedat 15.7 km over Tsukuba (36.1°N, 140.1°E) on June 28, 1991. From fall 1991, the Pinatuboaerosol particles were effectively transported from the tropical region into the northern hemisphere.The maximum values of integrated backscattering coefficient (IBC) above the tropopausewere obtained over Japan in February 1992. The IBC over Tsukuba in winter of 92/93decreased only about 40 percent compared with the IBC in 91/92 winter, but the IBC in winterof 93/94 decreased largely. The effects of the Pinatubo eruption on atmospheric minor constituentsand climate will be briefly denoted.
The concentration of stratospheric aerosols observed with a lidar in Alaska increased from December 1991 to March 1992. The measurements suggested that particulate matter injectedinto the stratosphere through the eruption of Pinatubo (Philippines, June 1991) was transportedto high latitudes and descended there. The speed of descending motion of the aerosollayer in Alaska was about 50 m/day, which is larger than the value in mid-latitudes. Decaytrend of aerosol load disturbed by Pinatubo volcanic eruption was approximately insimillarwith the trend observed in Japan suggesting that global distribution of volcanic aerosols isaffected by dispersion of particles from low and high latitudes and particle descending in highlatitudes.
The progress of lidar studies made in China is reviewed in the paper. The El Chichon volcaniccloud was measured with the ruby lidar in 1983. The large amounts of profile data of Pinatubovolcanic cloud have been obtained at Hefei and Beijing. The first Chinese ozone-DIALsystem has been constructed by Hu's group at AIOFM in 1993. Another ozone-DIAL lidarsystem is being constructed by Qiu's group at IAP. Chinese scientists have done many researchworks in the lidar measurements of the troposphere, such as plume dispersion, aerosol extinctioncoefficient profile, cloud base height, horizontal and slant visibility. Two lidar systems arebeing constructed for the middle atmosphere in China. The lidar, applied in measurements ofsea water temperature, and oil slicks at sea surface, has been developing in China also. Inaddition, Chinese scientists also did many theoretical studies in the methodologies of lidarmeasurements, multiple scattering in lidar measurement.
Lidars orbiting the Earth offer unique capabilities to scientists studying our atmosphere andclimate, and perhaps the Earth's surface properties. For example, a lidar with itsinherentsmall laser divergence and, therefore, small footprint can be used to probe the troposphere, aregion very difficult to measure with passive sensors. Further, the short pulse length of lasersprovide exceedingly high vertical resolution, while their monochromatic nature allows narrowband filtering and high signal-to-noise detection. Lidars have already shown theirunique capabilitiesfrom Earth-based or airborne platforms to accurately measure such critical constituentsand properties as ozone, water vapor, density, clouds and aerosols. This issue describes thoseand other capabilities. This paper discusses the historic background of spaceborne flight and, in addition, describes the first flight of a lidar in Earth orbit, that of LITE on the Space Shuttle Discovery in September 1994. Some early results illustrate the power of this new technique tocontribute to our knowledge of critical aspects of our Earth's environment.
Active laser remote sensing from space is recognised to be one of the most promising newmeans of obtaining essential atmospheric parameters on a global scale. This report gives anoverview on the ESA activities and will focus in the second part on the ongoing Doppler lidarsdevelopment at the German Aerospace Research Establishment (DLR).
The paper describes the activity on laser remote sensing of the atmosphere developed at the Institute of Atmospheric Optics Siberian Branch of USSR Academy of Sciences (now SB of Russian Academy of Sciences) including all the basic aspects of the problem, namely, developmentof sounding techniques; creation of surface-based, ship-, air- and spaceborne lidars; coverage of maximum number of the parameters being sounded; solution of corresponding inverseproblems; the use of all main phenomena of interaction of laser radiation with the atmosphere for the purposes of the remote sensing of atmospheric parameters; the use of different lasers for remote sensing in the IR, visible and UV ranges. The most important originalresults obtained at the Institute of Atmospheric Optics in the last 25 years of its existenceand directly related to the problems of laser remote sensing of the atmosphere are presented. Our consideration is restricted to the purpose and original special features of lidars and lidarcomplexes which has been developed, created and used at the Institute of Atmospheric Opticsincluding ground-based stationary lidars (the double-wavelength polarization lidar, the Ramanlidar, the DIAL lidar, the lidar station for sounding of aerosols, the multiwavelength lidar station), mobile lidars (the aerosol lidar “LOZA”, the spectrochemical lidar using a laser sparkgeneration in the atmosphere for analyzing the element composition of atmospheric aerosols, the Raman lidar, the lidar for sounding of wind velocity and its direction, and the long-path DIAL lidar) and also spaceborne lidars (“BALKAN-1”and “BALKAN-2”). The developedmost complicated technology of crystal growth which provides obtaining of nonlinear elementswith unique characteristics for the use in lidars is considered.