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Synonyms
Ladar; Laser radar
Definitions
Lidar. LIght Detection And Ranging
Ladar. LAser Detection And Ranging
Introduction
A lidar system in the strictly defined sense of the acronym measures range to a “target” that provides a signal that can be detected. Thus, the lidar system includes both a transmitter and a receiver. Ranging is accomplished using time-of-flight methods. The target can be a “hard” target that is essentially opaque to the lidar wavelength, not allowing measureable penetration beyond its range, or a “diffuse” scattering medium that allows penetration and range gating. Examples of the former are the surfaces of the Earth and other planets, or man-made objects. Examples of the latter are atmospheric aerosols and gases. In reality, these are terms that are commonly used but do not have strict, universally accepted definitions. In fact, the term “lidar” itself is commonly applied to systems that contain transmitters and receivers but do not have inherent range measurement capability. The lidar community is inclusive in this regard.
Following David Tratt’s introductory entry (“Emerging Technologies: Lidar”), which describes lidar basics and various classes or categories of lidar, we provide here a summary of the current capabilities in these various lidar applications areas. Our lidar categories are altimetry and mapping systems, backscatter systems, Doppler systems, and differential absorption systems. Comments on emerging technologies and methods are included. Lidar/ladar applications cover a wide range of activities and interests. The 3D imaging applications are a growth area with strong support from the defense community. System developments in this area are included only in brief overview mode. The balance in this entry is tilted more toward systems developed for scientific investigations.
Altimetry and mapping systems
Laser altimetry is relatively mature, with heritage in aircraft instruments, followed by Earth-orbiting, Mars-orbiting, and Lunar-orbiting systems. The early altimeter/mapping instruments used a form of threshold detection to trigger a circuit that enabled range measurement to a “first return” scattering surface. The implementation of fast waveform recovery, or multistop detection circuits, increases data rates but provides structure information in the line-of-sight dimension. The Geoscience Laser Altimeter System (GLAS) on the Earth-orbiting ICESat (Abshire et al., 2005) provided structure detail in the time domain, a capability that is essential for future use of laser 3D mappers in obtaining global estimates of biomass. High-resolution 3D imaging with very high depth resolution (∼1 mm) can be achieved at km distances using fiber lasers and high bandwidth waveform encoding and decoding techniques (Buck et al., 2007). The current and next-generation systems combine multi-beam transmitter patterns with structural detail in the range dimension. The laser altimetric observational method provides line-of-sight detail that complements radar methods as well as higher spatial resolution in the cross dimensions. Spatial coverage is a challenge, however. The advent of avalanche photodiode (APD) arrays and photon-counting receivers (e.g., Aull et al., 2002; Albota et al., 2002), combined with optical methods for simultaneous transmission of multiple beams, have greatly increased the mapping efficiencies of airborne and space systems. The use of statistical methods in a photon-counting mode has allowed the use of compact, high pulse-repetition frequency (prf), low pulse energy laser transmitters in various imaging and mapping systems (Degnan et al., 2008; Steinvall et al., 2008). The Lunar Orbiter Laser Altimeter (LOLA) instrument, scheduled to launched in June, 2009, uses a Diffractive Optical Element (DOE) to produce a 5-beam pattern for provision of more spatial coverage than with prior space laser altimeters (Ramos-Izquierdo et al., 2009; Smith et al., 2010). The DOEs have found use in various airborne laser 3D mappers.
An alternative to the use of scanners or elements such as DOEs, matched with APD arrays, is a flash lidar/ladar. The images in this type of system record the intensity reflected by the scene when flood-illuminated by the laser transmitter pulse. The laser transmitter irradiates the entire field of view of the receiver camera pixel array, and each pulse generates an entire frame of data (Stettner et al., 2005). The array elements are high-speed detectors that are periodically sampled in time at nanosecond timescales. The advances in hybridizing the focal planes with silicon CMOS read-out integrated circuits (ROICs), utilizing steady improvements in high-speed circuitry, provide the potential for growth with this approach. Laser sources can include semiconductor lasers and fiber lasers mated to power amplifiers.
Backscatter lidars
Here we include elastic backscatter lidars and various types of inelastic backscatter lidars (e.g., Raman, fluorescence). The emphasis is on atmospheric studies using these systems. The intensity or energy in the return signal is important with backscatter lidar measurements. Some method of calibration and/or normalization must be used in order to turn the data into useful observations. In the visible, the molecular density, if known sufficiently well, can be used to provide a Rayleigh backscatter intensity that effectively calibrates at least the range dependence of the lidar efficiency factor, or the efficiency factor itself at a particular atmospheric altitude where particle scattering is assumed negligible. This is not a viable technique at longer wavelengths in the infrared, due to the rapid decrease of the Rayleigh scattering cross section with increasing wavelength.
Backscatter lidars for cloud and aerosol studies date back to the early years of lidar, when ground-based lidars operating at visible wavelengths probed the stratospheric aerosol layers (e.g., Fiocco and Grams, 1964). The first Earth-orbiting lidar used for atmospheric studies was an elastic backscatter lidar (LITE, launched in 1994). GLAS operated both as an altimeter and an atmospheric lidar (Spinhirne et al., 2005). Currently the CALIPSO lidar is in Earth orbit, being used for cloud and aerosol studies. The CALIPSO transmitter is a diode-array pumped Nd: YAG laser, by far the most commonly used in backscatter lidars. A wide dynamic range of pulse energies and pulse-repetition frequencies are available in this laser medium. The compact micropulse lidars, which emit pulses in the micro-Joule range, are deployed around the globe in networks such as the MPLNET (Micropulse Lidar Network) (Campbell et al., 2008). Cloud and aerosol detection, characterization, and monitoring algorithms continue to improve for these compact lidars, making them more useful for deployment. The underlying technologies are robust. The vertical profiling capabilities of these lidars cannot be duplicated with passive instruments. Recently, a compact backscatter lidar was deployed on the surface of Mars as part of the Phoenix mission (Whiteway et al., 2008).
A variant of the elastic backscatter lidar that is taking center stage in current and future atmospheric investigations is the High Spectral Resolution Lidar or HSRL. Although Doppler lidars are the ultimate high spectral resolution lidars, the term “HSRL” is commonly used in the lidar community to refer to a system that can separate the molecular Rayleigh backscatter signal from the aerosol backscatter signal. This obviates the need to assume a “lidar ratio” (i.e., aerosol extinction-to-backscatter ratio) when interpreting the range-dependent backscatter signals to deduce aerosol optical properties, thereby achieving more robust estimates of aerosol extinction coefficients. A progression in HSRL implementation has gone from early 1980s laser technology such as fragile dye laser systems (Shipley et al., 1983) to more robust solid-state lasers (Grund and Eloranta, 1991). Iodine vapor filters offer simplicity compared with the etalon filters in the HSRL receiver (Hair et al., 2001). More recently, airborne HSRL has been developed, and measurement results have been reported (Hair et al., 2008). The next-generation Earth-orbiting backscatter lidar for cloud and aerosol studies will likely be an HSRL. In fact, the European Space Agency’s Atmospheric Laser Doppler Instrument (ALADIN), a Doppler lidar in the Atmospheric Dynamics Mission (ADM) with atmospheric wind field measurements as its primary objective, is fundamentally an HSRL and will be used for investigations of aerosol optical properties (Ansmann et al., 2007) (see www.esa.int for further information).
Resonance fluorescence lidars have been in use for decades to study dynamics and thermal properties of the middle atmosphere, particularly the mesosphere. Lidars built to measure alkalis in the upper mesosphere were also used as Rayleigh backscatter lidars to measure density and temperature profiles in the stratosphere and mesosphere (Hauchecorne and Chanin, 1980). Developments in solid-state laser technology and injection seeding methods have resulted in systems that are more amenable to transportation and operation at remote sites (e.g., She et al., 2007). Systems that interact with a variety of metals in addition to sodium and the other alkalis are now in development for investigations over a wider range of altitudes (Gardner, 2004).
Raman lidars are now commonly used for water vapor profiling and for characterizing the optical and microphysical properties of atmospheric aerosol. The latter method was described 20 years ago (Ansmann et al., 1990) and has continued to evolve into systems that are being used for characterization of major dust plumes that are transported long distances (e.g., Asian dust, Saharan dust) and for calibration/validation exercises (Mona et al., 2007). The former method has a long history and has slowly evolved with the use of improved techniques for minimizing background light, improved algorithms, and improved understanding of sources of bias. The use of Raman lidar for water vapor profiling in the lower atmosphere continues to gain credibility as the level of accuracy continues to improve (Adam and Venable, 2007; Leblanc and McDermid, 2008).
Doppler lidars
The atmospheric gas molecules and aerosol particles are in bulk motion in the dynamic atmosphere, and backscattering of laser radiation from the molecules and aerosol particles produces Doppler shifts in frequency. Doppler lidars detect these frequency shifts to deduce wind profiles. Two types of Doppler lidar have received attention over the years: direct detection and coherent detection lidars.
The coherent detection lidar is more sensitive and less difficult to implement at relatively longer wavelengths in the infrared, particularly at wavelengths longer than 1.5 μm, the so-called eye-safe region. The ultrahigh spectral resolution that is inherent with these systems makes coherent detection suitable for measuring Doppler-shifted backscatter from the atmospheric aerosol particles. The signal processing has similarities with Doppler radar. The use of rare-earth-doped solid-state laser technologies in the 2 μm wavelength region has been a popular choice for compact coherent detection systems. An example is the NOAA shipborne lidar, which has been used in many field campaigns (Tucker et al., 2009). Airborne systems date back to the mid-1980s when carbon dioxide gas laser transmitters were used (Bilbro et al., 1986). More recent, much more compact systems have also been deployed for measuring wind profiles with high spatial resolution (Hannon et al., 1999). Both the rare-earth-ion-doped solid-state crystal laser technology at 2 μm and the fiber laser technology developed primarily by the telecom industry have been employed in recent ground-based coherent Doppler lidars stationed at airports for airport safety enhancements. These lidar systems are being used for both wake vortex monitoring (e.g., Kopp et al., 2004) and wind shear detection and warning (e.g., Shun and Chan, 2008). Fiber laser technologies are being incorporated into current and future systems.
Direct detection lidar is the appropriate choice for regions of the atmosphere containing very low aerosol particle concentration in the size range that is useful for optical scattering. The predominant scattering is molecular Rayleigh scattering. An early example was the use of direct detection Rayleigh lidar, modified with the incorporation of twin Fabry-Pérot interferometer filters in the receiver, for measurements of horizontal winds in the middle atmosphere (Chanin et al., 1989). An airborne direct detection Doppler lidar was developed, for tropospheric wind field measurements (Gentry et al., 2007). It is designed for autonomous operation on a high-altitude aircraft. The European Space Agency’s ALADIN lidar is planned for launch in 2010, as the centerpiece instrument in the Atmospheric Dynamics Mission (ADM). ALADIN uses solid-state Nd: YAG laser transmitter technology, frequency-tripled to the 355 nm near-UV wavelength. It contains two receivers, one for the narrow-band Mie scattered radiation from the atmospheric aerosol particles (employing a multichannel Fizeau interferometer) and the other for the Rayleigh scattered radiation from the molecules (employing a double-edge Fabry-Pérot etalon). Accumulation CCD’s are used in both receivers (see www.esa.int for further information).
Differential absorption lidars
Differential absorption lidars require typically two carefully selected closely spaced transmit wavelengths and a laser transmitter subsystem that has either discrete or continuous tenability in the desired spectral region to interact with the species of interest. Early systems used dye lasers or nonlinear optics such as optical parametric oscillators to provide tunability. Atmospheric ozone and water vapor have been favorite measurement subjects for decades. More recent systems rely on solid-state laser technologies and modern techniques for generating tunable single-mode radiation with high spectral purity. Airborne systems have progressed in sophistication, with corresponding reductions in mass and dimensions as well. The LASE (Lidar Atmospheric Sensing Experiment) system was demonstrated in the 1990s as an autonomous operation water vapor differential absorption lidar on the high-altitude ER-2 aircraft (Browell et al., 1997). Currently, intercomparison campaigns involving multiple airborne water vapor systems with different designs are being planned and implemented in order to better understand the accuracies of measurement and quantify biases that might exist (Behrendt et al., 2007). Results to date show that measurement accuracies are in good agreement with expectations.
Currently, a major challenge for differential absorption lidar is the measurement of atmospheric CO2. Measurements with very high accuracy over regional to global scales would improve understanding of fluxes between atmosphere, land surface, and ocean surface. The influence that increasing carbon dioxide mixing ratio has on climate change has spurred an interest in applying both passive and active remote sensing techniques to address this question. Desired mixing ratio accuracy levels of better than 1 % place great demands on a differential absorption lidar system itself and require the minimization of errors due to imperfect knowledge of the relevant atmospheric parameters (Menzies and Tratt, 2003; Ehret et al., 2008). Demonstrations of CO2 mixing ratio measurement capability using ground-based, coherent detection systems have been reported (Gibert et al., 2008; Koch et al., 2008). Airborne systems are now being tested in flight campaigns, using both the solid-state 2 μm laser technology and the 1.6 μm fiber laser technology (Abshire et al., 2010; Spiers, et al., 2011). Studies of Earth-orbiting lidar systems for CO2 measurements are being conducted under the sponsorship of European, US, and Japanese space agencies (ESA, NASA, and JAXA respectively).
Summary
Using an unofficial taxonomy of lidar systems, selected highlights of recent developments and future plans have been provided. Generally speaking, the future applications for altimetry and three-dimensional mapping will motivate increases in coverage within a given available time frame. This will most likely come from increases in total laser transmitter output power, along with optical technology. In other lidar application areas, engineering advances will be critical. For example, advances in compactness, electrical power efficiency, autonomy, and reliability will be essential for further use in hazard detection and monitoring, as well as expansion of regional and global networks for weather, climate, atmospheric composition, and environmental monitoring. Atmospheric greenhouse gas measurements, on a global scale, present high-precision measurement challenges. Nearly 50 years after the first demonstration of the laser, many lidar system applications are still driven by laser technology advances. For example, many applications still await the development of a wider range of laser sources in infrared spectral regions that are presently underutilized. The advent of the quantum cascade laser and other “bandgap-engineered” semiconductor laser technologies, as well as fiber laser/amplifier technologies, are good examples of continuing laser technology advances.
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Acknowledgment
This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the NASA.
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Menzies, R. (2014). Lidar Systems. In: Njoku, E.G. (eds) Encyclopedia of Remote Sensing. Encyclopedia of Earth Sciences Series. Springer, New York, NY. https://doi.org/10.1007/978-0-387-36699-9_85
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