Special Section on Active Electro-Optical Sensing: Phenomenology, Technology, and Applications

Laser radar: historical prospective—from the East to the West

[+] Author Affiliations
Vasyl Molebny

Academy of Technological Sciences of Ukraine, 42 Glushkov Avenue, Kiev 03187, Ukraine

Paul McManamon

University of Dayton, 4161 Spruce Pine Court, Dayton, Ohio 45424, United States

Ove Steinvall

FOI (Swedish Defence Research Agency), Department of Electro-Optics, Olaus Magnus Väg 42, 581 11 Linköping, Sweden

Takao Kobayashi

University of Fukui, Higashiyama 1031-6, Gotenba, Shizuoka 412-0024, Japan

Weibiao Chen

Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Qinghe Road, No. 390, Jiading, Shanghai 201800, China

Opt. Eng. 56(3), 031220 (Dec 28, 2016). doi:10.1117/1.OE.56.3.031220
History: Received August 11, 2016; Accepted November 11, 2016
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Abstract.  This article discusses the history of laser radar development in America, Europe, and Asia. Direct detection laser radar is discussed for range finding, designation, and topographic mapping of Earth and of extraterrestrial objects. Coherent laser radar is discussed for environmental applications, such as wind sensing and for synthetic aperture laser radar development. Gated imaging is discussed through scattering layers for military, medical, and security applications. Laser microradars have found applications in intravascular studies and in ophthalmology for vision correction. Ghost laser radar has emerged as a new technology in theoretical and simulation applications. Laser radar is now emerging as an important technology for applications such as self-driving cars and unmanned aerial vehicles. It is also used by police to measure speed, and in gaming, such as the Microsoft Kinect.

Laser radar (also called ladar for laser detection and ranging, lidar for light detection and ranging, or opdar for optical detection and ranging) started most of its development in the early 1960s, shortly after the invention of the laser. There had been some earlier lidar development prior to the invention of the laser, but the laser has been a real enabler. Laser radar has become relatively inexpensive and reliable, and has very rich phenomenology, making laser radar competitive compared to alternative sensor technologies, such as passive electro-optical sensors or microwave radar. Laser radar started operating in the visible region (ruby laser) and then appeared in the near infrared (Nd:YAGlasers) to the thermal infrared (CO2 laser). Many laser radars are now being developed in the eye safe short-wave infrared region (1.5  μm).

Numerous publications accompanied the maturing of laser radar, bringing to life new journals, new professional meetings, symposia and conferences, such as “Laser Radar Technologies and Applications” managed by the SPIE as part of the Defense and Commercial Sensing symposium, which includes special laser radar courses for participants.1 Laser radar became the topic for new fundamental books25 and reports,6 as well as a subject taught at the universities.7 New technologies appeared based on laser radar principles, such as optical coherence tomography (OCT) and digital holography. The range of most laser radar applications is from micrometers8 to tens of kilometers.

Overviews on the history of laser radar development appeared concerning Europe,9 the United States,10 the former Soviet Union (FSU),11,12 Japan,13 and China.14 This paper combines and updates those histories. We have limited our examples to a sample of laser radar techniques and applications instead of trying to cover the whole field. It is still a daunting organizational task to cover the world history of laser radar technology for more than 50 years.

First Steps of Range Finders

A laser range finder is the simplest kind of laser radar. It uses a single detector to determine the range to a target based on the round-trip time-of-flight of a laser pulse to and from the object. Because we know the speed of light, we can calculate range. The idea to use short pulses of light to measure distance was brought out by Lebedev.15 Short pulses allow excellent range resolution. The prototype used a specially developed interference modulator to obtain short pulses of light in 1936. A range up to 3.5 km was measured with the accuracy of 2 to 3 m. In 1963 to 1964, laser-based rangefinders were developed using ruby and gallium arsenide in the same lab used by Lebedev in the 1930s, the Vavilov Optics State Institute (GOI) in Leningrad (now St. Petersburg).

Range finders, proximity fuzes, and weapon guidance were the first military laser systems in the late 1960s and early 1970s. The early ruby lasers were high cost, with poor efficiency, and with eye safety issues. Later short pulse, high-energy, and highly collimated monochromatic beams became available as Q-switched lasers revolutionized laser radar capabilities.

In Sweden, laser research with a ruby laser started at Swedish Defense Research Establishment (FOI) in 1961. In the industry, the pioneers were ASEA and LM Ericsson. In 1968, Ericsson delivered laser range finders to the Swedish Coastal Artillery for operational use. Bofors developed laser range finders for the infantry canon vehicle IKV 91 and for the BOFI system in co-operation with Hughes Aircraft. Later, ASEA developed cloud altimeters for the civilian and military markets. During the early 1970s, Bofors developed a successful antiaircraft missile beam rider system, the RBS 70, which later was modified into the RBS 90 and sold worldwide. The world’s first laser beam rider surface to air missile, SAM, to enter service was developed at Bofors and contained laser sensing. During the late 1970s, Ericsson developed laser-based proximity fuses for the Sidewinder missile. Ericsson also developed a laser tracking system, but that laser application was soon overtaken by video trackers.

In Norway, the Norwegian Defense Research Establishment transferred their knowledge to Simrad Optronics, which became famous for their laser range finder built-in to a handheld binocular. More recently, the company developed a family of range finders for target location and fire control.

In the United Kingdom, Royal Signals and Radar Establishment (RSRE) pioneered military laser development. An excellent review on the early laser range finder development was published by Forrester and Hulme.16 They claim that the LF-2 ruby tank laser sight was the first laser system in the world to be in large quantity production. It was developed primarily for use with the Chieftain main battle tank for the British army, but was widely used on other tanks, such as Vickers MBT, Centurion, Scorpion, and Chieftain derivatives. The original equipment used was a ruby laser with a spinning prism Q-switch, although a later version used a passive Q-switched YAG (LF-11).

In 1968, Ferranti (now Leonardo Finmeccanica) developed the world’s first fully stabilized laser system incorporating a Nd:YAG laser range finder and marking target seeker. The marking target seeker is a unit on the aircraft which locks onto a laser designated target. This equipment was a part of the weapon aiming systems of Jaguar, Harrier, and Tornado aircraft. The 1.06-μm laser transmitter power was generated by an electro-optically Q-switched Nd:YAG laser capable of operating at 10- or 20-Hz repetition rate.

One of the first Soviet laser range finders, BD-1 (Fig. 1), was described in a later SPIE publication.17 Another example is the KTD 2-2. The technology of laser range finders and laser designators is represented by numerous operational instruments described in a later publication.18 They are installed on SU and MIG airplanes as well as helicopters (see Fig. 2). In France, Thales Research and Technology delivered a Nd:YAG laser range finder to the French Official Services in 1967. The laser range finder was installed on the AMX 13 tank for field tests.19 In the 1970s, Thomson and Cilas developed an airborne target illumination system that demonstrated the ability to be used on a single seat airplane fighter, and it was later developed into the ATLIS targeting pod.

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Fig. 1
F1 :

Examples of early laser range finder (LRF) development. (a) Test equipment for the first Ericsson LRF (1965), (b) LRF for the Swedish Coastal Artillery (1968), (c) Simrad handheld LP-7, (d) LRF KTD 2-2 (Polyus, USSR), (e) LRF BD-1 (Institute # 801, USSR), and (f) Ferranti CO2 TEA LRF.

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Fig. 2
F2 :

Examples of the Soviet/Russian range finder/designator laser radars: (a) Samsheet-50 range finder/designator for Ka-52 helicopter, (b) 31E-MK electro-optic system for Su-30 fighter, (c) Shkval series for Su-25T, Su-25TM/Su-39, and Ka-50, and (d) pod mounted Sapsan-E for air-to-surface MiG and Su missions.

Germany also developed laser range finders and target designators based on a Nd:YAG laser by Carl Zeiss and Eltro. Military laser research was performed by FGAN-FfO. Many of their early publications were dedicated to laser propagation in the atmosphere.20

Long Distance Range Finders

The first laser ranging to the moon was done by MIT Lincoln Laboratory (MIT/LL) in 1962 using a 50  J/pulse Ruby laser. Precise laser ranging to the moon was then done in 1969 from NASA Goddard Space Flight Center in Greenbelt, Maryland. It used a retroreflector positioned on the Moon by the Apollo 11 astronauts. By beaming laser pulses at the reflector from Earth, scientists have been able to determine the distance to that spot on the moon to an accuracy of about 3 cm. Additional retroreflector packages were landed on the lunar surface by NASA during the Apollo 14 and Apollo 15 missions. French-built retroreflector packages were soft-landed on the lunar surface by Soviet landers.21

A retroreflector package was also used in the Japanese Retroreflector in Space (RIS) project, installed on the ADEOS satellite (National Space Development Agency of Japan, present: Japan Aerospace Exploration Agency) and launched in August 1996. The retroreflector had a curved mirror surface to compensate for the velocity aberration due to the satellite movement. A ground-based transmitter–receiver system used a transversally excited atmospheric pressure (TEA) CO2 laser and a HgCdTe detector having a 1.5-m diameter high-precision tracking telescope at the Communication Research Laboratory (now called the National Institute of Information and Communications Technology) in Kokubunji, Tokyo. The absorption spectra of atmospheric ozone were successfully measured in the 10-μm wavelength region using the reflection from the RIS.22

In the FSU, long-distance measurements with lasers started in 1962 by the Vympel Design Bureau, in co-operation with Lebedev Physics Institute. Its first experiments in 1967 allowed laser ranging of a Tu-134 airplane, equipped with optical retroreflectors.9 Ten years later, a giant laser radar LE-1 was tested at Sary-Shagan, and its main goal was antimissile defense. It tracked the satellite Molniya and measured the distance to it without any retroreflector.

The FSU LE-1 had a multichannel transmitter (49×4) and a multichannel receiver with an array of 196 range-gated photomultipliers, each having its own optical system. The switch is made of a block of four optical wedges rotating at 80 Hz. The LE-1 multibeam was controlled by means of a fast 2-D scanner consisting of two mirrors driven by stepping motors. The mirrors kept the beam stable during the transmit–receiver cycles. Each laser consisted of a master oscillator/power amplifier with identical ruby crystals and a KDP electro-optical switch. Output pulse energy was 1 J, pulse repetition frequency (PRF) of each laser was 10 Hz, and pulse duration was 30 ns. The optical train was designed by Geofizika Central Design Bureau in cooperation with Vavilov Optical Institute. The Cassegrain telescope with 1.2 m of main mirror aperture and 21 arc min field-of-view provides target tracking in the upper hemisphere. It was designed and manufactured by LOMO. The telescope drives the operation with an angular velocity up to 5  deg/s and angular acceleration up to 1.5  deg/s2 with a 5′ dynamic error. Velocity aberration is compensated by the tilt of a mirror. The LE-1 laser radar could detect a 1-m2 target at 400 km.

Laser Altimeters

Time-of-flight measurement is a keystone of laser radar altimeters. The lunar orbiter laser altimeter (LOLA), built by United States NASA, was designed to characterize landing sites and to provide a precise global geodetic grid on the Moon.23 LOLA’s primary measurement is surface topography. The instrument provides ancillary measurements of surface slope, roughness, and reflectance. LOLA is a multibeam laser altimeter that operates at a wavelength of 1064.4 nm with a 28-Hz pulse repetition rate. A single laser beam is split by a diffractive optical element into five output beams [Fig. 3(a)], each of which has a 100-mrad divergence and illuminates a 5-m diameter spot from the mapping orbit, resulting in a total sampling rate of the lunar surface of 140  measurement/s. Backscattered pulses are detected by the receiver, which images the five-spot pattern onto separate optical fibers, each of which relays the received signal to a distinct silicon avalanche photodiode (APD) detector. An example of lunar profiles is shown in Fig. 3(b).

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Fig. 3
F3 :

(a) Lunar orbiter laser altimeter: five output beams, (b) lunar profiles, and (c) topographic map of the Moon.

The laser spots form a cross pattern on the lunar surface, with each beam separated by an angle of 500 mrad and rotated 26 deg about the nadir axis with respect to the spacecraft forward velocity vector. The sample pattern permits calculation of surface slopes along a range of azimuths. A topographic map of the Moon is shown in Fig. 3(c).

To raise the sensitivity of laser radar for automotive applications, Inoue et al.24 from Toyota used fiber amplifiers both for transmitter and receiver, having designed an instrument with a sensor head 2  cm2 in size. The transmission optical system consists of a pulsed fiber laser. The peak output power is 10 kW, and the pulse width is 4 ns. The diameter of the scanning mirror is 10 mm. The optical fiber amplifier has a mid-way isolator, and a band-pass filter, to cut down spontaneous emission and to improve the conversion efficiency and noise figure.25 The resonant frequency of the scanning mirror is 100 Hz and the scanning angle is 40 deg.

The Institute of Space and Astronautic Science (ISAS), Japan, developed various laser altimeters for space and astronautical science use. Figure 4 shows some details of the Hayabusa mission using the Hayabusa lidar launched in 2005.26,27Figure 4(a) is a picture of the asteroid Itokawa (size: 540  m×270  m×210  m) taken by an imager, and Fig. 4(b) shows a flight model of ISAS. It operates at 1064 nm with 8 mJ, 1 Hz, 12.5-cm aperture, 3.7 kg, 24  cm×23  cm×25  cm). Figure 4(c) shows relative elevation measured by the Hayabusa laser radar. It has a range resolution of ±1  m. The precise elevation measurement of the asteroid was the first in the world. The Hayabusa-2 laser radar was launched in 2015 targeting a near-Earth asteroid named 1999 Ju3.

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Fig. 4
F4 :

“Hayabusa” mission using the Hayabusa lidar. (a) A picture of the asteroid “Itokawa” taken by an imager, (b) a flight model, and (c) relative elevation and horizontal distance measured by the Hayabusa lidar.

Laser Designators

The first laser designator is shown in Fig. 5(a). It is a part of the Paveway series of laser guided bombs [Fig. 5(b)]. Since its inception in 1968, the Paveway has revolutionized tactical air-to-ground warfare.8 These semiactive laser guided munitions, which home in on reflected energy directed from the target, not only drastically reduce the number of munitions required to destroy a target but also feature accuracy, reliability, and cost effectiveness previously unattainable with conventional weapons. NATO forces successfully employed Paveway during the operation Desert Fox and the subsequent patrol of the “no-fly” zones over Iraq, because of its pin-point accuracy and reduced chances of collateral damage. The Paveway III is the third generation providing the optimum operational flexibility through the use of an adaptive digital auto pilot, large field-of-regard, and highly sensitive seeker. It adapts to conditions of release, flies the appropriate midcourse, and provides trajectory shaping for enhanced warhead effectiveness. When used in conjunction with the BLU-109 or BLU-113 penetrator warheads, Paveway not only optimizes the trajectory and impact angle but also the angle-of-attack.

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Fig. 5
F5 :

(a) The first laser target designator (1969) and (b) the first laser guided bomb.

Laser Range Finders and Designators as System Components

After the initial development of ruby and Nd:YAG laser range finders and target designators, questions about range finder compatibility with FLIR in the 8 to 12  μm region were raised as well as the issue of eye safety. It was desired to measure the range of any target, which can be distinguished by the thermal imager, including, for example, an ability to penetrate mist and smoke. These considerations led to the development of pulsed range finders based on the pulsed CO2 TEA laser. A CO2 TEA prototype range finder was developed by RSRE and Ferranti.28

The CO2 TEA range finder was, however, not a success. Only a very few became operational due to a series of problems; among them, the “wet target problem” meaning range loss from wet targets due to low reflectivity at 10.6  μm, the necessity of cooled detectors, expensive optics, and laser life time problems. The main thrust in military laser range finding was soon centered around the wavelength of 1.5  μm utilizing Raman-shifted optical parameter oscillator or high-pressure gas technology, shifting Nd:YAG from 1.06 to 1.55  μm. Other schemes involved erbium glass or other materials. Starting with an efficient Nd:YAG laser as a pump source, the concept of multifunctional lasers was evolved.29 The idea was to combine 1.06-μm range finding/designation with laser radar, laser jamming, battlefield identification “friend or foe” and other functions into a compact system centered around one transmitter. In Fig. 6, examples of systems developed in the FSU are shown, incorporating the laser range finders and designators.

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Fig. 6
F6 :

FSU electro-optical systems with laser radars. (a) One of two on-deck devices of the Navy laser radar system (Kvant, Kiev), (b) multichannel CM with laser radar surveillance and fire control system (Kvant, Kiev), and (c) precision ranging and angular measurement system (Altai Optics-Laser Center).

Noncoherent Wind Measurement

Low altitude wind profile measurements with a noncoherent laser rangefinder were demonstrated using a simple balloon tracking system, with small (0.25-m diameter) lightweight balloons.30 Experiments on balloon trajectories demonstrate that laser range detection (±0.5  m) combined with azimuth and elevation measurements is a simple, accurate, and inexpensive alternative to other wind profiling methods. To increase the maximum detection range to 2200 m, a retroreflector tape was attached to the balloons. Night-time tracking was facilitated by low-power light-emitting diodes (LEDs).

Another example of noncoherent Doppler wind measurement is demonstrated in the paper of Liu et al.31 Liu is from the Ocean University of Qingdao, China, and some of his collaborators are from NASA Langley Research Center, Hampton, Virginia. A schematic of the lidar transmitter, receiver, and frequency control is shown in Fig. 7. The master oscillator of the system is the two-wavelength diode pumped continuous wave (CW) single-mode tunable Nd:YAGseed laser. The output at 1064 nm is used to seed a Continuum Powerlite 7000 Nd:YAG pulsed laser. The 532-nm output of the seed laser is sent to an iodine filter (cell 1) to control and lock the seed laser frequency. With this setup, the frequency precision is maintained to within 0.2 MHz, corresponding to a wind measurement uncertainty of 5.0  cm/s.

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Fig. 7
F7 :

Schematic diagram for injection-seeded Q-switched pulsed Nd:YAG laser transmitter.26

The mobile direct-detection Rayleigh-scatter Doppler lidar (Fig. 8) was developed by the University of Science and Technology of China32,33 to measure wind fields at a range of 15 to 70 km with a height resolution of 0.2 km below 40 km and 1 km above. This nonscanning system operates at an eye-safe wavelength of 354.7 nm using a frequency tripled 50-Hz Nd:YAG laser. A triple channel is used as a frequency discriminator to determine the wind velocity, two of them being double-edge channels located in the wings of the thermally broadened molecular backscattered signal spectrum, while the third one locks the frequency of the outgoing laser at the cross-point of the double-edge channels.34 The scanning system can detect the horizontal wind in four directions (north, south, east, west) by scanning the emitting–receiving system.

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Fig. 8
F8 :

Perspective view of two lidars.28

Airborne Applications

Coherent laser radars make use of the spatial and temporal coherence properties of laser radiation. By mixing the received signal with an optical local oscillator, the full field can be measured, including both phase and amplitude information, as compared to direct detection, where we only measure the amplitude (intensity) of the return laser signal. A typical application for coherent laser radar is velocity measurement, because by measuring phase, we can directly measure the Doppler frequency shift.

Airborne coherent laser radars have been used for ground imaging, obstacle warning, terrain following, as well as wind sensing including backscatter measurements. CLARA was a French-UK system developed for hard target (cables, ground surface, and so on) measurements.35 This equipment followed successful trials of the “laser obstacle and cable unmasking system” pulsed CO2 laser radar jointly developed by two groups within the former GEC Marconi. In another project, Société française d'équipements pour la navigation aérienne in France, a frequency-modulated CW (FMCW) laser radar was demonstrated for terrain following and terrain avoidance of combat aircraft.36 The LATAS airborne lidar developed by RSRE was mainly used for true air speed and atmospheric backscatter measurement at 10.6  μm, but also demonstrated intensity imaging of natural terrain and manmade objects.37

Other applications using coherent laser radar have been precision navigation to a designated landing site on Earth or on extraterrestrial objects, and rendezvous and docking with orbiting spacecraft require accurate information on the vehicle relative velocity and altitude. A Doppler lidar was developed by NASA under the ALHAT project.38 The lidar precision vector velocity data enabled the navigation system to continuously update the vehicle trajectory toward the landing site.

Figure 9 illustrates the configuration of an all-fiber lidar. Its waveform is generated from a very narrow linewidth fiber laser, is frequency modulated, and is directed through a single-mode fiber to a high-power fiber amplifier. The output of the fiber amplifier is split into three components in order to distribute the power to three optical channels corresponding to the velocity vector components (Fig. 10).

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Fig. 9
F9 :

Doppler lidar system configuration.

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Fig. 10
F10 :

Unit vectors describing the sensor geometry.

Atmospheric Wind Sensing

Wind sensing is a valuable application for coherent laser radars.39 Measurements include various ground-based programs, such as local wind field measurement and wake vortex investigation at airfields (Fig. 11). Airborne systems are used to measure true airspeed, pressure error, wind shear warning, and to collect atmospheric backscattering over the North and South Atlantic. In the 1990s, the European Space Energy supported a space-borne wind lidar in the Atmospheric Laser Doppler Instrument program. Pioneering wind lidar work in Europe was performed at RSRE (United Kingdom), DLR (Germany), and at the Laboratoire de Meteorologie Dynamique, Ecole Polytechnique (France). For a review on early coherent laser radar work in Europe, we refer to the paper by Vaughan et al.40 In the book of Killinger and Mooradian (editors), there are several articles dedicated to the first coherent laser radars in the United States.41

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Fig. 11
F11 :

Wind sensors: (a) NASA optical air turbulence sensor (2  μm, 1 mJ, 1 kHz, 5 cm aperture, chiller), (b) NASA ACLAIM turbulence warning (2  μm, 8 to 10 mJ, 100 Hz, 10 cm aperture, chiller), and (c) MAG-1A WindTracer (2  μm, 2 mJ, 500 Hz, 10 cm aperture, heat exchanger).

Coherent Doppler lidar (CDL) appeared to be a useful instrument for atmospheric wind sensing, from ground, air, or space. CDLs observe large volumes of atmosphere with high spatial and temporal resolution, making these data important for many applications.

The first CDL was reported by Huffaker et al.42 for wake vortex detection in 1970 using a 10.6-μm CW CO2 laser. In the late 1990s, Mitsubishi Electric commercialized eye-safe compact fiber-based lidars and middle-range (8 km) wind lidars (Fig. 12), having developed a high power fiber amplifier of Er, Yb:glass with a 3.3-kW output power, 580-ns width, and repetition rate of 4 kHz.43 Also, a ultra-long-range wind lidar exceeding 30 km was realized using this amplifier, with a range resolution of 300 m.44 An all-fiber CDL was also developed based on a new concept of the automatic parameter control adaptive to atmospheric condition. These systems have been used in various industrial applications, such as meteorological monitoring, wind survey for wind power generation and aviation safety, and so on.

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Fig. 12
F12 :

(a) CDL, (b) displayed diagrams of signal noise and Doppler velocity, (c) all-fiber Doppler lidar system, and (d) its data display unit.

In the late 1990 and early 2000, Coherent Technologies, Inc. (CTI) led the charge to develop wind sensing, with sponsorship mostly from the Air Force and NASA. CTI also developed commercial products like the wind tracer. Multiple WindTracer systems have been deployed at airports and research facilities for monitoring the wind shear in the airport area and for study of aircraft wake vortices.

Another application for this technology has recently been the monitoring of winds for wind energy generating platforms. These units are placed on the tower and monitor the wind and turbulence for better operation of the wind power generators. Halo Photonics in United Kingdom and Leosphere in France offer these units commercially.

The US Air Force has interest in wind sensing for air drop, gun ship, and dropping dumb bombs. The flight version of ballistic winds was flown in a near prototype C-130 Pod System. It was 15  ft3 and 1000 lbs. It used a solid-state, 15-mJ, 2-μm laser.

Adding Range Finding to Velocity Measurements

Coherent laser radars have the potential of adopting similar methods as in microwave radar to combine range finding with Doppler sensing. One of the concepts uses the principle of the FMCW signal. An example is the range finder developed at RSRE and described by Hulme et al.45 The laser uses a few watts of output power. It is modulated by an acousto-optic modulator generating “chirp” pulses of the type familiar in microwave radar. Other examples are given in Refs. 464748. At FOI, the systems were developed49,50 using FMCW signals from CO2 and semiconductor lasers.

Vibrometry

One of the attractive applications of the coherent laser radar is vibrometry based on the Doppler effect. Remote noncontact measurements offer potential for civil and military instrumentation. The vibrometers typically operate at 1.5, 2, or 10.6  μm. To get spatially resolved vibrational information, scanning and multibeam laser vibrometers are used. Reference 51 focused on applications in the field of defense and security, such as target classification and identification, including camouflaged or partly concealed targets, and the detection of buried land mines, with some examples of civil medium-range applications. The vibration spectrum of a target was acquired as an important and robust feature necessary for classification and identification purposes. The small target in Fig. 13(a) is an ordinary black rubber boat with an outboard engine. The boat is 4×2×2  m3. The range was 1  km. The diagram in the middle displays the frequency spectrum. The vibration frequencies originate from different parts of the target. To the right is the related spectrogram over 5 s.

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Fig. 13
F13 :

Vibration measurements from a rubber boat. (a) Visible image with marked laser beam, (b) frequency spectrum of the velocity, and (c) frequency spectrum over time.

Based on a spectrogram approach, vibration signatures were obtained from the LACE satellite in the course of ground-based laser radar measurements by using a coherent CO2 laser.5254 The satellite was equipped with IR germanium retroreflectors on deployable/retractable booms to enhance ground-based IR laser radar measurements of on-orbit boom vibrations. The data were acquired during, and subsequent to, one of the maneuvers (boom retraction). They indicated the presence of a complex time-varying mode structure. The power spectra of vibrations are shown in a Doppler-time-intensity format in Fig. 14(b). Here, the power spectra are aligned and displayed along the vertical time axis. The horizontal axis corresponds to Doppler frequency (i.e., velocity).

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Fig. 14
F14 :

(a) LACE satellite. Relative vibration was measured between the germanium retroreflector located on the satellite body and the retroreflector located on the retroreflector boom tip. (b) Doppler-time-intensity representation of data aligned to the peak return.

Besides the CO2 lasers at 10.6  μm, a number of solid-state laser sources based on neodymium at 1.06  μm, semiconductors or erbium fiber at 1.5  μm, and holmium at 2.1  μm have been used successfully as laser vibrometers. There are, however, often environmental conditions associated with poor visibility, turbulence, or high humidity, where it might be desirable to operate in the mid-IR band to improve system performance. Such conditions are not uncommon at low altitudes and in marine environments. Coherent laser radar systems in the mid-IR wavelength region can have advantages in low-altitude environments because they are less sensitive to scattering, turbulence, and humidity, which can affect shorter- or longer-wavelength systems. A monostatic coherent laser radar at 3.6  μm based on a single-frequency optical parametric oscillator was described in Ref. 55. It operated over short ranges outdoors, using two different stationary trucks with the motors running as targets. The system provided micro-Doppler measurements that were processed to give surface vibration spectra of the stationary, but running, trucks.

To get a spatial distribution of vibrations on the surface of the investigated object, a scanning vibrometer can be used. Scanning laser vibrometers allow analysis of the structure with a very fine spatial resolution, not modifying its dynamic behavior, decreasing the testing time if a large number of measurement points are requested. The problem of in-flight measurements was investigated using a scanning laser Doppler vibrometer to measure vibrations inside the cabin’s mock-up of the Agusta A109MKII.56 The whole area viewed was 430×315  mm2. In the scanning tests, a 30×20 grid was used. The comparison was also made with the vibrograms measured when the vibrometer was placed outside the mock-up. Figure 15 is a summary of the tests at different resonances. They all refer to instantaneous amplitude of the velocity component at a certain frequency orthogonal to the surface with the bandwidth ±10  Hz for the frequencies up to 1000 Hz and ±30  Hz for the frequencies up to 5000 Hz. Vibration sensing was also examined for buried mine detection.57

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Fig. 15
F15 :

Resonance vibration frequencies registered in the cabin of the helicopter mock-up.

Early Steps

In parallel to military laser sensing, the laser radar research community started to search for applications in atmospheric and ocean sensing. For example, laser radar observations of the mesosphere were made using a ruby laser as early as 1963 by Fiocco and Smullin.58 Spatial distribution of aerosols in the troposphere was reported by Collis et al.59 In the United States, vertical water vapor distribution was studied60 using a temperature tuned ruby laser; it was the first experiment using differential absorption lidar (DIAL). In Japan, a Mie scattering laser radar was first developed by utilizing a homemade Q-switched ruby laser. Basic relations were analyzed in 1968 on the scattering, extinction, and visibility by Inaba et al.61 at Tohoku University. In the FSU, the atmosphere temperature was studied by Arshinov et al.62 Laser sensing of the humidity profile of the atmosphere was studied by Zuev et al.63 Examples of lidar systems64,65 are shown in Fig. 16. As reported by Hu and Qiu,66 the first Chinese atmospheric laser radar was completed in 1965. Since then, experimental studies were carried out to investigate stratospheric aerosol and volcanic cloud, stratospheric ozone profile, tropospheric aerosol (including smoke plume density, aerosol extinction coefficient, atmospheric turbulence), sodium layer and Rayleigh scattering in middle atmosphere, seawater temperature, oil slicks on sea surface, and so on. Numerous theoretical studies were also made. Svanberg67 gives many examples of early laser radar monitoring of pollutants. Atmospheric and ocean laser radars involve all kinds of lasers and detectors depending on the goal of the laser radar. Possible goals might be aerosol or gas sensing, or ocean sensing, such as bottom profiling or water sensing (turbidity, plankton, and so on). The limited space in this paper does not allow us to go deeper into the atmospheric and ocean lidar development. Instead, we refer to the textbooks and reviews for further reading.68,69 Reference can be made to two other books discussing the propagation of laser radiation in water medium.70,71 Studies of the environmental laser radars in the FSU resulted in the seminal monographs on propagation of laser radiation in the atmosphere by Tatarskiy72 and by Zuev.73 They were followed by other monographs from the Tomsk scientific school.74,75 Practical problems connected to the applications in meteorology were discussed in the book for meteorologists.76 Achievements of the CW FM technologies are described in the book by Agishev.77

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Fig. 16
F16 :

Lidars for atmosphere investigation: (a) all-fiber coherent multifunctional CW laser radar for range, speed, vibration, and wind measurements at 1.55  μm (FOI, 2000), (b) Lidar automatic system for remote air pollution monitoring in large industrial areas (former Institute of Precision Instrumentation, Moscow), and (c) multifunctional lidar system (Astrofizika Corporation, Moscow).

Multiwavelength Lidars

Penn State University78,79 made significant progress in Raman laser radar. They enabled measurements of the optical and meteorological properties of the atmosphere based upon vibrational and rotational energy states of molecular species, such as water vapor and ozone, temperature, optical extinction, optical backscatter, multiwavelength extinction, extinction/backscatter ratio, aerosol layers, and cloud formation/dissipation (Fig. 17). An angular scattering technique enhances the information by measuring the scattering phase function for aerosols, including the polarization ratio of the scattering phase function, number density versus size, size distribution, identification of multicomponent aerosols, index of refraction, and so on. Multistatic aerosol laser radar and multiwavelength multistatic laser radars are good candidates for prospective studies.

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Fig. 17
F17 :

(a) Lidar at the Pennsylvania State University and (b and c) dynamics of temperature and extinction.

Laser Radar Sensing in China

Studies of yellow sand storm events were one of the practical applications of Mie scattering laser radar in China, monitoring the transport of dust, aerosol extinction coefficient profiles, regional air pollutant transport, and the urban mixed layer.8082 In addition, Anhui Institute of Optics and Fine Mechanics (AIOFM) has joined national and international projects on lidar monitoring of the atmosphere.83,84 These projects include the Asian Dust lidar observation NETwork,85 Aeolian Dust Experiment on Climate impact, etc. A double wavelength polarization Mie laser radar was implemented,86 and a 12-year observation of aerosol in Hefei was reported.87

The first Mie scattering laser radar was implemented to measure stratospheric aerosol in 1980s at the Institute of Atmospheric Physics (IAP). The stratospheric aerosol enhancement by volcanoes El Chichon (1982) and Pinatubo (1991) was monitored in Beijing. In the early 1990s, AIOFM started to develop Mie scattering lidar based on a Nd:YAG laser. A large amount of profile data of the Pinatubo volcanic cloud were obtained in Hefei and Beijing.88

In 1992, IAP developed a multiwavelength laser radar to observe stratosphere ozone and aerosols and the high altitude clouds. The laser radar used a XeCl excimer laser and a Nd:YAG laser, a 1-m diameter telescope, and multichannel detector. The Nd:YAG laser had outputs of 1 J, 300 mJ, and 150 mJ at 1064-, 532-, and 355-nm wavelength, respectively. The PRF was 10 Hz. The excimer laser had an output of 140 mJ at 308 nm and the PRF was 100 Hz.89 In 1994, AIOFM also developed a laser radar using the second and third harmonics of Nd:YAG and an excimer laser for stratospheric ozone profile monitoring.90 A DIAL for pollutant gas, such as SO2, O3, NO2 observation, was developed in AIOFM.91,92 A CH4 and D2 gas cell was pumped by the fourth harmonic of Nd:YAG.

The vibrational–rotational Raman laser radar was developed at Xi’an University of Technology.93 A schematic diagram of the vibrational–rotational Raman laser radar is presented in Fig. 18. The system employs a pulsed Nd:YAG laser as a light source, operating at a frequency tripled wavelength of 354.7 nm with a 20-Hz repetition rate and an energy output of 250 mJ with a 9-ns pulse duration. Returned signals are collected with a 600-mm Newtonian telescope, and then coupled into a multimode optical fiber and guided into the spectroscopic box. The laser radar was used to measure profiles of water vapor mixing ratio by Raman scattering as well as the aerosol extinction coefficient by Mie scattering.94,95 High accuracy temperature profiles can be achieved up to a height of 25 km.96 The effective measurement for atmospheric water vapor can be achieved up to a height of 16 km.97 Examples of extinction and temperature profiles are given in Fig. 18.

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Fig. 18
F18 :

(a) Vibration-rotational Raman lidar system developed at Xi’an University of Technology and (b) extinction and temperature profiles.

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Fig. 19
F19 :

(a) Early Raman scattering lidar systems: at the observatory of Tohoku University and (b) mobile scanning Raman lidar.

The first sodium laser radar for the middle atmosphere in China was developed in 1996 at Wuhan Institute of Physics and Mathematics.98 Rayleigh and sodium lidar was developed by Wuhan University.99 The laser radar transmits both in 532- and 589-nm wavelengths and detects Rayleigh backscattering, Raman signal of N2 and H2O, and sodium fluorescence. Similar sodium laser radars were also developed at the University of Science and Technology of China,100 and Wuhan University.101 Wuhan University developed an iron Boltzmann lidar to measure the mesopause temperature.102 National Space Science Center developed a sodium fluorescence Doppler lidar to observe both temperature and wind.103 At present, five laser radars (in Wuhan, Hefei, Qingdao, Beijing, and Hainan) are operating to observe the middle atmosphere.

Laser Radar Sensing in Japan

In Japan, a Raman scattering laser radar for sensing of air pollution over Japanese industrial areas was proposed by Inaba and Kobayashi104 at Tohoku University. Raman scattering laser radars are shown in Fig. 19. A nitrogen molecular laser at ultraviolet 337.1-nm wavelength was developed for the Raman laser radar. Molecular vibrational Raman spectra of CO2, O2, N2, and H2O were observed in the clear air and it was shown for the first time that major air molecules could be separately detected. Also, various molecules, such as O3, CO, CH4, liquid and vapor H2O, were identified from automobile exhaust gas in air. After this experiment, the mobile scanning Raman laser radar system was developed by Nakahara et al.105 at Mitsubishi Electric Co. Ltd. using a Nd:YAG laser second harmonic beam at a wavelength of 532 nm. It was shown that SO2 molecules in stack effluent plume were detectable with 1000 ppm concentration sensitivity at the slant range of 220 m. The Raman laser radar was also used for humidity sensing by water vapor Raman spectroscopic detection and was gradually extended into sensing of extinction coefficients of atmospheric aerosols by simultaneous measurement of Mie and nitrogen Raman spectra.

After the demonstration of high sensitivity of resonant scattering laser radar in 1969,106 several Na (sodium) resonant scattering lidars were developed and reported by Aruga et al.107 at Tohoku University and by Nagasawa et al.108 at Kyushu University.

Stratospheric ozone was measured by Uchino et al.109 at Kyusyu University in 1978 employing a discharge-pumped XeCl laser at 308 nm wavelength based on the differential-absorption technique. In 1988, the Meteorological Research Institute developed a mobile laser radar for simultaneous measurements of ozone, temperature, and aerosols in the stratosphere.110 They used three Stokes lines (276, 287, and 299 nm) of stimulated raman scattering from a carbon dioxide gas cell pumped by a Nd:YAG laser (266 nm).111 The National Institute for Environmental Studies (NIES) also developed the same type of ozone DIAL.

To clarify the ozone loss mechanisms in the polar stratosphere known as the “ozone hole,” a XeF excimer laser, with 200-mJ pulse-energy and 80-Hz repetition rate at 351- and 353-nm wavelengths, was developed. It was used in 1986 for observation of molecular density and temperature in altitude range of the middle atmosphere by T. Shibata et al. at the Kyushu University.112

To examine the potential of Mie scattering, a large scanning laser radar was built in 1979 at the National Institute for Environmental Studies, NIES, in Japan with a pulse energy of 400 mJ at 532 nm and repetition rate of 25 Hz (Fig. 20). The laser radar was used in various studies on Mie scattering for measuring the structure of the atmospheric boundary layer,113,114 aerosol distribution, and optical characteristics.115

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Fig. 20
F20 :

Large scanning lidar system (NIES): (a) schematic diagram, (b) photo of the lidar, and (c) structure of sea breeze front.

Early Laser Radar Imaging