development of coherent laser radar at lincoln laboratory · • gschwendtner and keicher...

• GSCHWENDTNER AND KEICHERDevelopment of Coherent Laser Radar at Lincoln Laboratory

VOLUME 12, NUMBER 2, 2000 LINCOLN LABORATORY JOURNAL 383

Development of Coherent LaserRadar at Lincoln LaboratoryAlfred B. Gschwendtner and William E. Keicher

■ The invention of the laser in 1960 created the possibility of using a source ofcoherent light as a transmitter for a laser radar. Coherent laser radars sharemany of the basic features of more common microwave radars. However, it isthe extremely short operating wavelength of lasers that introduces new militaryapplications, especially in the area of target identification and missile guidance.This article traces laser-radar development at Lincoln Laboratory from 1967 to1994. This development involved the construction, testing, and demonstrationof two laser-radar systems—the high-power, long-range Firepond laser-radarsystem and the compact short-range Infrared Airborne Radar (IRAR) system.Firepond addressed strategic military applications such as space-objectsurveillance and ballistic missile defense, while IRAR was used as a test bed forairborne detection and identification of tactical targets.

T development revealsthat radar advances and innovations are oftendriven by the availability and quality of high-

power signal sources. The British invention of thehigh-power microwave magnetron allowed scientistsand engineers at the MIT Radiation Laboratory, inCambridge, Massachusetts, to develop airborne mi-crowave radar during World War II. As the temporalcoherence of signal sources improved, new signal pro-cessing techniques became available. Finally, with thedevelopment of computers and high-speed digital sig-nal processing, new ways to detect targets and extracttarget information surfaced. The development of thelaser radar mirrors this paradigm.

The laser’s high operating frequency plus temporaland spatial coherence properties provided the basisfor developing unique laser radars at Lincoln Labora-tory from 1967 to 1994. This article discusses twoLaboratory laser-radar systems developed by theOptics division. The long-range Firepond laser-radarsystem, operating at wavelengths of 10.59 µm and11.17 µm for the carbon-dioxide (CO2) laser and1.064 µm for the neodymium-doped yttrium-alumi-num-garnet (Nd:YAG) laser, was used to develop

strategic military applications of laser radars. Thecompact Infrared Airborne Radar (IRAR) system, op-erating at wavelengths of 10.59 µm for the CO2 laser,0.85 µm for the gallium-arsenide (GaAs) laser, and1.064 µm for the Nd:YAG laser, was used to developtactical military applications of laser radars.

Coherent Laser Radars atLincoln Laboratory: 1967 to 1971

The CO2 laser, invented in 1964 by C.K.N. Patel,operated in the infrared spectrum at wavelengths withminimal absorption [1]. The CO2 laser rapidlygained efficiency and power compared to other lasersystems. In 1966, using the construction techniquesof MIT professor Ali Javan, who pioneered the he-lium-neon laser, Charles Freed at Lincoln Laboratorybuilt a CO2 laser with a temporal coherence that ex-ceeded any previously reported CO2 laser by a factorof at least 100 [2]. This development set the stage forLincoln Laboratory to demonstrate the first coherentCO2 laser radar in 1967. Yet another hundredfold im-provement in laser frequency stability was demon-strated in 1968. That same year, researchers in Lin-coln Laboratory’s Solid State division had developed a


384 LINCOLN LABORATORY JOURNAL VOLUME 12, NUMBER 2, 2000

wide-bandwidth (1.5 GHz) copper-doped germa-nium photoconductor for use as a photomixer with alaser-radar receiver that could observe Doppler fre-quency shifts in satellite echoes.

In parallel with Lincoln Laboratory’s efforts, Ray-theon built a 1000-W, continuous-wave (CW) CO2laser oscillator with funding from the Defense Ad-vanced Research Projects Agency (DARPA) and theOffice of Naval Research. The oscillator was given tothe Laboratory for laser-radar experiments afterRaytheon evaluated it. The Firepond Optical Re-search Facility (shown in Figure 1), near the MillstoneHill radar site in Westford, Massachusetts, was com-pleted by the Laboratory in late 1968 to permit long-range laser-radar measurements on terrestrial targets,aircraft, and satellites (the name Firepond derivesfrom the adjacent pond that serves as a source of wa-ter in case of fire). By the end of 1971, a 1.2-m-diam-eter telescope had been installed at Firepond andangle-resolved images of various targets were collectedwith a flying-spot scanner. The results of these earlymeasurements encouraged researchers to examinemilitary applications of laser radar.

Laser Radars for Strategic Defenseat Firepond: 1972 to 1993

In 1972, Robert S. Cooper of Lincoln Laboratory in-vestigated the utility of a wideband, high-power,range-Doppler laser radar for space surveillance. The

study, seen as the start of a ten-year effort, generatedspecifications for a 200-kW laser amplifier with abandwidth of 1 GHz, the most powerful coherent la-ser radar yet conceived. Many critical componentshad to be invented. Although key technologies andsignificant milestones were achieved, the work endedwithout successfully developing an imaging laser ra-dar. The main shortfall was the failure to achieve the200-kW high-power amplifier with a continuouslycirculating CO2 medium.

Advances continued at Lincoln Laboratory duringthis ten-year period in laser-radar component devel-opment. David Spears built wideband mercury-cad-mium-telluride (HgCdTe) photodiode photomixersfor monopulse laser angle tracking [3]. Charles Freedcontinued basic research in laser physics and eventu-ally cataloged the lasing frequencies of nine out ofeighteen possible CO2 laser isotopic combinations[4]. In 1976, because optical modulators were re-quired to incorporate the wideband FM waveform onthe laser beam, Lincoln Laboratory developed a wide-band, double-sideband, GaAs electro-optic modula-tor that was used to form the first CO2 laser-radarrange-Doppler images of a pair of moving retroreflec-tors on a ground range.

Although the radar power goal of 200 kW was notachieved, the laser amplifier still was among the mostpowerful in the world. When driven by the original1-kW narrowband laser amplifier, it had a maximumpeak pulse power close to 11 kW. Without an effi-cient wideband electro-optic modulator or a wide-band laser preamp, however, the power was availableonly in the narrowband mode.

In 1977, narrowband monopulse laser trackingwas demonstrated in experiments on aircraft and sat-ellites. The monopulse experiments resulted in track-ing errors of approximately 1 µrad root mean square(rms) for targets equipped with retroreflectors [5].Other experiments included the light detection andranging (lidar) measurement of high-altitude windsin 1978.

In 1981 the high-power laser-radar amplifier sys-tem was installed in the Firepond Optical ResearchFacility. The laser-radar power amplifier (LRPA) filleda large room. The pond adjacent to the facility wasused to cool the LRPA. Figure 2 shows the Firepond

FIGURE 1. The Lincoln Laboratory Millstone Hill radar site inWestford, Massachusetts. The Firepond Optical ResearchFacility is located in the left foreground.



high-power, narrowband, coherent laser-radar systemin 1981. The 10.59-µm wavelength and 1.2-m aper-ture produced a beamwidth of about 10 µrad.

Many modifications were required to achieve reli-able operation at typical peak powers of 4 to 5 kW in4-msec pulses. For example, contamination of the la-ser gas during operation of LRPA required continu-ous replacement of the laser gas.

The sharp Doppler-frequency resolution allowedresearchers to collect Doppler time intensity (DTI)measurements of satellites. In 1981, Lincoln Labora-tory successfully generated a detailed DTI plot of aslowly tumbling space object—an Agena D rocketbooster, at a slant range of 1350 km. Figure 3 illus-trates three Doppler spectra obtained with the Fire-pond laser radar from an orbiting Agena D rocketbooster. The laser radar demonstrated a capability foracquiring and monopulse-angle-tracking unen-hanced targets in low- and medium-altitude earth or-bits. Space objects were automatically tracked in fre-quency while Doppler data were recorded, and DTIplots were generated on a wide variety of targets.However, the failure to produce high-resolution,range-Doppler images and meet the average-powergoal led to an interruption of the radar program inthe early 1980s.

In 1984 the Strategic Defense Initiative Organiza-tion (SDIO) recommended the use of an orbiting la-ser-radar sensor to discriminate warheads from de-coys during the post-boost phase of an ICBM’s flight.Lincoln Laboratory responded with the Optical Dis-crimination Study, funded by DARPA, to further de-fine the requirements for laser radars for ballistic mis-sile defense. Completed in January 1985, this studyled to the Optical Discrimination Technology pro-gram in February 1985. The Laboratory resumed thehigh-power laser-radar effort with a reinforced em-phasis on high-resolution, range-Doppler imaging.

Because atmospheric CO2 (principally the com-mon isotope 12C16O2) has some narrowband absorp-tion at 10.59 µm, models of atmospheric propagationpredicted a significant nonlinear frequency dispersionand, therefore, a distortion of the wideband laser-ra-dar signal. In the new laser radar, all of the lasers useda rare form of carbon dioxide (13C16O2) and thus hadto operate in a sealed-off mode to conserve the gas.The resultant output wavelength, 11.17 µm, was not

FIGURE 2. Firepond high-power, narrowband, coherent la-ser-radar system in 1981. The building on the left housed thelaser-radar power amplifier (LRPA) and its supportingequipment; the building on the right housed the carbon di-oxide (CO2) laser amplifier.

FIGURE 3. Three successive single-pulse Doppler spectraof a tumbling Agena D rocket booster (object 3892) col-lected by the Firepond CO2 laser radar (10.59 µm) in 1981.The orbiting booster is at a slant range of 1350 km. The Dop-pler frequency resolution of the narrowband laser radar is250 Hz, which corresponds to a Doppler velocity resolutionof approximately 1.5 mm/sec. This resolution is due to theextremely high operating frequency (approximately 28,200GHz) of the laser radar.

Master- and local- oscillator lasers

CO2 laser amplifierCompressor

Heatexchanger

Laser-radarpower amplifier

–40 –20 0 20 40

Relative frequency (kHz)

~4 m

Agena Drocket booster

Tim

e



significantly absorbed or distorted by propagationthrough the atmosphere.

A master-oscillator/power-amplifier (MOPA) con-figuration was chosen for the wideband laser radar;however, a short-pulse design was used to maximizethe laser-amplifier gain. Lincoln Laboratory devel-

oped major components for this radar: the program-mable wideband waveform generator (which madethe wideband linear-FM multiple-chirp waveform),and the wideband laser receiver and analog stretchprocessor. A wideband, efficient, single-sidebandelectro-optic modulator was finally developed [6].

FIGURE 4. High-power, wideband, coherent laser-radar amplifier. The electron-beam-sustained, electric-discharge, isotopic-CO2 laser amplifier had a bandwidth of over 2 GHz with pulse energies of up to 100 J.The amplifier was developed by Rockwell International and Spectra Technologies.

FIGURE 5. The Firepond high-power, wideband, coherent laser-radar system and other electro-opticsubsystems in 1992. Several laser radars are indicated, including the high-power, wideband imagingCO2 laser radar (11.17 µm); the long-pulse, narrowband CO2 laser radar (10.59 µm); the photon-countingfrequency-doubled neodymium-doped yttrium-aluminum-garnet (Nd:YAG) laser radar (0.532 µm); andthe argon-ion-laser (0.514 µm) and ruby-laser (0.694 µm) illuminators. The optical path of the imaging-laser-radar transmitter signal is traced in red and the receiver signal path is traced in blue.

Laser gain module

Beam spatial filter assembly

Outputbeam

Inputbeam

Optical isolator

Wideband CO2laser amplifier

Master- and local-oscillator lasers

CO2 long-pulselaser amplifier

Nd:YAGlaser

Narrow-bandwidthCO2 laser amplifier

OpticalFM modulator

Argon laserReceiver

Radar controlroom

Computerbuilding

Ruby laser

Opticalisolator

Nd:YAG activetrackingreceiver

Passive sensor



The wideband imaging laser radar had a pulse-rep-etition frequency of 8 Hz with a pulse duration of 32µsec. The laser waveform consisted of multiple linear-FM chirps, each with a bandwidth of 1 GHz. Themaximum output energy achieved with the wide-band, coherent radar amplifier was 100 J/pulse (3.1MW peak power). The laser radar initially operated atan energy of 24 J/pulse, then at an energy of 40 to 60J/pulse during typical imaging experiments [7]. Fig-ure 4 illustrates the high-power wideband coherentlaser-radar amplifier; Figure 5 illustrates the Firepondhigh-power, wideband, coherent laser-radar systemand other electro-optic subsystems in 1992.

On 4 March 1990, the wideband laser radar suc-cessfully collected the first range-Doppler images ofan orbiting satellite. Laser-radar images of Sea Satel-lite (Seasat) were collected at 800 to 1000 km while avisible-light tracker performed precision angle track-ing. The plan to build a laser radar for space-objectsurveillance, originally developed in the 1972 study,was fulfilled eighteen years later.

For the next two years, wideband range-Doppler

images of satellites at ranges up to 1500 km were col-lected. Although most range-Doppler images of satel-lites are classified, Figure 6 shows a photograph andtwo range-Doppler images of the retroreflector-equipped Laser Geodynamics Satellite (LAGEOS).The range-Doppler images of a single scatterer werecollected with two different waveforms. The high-resolution waveform relied on a 1-GHz-bandwidthlinear-FM chirp, while the low-resolution image useda waveform with a bandwidth of 150 MHz.

Only twenty-five days after the laser radar first be-gan imaging operations, the Firefly sounding-rocketexperiment for the SDIO was successfully completed.This exercise involved sounding-rocket launchesfrom the NASA Wallops Island Space Flight Facilityin Virginia, on 29 March 1990 (Firefly I), and 20 Oc-tober 1990 (Firefly II). In both tests, the flight path,aimed toward the east, was projected to reach an apo-gee of 460 km at an elevation angle of 36° from Mill-stone Hill, resulting in a range of 700 km. The actualapogee and range were 462 km and 743 km, respec-tively, for the Firefly I test, and 456 km and 724 km

FIGURE 6. (a) The Laser Geodynamics Satellite (LAGEOS), a 60-cm aluminum sphere with a brass core. The satel-lite has a total mass of 406 kg and is in a near circular orbit at an altitude of approximately 5900 km. LAGEOS has 426silica retroreflectors and 4 germanium retroreflectors to serve as a laser-radar target. (b) Range-Doppler image ofthe LAGEOS satellite collected by the wideband CO2 laser radar at Firepond. The coarse image was made with a sig-nal bandwidth of 150 MHz, while the fine image was made with a bandwidth of 1 GHz. Doppler velocity resolution isapproximately 30 cm/sec. Color in the image represents relative signal amplitude. The range and Doppler sidelobesare better than 20 dB below the main lobe. The range-Doppler images are the point-target responses of the imaginglaser radar produced by different bandwidths.

Coarse image

DopplerR

ange

Incr

easi

ng s

igna

l am

plitu

de

Fine image

Doppler

(b)(a)

Ran

ge



for the Firefly II test. By utilizing angle-tracking datafor initial acquisition from the NASA C-band andHaystack X-band radars, the Firepond laser radarangle tracked the deployed target to sub-µrad preci-sion. The laser radar collected real-time, high-resolu-tion, range-Doppler images of the ejection and infla-tion of the replica decoy [8]. Figure 7 illustrates theplanned trajectory of the Firefly experiment.

After the first Firefly experiment, satellite measure-ments continued with the Raytheon narrowband,1-kW CO2 laser radar. These experiments focused onthe measurement of vibrations of the boom structureof the Laser Atmospheric Compensation Experiment(LACE) satellite for the Naval Research Laboratory.Lincoln Laboratory researchers were able to measurethe extremely low-frequency vibrations of this ret-roreflector-equipped satellite.

With the success of the Firefly experiments, theU.S. Air Force Brilliant Eyes program office wantedto investigate the use of a low-power, diode-laser-pumped Nd:YAG laser radar for ranging on rocketboosters and post-boost vehicles. This concept in-volved the use of photon-counting ranging on thetarget, which is discussed in the sidebar entitled“Photon-Counting Optical Receivers.” The Labora-tory developed a 30-mJ/pulse diode-laser-pumped,Nd:YAG laser transmitter, and the associated laser-ra-dar tracker.

A second series of flight tests designated Firebird(for Firepond bus imaging radar demonstration)demonstrated sophisticated laser-radar discrimina-tion techniques and laser-radar countermeasures. Inthese tests, the Millstone and Haystack radars wereoperated to support the Firepond laser radar in ac-

FIGURE 7. The Firefly CO2 imaging-laser-radar experiment. This experiment successfully demonstrated that an imaging laserradar could observe the deployment and inflation of a replica reentry-vehicle decoy at a range of over 700 km. The Firefly tests,held on 29 March and 20 October 1990, utilized a Terrier-Malemute sounding rocket equipped with an inflatable conical balloon.The rocket was launched from NASA Wallops Island Space Flight Facility in Virginia and observed by the Firepond CO2 laserradar in Westford, Massachusetts. The decoy was ejected as a spinning canister that inflated into a 2-m-long cone.

Terrier-Malemutesounding rocket

WallopsIsland

350Time (sec)

0 700Impact 400 km

east of WallopsIsland

Firepond

460-kmaltitude

Rocket trajectory

Firepond CO2laser radar

Laser-radar observationof the decoy

deployment process

Decoyinflated

Spin up

Coninginduced



quiring and tracking the targets. The Firebird experi-ments used a high-performance Talos-Minuteman IStage II guided booster to deploy a dozen targets to beacquired by the Millstone Hill sensors and other air-borne and ground-based sensors scattered along theU.S. east coast. Test objectives for Firebird 1 includedlaser-radar deployment discrimination based on therocket-plume interaction of the bus with decoys andother deployment dynamics. Firebird 1B addedcountermeasure-complex surface signatures, photon-counting Nd:YAG laser-radar bus tracking, and pas-sive stereo tracking by using dispersed ground andairborne sensors.

The Firebird 1 rocket was launched on 12 April1991. During the Firebird 1 experiment, theFirepond, Haystack, and Millstone Hill radars plusthe Cobra Eye aircraft sensor and infrared sensors atNASA Goddard collected data. Firebird 1B waslaunched on 13 April 1992. The following sensorsalso collected data during the flight: Utah StateUniversity’s infrared sensor located at Firepond andthe SDIO Airborne Surveillance Testbed (AST) infra-red-sensor aircraft, the Position and Velocity Extrac-tion (PAVE) Phased Array Warning System (PAWS)UHF radars in Massachusetts and Georgia, and opti-cal sensors at Malabar, Florida. Figure 8 shows the

P H O T O N - C O U N T I N G O P T I C A L R E C E I V E R S

must be much less than one dur-ing the period of observation. Aphotomultiplier gain of 106 isnot unusual and is required toovercome noise associated withthe amplifier following the pho-tomultiplier. A single photoelec-tron can produce 106 signal elec-trons. A high receiver threshold isset such that the current pulserepresenting an individual pho-ton or a rare thermally generatednoise electron originating in thephotocathode easily crosses thethreshold, while amplifier noisecurrents are orders of magnitudeless than the threshold. Thus in-dividual photons are detected.

Using very short laser pulseswith range gating allows this typeof detection to be used to per-form range measurements. As-suming a unity conversion ofphotons to photoelectrons and anonfading target, the probabilityof detecting at least one or more

photons out of three photons re-ceived is almost 95%. Three pho-tons of green light correspond toa received energy of approxi-mately 10–18 J. The false-alarmrate would depend on back-ground and dark-current levels.A conventional radar having a3-dB noise figure and requiring asingle-pulse IF signal-to-noise ra-tio of 13 dB (Pd = 95%, Pfa =1.25 × 10–5) would require about1.66 × 10–19 J per Hz of receiverbandwidth. For a microwave ra-dar receiver with a 1-MHz band-width and no additional process-ing, 1.66 × 10–13 J of energywould be required for a singlemeasurement. The performanceof a coherent laser radar operat-ing at a green wavelength (0.5µm) would require an additionalfactor of 90 in energy (the ratioof photon to phonon energies,hν/kT ) above the requirementsfor the microwave radar.

performing co-herent detection and conven-tional energy (direct) detection,special optical receivers can alsodetect individual photons undercertain circumstances. This formof detection is the ultimate in op-tical-receiver sensitivity. A pho-ton-counting detector typicallyhas extremely high gain associ-ated with the photoelectron-gen-eration process. Examples of thistype of photodetector includevery high-gain photomultipliersand very high-gain avalanchephotodiodes. Photomultipliertubes have the added advantageof a minimal noise figure associ-ated with the gain process.

The conditions for photoncounting require a minimal opti-cal background level and mini-mal photocathode dark current;essentially, the mean number ofbackground photoelectrons andthermally generated electrons



Firebird 1B flight-test scenario. Although the Firebirdtest data remain classified, the flight tests gave excel-lent results and completed the experimental investiga-tion of the laser-radar discrimination techniquesdescribed in the 1984 SDIO study and other optical-discrimination studies [9]. In October 1993, high-power laser-radar research at Lincoln Laboratoryended with the completion of the Optical Discrimi-nation Technology program.

Tactical Laser Radars—The Infrared AirborneRadar System: 1975 to 1994

In 1975, Robert J. Keyes of Lincoln Laboratory led astudy of the potential utility of a coherent CO2 laserradar for ground-target surveillance, acquisition, andfire control. It was envisioned that a ground-attackaircraft such as the A-10 would use this system, nightor day, for very low-altitude (100 m or less) attack, to

FIGURE 8. Firebird (Firepond bus imaging radar demonstration) 1B flight-test scenario. TheFirebird test involved deployment of numerous passive infrared, laser-radar, and microwave-radar decoys and calibration spheres. It is noteworthy that the favorable weather along the en-tire east coast of the United States permitted the collection of optical, infrared, and laser-radardata from Florida to Massachusetts, and from high above West Virginia to high above the At-lantic Ocean south of Rhode Island. UHF and microwave data were collected at sites in Mas-sachusetts, Virginia, and Georgia. Data were successfully collected on the deployment dy-namics and signatures of several classes of reentry-vehicle decoys.

Haystack

Firepond

Millstone

Millstone Hill:plume interactions,infrared signature

measurements, andreplica dynamics

Range

NASA Wallops Island Space Flight Facility:tracking and telemetry data

142 km

Alti

tude

423 km

NASAGoddard

Plumedynamics

Plumedynamics

Replicaexperiment

Cobra Eye:closely spaced tracking,

infrared signaturemeasurements

Airborne SurveillanceTestbed: infrared signature

measurements



insure that the laser radar would be below clouds andmost weather. The extreme low-altitude environmentfor the platform would also frustrate certain classes ofair-defense systems. The system was sized for a 10-Wtransmitter developed by Stephen Marcus [7] and a15-cm aperture. The follow-on study by Richard J.Becherer [10] added further refinements and require-ments. An internal research program on tactical lasersbegan in 1976.

A number of singular technologies developed inthe course of the program include coherent detectorarrays, binary optics that provided local-oscillatorbeams with the proper phase and amplitude on thecoherent detector arrays [11], and surface-acoustic-wave devices for Doppler processing. In addition, im-age-processing schemes automated the understandingof the multiple data returns available from a laser ra-dar, especially when operated in synchronism with apassive infrared detection system that provides a mea-surement of target and background temperatures.

A development goal to increase the image framerate led to the specification of a twelve-element, co-herent HgCdTe detector array to provide sufficientautonomous search and identification capability forthe laser radar. Subsequently, a passive infrared arraywas placed in the same dewar with the active array,sharing the same optics train so that near simulta-neous measurements could be made of target andbackground temperatures to assist in target detectionand identification, as described in a report by RobertC. Harney [12].

Under the technical direction of Robert J. Hulland Theodore M. Quist, the Infrared Airborne Radar

(IRAR) development proceeded through severalphases: technology development and laboratory dem-onstration, operation of the laboratory system in atruck from which targets of interest could be observedat various military locations, and, finally, flight test[13]. The truck-transportable system involving a laserradar and passive infrared sensors is shown in Figure9. The laser-radar and passive infrared systems wereboresighted and pixel-registered with the data streamsdigitally recorded. Figures 10 and 11 show imagesfrom the transportable system. Figure 10 displays la-ser-radar range images of a tank and truck at a range

FIGURE 9. Transportable multisensor measurement systemconsisting of a coherent CO2 laser radar with a boresightedand pixel-registered 8-to-12-µm passive infrared sensor anddigital recording system.

FIGURE 10. Coherent laser-radar, color-coded intensity images of tank (left) and truck (right)taken with the transportable system at a range of 2.7 km.



of 2.7 km. Figure 11 is a Doppler-velocity image of ahelicopter executing a rotational maneuver. TheGulfstream G-1 airborne multisensor aircraft, whichcarries the IRAR, is shown in Figure 12. All of the sys-tems utilized an optical turret attached to the under-side of the aircraft for beam scanning with the majorelectronic and recording systems within the aircraftfuselage. In Figure 13 hangars with aircraft and otherbuildings and fences in the foreground are imaged inpassive infrared, laser range, and laser intensity. Theseimages were taken from the airborne system duringearly ground tests. In the passive infrared image,white is warm and black is cold. In the laser-range im-age, objects farthest away are in yellow and nearest inblue. In the laser-intensity image, black and white de-note the absence or presence, respectively, of strongsignals. In Figure 14, IRAR measurements of theBourne Bridge by the forward-looking laser radar areshown in color to denote the relative ranges of bridge

components. The inset image shows that, since abso-lute range is being measured, the laser-radar rangeimage can be transformed to views other than the oneat which the image was taken. Laser-radar range dataalso allow similar transformations on the passiveinfrared image. This experiment was a seminal inves-tigation of what later was to be called multidimen-sional image processing. The Doppler-image capabil-ity of the airborne forward-looking laser radar isdemonstrated in Figure 15, which shows cars travel-ing on Interstate 495 near Boston.

FIGURE 11. Doppler-velocity image of a UH-1 helicopter ex-ecuting a rotational maneuver. The image is an angle-angle-Doppler-intensity image collected by the truck-transportableCO2 laser radar. The Doppler shift of each of the approxi-mately 16,000 pixels in the image was extracted by a surface-acoustic-wave processor at a frame rate of 1 Hz. Velocity ismapped into color as shown. Laser radars, by virtue of theirvery short wavelengths, simultaneously permit high angularresolution (equivalent to human vision in this image) andhigh Doppler resolution (approximately 1 m/sec in this im-age). The ability to sense moving parts on a vehicle providesa powerful means to discriminate targets from clutter.

FIGURE 12. Gulfstream G-1 aircraft with the Infrared Air-borne Radar (IRAR) optical aperture located in the ventralfairing on the aircraft. The IRAR test bed carried a wide vari-ety of active and passive sensors over a ten-year period.

FIGURE 13. Simultaneous passive infrared, CO2 laser-range,and laser-intensity images of Hanscom Air Force Base han-gars and buildings, collected by the IRAR system. The rangeaccuracy of the range image is 1 m.

70350

Doppler velocity (mph)

Passive

Range

Intensity



After a decade of research, development, and test-ing of a forward-looking multidimensional laser radarbased on a CO2 laser and active and passive HgCdTedetector arrays, sponsors at DARPA and other groupsbecame interested in laser-radar operation at verticalor near vertical viewing conditions relative to theground. Two potential applications were to use highangle and range resolution to penetrate foliage and

camouflage and to use laser radars as acquisition andguidance sensors for vertical attack by smart weapons.The forward-looking sensor suite was augmented by ahigh-range-resolution millimeter-wave radar bore-sighted to the optics for further multidimensional in-vestigations. Downlooking GaAs and Nd:YAG laserradars were developed that could operate at adownlooking range of about 100 m and have a cross-range spatial resolution of 15 cm combined with arange precision of 15 cm. The first high-resolutiondownlooking sensor, developed by Perkin Elmer, used

FIGURE 14. IRAR CO2 (10.59 µm) laser-radar images of theBourne Bridge, which spans Cape Cod Canal in Massachu-setts. Range to each picture element is coded in color. Thedata collected in the original oblique view are transformedinto an overhead view, as shown in the inset image. Thisview may be useful for missile seekers that use terrain fea-tures for targeting.

FIGURE 15. Angle-angle-Doppler image taken with theIRAR in flight over Interstate 495 near Boston, Massachu-setts. Velocity is represented by color. Automobiles areclearly visible on the highway. The background can be sup-pressed by simple velocity thresholding.

FIGURE 16. (a) Visible-light photograph and (b) GaAs (0.85 µm) laser-radar angle-angle-range image of a tank concealed by acamouflage net. The laser radar utilizes a high-accuracy, sinusoidal, amplitude-modulated waveform while observing the tankin a downlooking scenario. The camouflage net was readily gated out of the image to leave the tank image. The tracks left by thetank in the soil are also visible in the laser-radar image.

Transformation of 3-D rangeimagery

Overhead view

Original forward view

605040

0 mph

50

60

(a) (b)



GaAs laser-diode technology that resulted in a relativerange precision of 15 cm and a range-ambiguity inter-val of 30 m. The capability of this system is demon-strated in Figure 16, in which a tank can be seenclearly under a visible-light camouflage net. A Lin-coln Laboratory sensor system called the Multispec-tral Active/Passive Sensor (MAPS) was subsequentlydeveloped that also incorporated a 10.59-µm-wave-length relative range-measuring laser radar and an8-to-12-µm passive infrared imager. These sensorswere all boresighted to provide near simultaneousmeasurements of targets and backgrounds. Figure 17shows an example of the data obtained by using a shipas a target.

As noted previously, the GaAs system had highrelative range precision, but the range-ambiguity in-terval presented difficulties in areas where it was re-quired to look down through tall trees. An absolute-ranging laser radar based on the Lincoln LaboratoryNd:YAG microchip laser was developed to replacethe GaAs radar in the MAPS. With a range precisionof 15 cm, development of a real-time processor wasthe pacing technology. Preliminary processing hasdemonstrated that height thresholding with this sys-tem can be used to search for objects hidden undertrees where there are holes in the tree cover.

In 1994, near the termination of the program,laboratory investigations were also made of the use of

FIGURE 17. Multiple, simultaneous downlooking images of the fast frigate USS Connole (FF-1056) collected by using the Multi-spectral Active/Passive Sensor (MAPS) system. The MAPS system includes a GaAs laser radar (0.85 µm), a coherent CO2 la-ser radar (10.59 µm), and an infrared imager (8-to-12 µm) mounted in a Gulfstream G-1 aircraft. Registered image pixel range,scene reflectivity at 10.59 µm and 0.85 µm, and apparent temperature provide unique data for target classifier algorithms.

GaAs laser radar (0.85 m)

Passive infraredimager (8–12 m)

Intensity

Range

CO2 laser radar (10.59 m)µ

µ

µ



the microchip laser for development of an opticalsynthetic aperture. This technology holds promise forspace-based surveillance of space objects, where spa-tial resolution would be independent of range.

Summary

The CO2 laser-radar research that took place from1966 to 1994 developed many novel technologies,components, and techniques that have yet to be fullyutilized by the Department of Defense. Other pro-grams at the Laboratory have focused on the use ofruby and Nd:YAG solid state lasers, and new applica-tions for CO2 and solid state lasers continue to be in-vestigated for a number of applications. However, theoverall goal of the Laboratory program was to encour-age the Department of Defense to exploit the greatvalue of coherent laser radar for a variety of applica-tions, and this goal was successfully realized. Manytechnical firsts were achieved during this time. Thework stands as a tribute to the ingenuity, hard work,and dedication of the people at Lincoln Laboratorywho defined the field of laser radar.

R E F E R E N C E S1. C.K.N. Patel, “Continuous-Wave Laser Action on Vibra-

tional-Rotational Transitions of CO2,” Phys. Rev. 136 (5A),1964, pp. A 1187–A 1193.

2. C. Freed, “Design and Short-Term Stability of Single-Fre-quency CO2 Lasers,” IEEE J. Quantum Electron. 4 (6), 1968,pp. 404–408.

3. D.L. Spears, “Planar HgCdTe Quadrantal Heterodyne Arrayswith GHz Response at 10.6 µm,” Infrared Phys. 17 (1), 1977,pp. 5–8.

4. C. Freed, A.H.M. Ross, and R.G. O’Donnell, “Determinationof Laser Line Frequencies and Vibrational-Rotational Con-stants of the 12C18O2, 13C16O2, and 13C18O2 Isotopes fromMeasurements of CW Beat Frequencies with Fast HgCdTePhotodiodes and Microwave Frequency Counters,” J. Mol.Spectrosc. 49 (3), 1974, pp. 439–453.

5. L.J. Sullivan, “Infrared Coherent Radar,” SPIE 227, 1980, pp.148–161.

6. N.W. Harris, J.G. Grimm, and R.S. Eng, “Wideband Long-Pulse Operation of an Efficient Electro-Optic Modulator at10.6 µm,” Opt. Lett. 15 (20), 1990, pp. 1156–1158.

7. I. Melngailis, W.E. Keicher, C. Freed, S. Marcus, B.E.Edwards, A. Sanchez, T.Y. Fan, and D.L. Spears, “Laser RadarComponent Technology,” Proc. IEEE 84 (2), 1996, pp. 227–267.

8. “ ‘Firefly’ Laser Experiment Successful in Measuring InflatableDecoy Motion,” Aviat. Week Space Technol. 132, 23 Apr. 1990,pp. 75.

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. joined Lincoln Laboratory in1964 after receiving B.S. andM.S. degrees in physics fromPennsylvania State University.He became an assistant groupleader in 1969 and then groupleader of the Opto-RadarSystems group in 1971, wherehe served until 1995. Duringthis period, laser-radar andmultidimensional systemsconstituted a major portion ofthe group’s research efforts. In1997 he became a member ofthe senior staff in the Aero-space division, where he isinvolved with a variety ofprograms and Department ofDefense advisory panels.

. is the leader of the OpticalCommunications Technologygroup at Lincoln Laboratory,where he is involved in analogand digital optical communi-cations research. From 1993 to2000 he was an associategroup leader in the TacticalDefense Systems and AirDefense Systems groups,evaluating air defense systemconcepts. From 1985 to 1993he led the Laser Radar Mea-surements group, which builtand operated the Firepondlaser-radar system. His earlierassignments included servingas assistant leader of the Opto-Radar Systems group (1983–1985) and member of thetechnical staff (1975–1983) inthe Opto-Radar Systemsgroup. His assignments at theLaboratory have includedresearch in active and passiveelectro-optic sensors as well asmillimeter and microwaveradar and communicationsystems. Prior to joiningLincoln Laboratory, he workedon spatial light modulators atCBS Laboratories. He earnedB.S. (1969), M.S. (1970), andPh.D. (1974) degrees in elec-trical engineering at Carnegie-Mellon University. He is asenior member of the IEEEand a member of the OpticalSociety of America, the Asso-ciation of Old Crows, and EtaKappa Nu.

development of coherent laser radar at lincoln laboratory · • gschwendtner and keicher...

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