Three-Dimensional Imaging Laser Radars with Geiger-Mode Avalanche Photodiode Arrays

• ALBOTA ET AL.Three-Dimensional Imaging Laser Radars with Geiger-Mode Avalanche Photodiode Arrays

VOLUME 13, NUMBER 2, 2002 LINCOLN LABORATORY JOURNAL 351

L three-di-mensional (3D) laser radars (ladars) with at-tractive features that include capture of an

entire 3D image with a single laser pulse, image reso-lution of tens of thousands of pixels, range resolutionof a few centimeters, and small size. The laser tech-nology for the ladar is based on diode-pumped solid-state microchip lasers that are passively Q-switched[1]. The detector technology is based on arrays of ava-lanche photodiodes (APDs) operating in Geigermode, with integrated timing circuitry for each pixel

[2]. (In Geiger mode, an electron-hole pair generatedby a single photon initiates an avalanche process thatresults in a large current pulse.)

Figure 1 shows the 3D ladar concept. Light from asingle laser pulse is diverged to illuminate the entirescene of interest. An auxiliary detector marks the timeat which the laser pulse is emitted. The reflected lightis imaged onto a two-dimensional array of detectors.Rather than measuring intensity, as in a normal cam-era, these detectors measure the time of flight of thereflected light. The time of flight is proportional to

Three-Dimensional ImagingLaser Radars with Geiger-ModeAvalanche Photodiode ArraysMarius A. Albota, Brian F. Aull, Daniel G. Fouche, Richard M. Heinrichs,David G. Kocher, Richard M. Marino, James G. Mooney, Nathan R. Newbury,Michael E. O’Brien, Brian E. Player, Berton C. Willard, and John J. Zayhowski

■ Lincoln Laboratory is developing three-dimensional (3D) imaging laserradars (ladars) that feature few-centimeter range resolution, thousands of pixels,and capture of an entire 3D image on a single laser pulse. The transmittersare based on diode-pumped solid-state microchip lasers that are passivelyQ-switched. The receivers are based on microelectronic arrays of avalanchephotodiodes (APDs) operating in Geiger (photon counting) mode, withintegrated timing circuitry for each pixel. Ladars using these laser and detectortechnologies offer not only high performance, but also compact packaging,ruggedness, and high efficiency. Potential applications include advancedinterceptor seekers, combat identification, navigation of autonomous robots andvehicles, surveillance, and topography. Early experiments with a single detectorshowed the feasibility of using a Geiger-mode APD for 3D imaging.Subsequently, we constructed a first-generation (“brassboard”) 3D ladarincorporating a 4 × 4 array of Geiger-mode APDs and a microchip laser.Experiments with the brassboard and a low-light-level camera showed that insome important situations targets can be better detected and identified from3D ladar images than from intensity images. We present results from both thesingle-detector and brassboard experiments.


352 LINCOLN LABORATORY JOURNAL VOLUME 13, NUMBER 2, 2002

the distance between the point of reflection on thetarget and the sensor system. With range measuredfor each pixel, the ladar produces a 3D (angle-angle-range) image.

Discrimination and aimpoint selection for ad-vanced interceptor seekers are potential applicationsfor this type of ladar. The ladar is also well suited forcombat identification, navigation of autonomous ve-hicles and robots, surveillance, and topography. Inaddition, as demonstrated by the 3D images shown inthis article, the ladar can be used to find targets hid-den by camouflage nets and foliage.

In this article, we provide a brief history of this ef-fort and a description of more recent laser and detec-tor technologies; detailed information is availableelsewhere [1–13]. Next we describe a transportable“brassboard,” or first-generation, 3D ladar that incor-porates a microchip laser and a Geiger-mode APD ar-ray, and we show a few example images. Last, we showbrassboard images of vehicles hidden by a camouflagenet or trees. The 3D images are compared with thosefrom a low-light-level intensity camera.

Advanced Discriminating Interceptor Seekers

In 1990, the Ballistic Missile Defense Organization(BMDO) initiated a series of programs to investigatethe utility and feasibility of small ladars, which couldbe integral to future advanced discriminating inter-ceptor seekers. The additional information providedby ladar technology was thought necessary to dis-criminate a warhead target from advanced and so-phisticated decoys. The goal was to design a seekersystem that featured both a passive infrared imagingsensor with a wide field of view and a high-resolutionladar sensor with a narrow field of view. A variety ofladar architectures and designs were investigated withcompeting teams. These included coherent mode-locked pulse-train ladars that could measure high-resolution range-Doppler images. The mode-lockedwaveform was investigated with both CO2 (carbondioxide) gas waveguide and solid-state Nd:YAG(neodymium-doped yttrium aluminum garnet) lasersoperating at 10.6-µm and 1.06-µm wavelengths, re-spectively. Also studied was a direct-detection system

FIGURE 1. Concept for three-dimensional (3D) laser radar (ladar) that creates a 3D (angle-angle-range) im-age from a single laser pulse. The architecture for this active sensor is simple. A short-pulse laser is used toflood-illuminate the target scene. The receiver collects the backscattered light and directs an image of thetarget scene onto a two-dimensional array of avalanche photodiodes (APDs). Each pixel of the array mea-sures the range to its associated target surface element by measuring the laser pulse two-way time of flight.

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that illuminated the target scene with a short-pulse,solid-state, frequency-doubled Nd:YAG laser andsplit the receive signal into two channels. One chan-nel was for a visible charge-coupled device (CCD)imager for high-resolution angle-angle-intensity data;the other was for a single fast APD detector to mea-sure pulse-height intensity and range to target viatime of flight.

We provided an independent analysis and assess-ment of the expected performance of these variousladar designs. Both coherent heterodyne and direct-detection sensor designs could provide high-resolu-tion range data. Like the wideband coherent CO2 la-dar demonstrated earlier at Lincoln Laboratory’sFirepond Infrared Research Facility in Westford,Massachusetts [3], the coherent heterodyne architec-tures also provided exquisite Doppler or velocity in-formation. But these architectures were somewhatmore complicated and heavier than the direct-detec-tion sensor designs, which could also provide angle-angle-intensity data. One of the most stressing re-quirements for this advanced seeker was to makediscrimination-supporting measurements as early aspossible to allow enough time to divert and interceptthe selected target. The sensor’s maximum effectiveranges were largely determined by the need to fitwithin the strict weight, volume, and size limits of theladar design.

During these studies, it became clear that an alter-nate architecture has distinct merits. Our analysisshowed that a direct-detection ladar that transmitsshort pulses and receives the backscattered signal byusing a two-dimensional focal-plane array of photon-counting Geiger-mode APDs can measure the 3Dspatial structure of a remote object [5]. The advan-tages of this design include the simplicity of a single-channel direct-detection receiver and the potential forextremely high sensitivity. In fact, when the effects ofcoherent interference (intensity variations due tospeckle) are accounted for, photon counting with in-coherent direct-detection systems can have superiordetection statistics over those of coherent heterodynesystems. Furthermore, when APDs are operated inGeiger mode, they behave like binary or digital de-vices, and subsequent signal processing can be greatlysimplified with this photon-to-digital converter.

Our analysis also showed that for a given laser-power-times-aperture-area product, 3D imaging withGeiger-mode APD arrays can provide important dis-crimination information at ranges greater than theother approaches being evaluated. Subsequent labora-tory experiments verified the feasibility of high-reso-lution ranging with photon-counting Geiger-modeAPD arrays. Lincoln Laboratory later embarked on aprogram to develop 4 × 4 and 32 × 32 arrays of sili-con (Si) Geiger-mode detectors. Recently, these de-tectors have been successfully bonded to arrays ofdigital complementary-metal-oxide-semiconductor(CMOS) timing circuits. This unique detector tech-nology is described in more detail in a companion ar-ticle in this issue by B.F. Aull et al., entitled “Geiger-Mode Avalanche Photodiodes for Three-DimensionalImaging” [2]. While this sensor technology still hasapplication to advanced discriminating interceptorseekers, it also shows great promise for many othermilitary and commercial applications.

Initial Feasibility Experiments

The idea of operating a photodiode with a bias volt-age above its breakdown voltage and counting pho-tons differs significantly from linear sensor concepts.Consequently, the first experiments were designed tocompare linear versus Geiger-mode operation of asingle avalanche photodiode. Figure 2 shows a dual-mode photon-counting detector device that was de-veloped by EG&G Canada for this purpose. It in-cludes an active quenching circuit to limit the currentof the Geiger-mode avalanche and to reset the detec-tor within 15 nsec. Figure 3 shows an example of atransistor-transistor logic (TTL) pulse triggered bythe Geiger-mode detection of a single photon. Al-though the pulse width is approximately 10 nsec, jit-ter in the leading edge is what determines the rangingprecision and effective resolution. Laboratory experi-ments demonstrated range precision of under 3 cm.

We developed a laboratory experiment to investi-gate the performance of single-photon ranging and tovalidate the concept of 3D imaging with Geiger-mode APDs. Test were performed with various SiGeiger-mode detector arrays from Radiation Moni-toring Devices, Inc. The setup included a LincolnLaboratory–developed short-pulse (<1 nsec), fre-



jects begin to emerge in the image. Even with ten or ahundred times as many received photons per frame, itis difficult to recognize any of the objects in the inten-sity image.

These early experiments verified the utility of thephoton-counting Geiger-mode APD as a unique

FIGURE 3. A TTL pulse generated by the Geiger-mode de-tection of a single photon. Jitter in the leading edge of thepulse width determines the range measurement precision.While the TTL pulse width is approximately 10 nsec fullwidth half maximum (FWHM), the pulse-to-pulse jitter in theleading edge is less than 200 psec.

quency-doubled, Nd:YAG, microchip laser transmit-ting at 532-nm wavelength. The laser transmitter wasused to flood-illuminate the scene, while the receiverfield of view was scanned to generate a 32 × 32-pixelimage of a variety of test objects. Figure 4 shows a dia-gram of the laboratory setup. Figure 5 shows example3D ladar images of a cone with two spheres in thebackground. These are false color images, with thered end of the spectrum denoting the closest points inthe scene and the blue end the most distant points.Figure 5 also shows image segmentation based onrange. A simple thresholding operation separates fore-ground objects from background objects. Accom-plishing such segmentation with a conventional in-tensity image would be much more difficult,especially if reflectivity contrast was low.

Figure 6 compares angle-angle-intensity imagesfrom a low-noise CCD with 3D spatial angle-angle-range images, all based on the scene consisting of acone and two spheres shown in Figure 5. Both sets ofimages were generated by using the same illuminationlaser. The CCD pixels were binned to make a 32 ×32-pixel image to facilitate a fair comparison. Notethat even with an estimated 700 received photons perframe (about 0.7 photon per pixel), recognizable ob-

FIGURE 2. An early photon-counting detector system used to compare linear versus Geiger-mode operation of a single silicon (Si) APD. At left is a dual-mode single-element detectormodule, which includes an active-quenching circuit that can reset the Geiger-mode detectorwithin 15 nsec. The detector could be operated in either a linear or a Geiger mode. In Geigermode, the output of the APD was used to trigger a digital transistor-transistor-logic (TTL)pulse. At right is a control module used to set and control the detector operating conditions.

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technology for efficient 3D imaging, and indicatedthat this method of 3D imaging is an enabling tech-nology for remote sensing and automatic targetrecognition [8].

Microchip Lasers

Passively Q-switched microchip lasers are constructedby diffusion-bonding a short piece of laser-gain me-dium, in this case Nd:YAG, to a similarly short pieceof saturable absorber, in this case chromium-dopedYAG (Cr4+:YAG) [9, 10]. The pump-side face of thecomposite structure is coated to transmit the pumplight (808 nm from a diode laser) and reflect the laserlight (1064 nm). The output face, which is coated tobe partially reflecting at the lasing frequency, provides

FIGURE 4. Diagram of setup that proved the feasibility of photon-counting 3D imaging. A Lincoln Laboratory–built microchip laser was used to flood-illuminate an indoor scene with short pulses of less than 1 nsec FWHM.Attenuating filters reduced the laser intensity so that the detector was receiving on average less than one photonper pulse. The laser-pulse transmit time was measured with a second optical detector and used to start the high-precision time-interval counter. Two single-axis scan mirrors were used to raster-scan the receive field of viewover the target scene. One laser pulse was transmitted for each image pixel or receiver pointing location. Anarrowband spectral filter reduced background light reaching the detector. We performed separate tests withthree different APD detector boards: the dual-mode detector module (MIT-1), a commercial Single Photon De-tection Module (SPCM) from EG&G Canada, and a single element of an experimental 4 × 4 detector array fromRadiation Monitoring Devices, Inc. A computer was used to control and interface to the time-interval counter(Model SR620 from Stanford Research) through a general-purpose interface bus (GPIB, or IEEE-488 standard).

both feedback and laser output. The devices, typicallya few millimeters long, are longitudinally pumpedwith the output of a multimode optical fiber. Thepump diode is at the opposite end of the fiber. Figure7 shows the simplest embodiment of the laser. In op-eration, the intracavity saturable absorber preventsthe onset of lasing until the average inversion densitywithin the cavity reaches a critical threshold. At thatpoint, the onset of lasing produces a high intracavityoptical field that saturates the absorber, resulting in aQ-switched output pulse a few hundred picosecondslong. The pulse repetition rate depends on pumppower and is typically several thousand per second.The output beam has a transform-limited frequencyspectrum, fundamental transverse mode with diffrac-

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tion-limited divergence, and linear polarization.For high pulse energies, the input and output faces

of the composite structure shown in Figure 7 arecapped with short pieces of undoped YAG in order toincrease the damage threshold [11]. In separate de-vices, we have obtained 250 µJ per pulse at 1000pulses/sec and 110 µJ per pulse at 5000 pulses/sec.Both lasers had 380-psec pulse widths. If higher en-ergy is required at the same short pulse width, theoutput from such lasers can be coupled to opticalamplifiers.

The high intensity of the output beam allows itsfrequency to be doubled by simply placing a smallpiece of titanate phosphate (KTP) near its outputface. The energy efficiency of frequency doubling tothe green (532 nm) is typically 50 to 60% [9–11]. Ifdesired, a second nonlinear crystal adjacent to thefirst can convert the green light to ultraviolet at 355or 266 nm [12], with an expected energy efficiency ofabout 25%. The addition of a small lens ahead of thesecond crystal can improve green-to-ultraviolet en-ergy efficiency to about 50%.

Arrays of Geiger-Mode APDs Integrated withCMOS Timing Circuitry

The basic structure of the detector arrays [3, 6, 7],shown in Figure 8, consists of a two-dimensional ar-ray of APDs bonded to a commensurate array of tim-ing electronics. The APDs are fabricated in Si andhave a circular active area with a diameter of 30 to 50µm and a pitch of 100 µm. The pitch is determinedby area requirements of the timing electronics. TheAPDs are biased to operate in Geiger mode, in whichan electron-hole pair generated by a single photoninitiates an avalanche process that results in a largecurrent pulse. This fast, high-amplitude current pulsetriggers high-precision timing circuitry.

APD arrays with up to 32 × 32 detectors have beenproduced in several fabrication runs at the LincolnLaboratory Microelectronics Laboratory. Measure-ments show that the 32 × 32 APDs have crosstalk ofless than 1%, timing jitter of less than 150 psec, and40% detection probability at 532 nm for uncoated,front-illuminated devices. Detection probability ofabout 80% is expected for optimally designed, anti-reflection-coated, backside-illuminated devices. The

FIGURE 5. 3D ladar images of a cone and two spheres,taken in the laboratory by transmitting multiple flood-illumi-nating laser pulses while scanning the field of view of asingle commercial APD operated in Geiger mode. The lowertwo images show how a simple thresholding operation ap-plied to the measured range data can be used to segmentthe image, excluding either the more distant objects or thecloser ones.

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measured dark-current rate of about 10 counts permsec is negligible for typical 3D-ladar applications;moreover, we expect to reduce it to about 1 count permsec with a different material-processing technique.A detailed description of these detector developmentsappears in the companion article by Aull et al.

Figure 9 is a block diagram of a 17-bit, single-pixeltiming circuit. A common clock broadcast to all thepixels controls timing. The clock runs a feedback shiftregister that counts through a predetermined se-quence. When the APD fires, the resulting signal trig-

gers a latch, which stops the timing register. The dataare then read out from each of the pixels in serial fash-ion. Also read out are two vernier bits representingthe phases φ1 of the master clock and φ2 of a second-ary clock, which is delayed in phase by 90° to providehigher temporal resolution. The first-generation de-vices produced in a 0.35-µm MOSIS CMOS processshowed complete end-to-end operation down to 0.7-nsec timing resolution. The second-generation de-vices are expected to have resolution of 0.5 nsec orbetter [3, 6, 7].

FIGURE 6. Comparison of angle-angle-intensity images (left) from a low-noise charge-coupled device (CCD) with spatialangle-angle-range images (right) from a 3D Geiger-mode APD, all of the same objects imaged in Figure 5. All images were ob-tained by active illumination of the scene by a laser, with the laser-pulse energy varied. The total number of detected photons inall 1024 pixels is indicated above each image. Note how much easier it is to recognize objects in the 3D APD images, even whenthe illumination is much less. All images have been normalized to identical quantum efficiencies.

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One of our major challenges is bonding the APDarray to the timing-circuit array. We have developed atechnique that we call bridge bonding. The CMOSchip is epoxied to the APD wafer, then sloped vias areetched through the epoxy and metallized to make anelectrical connection between the APD and the tim-ing circuit. An intermediate step, done after epoxyingand before etching, is to thin the substrate of theAPD wafer. Thinning enables the bridge-bondingprocess and is necessary to achieve a high detectionprobability, because the substrate is opaque to visibleand near-infrared light. We have recently completedseveral integrated 32 × 32 APD/CMOS devices, and

have developed working 32 × 32 arrays of detectorsintegrated with timing circuits. Scaling up to largerarrays should be straightforward.

3D-Ladar Brassboard

Lincoln Laboratory developed a transportable 3D-ladar system. This first-generation system, called abrassboard, was built to test the interaction of thevarious ladar components and to gather 3D images offull-scale targets for subsequent analysis.

Figure 10 shows a diagram of the optical layout ofthe brassboard. The pulsewidth is 380 psec. The 532-nm microchip laser emits 30 µJ per pulse at 1000

FIGURE 7. Simplest embodiment of passively Q-switched microchip laser. The microchip laser is attached to the end of anoptical fiber, through which it is pumped with an 808-nm diode laser. The Nd:YAG (neodymium-doped yttrium aluminumgarnet) crystal acts as the gain medium. The microchip laser is passively Q-switched by the saturable absorber of chro-mium-doped YAG (Cr4+:YAG).

FIGURE 8. Basic structure of focal-plane array consisting of Geiger-mode APDs bonded to complemen-tary-metal-oxide-semiconductor (CMOS) timing circuitry. A photon of energy hν is absorbed in the APDactive area. The gain of the APD resulting from the electron avalanche is great enough that the detectorgenerates a pulse that can directly trigger the 3.3-V CMOS circuitry. No analog-to-digital converter isneeded. A digital logic latch is used to stop a digital timing register for each pixel. The time of flight is re-corded in the digital value of the timing register.

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pulses/sec. The Geiger-mode APD detector array is a4 × 4 prototype. The timing electronics are external(not integrated) and have a least-significant bit of 75psec. Separate time-to-digital converters measure thetimes of the outgoing laser pulse and of the APD cur-rent pulse. We have measured single-pixel range pre-cisions of 2.0 cm at high detection probabilities. The4 × 4 array is scanned across a target to generate im-ages up to 128 × 128 pixels. The frame rates for 32 ×32 and 128 × 128 images are 4.5 per sec and 0.6 persec, respectively. The adjustable range gate (i.e., therecording interval beginning when the APDs areoverbiased to Geiger mode) is typically 100 nsec andstarts at a selectable time after the laser pulse is de-tected in the transmit optical path. The entire systemfits into a van and can be taken to field sites.

To date, a variety of static and dynamic images ofpeople, cars, trucks, planes, boats, trees, wires, foliage,and other objects have been collected with the brass-board. Figures 11 through 13 show examples of these

FIGURE 9. Block diagram of single-pixel timing circuit with17-bit resolution. A common clock broadcast to all the pixelscontrols timing. The clock runs a feedback shift register thatcounts through a predetermined sequence that uses a 15-bitcounter. When the APD fires, the resulting signal triggers alatch, which stops the timing register. Data are then read outfrom each of the pixels in serial fashion. Also read out aretwo vernier bits representing phases φ1 of the master clockand φ2 of a secondary clock, which is delayed in phase by 90°to provide higher temporal resolution.

FIGURE 10. Diagram of the optical layout of the brassboard developed to test the 3D-ladar con-cept. A Lincoln Laboratory–built microchip laser transmits short pulses to a target scene. The la-ser beamwidth is matched to the instantaneous field of view of a 4 × 4 Geiger-mode APD arraywith 100-µm pitch and 30-µm active diameter. Two single-axis scan mirrors raster the 4 × 4 instan-taneous field of view over the target field of regard to generate images with up to 128 × 128 pixels.The outputs of the sixteen detectors are recorded for each laser pulse. A narrowband (NB) filterreduces background light falling on the detector array. A polarizing beamsplitter serves as atransmit-receive switch. Infrared and visible-based cameras are auxiliary sensors.

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FIGURE 11. Image of a Chevrolet Astro van obtained from the 3D-la-dar brassboard. This is a single 128 × 128-pixel image recorded withthe ladar looking at the front and right side of the van from a distanceof 60 m. In the upper left is a 3D model rendered from the angle-angle-range data. The other three renditions are point clouds thathave been computer-rotated to view the van from different aspects.In a point cloud, each pixel is assigned a point with x- and y-coordi-nates corresponding to the pixel position in the array and a z-coordi-nate corresponding to its range. The points, colored according torange, are then projected onto a plane representing the viewingscreen. Rotating the image in software better reveals shapes, sizes,and relative positions of different parts of the van.

FIGURE 12. 3D image of a Ford Bronco with overhead wires shownafter two different rotations. The 3D position of the wires with re-spect to the Bronco and to the brassboard can be determined fromthe angle-angle-range data.



images, which were all recorded in the daytime from asingle viewing point. The 3D quality of the images,which is difficult to appreciate from these 2D pro-jected renderings, is enhanced when they are rotatedin software and displayed dynamically on a monitor.With the 3D data, we can determine the separation ofthe headrests from the back door of the van of Figure11, the relative positions of the overhead wires in Fig-ure 12, and the 3D structure of trees in Figure 13.

Comparison of 3D and Intensity Images

As shown in Figure 6, targets can sometimes be betterrecognized from 3D images than from intensity im-ages. This was demonstrated further in a series ofmeasurements using the 3D-ladar brassboard and alow-noise CCD camera, both looking at the samescene. For all of the results presented here, the sensorswere co-located and had the same field of view, andboth provided image frames composed of 128 × 128pixels.

The Lincoln Laboratory–built CCD camera hasvery low noise, only five to six electrons root meansquare (rms) per pixel at our operating conditions. Itssignal is digitized to 12 bits, with the least-significantbit corresponding to fewer than 5 electrons. For re-cordings of the camera images, the scene was illumi-nated by a 1-W, continuous-wave (CW), 808-nm di-ode laser. At this wavelength, the quantum efficiencyof the camera is 55%.

For camera and ladar images that are presentedlater, we calculated the average number of photoelec-

trons created per pixel per frame. For the camera, weused the measured calibration factor between numberof photoelectrons and digital counts. For the ladar, wedetermined the detector firing rate for a given pixeland converted that to average number of photoelec-trons by using the probability for a Poisson process.

Figure 14 shows the first results—both intensityand 3D images of a cone and a cylinder (with sur-rounding mounting structure) viewed nearly head onover a 65-m indoor range. Each image is from a singlerecorded frame. Because the camera has a 12-bit dy-namic range, whereas displayed gray scale has only 8bits, we show a single intensity image in three ways;namely, with (from left to right) the lower, middle,and upper 8 bits of the data mapped into the grayscale. Even with three versions of the intensity image,it is difficult, if not impossible, to tell what the objectsare. In contrast, the shapes are evident from the 3Dimage, which also is shown in three different versions,as described in the figure caption. The 3D image wasmade with a very low 2.2 photoelectrons per pixel onaverage, and the intensity image at the top was madewith 869 photoelectrons per pixel on average. There-fore, the ladar image was made with a signal lower bya factor of 400. Even so, it better reveals the shapes ofthe cylinder and cone.

Next, the sensors were placed about 10 m aboveground level in the dome of the Lincoln LaboratoryFirepond facility in Westford, Massachusetts. The in-tensity camera looked through an aperture only a fewinches from that of the 3D-ladar brassboard. Another

FIGURE 13. 3D image of a truck in front of two trees. Laser light penetratedthe outer envelope of the trees. The point cloud is shown after two differentamounts of computer rotation to better reveal the 3D structure of the trees.



FIGURE 14. Intensity (top) and 3D (bottom) images of cone and cylinder viewed nearly head on, withmounting structure behind. These are single-frame images displayed in different ways. The 12-bit in-tensity image is shown (from left to right) with the lower, middle, and upper eight bits mapped intothe 8-bit gray scale available from the printer. The 3D image at lower left is shown as a point cloud inits originally recorded orientation, with color coding according to range. At lower middle, the same3D image is shown after computer rotation to show the view as if from above. At lower right is a sur-face-plot version of the 3D image with a different amount of computer rotation. Although the aver-age number of photoelectrons per pixel is 869 for the intensity image and only 2.2 for the 3D-ladar im-age, the 3D image more readily reveals the shape difference between the cylinder and cone.

FIGURE 15. Photographs of the 500-m propagation range at the Lincoln Laboratory Firepond facility inWestford, Massachusetts. On the left is an aerial view with the Firepond dome at top right and the target padat bottom right. The middle image shows the view from the dome toward the target pad, and the right imageshows the view from the pad to the dome. The 3D-ladar brassboard, low-light-level intensity camera, and di-ode laser were housed within a few inches of one another in the dome. At nighttime, the diode laser illumi-nated the scene for the camera.



few inches away was the aperture of the 808-nm di-ode laser that illuminated the scene for the camera.The sensors looked at objects placed at the far end ofa 500-m path that was cleared of trees and brush, asshown in Figure 15.

In a typical setup at the 500-m site, two vehicleswere arranged side by side, slightly facing one anotherand seen nearly head on from the dome. A few metersin front of one of the vehicles was either a camouflagenet or a line of evergreen trees. The trees were freshlycut and were leaned against a suspended rope. Figure16 shows the vehicles, camouflage net, and trees. Thecamouflage net is colored in drab shades of green andbrown.

The fields of view of the 3D ladar and intensitycamera were aligned to each other and were about9 m across at the 500-m site. Because each sensor has128 × 128 pixels, each had a pixel size of about 7 cm.The diffraction-limited resolution for the sensors,which had receiving apertures of about 5 cm, wasabout 1 cm. The diode laser for the camera illumi-nated a circular area with a diameter of about 9 m.The range gate of the 3D ladar, about 15 m deep, en-compassed the scene of interest. For each 500-m

setup, we recorded 100 to 200 frames of data fromeach sensor.

In Figures 17 and 18, the images are presented af-ter multiple-frame processing. To process the cameradata, we averaged N frames (i.e., averaged the N val-ues for each pixel) and displayed the average framewith an 8-bit gray scale. The intensity range of thedata does not exceed 8 bits. We tried various map-pings of intensity into gray scale in an attempt to re-veal the details of interest. (The signal-to-noise ratioobtained by averaging 200 camera frames recordedover a time t is lower than that which would be ob-tained by allowing the camera to integrate for the en-tire time t before reading out a single frame. The rea-son for this is that readout noise is suffered 200 timesinstead of only once. Even so, the data show thatactual signal-to-noise ratios were very high and werenot the factors limiting recognizability of imagedobjects.)

For the 3D ladar, we recorded N frames and thencreated a range histogram for each pixel. A range his-togram is a graph of the number of times that eachrange is measured versus range. The range scale wasdivided into bins of width 1.1 cm, the size of the

FIGURE 16. Photographs of objects imaged by the ladar and camera. The vanhas a pod on top, the utility vehicle is camouflage painted, the net is camouflagecolored, and the trees are evergreens.



FIGURE 17. Intensity (left) and 3D (right) images of an exposed, camouflage-painted utility vehicle and aline of trees behind which is the white van. The intensity image is shown with two different mappings ofintensity to gray scale, and the 3D image is shown after computer rotations by two different angles. Onehundred frames recorded on a moonless night were processed. The average number of photoelectronsassociated with the right-side utility vehicle is roughly 600 per pixel per frame for the intensity image and0.05 per pixel per frame for the 3D image. Even so, the 3D image reveals the white van with its rooftoppod behind the trees, while the intensity image fails to do so.

least-significant bit in the timing electronics. For apixel imaging part of a solid object, the recordedranges accumulated in the bin corresponding to therange to the object. A few neighboring bins also havedata points because of timing jitter in the brassboardand because of the depth of the object within thepixel, if any. Other data points were scattered about inmore distant range bins because of noise. We locatedthe bin with the maximum number of data pointsand calculated the first moment of the range over aninterval encompassing a few range bins on either sideof the maximum bin. We took this moment as theobject’s range. When a particular pixel covered partsof two objects separated in range, the range histogramexhibited two peaks. In this case, we chose to displayeither peak or both.

Figure 17 shows intensity and 3D images of thecamouflage-painted utility vehicle and white van.The van is a few meters behind the line of trees on theleft side of the images, and the utility vehicle is on theright. One hundred frames recorded on a moonlessnight were processed.

For the intensity image on the left side of Figure17, the average number of photoelectrons associated

with the right-hand vehicle is roughly 600 per pixelper frame. The corresponding signal-to-noise ratio isapproximately 1000, given that the noise is approxi-mately 6 electrons and that the signal-to-noise ratioscales as the square root of the number of frames aver-aged. The camera image is shown with two differentmappings of intensity to gray scale. The maximumand minimum intensities differ by 8 bits, and in theupper image all 8 bits are shown. In the lower image,the bottom 7.5 bits of the data are mapped into the 8-bit gray scale to provide more contrast in the tree re-gion at the expense of saturation in other regions.

The range gate of the ladar was set to open be-tween the trees and the van. For the 3D image, thenumber of photoelectrons associated with both ve-hicles is roughly 0.05 per pixel per frame on average,the attenuation by the trees being offset by the higherreflectivity of the van. The 3D image is shown aftercomputer rotations of 10° (upper image) and 45°(lower image).

In Figure 17, the van behind the trees cannot bediscerned in the intensity image, despite having anaverage signal-to-noise ratio of approximately 1000.Yet it can be readily discerned in the 3D-ladar image,



which was made with four orders of magnitude fewerphotoelectrons than was the intensity image. Thetrees were thick enough in a few spots to block all la-ser light to the van, so the 3D image has a few holes init. Even so, it is clear (especially when the point cloudis rotated via software) that a vehicle with a pod ontop is behind the trees. The vehicle on the rightclearly does not have a pod and has a different shape.The ability to create a high-quality 3D image withextremely low light levels attests to the exquisite sensi-tivity of the Geiger-mode APDs.

In Figure 18, the line of trees has been replaced bya camouflage net, and a second camouflage-paintedutility vehicle is behind the net. We have conclusionssimilar to those above: the 3D image shows that thesize and shape of the vehicle behind the net are thesame as for the unobscured vehicle, while the inten-sity image, with two orders of magnitude more pho-toelectrons, fails to reveal even that a vehicle is behindthe net.

In Figures 17 and 18, the vehicles behind the netand trees are difficult to recognize from the intensityimages, even though signal-to-noise ratios are high.Part of the reason for this may be low intensity con-trast between the vehicles and obscurants. Low con-trast can be caused by similar reflectivities at thesensed wavelength (808 nm) and by the mixingwithin a pixel of photons from both the obscurantand the vehicle. Contrast might have been better ifsome other wavelength had been used, or if multiplewavelengths and special processing had been used, orif the pixels had been smaller to reduce mixing. Also,higher signal-to-noise ratios and finer intensity reso-lution in displays would help distinguish low-contrastobjects. Perhaps more important than low contrast inlimiting recognizability are the spatial variations inintensity impressed upon the image by the obscurant.The variations have spatial frequencies similar tothose of features on the vehicles and thereby hide thedistinguishing features. The 3D data do not suffer

FIGURE 18. Intensity (left) and 3D-ladar (right) images of two camouflage-painted utility vehicles, onebehind a camouflage net. The intensity image is shown with two different mappings of intensity to grayscale, and the 3D image is shown from two different aspect angles. Two hundred frames, recorded on amoonless night, were processed. The average number of photoelectrons associated with the right-side ve-hicle is roughly 120 per pixel per frame for the intensity image and only 0.4 per pixel per frame for the 3D im-age. The vehicle behind the net shows up clearly in the 3D image after computer rotation, but it cannot bediscerned in the intensity image.



from these limitations; they are insensitive to inten-sity contrast variations, and they provide range so thatthe transverse features of the obscurant and vehiclecan be separated. In addition, the 3D data reveal sizeand shape. These characteristics of the 3D data makethe vehicles recognizable in the 3D images even whenthe signal levels are much lower than those for the in-tensity images.

Future Development of 3D Ladar

We are currently developing more compact 3D imag-ing ladar systems, which will use the recently devel-oped 32 × 32 APD/CMOS arrays [2]. Given successwith this size, we expect that scaling up to 128 × 128elements or greater will be straightforward.

Because the quantum efficiency of Si detectors de-creases rapidly for wavelengths beyond about 900nm, we are developing APDs with Class III-V semi-conductor materials that will respond to longer wave-lengths, such as those around 1500 nm in the eye-saferegime.

The energy per pulse (up to approximately 100 µJat 532 nm) and repetition rate (greater than 1000pulses/sec) from existing microchip lasers are suffi-cient for many applications. If an application requiresmore energy, it is straightforward to couple a micro-chip laser to a diode-pumped, solid-state amplifier.Eye-safe wavelengths around 1500 nm can be pro-duced by using microchip lasers to pump opticalparametric oscillators [13].

With microchip lasers and microelectronic arraysof detectors and timing circuitry, we expect that anentire 3D ladar can fit into a package small enough(the size of a coffee can) and light enough (two orthree kilograms) to be handheld. Because the ladarcould work at greater than 1000 frames per second,hundreds of frames could be processed in a fraction ofa second in order to see through obscurants, to freezemotion in a dynamic scene or in the ladar platform,or to extend the range. The range can also be greatlyextended by coupling the microchip laser to an am-plifier. After development, such a ladar could operateat an eye-safe wavelength near 1500 nm. As shown bythe images in this article, a 3D-imaging ladar withGeiger-mode photon-counting arrays can providevaluable target and scene information at very low sig-

nal levels. This information is difficult to obtain fromintensity cameras at even much higher signal levels.

Acknowledgments

In addition to the authors, many others have contrib-uted to the development and maturation of thesenovel and interesting imaging technologies. Paul F.McManamon of the Air Force Research Lab SensorsDirectorate and Walter Dyer of the Missile DefenseAgency were early supporters of this activity andhelped initiate the pioneering concepts and experi-ments. The MIT Lincoln Laboratory AdvancedConcepts Committee provided support for the earlylaboratory experiments in 3D imaging with photon-counting detectors (ACC Project 199). Through theircareful and creative efforts, Salvatore DiCecca, JuanOchoa, and Antonio Rubio-Sanchez assembled andoperated the first laboratory proof of principal. Thefirst 3D ladar images from an array of detectors wereaccomplished with generous support from StephanVasile, then of Radiation Monitoring Devices, Inc.,who provided samples of his novel 4 × 4 silicon Gei-ger-mode APD arrays for testing. These encouragingresults have led to Lincoln Laboratory developmentof the brassboard ladar system as well as the 4 × 4 and32 × 32 arrays of detectors and digital timing circuitsdescribed in the companion article in this issue byBrian F. Aull, et al. We also acknowledge the supportfrom DARPA, SDIO, BMDO, and the U.S. Army,U.S. Navy, and U.S. Air Force.



R E F E R E N C E S1. J.J. Zayhowski, “Microchip Lasers,” Linc. Lab. J. 3 (3), 1990,

pp. 427–446.2. B.F. Aull, A.H. Loomis, D.J. Young, R.M. Heinrichs, B.J.

Felton, P.J. Daniels, and D.J. Landers, “Geiger-Mode Ava-lanche Photodiodes for Three-Dimensional Imaging,” Linc.Lab. J., in this issue.

3. W.E. Keicher, W.E. Bicknell, R.M. Marino, T. Stephens, W.R.Davis, and S.E. Forman, “Laser Radar Technology for BallisticMissile Defense,” Linc. Lab. J. 13 (1), 2001, pp. 205–228.

4. R.M. Heinrichs, B.F. Aull, R.M. Marino, D.G. Fouche, A.K.McIntosh, J.J. Zayhowski, T. Stephens, M.E. O’Brien, andM.A. Albota, “Three-Dimensional Laser Radar with APDArrays,” SPIE 4377, 2001, pp. 106–117.

5. R.M. Marino, R.M. Spitzberg, and M.J. Bohrer, “A PhotonCounting 3-D Imaging Laser Radar for Advanced Discrimi-nating Interceptor Seekers,” 2nd Ann. AIAA SDIO InterceptorTechnology Conf., 6–9 June 1993, Albuquerque, N.Mex.

6. B.F. Aull, A.H. Loomis, J.A. Gregory, and D.J. Young, “Gei-ger-Mode Avalanche Photodiode Arrays Integrated withCMOS Timing Circuits,” Mtg. of the Boston Chap. of theIEEE Electron Devices Society, Lexington, Mass., Nov. 1998.

7. B.F. Aull, “Geiger-Mode Avalanche Photodiode Arrays Inte-grated with CMOS Timing Circuits,” 56th Ann. Device Re-search Conf. Dig., Charlottesville, Va., 22–24 June 1998,pp. 58–59.

8. R.M. Marino, “Method and Apparatus for Imaging a SceneUsing a Light Detector Operating in Non-linear Geiger-mode,” U.S. Patent No. 5,892,575 (6 Apr. 1999).

9. J.J. Zayhowski and C. Dill III, “Diode-Pumped Passively Q-Switched Picosecond Microchip Lasers,” Opt. Lett. 9 (18),1994, pp. 1427–1429.

10. J.J. Zayhowski, “Passively Q-Switched Microchip Lasers andApplications,” Rev. Laser Eng. 29 (12), 1988, pp. 841–846.

11. J.J. Zayhowski, C. Dill III, C. Cook, and J.L. Daneu, “Mid-and High-Power Passively Q-Switched Microchip Lasers,”OSA TOPS 26, Advanced Solid-State Lasers, Topical Mtg., 31Jan.–3 Feb. 1999, pp. 178–186.

12. J.J. Zayhowski, “Ultraviolet Generation with Passively Q-Switched Microchip Lasers,” Opt. Lett. 21 (8), 1996, pp. 588–590; errata, Opt. Lett. 21 (19), 1996, p. 1618.

13. J.J. Zayhowski, “Periodically Poled Lithium Niobate OpticalParametric Amplifiers Pumped by High-Power Passively Q-Switched Microchip Lasers,” Opt. Lett. 22 (3), 1997,pp. 169–171.



. is an assistant staff member ofthe Optical CommunicationsTechnology group, researchingnonlinear and quantum optics,optical communications, andimaging. Before this assign-ment, Marius was involved inthe initial development andfielding of the 3D imaginglaser radar with the Laser andSensor Applications group.Prior to joining Lincoln Labo-ratory in 1997, he was a re-search assistant working onmultiphoton microscopy atCornell University. He re-ceived a B.S. degree in engi-neering physics from CornellUniversity and an S.M. degreein electrical engineering andcomputer science (EECS)from MIT. He is working onhis Ph.D. degree in EECS atMIT through the LincolnScholars Program with advi-sors Franco N.C. Wong andJeffrey H. Shapiro.

. is a senior staff member of theLaser and Sensor Applicationsgroup. He joined LincolnLaboratory in 1972 afterreceiving an A.B. degree inengineering and applied phys-ics from Harvard Universityand a Ph.D. degree in appliedquantum physics and quan-tum electronics from YaleUniversity. His doctorateconcerned inelastic lightscattering from molecules. Atthe Laboratory, he has contrib-uted to systems studies, hard-ware development, and experi-mentation in severalelectro-optical areas, includinghigh-energy-laser propagation,precision tracking, adaptiveoptics, passive IR imaging,nonlinear optics, and laserranging. Recently, he hasfocused on 3D imaging laserradars.

. develops 3D imaging andphoton counting focal planesby using Geiger-mode ava-lanche photodiodes as a staffmember in the AdvancedImaging Technology group.He joined the Laboratory in1985, after earning a Ph.D.degree in electrical engineeringfrom MIT. During his firstdecade at the Laboratory heled teams that demonstratedmultiple-quantum-well spatiallight modulators with gallium-arsenide/aluminum-gallium-arsenide (GaAs/ AlGaAs)charge-coupled device (CCD)arrays for electrical addressingand developed optoelectronicswitching and neural process-ing devices by using quantum-well modulators and double-barrier resonant tunnelingdiodes. During the past twoyears Brian has developedarrays of Geiger-mode ava-lanche photodiodes integratedwith high-speed CMOS tim-ing circuits, which are beingintegrated into a 3D ladarsystem. He also holds a B.S.degree in electrical engineeringfrom Purdue University.

. leads the Laser and SensorApplications group, whichstudies direct detection andcoherent laser remote sensing.Rick joined Lincoln Labora-tory in 1986, after completinga postdoctoral position innonlinear fluid dynamics atthe University of California inSanta Barbara. He receivedS.B. degrees in physics andelectrical engineering and anS.M. degree in electrical engi-neering from MIT, and aPh.D. degree in physics fromthe University of Massachu-setts. He will chair the 2003Coherent Laser RadarConference.



. studies nonlinear effects infiber optics for the Opto-electonics division at theNational Institute of Standardsand Technology (NIST). Hepreviously worked at LincolnLaboratory on biological agentdetection and ladars. Heearned a B.A. degree in physicsfrom Williams College and aPh.D. degree in physics fromPrinceton University.

. is an associate staff member ofthe Laser and Sensor Applica-tions group, working onanalog and digital electronicdesign related to the detectionof biological agents and 3Dlaser imaging systems. Jimjoined the Laboratory in 1967as an electronic technician. Ayear later, he joined the U.S.Navy and trained to work inelectronic countermeasures.Returning to the lab in 1972,he worked on a number ofadaptive optics systems usedfor high- energy laser propaga-tion and atmospheric compen-sation.

. received a B.S. degree inphysics from Cleveland StateUniversity in 1979, and anM.S. degree in physics and aPh.D. degree in high-energyphysics from Case WesternReserve University in 1983and 1985, respectively. In1985 he joined Lincoln Labo-ratory as a staff member in theLaser Radar Measurementsgroup. In 1993 he joined theSystems and Analysis groupwithin the Air Defense Tech-nology division. One of hismost significant achievementshas been his pioneering leader-ship in the development of a3D imaging laser radar withphoton-counting sensitivity.From 1995 to 1997 he workedat the Millimeter Wave Radar(MMW) and the ARPA-Lincoln C-band ObservablesRadar at the Kwajalein MissileRange in the Marshall Islands.While there, he was a missiontest director at MMW andworked on Range Moderniza-tion Plans. In 1997 he joinedthe Sensor Technology andSystems group of the Aero-space division and relocated itsSilver Spring, Maryland,location to join the NationalPolar-Orbiting OperationalEnvironmental Satellite Sys-tem (NPOESS)/IntegratedProgram Office (IPO). At theIPO, he was lead technicaladvisor for the NPOESSCross-Track Infrared Atmo-spheric Sounder Instrument(CrIs). He returned to LincolnLaboratory in Lexington in1999 and is again working onthe development of 3D imag-ing laser-radar systems.

. received a B.S. degree inelectrical engineering from theUniversity of Illinois in 1957.Subsequently he received theS.M., E.E., and Sc.D. degreesin electronics from MIT. Hejoined Lincoln Laboratory in1963 as a staff member, be-came a senior staff member in1988, and currently is in theLaser and Sensor Applicationsgroup. At Lincoln Laboratory,he has specialized in electronicand optical instrumentationand measurements supportingthree areas of defense-relatedresearch: optical measurementof missile reentry phenomena,the stabilization and control ofhigh-energy laser beams, andthe use of laser radars forremote sensing and measure-ment. In connection with thiswork, he has developed opticalinstrumentation for airbornesensors and for identifying thephenomena limiting theangular resolution of airbornetelescopes. He developed anautomatic optical focus sensorand a fifty-two-channel adap-tive optics sensor for use withhigh-energy laser beams andled the teams that performedfield tests of systems usingthese sensors. He also designedwaveform generation anddemodulation electronics for awideband range-Doppler laserradar, receiver electronics fortwo single-photon detectinglaser radars, and a laser illumi-nator used for missile tracking.



. supports the Laser and SensorApplications group as subcon-tract engineer throughLockheed Martin. He special-izes in optical detectors andpreamplifiers used in directdetection and coherent laserremote sensing. Before joiningthe Laboratory as a subcon-tractor in 1974, Brian con-ducted radiometric measure-ments to support reentryphysics for Avco Everett Re-search Laboratory in Everett,Massachusetts. He received aB.A. degree in physics fromNortheastern University.

. is a senior staff member of theQuantum Electronics group,where he develops robustminiature solid-state lasersystems and their applications.John holds fourteen U.S.patents in this area and hasworked closely with industryto transfer the technology tothe commercial sector. Appli-cations enabled by John’s workinclude the Lincoln Labora-tory bio-aerosol warningsensor (BAWS) and high-resolution direct-detection 3Dimaging ladar. John is a co-recipient of a 1998 R&D 100award for the development ofCyraxTM-Portable, 3-DLaser-Mapping and ImagingSystem. He is a member andformer chairman of the Ad-vanced Concepts Committeeand a member of the NewTechnology Initiatives Board.John is the organizer andchairman of the subcommitteeon Active Optical Sensing forthe Conference on Lasers andElectro-Optics, to be held inBaltimore, Maryland, June1–6, 2003; and the programchair for the Topical Meetingon Advanced Solid-StatePhotonics, to be held in SanAntonio, Texas, February 2–5,2003. Before joining theLaboratory in 1986, heworked for the Texas Instru-ments Central Research Labo-ratory in Dallas, Texas. Johnearned joint S.M. and S.B.degrees in electrical engineer-ing and computer science anda Ph.D. degree in electricalengineering, all from MIT.

. is a staff member of the Laserand Sensor Applicationsgroup, performing all of itsoptical systems engineering.His work focuses on 3D laserradars, laser vibrometers,adaptive optics, infrared imag-ing optics, and wake vortexmonitors. He holds a B.S.degree in optics engineeringand an M.S. degree in optics,both from the University ofRochester.

. ’is an associate staff member ofthe Laser and Sensor Applica-tions group. His work focuseson the simulation of 3D laserradar systems and the process-ing of 3D laser radar data. Hereceived a B.S. degree inmathematics from RensselaerPolytechnic Institute and anM.S. degree in applied math-ematics from WorcesterPolytechnic Institute.

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Three-Dimensional Imaging Laser Radars with Geiger-Mode Avalanche Photodiode Arrays