Improvement of range precision in laser detection and ranging system by using twoGeiger mode avalanche photodiodesTae Hoon Kim, Hong Jin Kong, Sung Eun Jo, Byoung Goo Jeon, Min Seok Oh, Ayoung Heo, and Dong Jo Park

Citation: Review of Scientific Instruments 84, 065112 (2013); doi: 10.1063/1.4811459 View online: http://dx.doi.org/10.1063/1.4811459 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/84/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Multihit mode direct-detection laser radar system using a Geiger-mode avalanche photodiode Rev. Sci. Instrum. 81, 033109 (2010); 10.1063/1.3374109 Ge–Si separate absorption and multiplication avalanche photodiode for Geiger mode single photon detection Appl. Phys. Lett. 93, 183511 (2008); 10.1063/1.3020297 Room temperature single ion detection with Geiger mode avalanche diode detectors Appl. Phys. Lett. 93, 043124 (2008); 10.1063/1.2967211 Numerical analysis of single photon detection avalanche photodiodes operated in the Geiger mode J. Appl. Phys. 99, 124502 (2006); 10.1063/1.2207575 Comparing leakage currents and dark count rates in Geiger-mode avalanche photodiodes Appl. Phys. Lett. 80, 4100 (2002); 10.1063/1.1483119

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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 065112 (2013)

Improvement of range precision in laser detection and ranging system byusing two Geiger mode avalanche photodiodes

Tae Hoon Kim,1,a) Hong Jin Kong,1 Sung Eun Jo,1 Byoung Goo Jeon,1 Min Seok Oh,1

Ayoung Heo,2 and Dong Jo Park2

1Department of Physics, KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea2Department of Electrical Engineering, KAIST, 373-1 Guseong-dong, Yuseong-gu,Daejeon 305-701, South Korea

(Received 1 November 2012; accepted 5 June 2013; published online 26 June 2013)

In this paper, the improvement of range precision in a laser detection and ranging (LADAR) systemby using two Geiger mode avalanche photodiodes (GmAPDs) is described. The LADAR systemis implemented by using two GmAPDs with a beam splitter and applying comparative process totheir ends. Then, the timing circuit receives the electrical signals only if each GmAPDs generateselectrical signals simultaneously. Though this system decreases the energy of a laser-return pulsescattered from the target, it is effective in reducing the range precision. The experimental resultsshowed that the average value of standard deviation of time of flights was improved from 61 mm to37 mm when the pulse energy is 0.6 μJ. When the time bin width is 0.5 ns, the single-shot precisionerror of the LADAR system was also improved from 280 mm to 67 mm. © 2013 AIP PublishingLLC. [http://dx.doi.org/10.1063/1.4811459]

I. INTRODUCTION

LIDAR (Light Detection And Ranging) was first used in1953, and in 1962, high-energy or Q-switched pulsed laserswere developed and used for LIDAR application. In 1963,Fiocco and Surllival published work on atmospheric obser-vations using a ruby laser.1 In 1969, a laser range finder anda target mounted on the Apollo-11, space craft, was usedto measure the distance from Earth to the Moon.2 In 1985,laser range finders reached the stage of mature technologiesand have been separated from the LIDAR technology indus-try. In 2004, the National Institute of Standards and Technol-ogy (NIST) adopted the term LADAR (Laser Detection AndRanging) for laser-based RADAR-type systems.3, 4 There arevarious methods to measure a distance with a laser source [in-terferometry, amplitude modulation, frequency modulation,and time-of-flight (TOF) methods].5 For higher range capa-bility, a pulsed 3D imaging LADAR system using the TOF issuitable because the light energy is intensively compressed inthe form of a pulse in the time domain.

Because shorter laser pulses provide better range preci-sion, typical LADAR systems transmit short pulse of laserlight. The generation of a laser pulse is often accomplishedusing a component within the laser cavity called a Q-switch.However, the shorter pulses demand more sophisticated laserQ-switches.

These engineering issues place practical limits on howshort a laser pulse can be made. Current LADAR systems usea few nanoseconds long pulsed laser as a light source.

The shot noise is the randomness in time between photonarrivals. Shot noise sources are background photons, dark cur-rent, and received laser pulse. The background photons andthe dark current are randomly generated in overall time do-

a)Author to whom correspondence should be addressed. Electronic mail:kimth@kaist.ac.kr. Tel.:+82-42-350-2561. Fax: +82-42-350-2561.

main. The signals, which are generated by laser-return pulsesscattered from the target, are spread over several time binsbecause of the randomness in times between photon arrivals.These phenomena increased the range precision. And theelectronic jitter also increased the range precision. Therefore,the range precision of a LADAR system is limited by shotnoise and timing jitter of the electronics.9 The shot noise andthe timing jitter of electronics are sources of random error ina LADAR system. By making the same measurements manytimes, the randomness of the measurements can be reduced.But, it is time-consuming to repeat the measurements manytimes (typically 104–106).6–10

Therefore, we developed a LADAR system to improvethe range precision with a small number of measurements.The LADAR system is implemented by using two GmAPDswith a beam splitter and applying a comparative process totheir ends. Then, the timing circuitry receives the electricalsignals only if each GmAPDs generates the electrical signalssimultaneously. Though this system decreases the energy ofa laser-return pulse scattered from the target, range precisionwith fewer measurements was improved. Additionally, it hasthe advantage of obtaining a clear 3D image in the acquisitionstage of raw TOF data.

In this paper, the improvement of the range precision andreduction of precision error is described in addition to the ac-quisition of clear 3D image. This system may be useful foraccurate object recognition in 3D image processing.

II. RANGE PRECISION OF THE LADAR SYSTEM

Measurement errors are classified into two major cat-egories: Systematic errors and random errors. Systematicerrors are errors that will make our results different formthe “true” values with reproducible discrepancies. There-fore, systematic errors determine the measurement accuracy.

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065112-2 Kim et al. Rev. Sci. Instrum. 84, 065112 (2013)

Systematic errors are difficult to detect and cannot be ana-lyzed statistically. They must be estimated from an analysis ofthe experimental conditions, techniques, and physical interac-tions. Generally, the systematic errors are sufficiently stablethat they can be removed by calibration. Random errors arefluctuations in observations that yield different results eachtime the experiment is repeated. Random errors determine themeasurement range precision. The possible sources of ran-dom errors are shot noise associated with photon-countingsystem, pulse width of laser, and electronic jitter, etc.

The precision is determined by several factors, whichare the pulse width of laser pulse (�σ laser), the timing res-olution of detector (�σ detector), the timing resolution of TDC(�σ TDC), which is a timing circuit, the jitters of timing systemtiming jitters generated by electronics (�σ electronics), the cor-relation factors of the timing system (�σ jitter = �σ electronics

+ �σ correlation), which are the timing jitters of electronics gen-erated by electronics (�σ electronics) and the correlation factorsof the timing system (�σ correlation), and they are uncorrelated,and the randomness of noise (�σ noise), which is generated bybackground photon and dark current. Therefore, the precision(�σ precision) is given by

�σ precision =√√√√ (�σlaser)2 + (�σdetector)2

+(�σTDC)2 + (�σjitter)2 + (�σnoise)2. (1)

If the threshold processing is performed, the precisionwill be more decrease due to the reduction of �σ noise. There-fore, the precision (�σ precision) of the LADAR system usingsingle GmAPD is approximately 897 ps (�σ laser = 800 ps,�σ detector = 40 ps, �σ TDC = 50 ps, and �σ jitter = 400 ps).Then, the range precision is approximately 134 mm in dis-tance. If the pulse energy of laser is increased, �σ laser is ap-proximately zero. Therefore, �σ precision of the LADAR sys-tem using single GmAPD is 405 ps (�σ laser = 0 ps, �σ detector

= 40 ps, �σ TDC = 50 ps, and �σ jitter = 400 ps). The rangeprecision is approximately 61 mm in distance.

The range precision error caused by photon noise is im-proved by integrating more measurement. If the error on asingle timing measurement is σ , then the error, σ N, on themean of N measurements, assuming random errors, will begiven by3

σN = σ/N. (2)

If we use the LADAR system using two GmAPDs, theprecision error cause by photon noise can be improved by thefactor of 1/

√2 with single pulse operation. The GmAPDs are

a produces a fast electrical pulse of several volts amplitudein response to the detection of even a single photon. Thena correlation process, which is AND gate process, is con-ducted. It compares the arrival time of electrical pulses of twoGmAPDs. Then the timing circuitry measures the time delayof each electrical pulse. As a result, the information of stan-dard deviation of TOFs is obtained from the data of TOFs.

There are common error (�σ laser, �σ TDC) and differenterrors (�σ detector, �σ jitter, �σ noise) in the LADAR system us-ing two GmAPDs. Compare to common error, the effect ofdifferent errors is relatively small in our system. Therefore,

FIG. 1. Schematic diagram of the LADAR system using two GmAPDs.

�σ precision is approximately improved by the factor of 1/√

2by using two GmAPDs. In our system, the range precision isless than 84 mm in low pulse energy and 43 mm in case ofhigh pulse energy.

A. Range precision in LADAR system usingtwo GmAPDs

The schematic diagram of the LADAR system using twoGmAPDs is shown in Figure 1. A laser pulse is emitted by apulse laser and passes through the optical system, triggeringthe time-to-digital converter (TDC). The emitted laser pulseis scattered by the target; a part of the scattered laser pulseand background light in the field-of-view (FOV) is collectedby the optical system. Then, the collected laser-return pulseand background light are intensity-divided in half and routedto two GmAPDs. A correlation process, which is AND gateprocess, is conducted by comparing the arrival time of elec-trical pulses of two GmAPDs.

Figure 2 shows the optical system of the LADAR systemusing two GmAPDs. A diode-pumped passively Q-switchingmicrochip laser with second harmonic generation was used

FIG. 2. Optical system of the LADAR system using two GmAPDs.

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065112-3 Kim et al. Rev. Sci. Instrum. 84, 065112 (2013)

as a light source. Wavelength laser pulses, 532-nm, with afull width at half maximum of 1 ns, a beam divergence of 6mrad, and an energy of 3 μJ were emitted at a repetition ratethat varied in between 2 and 20 kHz, depending on the opticalpower of the pump light.8, 11, 14 Some of laser pulse energy waspassed through Mirror1 to generate the electrical start signalby the photodiode; the start signal initiated the TDC. Most ofthe laser pulse was collimated by lenses L1 and L2. Due tothe single polarization of the laser, a half-wave plate (HWP)is located before the polarization beam splitter (PBS) to con-trol both the transmission and reflection of the laser pulsesat the PBS. The emitted laser pulse is scattered by the tar-get; a part of the scattered laser pulse and background lightin the FOV is collected by the optical system. Then, the col-lected laser-return pulse and background light are intensity-divided in half and routed to two GmAPDs. A GmAPD wasused as a detector in a long-range LADAR system due to itshigh sensitivity.5, 12, 13 Two GmAPDs (Id Quantique Id100-20-ULN), which have a timing resolution of 40 ps, an afterpuls-ing probability of 3%, an output pulse width of 10 ns, a deadtime of 45 ns, a photon detection probability of 35% at 500 nmwavelength, and a measured mean dark count rate of less than1 kHz, were used.5 The TDC (Agilent U1051A), which hadsix channels and timing resolutions of 50 ps, received elec-trical stop signals from the GmAPDs.12, 15 A compact periph-eral component interconnect (cPCI) system, which includesTDC and an arbitrary waveform generator (United ElectronicIndustries PDXI-AO-8/16) was used for data acquisition andcontrolling a two-axis galvano scanner.16

The Lambertian-target of 99% reflectance was located at15 m apart from the LADAR system. Varying the energy oflaser pulse, the TOFs of laser pulse were acquired with 10 000laser pulses. The time bin width (TBW) is an important fac-tor for the correlation process in timing circuitry because theTWB has a connection with the range precision. Therefore,the TBW is also varied from 50 ps to 5 ns. The mean num-bers of firings generated by noise, NPE, were 15 kHz and39 kHz, respectively When applying the functionality of acorrelation process to the TOFs, calibration was needed dueto the different time-delay characteristics between GmAPD1and 2. In the case of this experimental setup, there was a time-delay difference of 5.16 ps. To check the comparison of rangeprecision, the LADAR system with single GmAPD systemwas also examined by controlling the HWP2 in Figure 2.

The standard deviation (STD) of TOFs is defined as thestandard deviation of TOFs,

STD of TOFs

= c

2

√1

N

∑N

i=1(TOFi − TCOM)2,

(3)

where c is the speed of light ( = 3 × 108 m/s2), the N is thetotal number of TOFs, the TOFi is the time-of-flight data onith measurement, the TCOM is the mean value of TOFs usingcenter of mass method algorithm

TCOM =∑

Target bins Pbin(j)τj∑Target bins Pbin(j)

, (4)

FIG. 3. (a) STD of TOFs versus pulse energy of laser, (b) target detectionprobability versus pulse energy of laser.

where the Pbin(j) is the target detection on the jth time bin, τ j

is the time duration of a time bin.Figure 3 shows the STD of TOFs versus pulse energy and

target detection probability versus pulse energy. As the pulseenergy was increased, the STD of TOFs was decreased from134 mm to 61 mm in case of single GmAPD and from 66 mmto 32 mm in case of two GmAPDs. These results were similarto the theoretical values. As the pulse energy increased, theSTD of TOFs was decreased in Figure 3(a) because the factorof shot noise, which are generated by laser-return pulses scat-tered from the target, was reduced. And the reduction of sizeof TBW caused the decrease of STD of TOFs because the shotnoise was also reduced. However, the reduction of the size ofTBW causes the reduction of target detection probability asshown in Figure 3(b). As a result, when the TBW is 50 ps, theaverage difference of the STD of TOFs between the LADARsystem using single GmAPD and using two GmAPDs was30 mm.

The effect of TBW also appears in Figure 4 wherethe pulse energy is a constant value, which is 0.6 μJ and0.1 μJ. The average value of STD of TOFs was improvedfrom 61 mm to 37 mm when the pulse energy is 0.6 μJ; and

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065112-4 Kim et al. Rev. Sci. Instrum. 84, 065112 (2013)

FIG. 4. (a) STD of TOFs versus time bin width when the laser pulse energyis 0.6 μJ, (b) STD of TOFs versus time bin width when the laser pulse energyis 0.1 μJ.

58 mm to 41 mm when the pulse energy is 0.1 μJ. Figure 4shows that the STD of TOFs of the LADAR system using twoGmAPDs was always small and constant with some fluctua-tions compared to the LADAR system using single GmAPD.The small fluctuations are the effect of quantization in timedomain.

The experiment on range characteristics was conductedto identify the variation of the STD of TOFs with target loca-tion. The STD of TOFs was obtained with 10 000 laser pulseswith the target located between 10 m and 100 m distance, at10 m intervals. The results are shown in Figure 5(a). The vari-ation of STD of TOFs is 44 mm to 62 mm in the case of twoGmAPDs. And, the average difference of the STD of TOFsbetween the single GmAPD case and the two GmAPDs casewas 19 mm.

The single-shot precision error is defined as the STD ofa single measurement, while the precision error of N laserpulses is defined as the STD of the several number of the rep-resentative range acquired by N laser pulses.

To specify the precision error of the N laser pulses, theN laser pulses are measured (with N = 20 000, 10 000, 1000,

FIG. 5. (a) STD of TOFs versus the position of target, (b) precision errorversus the number of laser pulses.

100, 10). The target is located 10 m from the LADAR system.The pulse energy is 0.27 μJ. The mean numbers of firingsgenerated by noise, NPE, were 15 kHz and 39 kHz, respec-tively.

The reference position (RCOM) is determined using thecase of N = 20 000 in this experiment

RCOM = c

2TCOM, (5)

where the c is the speed of light ( = 3 × 108 m/s2), the TCOM

is the mean value of TOFs using center of mass method algo-rithm.

The experimental results of precision error are shown inFigure 5(b). The precision error of two GmAPDs decreased67 mm to 0.8 mm. The single-shot precision error of twoGmAPDs was 67 mm and the single-shot precision error ofsingle GmAPD was 218 mm. The ratio of precision error isdefined as the precision error in the case of single GmAPDdivided by the precision error in the case of two GmAPDs.The ratio of precision error was from 1.5 to 4.2. It means thatthe precision improved between 1.5 and 4.2 times greater inthe case of two GmAPDs.

As the results indicate, the precision error is reduced bymore repeated measurements. The precision error and single-shot precision error caused by shot noise and timing jitter gen-erated by electronics were improved by using two GmAPDs.

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065112-5 Kim et al. Rev. Sci. Instrum. 84, 065112 (2013)

FIG. 6. (a) 2D image of the tree and side view of 3D image in the case of(b) single GmAPD (c) two GmAPDs.

In addition, the same value of precision error could be ob-tained with a small number of measurements.

B. 3D image obtained by the LADAR system usingtwo GmAPDs

Figure 6 shows a 2D image of a tree and 512 × 512 pix-els 3D images were obtained when the location of the treewas 60 m from the LADAR system. The mean numbers of

firings generated by noise, NPE, were 2.9 MHz and 2.1 MHz.Considering the signal-to-noise ratio (SNR), the TBW wasset to 0.5 ns. Figures 6(b) and 6(c) show the 3D image rep-resented by point cloud method using raw data. Compared tothe case of the singe GmAPD, the image resolution was im-proved and few false alarms, which are caused by noise, weregenerated. The size of the data was 1.8 megabytes in the caseof two GmAPDs, but the size of the data was 25 megabytesin the singe GmAPD case. This was helpful in the reductionof 3D visualization because it reduces the size of memory re-quired in the computer. Therefore, the LADAR system usingtwo GmAPDs can obtain clear and high-precision 3D imagesquickly with a small number of measurements.

III. CONCLUSION

In this paper, we developed the LADAR system to im-prove range precision and reduce precision error with a smallnumber of measurements. The LADAR system is imple-mented by using two GmAPDs with a beam splitter and ap-plying comparative process to their ends. Then, the timingcircuitry receives the electrical signals only if each GmAPDsgenerates the electrical signals simultaneously. Though thissystem decreases the energy of a laser-return pulse scatteredfrom the target, it is effective in reducing shot noise and tim-ing jitter. The experimental results showed that the averagevalue of STD of TOFs was improved from 61 mm to 37 mmwhen the pulse energy is 0.6 μJ. When the TBW is 0.5 ns, thesingle-shot precision error of the LADAR system was alsoimproved from 280 mm to 67 mm by reducing the shot noiseand timing jitter. Additionally, it has the advantage of obtain-ing clear 3D images in the acquisition stage of raw TOF datawith a small size of data. This system results in a reductionof time. Additionally, this system will help accurate objectrecognition in 3D image processing.

ACKNOWLEDGMENTS

This research was supported by the Defense AcquisitionProgram Administration and Agency for Defense Develop-ment, Korea, through the Image Information Research Centerat Korea Advanced Institute of Science & Technology underContract No. UD100006CD.

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