Peltier-cooled and actively quenched operation ofInGaAs�InP avalanche photodiodes as photoncounters at a 1.55-�m wavelength

Ivan Prochazka

The performance of commercially available InGaAs�InP avalanche photodiodes as single-photon detec-tors at a 1.55-�m wavelength has been investigated. A new active quenching and gating circuit, tailoredfor operation of these diodes at temperatures in the range from room temperature to �60 °C andachievable by means of thermoelectrical cooling, has been developed. Careful tuning of the diodes’operating conditions resulted in a significant reduction of afterpulsing effects; it permitted operation ofthe detectors with high repetition rates. A noise-equivalent power of 7 � 10�16 W�Hz1�2 was obtainedat a 1.55-�m wavelength. © 2001 Optical Society of America

OCIS codes: 040.3060, 040.5570.

1. Introduction

Operation of commercially available InGaAs�InP av-alanche photodiodes as photon counters at high tem-peratures at a 1.55-�m wavelength warrants specialattention for numerous applications, including time-resolved spectroscopy, laser-induced fluorescence,optical time-domain reflectometry, quantum cryptog-raphy, eye-safe laser ranging, and imaging. Thiswavelength region is of special interest for two rea-sons: 1.55 �m corresponds to the third telecom win-dow for optical communication and quantumcryptography, and 1.54 �m is the so-called eye-safelaser ranging wavelength.1 At present, the mostpromising device for detecting a single photon by useof a solid-state detector is an avalanche photodiode�APD� operated in the Geiger mode. In this operat-ing mode the diode is pulse biased above its break-down voltage; no current flows until an avalanche istriggered by an incoming photon or a thermally gen-erated carrier. The current pulse’s rise time marksthe photon’s arrival time. An external electrical cir-cuit,2 either passive or active, is used to quench theavalanche and to reapply bias to the diode.

Previous research and development by the present

author and others3 in the field of single-photonavalanche diodes resulted in a large-aperture silicon-APD-based detector package with an active quench-ing circuit for applications in spectroscopy, laserranging, and fiber optics sensors.3 Recent develop-ments have yielded a custom-designed silicon APDwith an active area of 0.2-mm diameter, a timingresolution of 44 ps FWHM, and a dark-count ratebelow 10 kHz. The APD chip is cooled by a three-stage thermoelectrical cooler and enclosed in a min-iature evacuated package. The quantum efficiencycorresponds to that of silicon: It drops for a wave-length longer than 1.1 �m; for the 1.5-�m wavelengthregion a silicon APD is unusable. Several attemptshave been made to fabricate APDs suitable for photoncounting in the near infrared on the basis of theGe–Si monocrystal in the author’s laboratory; theirdevelopment is ongoing.

One of the candidates for a detector in the 1.5-�mregion is a germanium APD. The author and col-leagues4 developed a large-aperture germanium de-tector package for picosecond photon counting in thewavelength range from 0.5 to 1.6 �m. Although thisdetector package is performing well for satellite laserranging purposes, its use in other applications is lim-ited: A cryogenic environment is required, and thedark count is of the order of 1 MHz.

Separate absorption multiplication �SAM� APDsthat comprise InGaAs�InP is another candidate forsensing single photons.5,6 These type of diode hasbeen found to be the best choice as a photon-countingsensor from the point of view of quantum efficiency at

The author �prochazk@mbox.cesnet.cz� is with the Czech Tech-nical University, Brehova 7, 115 19 Czech Republic, Prague.

Received 20 February 2001; revised manuscript received 7 Au-gust 2001.

0003-6935�01�336012-07$15.00�0© 2001 Optical Society of America

6012 APPLIED OPTICS � Vol. 40, No. 33 � 20 November 2001

1.55 �m. It has been tested at a broad range oftemperatures ranging from room temperature to 77K; a single-photon detection efficiency of several per-cent and a time response from microseconds to 200 pshave been reported.5 All the reported results wereobtained with commercially available SAM APD In-GaAs�InP chips. Operational structures of SAMAPD InGaAs�InP chips specifically tailored for pho-ton counting have not yet been achieved. Cost andcomplexity of manufacturing technology are probablylimiting the development of an InGaAs�InP struc-ture. Additionally, most of the reported experi-ments with this type of diode were carried out in acryogenic environment at temperatures of 77–180 Kto achieve an acceptable dark-count rate and detec-tion sensitivity. Although compact closed-cyclecryocoolers are currently available, their size, powerconsumption, lifetime, complexity, and cost are notsuitable for most of the desired applications. Oper-ation of photon counters based on InGaAs APDs withthermoelectric cooling and a passive quenching cir-cuit was described recently.7

Considering the achievements listed above on onehand and the foreseen applications on the otherhand, I have constructed a compact, all-solid-statephoton counter operating near 1.55 �m that is basedon commercial InGaAs chips, thermoelectricallycooled with an active quenching and gating circuit.

2. Photon-Counting Detector Package

The main parameters of the photon-counting detectorare detection efficiency, timing resolution, dark-countrate, and maximum achievable detection repetitionrate. The main parameters of the photon-countingdetector depend, apart from the construction of thediode structure itself, on the operating conditions ofthe APD.

The operating temperature influences all the maindetector parameters. The number of thermally gen-erated carriers inside the semiconductors decreaseswith temperature. Thus a decrease in the diode’stemperature will result in a decrease in the dark-count rate of the detector. In general, reduction ofthe dark-count rate is one of the main reasons fordetector cooling. The dark-count rate is increasedby the effects of afterpulsing,5 i.e., the influence ofcarriers persisting in the detection structure after theprevious breakdown event. In general, the after-pulsing effects are less significant at higher temper-atures. The operating temperature influences thebreakdown voltage of the diode because of the com-plex structure of the SAM APD InGaAs�InP; the tem-perature also significantly influences detectionefficiency and timing resolution at 1.55-�m wave-length.5

Diode bias influences all the main detector param-eters: detection sensitivity, timing resolution, anddark-count rate. In general, when bias is increasedabove the breakdown point, photoncounting detec-tion sensitivity will increase and will be saturated ata specific voltage. Increasing the bias will improvethe timing resolution of the detector and saturate it

at some level. Increasing the bias above breakdownwill increase the dark-count rate of the detector; nosaturation can be observed in this case.

For the operation repetition rate, the rate withwhich the detector, operated in the gated mode, isrepeatedly activated, the maximum useful detectionrate is limited by the afterpulsing effect.

The construction of the quenching and gating cir-cuit determines the process of diode operation as aphoton counter. The circuit’s impedance influencesthe avalanche current’s buildup; the circuit’s internaldelays determine the time during which the ava-lanche builds up before being quenched. The totalavalanche charge, the number of generated carriers,strongly influences afterpulsing and hence both thedetector’s dark-count rate and the maximum detec-tion rate.

On the basis of the description above, one can con-clude that establishing settings for operating condi-tions and the design and construction of a quenchingand gating circuit represent a complex task. Theresultant solution is always a compromise tailored tothe specific application. The photon counter’s timeresolution, detection efficiency, dark-count rate, andmaximum detection rate are subject to a delicatetrade-off.

3. Quenching and Gating Circuit

The purpose of a quenching circuit is to control diodebias as a function of current flow across the diode.The circuit lowers the bias after buildup of the ava-lanche current and reapplies the original bias after afixed delay or before the arrival of the photon of in-terest �in a gated mode�. The simplest quenchingcircuit is a passive one; it is analogous to the Geigerscheme �see Fig. 1�. The diode structure is bridgedby a small capacitor of the order of picofarads and isbiased via a serial resistor of the order of mega-ohms.In a gated operation mode, the gate signal is appliedto the other end of the capacitor and the bias is ad-justed just below the breakdown voltage of the diode.

Fig. 1. Passive quenching and gating circuit of the avalanchephotodiode operating as a photon counter.

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In a nongated mode, the bias is adjusted above thebreakdown voltage of the diode. The advantages ofthe passive circuit are its simple arrangement andthe possibility that it can amplify the output signalvia an amplifier to a desired level. An interestingmodification of the Geiger circuit has been tested forquantum cryptography applications6: The serial re-sistor is omitted and the avalanche is quenched bythe end of the gate signal. This version of a passivecircuit may be used when the time of arrival of aphoton of interest is limited to a time window ofseveral nanoseconds only. A detailed analysis of thepassive quenching of the avalanche photodiodes op-erated as photon counters was made by Cova et al.,2who pioneered the concept of an active quenchingcircuit. This active quenching circuit was developedsubsequently for silicon APDs operated as photoncounters and modified subsequently for various ap-plications.3 A similar scheme of an active quenchingand gating circuit is suitable for the operation of aphoton counter based on a germanium APD and op-erated in a cryogenic environment.4 However, theexisting active quenching and gating schemes are farfrom optimum for operating InGaAs�InP photodiodesas photon counters. There are two main reasons forthis: First, the differential serial resistance of theInGaAs�InP diode above its breakdown is severaltimes higher than those of the silicon and germaniumdiodes.5 Thus the avalanche-sensing comparatormust be capable of responding to low-amplitudepulses. Second, owing to the strong influence of theafterpulsing, it is desirable to keep the total chargethat flows across the structure after a breakdownminimal; this means lowering the diode bias belowthe breakdown level as soon as possible. These tworequirements are in direct contradiction: More-sensitive �higher gain� comparators have higherpropagation delays and vice versa.

A new active quenching and gating circuit designedspecifically for the SAM InGaAs�InP diode structureshas been developed. The circuit was optimized fromthe point of view of avalanche-current sensitivity andthe total charge flowing through the diode afterbreakdown. A block schematic of the circuit is shownin Fig. 2. The APD avalanche current is sensed by

the comparator �CO�, its output is prolonged by themonostable circuit �MC� and its output is combined inthe logical OR with the gate signal. The output of theOR drives a pulse-forming circuit �PF�, which controlsthe APD bias. The pulse-forming circuit permits ad-justment of the slew rate at which the diode is biasedabove breakdown to minimize the transient effect—cross talk—of the gate on switching. The triggerthreshold is fine tuned by a resistor �R2�; the APDcapacitance is compensated for by an adjustable ca-pacitor �C�. The circuit is constructed on a printedboard of the size of 20 mm � 40 mm by surface mounttechnology. The circuit topology and the use of ul-trafast active components are essential.

To check the active quenching circuit speed, I re-placed the APD with a 100-pF capacitor. The gatingsignal was provided from a pulse generator. Thecapacitor simulated the APD break as a response tothe gate on signal. The electrical signal at point A ofthe circuit was monitored with a digitizing oscillo-scope �sampling rate, 2Gs�s; 400-MHz bandwidth;passive probe, 100�� externally triggered synchro-nously with the generator output. The recordedpulse length at point A corresponds to the time be-tween events when the APD breakdown current ex-ceeds the trigger level and the APD bias is loweredbelow breakdown. The recorded pulse length was3.0 ns. Taking into account the bandwidth of theoscilloscope and the probe, I evaluated the responsetime of the entire circuit to be 2.6 ns. This result isin good agreement with the value of 1.9 ns that wascalculated on the basis of the individual componentsand signal propagation delays. The circuit is capa-ble of responding to pulses with amplitudes as low as10 mV.

4. Experimental Setup

Two types of commercially available diode were used:Fujitsu FPD5W1KS diodes with a 30-�m-diameteractive area and EG&G C30644E diodes with a 50-�m-diameter active area. Both fiber pigtailed andunpacked �two samples each� Fujitsu chips weretested, together with unpacked EG&G diodes �twosamples�. The measured results were quite similarfor the same diode type. Average values are pre-sented for each diode type. The output pulse of theEG&G diode biased 0.5 V above breakdown is plottedin Fig. 3. The output pulse operating the diode inthe Geiger circuit plotted in Fig. 1 was measured.

The new active quenching and gating circuit wasused to operate the diodes as photon counters. Thechips were cooled and the temperature stabilized in atemperature range of �60 to �30 °C. The custom-built thermostat was cooled with liquid nitrogen, andthe desired temperature was maintained bycomputer-controlled temperature monitoring andelectrical heating within �0.1 °C. Water condensa-tion was prevented by the use of a dry-air environ-ment. The arrangement of the test setup is plottedin Fig. 4. The standard scheme for a time-correlatedphoton-counting experiment was used. The laser di-ode was driven by 400-ps-long optical current pulses

Fig. 2. Schematic of an active quenching and gating circuit. Ab-breviations are defined in text. The resistors R1 are the compar-ator input loads.

6014 APPLIED OPTICS � Vol. 40, No. 33 � 20 November 2001

at 1.55 �m with an optical peak power of 10 mW.The signal was attenuated by a neutral-density filter�ND� with 50% transmission. The EG&G time-to-amplitude converter had an ultimate time resolutionof 20 ps. The data from the time-to-amplitude con-verter were processed in a multichannel analyzercard in a personal computer �PC�. The gate signalwas generated by a pulse generator.

5. Detector Performance: Results and Discussion

An example of the output data is shown in Fig. 5, acurve that represents time-correlated photon count-ing. The EG&G InGaAs�InP diode was operatedat 0.5 V above breakdown, the diode was kept at atemperature of �48 °C, and the timing resolutionwas 160 ps�channel. The experiment was com-

pleted at a repetition rate of 10 kHz. Consideringthe photon-counting statistics, I computed a dark-count rate of 260 kHz; a timing resolution of 3.2 nswas obtained. As the laser diode output pulse’slength and the resolution of the timing electronicsare much shorter, this dark-count rate correspondsto the timing resolution of the photon-counting de-tector itself.

The photon-counting sensitivity was estimatedfrom the corrected useful-signal detection rate. Thephoton-detection probability in absolute figures wascomputed from the experimental energy balance:The laser output power was 10 � 3 mW; the laserpulse length was 400 ps. The laser output pulseshape was nearly square; its rise and fall times wereshorter than 100 ps. The total energy generated at1.55 �m was 4 � 10�12 J�pulse. The diodes testedwere illuminated with a large divergent beam. Tak-ing into account the experimental geometry, i.e., thedistance of the photon counter from the laser diode�300 mm�, the detector-active area, the laser dioderadiation pattern, and the transmission �50%� of theneutral-density filter, permits the average number ofphotons per laser shot hitting the detector-active areato be computed. The resultant average photon fluxdensities were 1 and 0.3 photons per laser shot fordetector-active area diameters of 50 and 30 �m, re-spectively. In the optimized configuration �see be-low�, at �48 °C and 0.5 V above the breakdown, withthe EG&G diode, the photon detection probabilityreached 12%. Considering the estimated photonflux on the diode aperture, I calculated a quantumefficiency of �12 � 3�% for the EG&G diode. ForFujitsu diodes operated at 0.5 V above breakdown at�61 °C, the detection probability was 3.3%. Consid-ering the estimated photon flux on the diode apertureof 0.3 photon�laser fire, I calculated a quantum effi-ciency of �11 � 3�% for the Fujitsu unpacked diode.

The dependence of the dark-count rate of the de-tectors on operating temperature is plotted in Fig. 6.The diodes were operated at 0.5 V above their break-down voltage. The dark-count rates of all the diodestested decreased with temperature at a rate of 6 or-ders of magnitude for every 10-K step. It is worthmentioning here that the dark-count rate of the

Fig. 3. Output pulse of the EG&G diode, 0.5 V above breakdown,with passive quenching and gating at room temperature, recordedon a digitizing scope with an analog bandwidth of 400 MHz. Thefirst �longer� pulse is cross talk of the gate pulse; the second pulse�triangle� is a response to avalanche buildup.

Fig. 4. Experimental setup for InGaAs�InP photon-counter tests.The neutral-density filter, at 50% transmission. PC, personal com-puter.

Fig. 5. Time-correlated photon-counting data: 1.55-�m wave-length; laser pulse length, 0.4 ns; horizontal scale, 160 ps�channel;FWHM timing resolution.

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EG&G diodes was more than 1 order less than that ofthe Fujitsu diodes.

The dependence of the signal-to-noise ratio and thephoton-counting detector’s time resolution �FWHM�on temperature is plotted in Fig. 7. For evaluationof the signal-to-noise ratio �SNR�, the signal countand the effective dark-count rates measured in thetime-correlated photon-counting experiments wereused. For the EG&G diode, the time resolution waspractically constant within the entire temperaturerange, with a steep increase in temperatures below�48 °C. The Fujitsu diode did not exhibit this ef-fect; its time resolution remained more or less con-stant over the entire temperature range. On the

basis of the results presented on Fig. 7 we can con-clude that the optimum operating temperatures ofthe EG&G and the Fujitsu diodes are �48 and�61 °C, respectively.

In Fig. 8 there is a plot of the SNR and of the timingresolution as functions of the bias above the break-down at the optimum temperature. For the EG&Gdiode, the SNR remained relatively high for biases of0.1–0.6 V above the breakdown and dropped signifi-cantly for biases of 0.7 V or greater above breakdown.The timing resolution was significantly increased forbiases higher than 0.4 V above the breakdown. Thismeans that there exists a clearly defined optimumbiasing of the diode for a given temperature. For�48 °C the optimum bias is 0.55 V above breakdown,with a tolerance of 0.1 V only. Considering the de-pendence of the breakdown voltage of the diode ontemperature of �0.16 V�°C, one can conclude that thediode chip’s temperature has to be stabilized with anaccuracy of 0.5 °C or better. The Fujitsu diode’stemporal resolution and SNR are plotted in Fig. 8,bottom. It is worth mentioning that the timing res-olution of the Fujitsu diode was greatly improved forhigher biases above the breakdown. However, thedark-count rate was of the order of megahertz whenthe rate was biased more than 1 V above breakdownat these temperatures. In general, the Fujitsu di-odes exhibited higher timing resolution and a muchhigher dark-count rate than the EG&G diodes in theobserved temperature range.

For numerous applications such as time-resolvedspectroscopy, fiber-optics diagnostics, and quantumcryptography, a high repetition rate of photon count-ing is desired. The operation of the active quenchedand gated APDs at high gate repetition rates is lim-ited by the afterpulsing effect: The dark-count rateincreased with the repetition rate of the gating of the

Fig. 6. Effective dark count as functions of temperature, activequenching, and a 10-kHz repetition rate at 0.5 V above breakdown.

Fig. 7. Timing resolution FWHM and SNR as functions of tem-perature, active quenching, and a 10-kHz repetition rate at 0.5 Vabove breakdown.

Fig. 8. SNR and timing resolution FWHM as functions of voltageabove breakdown with active quenching, a 10-kHz repetition rate,an EG&G diode at �48 °C and a Fujitsu diode at �61 °C.

6016 APPLIED OPTICS � Vol. 40, No. 33 � 20 November 2001

photon-counting detector. Figure 9 plots the dark-count rate for gate rates of 1 kHz to 1 MHz for anEG&G detector at �48 °C, biased 0.5 V above break-down. The increase in the dark count with the gatefrequency was small, thus making this detector con-figuration attractive for high-repetition-rate systems.This result is in contrast with the results reportedearlier5; the relatively high operational temperatureand a new design of an active quenching and gatingcircuit are responsible for this improvement. Mea-surement of the increase in the dark-count rate forhigh gating repetition rates was limited by the ex-perimental setup employed. The measurementchain that was used limited the maximum repetitionrate to values below 1 MHz. Even higher maximumrepetition rates are expected to be achievable by useof the existing active quenching and gating circuit inconnection with the EG&G diode operated in a tem-perature range of �48 to �30 °C.

On the basis of the measured quantum efficienciesand dark-count rates of the detectors in optimizedconfigurations, the noise-equivalent power �NEP�values were computed to permit comparison of thesedetectors with others. NEP values of 7 � 10�16

W�Hz1�2 and 1 � 10�15 W�Hz1�2 were computed forthe EG&G and Fujitsu detectors, respectively.

The recommended operating temperature for theEG&G diodes of �48 °C can be obtained by thermo-electrical Peltier cooling. Long-term experience hasbeen acquired with silicon-based photon counterscustom packed in the EG&G housing with built-inPeltier coolers �EG&G series JxxTE3-66G�.8 Thethree-stage Peltier cooling element was enclosed inan evacuated package with an optical window, andthe photon-counting APD was attached to the coolend of the Peltier stack, together with the tempera-ture sensor. An external electronic circuit moni-tored and stabilized the temperature by controllingthe current flowing to the Peltier coolers. The de-tector temperature could be stabilized within �0.1 °Cin the range �65 to �20 °C; the total heat dissipationof the cooler was well below 2 W. The evacuatedpackage minimized heat transfer and eliminated thewater-condensation problem. These cooled detec-tors have been used in satellite laser ranging systems

for more than seven years without any performancedegradation of the cooler or of the detector.

6. Conclusion

The parameters of EG&G and Fujitsu InGas�InPavalanche photodiodes operated in active quench-ing and gating mode as photon counters in a tem-perature range �60 to �30 °C were investigated indetail. The design and construction of the activequenching and gating circuit were optimized foroperation with these diodes. In terms of dark-count rate and afterpulsing, the EG&G diode hasbeen found to be superior to the Fujitsu diode. Theoptimum operating conditions for the EG&G diodeshave been identified. As a result, I have con-structed a compact all-solid-state photon-countingdetector based on the EG&G C30644E InGaAs�InPavalanche photodiode, which exhibits a NoiseEquivalent Power of 7 � 10�16 W�Hz1�2 and a tim-ing resolution of 3.2 ns. The dark-count rate in-creased less than twice for the gate repetition ratesreaching 1 MHz. The detection chip was cooled to�48 °C. This temperature was achievable withcommercial thermoelectrical coolers. The exacttemperature had to be maintained within 0.5 °C.Subnanosecond timing resolution was achievablewith the Fujitsu diodes when the chips were biasedseveral volts above the breakdown voltage at tem-peratures below �60 °C; however, the dark-countrate exceeded several megahertz, and the NEP in-creased above 1 � 10�15 W�Hz1�2 in this setup.The design of the new active quenching and gatingcircuit together with the optimization of the diodeoperating conditions has permitted construction ofa small, compact, and rugged photon-counting de-tector package for the wavelength region near1.5-�m based on commercially available compo-nents and technologies. The detector package maybe used in quantum cryptography, optical time-domain reflectometry, laser ranging, and time-resolved imaging at the eye-safe wavelength and inother applications in which photon counting withnanosecond timing resolution is required.

This research was performed in the laboratories ofthe Czech Technical University in Prague with thepartial support of grant 102�00�0820 from the grantagency of the Czech Republic. The author is grate-ful to Melatronik Nachrichtentechnik GmbH, Ger-many, for providing the Fujitsu chips.

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