Design and Implementation of a Compact Single-Photon Counting Module

electronics

Article Design and Implementation of a Compact Single-Photon Counting Module

Ming Chen 1,2 , Chenghao Li 2, Alan P. Morrison 3, Shijie Deng 1,*, Chuanxin Teng 1, Houquan Liu 1, Hongchang Deng 1, Xianming Xiong 1,* and Libo Yuan 1

1 Guangxi Key Laboratory of Optoelectronic Information Processing, School of Electronic Engineering and Automation, Guilin University of Electronics Technology, Guilin 541004, China; [email protected] (M.C.); [email protected] (C.T.); [email protected] (H.L.); [email protected] (H.D.); [email protected] (L.Y.) 2 School of Information and Communication, Guilin University of Electronics Technology, Guilin 541004, China; [email protected] 3 Department of Electrical and Electronic Engineering, University College Cork, Cork T12 K8AF, Ireland; [email protected] * Correspondence: [email protected] (S.D.); [email protected] (X.X.)

Received: 2 June 2020; Accepted: 9 July 2020; Published: 11 July 2020

Abstract: A compact single-photon counting module that can accurately control the bias voltage and hold-off time is developed in this work. The module is a microcontroller-based system which mainly consists of a microcontroller, a programmable negative voltage generator, a silicon-based single-photon avalanche diode, and an integrated active quench and reset circuit. The module is 3.8 cm 3.6 cm 2 cm in size and can communicate with the end user and be powered through a × × USB cable (5 V). In this module, the bias voltage of the single-photon avalanche diode (SPAD) is precisely controllable from 14 V ~ 38 V and the hold-off time (consequently the dead time) of the − − SPAD can be adjusted from a few nanoseconds to around 1.6 µs with a setting resolution of 6.5 ∼ ns. Experimental results show that the module achieves a minimum dead time of around 28.5 ns, giving a saturation counting rate of around 35 Mcounts/s. Results also show that at a controlled reverse bias voltage of 26.8 V, the dark count rate measured is about 300 counts/s and the timing jitter measured is about 158 ps. Photodetection probability measurements show that the module is suited for detection of visible light from 450 nm to 800 nm with a 40% peak photon detection efficiency achieved at around 600 nm.

Keywords: single-photon counting; compact module; bias voltage control; hold-oﬀ time setting

1. Introduction Single-photon counting techniques have been used in low-light sensing applications such as LIDAR [1,2], quantum key distribution [3], medical imaging technology [4], and 3D imaging technology [5,6]. Photomultiplier tubes have been the traditional solution for photon counting applications, but in recent years single-photon avalanche diode (SPAD) has become an alternative candidate due to its lower cost, lower operating voltage, higher sensitivity, and smaller size. The dead time in a SPAD is the off time when the SPAD is quenched after every avalanche event to dissipate trapped charge, thereby minimizing the “afterpulsing” phenomenon. As a result, a trade-off exists between the maximum photon counting rate and an acceptable level of noise. An accurate adjustment of the dead time is important to achieve an optimal dead time (and consequently optimal maximum photon counting rate) in a single-photon counting system. The bias voltage has a major effect on the SPAD’s important performance parameters such as dark count rate (DCR) and photon detection

Electronics 2020, 9, 1131; doi:10.3390/electronics9071131 www.mdpi.com/journal/electronics Electronics 2020, 9, 1131 2 of 11 Electronics 2020, 9, x FOR PEER REVIEW 2 of 11 edetectionfficiency (PDE).efficiency A higher(PDE). biasA higher voltage bias will voltage lead towill better lead PDE to better due toPDE the due increase to the in increase the avalanche in the triggeravalanche probability. trigger probability. However, theHowever, potential the drawbacks potential includedrawbacks higher include DCR higher and the DCR considerable and the afterpulsingconsiderable e afterpulsingffect. Therefore, effect. the Therefore, bias voltage the shouldbias voltage be adjustable should be and adjustable optimized and to optimized achieve best to overallachieve performance. best overall Severalperformance. SPAD-based Several single-photon SPAD-based countingsingle-photon modules counting have been modules developed have [been7–17 ] butdeveloped these modules [7–17] but are these limited modules in that are the limited dead in time that and thebias dead voltage time and of bias the detectorvoltage of are the di detectorfficult to change.are difficult This limitsto change. their This usefulness limits their in varying usefulne environmentsss in varying and environments for the case whereand for the the detector case where needs tothe be detector changed needs for di fftoerent be changed measurement for different purposes. measurement In addition, purposes. existing designsIn addition, are usually existing bulky designs and cumbersome,are usually bulky which and limits cumbersome, their usefulness which limits in applications their usefulness that require in applications compact that solutions. require compact solutions.In this work, a compact multi-parameter adjustable single-photon counting module is developed. The moduleIn this is work, a microcontroller-based a compact multi-parameter system mainly adjustable consisting single-photon of a programmable counting negative module voltage is generator,developed. a The silicon-based module is SPAD,a microcontroller-based an active quenching system and mainly reset integrated consisting circuit of a programmable (AQR-IC) and I/negativeO components. voltage generator, In this module, a silicon-based a custom SPAD, designed an active SPAD quenching was fabricated and reset and usedintegrated with circuit its bias voltage(AQR-IC) precisely and I/O controllable components. from In this14 Vmodule, ~ 38 Va usingcustom a programmabledesigned SPAD negative was fabricated voltage and generator. used − − Withwith theits bias AQR-IC, voltage the precisely hold-off controllabletime (consequently from −14 theV ~ − dead38 V time)using ina programmable the SPAD can negative be adjusted voltage from agenerator. few nanoseconds With the to AQR-IC, around the 1.6 µhold-offs with atime setting (consequently resolution the of dead6.5 ns. time) A microcontroller in the SPAD can of be the adjusted from a few nanoseconds to around 1.6 μs with a setting∼ resolution of ∼6.5 ns. A module is used to drive and control the SPAD’s bias voltage and hold-off time which can also be microcontroller of the module is used to drive and control the SPAD’s bias voltage and hold-off time connected to a PC through USB for the graphical user interface. With the control of the bias voltage and which can also be connected to a PC through USB for the graphical user interface. With the control high-resolution setting of the dead time, intelligent control can be added to the module for detection of the bias voltage and high-resolution setting of the dead time, intelligent control can be added to optimizations under varying environments that greatly improve the SPAD-based photon counting the module for detection optimizations under varying environments that greatly improve the SPAD- system’s robustness. In addition, the module developed is about 3.8 cm 3.6 cm 2 cm in volume, based photon counting system’s robustness. In addition, the module developed× is about× 3.8 cm × 3.6 allowing it to be used in compact photon counting systems. Experimental results show that the module cm × 2 cm in volume, allowing it to be used in compact photon counting systems. Experimental achieves a minimum dead time of around 28.5 ns which demonstrates a saturation counting rate results show that the module achieves a minimum dead time of around 28.5 ns which demonstrates of around 35 Mcounts/s. At a controlled reverse bias voltage of 26.8 V, the DCR measured is about a saturation counting rate of around 35 Mcounts /s. At a controlled reverse bias voltage of 26.8 V, the 300 counts/s and the timing jitter measured is about 158 ps. Photodetection probability measurements DCR measured is about 300 counts/s and the timing jitter measured is about 158 ps. Photodetection show that the module is suited for detection of visible light from 450 nm to 800 nm with a 40% peak probability measurements show that the module is suited for detection of visible light from 450 nm PDE at around 600 nm. to 800 nm with a 40% peak PDE at around 600 nm. 2. System Description 2. System Description Figure1 shows the block diagram of the single-photon counting module consisting of the following Figure 1 shows the block diagram of the single-photon counting module consisting of the parts: (1) A Microcontroller (STC89C52RC) circuitry which is used to communicate with the user following parts: (1) A Microcontroller (STC89C52RC) circuitry which is used to communicate with end (PC) via an USB to TTL chip and also used for the control of bias voltage and the hold-off time; the user end (PC) via an USB to TTL chip and also used for the control of bias voltage and the hold- (2) A programmable negative voltage generator which is used to generate a stable and controllable off time; (2) A programmable negative voltage generator which is used to generate a stable and negative high voltage for the SPAD; (3) An active quenching and reset integrated circuit (AQR-IC) for controllable negative high voltage for the SPAD; (3) An active quenching and reset integrated circuit the hold-off time setting in the SPAD and (4) A custom designed SPAD. (AQR-IC) for the hold-off time setting in the SPAD and (4) A custom designed SPAD.

FigureFigure 1.1. Block diagram of the single-photo single-photonn counting counting module module developed. developed.

2.1. Microcontroller Circuitry Electronics 2020, 9, x FOR PEER REVIEW 3 of 11 Electronics 2020, 9, 1131 3 of 11 In the microcontroller circuitry, a crystal oscillator is used to provide the clock pulses to the microcontroller chip which is powered through a USB cable. The circuitry is used to receive the 2.1.control Microcontroller information Circuitry (including hold-off time and bias voltage value) from PC (end user) through the USB-TTLIn the converter microcontroller chip and circuitry, sends athe crystal control oscillator signals isto usedrelated to circuitry provide thefor clockthe bias pulses voltage to theand microcontrollerhold-off time setting chip which in the isSPAD. powered The throughmicrocontroller a USB cable.sets the The eight circuitry binary ports is used S0~S to7 receiveof the AQR- the controlIC, from information 0 (“00000000”) (including to 255 hold-o (“11111111”)ff time andto achieve bias voltage the hold-off value) time from of PC 6.5 (end ns~1.6 user) μs throughin the SPAD. the USB-TTLIt also sends converter “ADJ” chip signal and to sends the programmable the control signals negative to related voltage circuitry generator for to the achieve bias voltage the required and negative bias voltage for biasing the SPAD. hold-off time setting in the SPAD. The microcontroller sets the eight binary ports S0~S7 of the AQR-IC, from 0 (“00000000”) to 255 (“11111111”) to achieve the hold-off time of 6.5 ns~1.6 µs in the SPAD. It2.2. also Programmable sends “ADJ” Negative signal to Voltage the programmable Generator negative voltage generator to achieve the required negativeFigure bias 2 voltage shows forthe biasing schematic the SPAD.of the programmable negative voltage generator. In the circuit, a boost circuit (consists of a bipolar transistor, an inductor, a diode and two capacitors) is used to 2.2. Programmable Negative Voltage Generator generate high negative output voltage and a MAX749 chip (digitally adjustable bias regulator) [18]- basedFigure circuit2 shows is used the to schematic drive the ofboost the programmablecircuit, detect the negative feedback voltage signal, generator. and maintain In the the circuit, output a boostvoltage circuit at a (consistsset value. of The a bipolar output transistor, of the programmable an inductor, a diodenegative and voltage two capacitors) generator is usedis determined to generate by highthe negativeinternal reference output voltage current, and IREF a MAX749 of the chip chip which (digitally is setadjustable by the microcontroller bias regulator) (through [18]-based signal circuit line, isADJ). used When to drive the the voltage boost generator circuit, detect is operating, the feedback if the current signal, flowing and maintain through the the output feedback voltage resistance at a set(R value.FB), IS , Theis greater output than of the the programmable reference current, negative IREF, the voltage boost generator circuit will is determinedbe stopped and by the the internal absolute referencevalue of current,the output IREF voltageof the will chip drop which and is the set I byS will the decrease. microcontroller If IS falls (through below IREF signal, the boost line, ADJ).circuit Whenwill be the driven voltage to generator continue is to operating, boost the if output the current and flowingthe absolute through value the of feedback the output resistance voltage (R FBwill), ISincrease, is greater and than the theIS will reference increase. current, In this IREF way,, the IS boostcan be circuit set equal will to be I stoppedREF and the and output the absolute voltage value −VOUT ofcan the be output controlled voltage as follows: will drop and the IS will decrease. If IS falls below IREF, the boost circuit will be driven to continue to boost the output and the absolute value of the output voltage will increase −V =I ×R (1) and the I will increase. In this way, I can be set equal to I and the output voltage V can be S S REF − OUT controlledIn this as follows:design, the feedback resistance RFB is set to 2 MΩ and the reference current can be set from around 7 μA to 19 μA. This makesV OUTthe =settingIREF rangeRFB of the programmable negative voltage(1) generator to be around −14 V ~ −38 V. − ×

FigureFigure 2. 2.Schematic Schematic of of programmable programmable negative negative voltage voltage generator. generator.

2.3.In SPAD this design,and AQR-IC the feedback resistance RFB is set to 2 MΩ and the reference current can be set from around 7 µA to 19 µA. This makes the setting range of the programmable negative voltage generator to be aroundThe SPAD14 and V ~ AQR-IC38 V. used in the module are previously designed chips [19–22]. The SPAD is a 20 μm diameter− planar− SPAD which is based on a shallow p-n junction. It is manufactured in p- type epitaxially grown bulk silicon using a 1.5 μm complementary metal–oxide–semiconductor (CMOS) compatible process and suited for detection of visible wavelengths (from 400 nm to 850 nm) [19,20]. In the SPAD, a p-substrate with a p-type doping forming the central active area and the N+ Electronics 2020, 9, 1131 4 of 11

2.3. SPAD and AQR-IC The SPAD and AQR-IC used in the module are previously designed chips [19–22]. The SPAD is a 20 µm diameter planar SPAD which is based on a shallow p-n junction. It is manufactured in p-type epitaxially grown bulk silicon using a 1.5 µm complementary metal–oxide–semiconductor (CMOS) compatible process and suited for detection of visible wavelengths (from 400 nm to 850 nm) [19,20]. Electronics 2020, 9, x FOR PEER REVIEW 4 of 11 In theElectronics SPAD, 2020 a, p-substrate9, x FOR PEER withREVIEW a p-type doping forming the central active area and the N+ doping4 of 11 overlapsdoping theoverlaps active the area, active forms area, the diodeforms cathode,the diode and cathode, contact and to metalcontact layers. to metal The layers. overlapping The doping overlaps the active area, forms the diode cathode, and contact to metal layers. The guardoverlapping ring, and guard the diode ring, and forms the with diode the forms p-substrate, with the prevents p-substrate, edge prevents breakdown edge ofbreakdown the central of activethe overlapping guard ring, and the diode forms with the p-substrate, prevents edge breakdown of the area.central The active AQR-IC area. is usedThe AQR-IC to actively is used quench to actively the SPAD quench after eachthe SPAD avalanche after eventeach avalanche (lower its event reverse central active area. The AQR-IC is used to actively quench the SPAD after each avalanche event bias(lower voltage its reverse below itsbias breakdown voltage below voltage), its breakdown keep it in voltage), “OFF” statekeep forit in a “OFF” period state of user-defined for a period timeof (lower its reverse bias voltage below its breakdown voltage), keep it in “OFF” state for a period of (hold-ouser-definedff time) time and (hold-off set the reversetime) and bias set the voltage reverse back bias to voltage its original back to levelits original for the level next for avalanchethe next user-defined time (hold-off time) and set the reverse bias voltage back to its original level for the next detection.avalanche The detection. hold-o ffThetime hold-off (and thetime dead (and time)the dead is controlled time) is controlled by the end by the user end through user through the PC the and avalanche detection. The hold-off time (and the dead time) is controlled by the end user through the thePC microcontroller and the microcontroller chip. With chip. the developedWith the develo activeped quenching active quenching and reset and IC, reset the maximumIC, the maximum counting PC and the microcontroller chip. With the developed active quenching and reset IC, the maximum counting rate of the module can be greatly extended and the after pulsing effects can be effectively ratecounting of the module rate of the can module be greatly can extendedbe greatly andextend theed after and pulsingthe after epulsingffects can effects be e ffcanectively be effectively reduced. reduced. The photos of the fabricated SPAD and AQR-IC can be seen in Figure 3a,b, respectively. Thereduced. photos The of the photos fabricated of the SPADfabricated and SPAD AQR-IC and can AQR-IC be seen can in be Figure seen 3ina,b, Figure respectively. 3a,b, respectively.

(a) (b) (a) (b) Figure 3. Photos of the custom designed and fabricated (a) SPAD and (b) AQR-IC. FigureFigure 3. 3.Photos Photos of of the the custom custom designed designed andand fabricatedfabricated ( a)) SPAD SPAD and and ( (bb)) AQR-IC. AQR-IC. 3. Experimental3. Experimental Results Results 3. Experimental Results FigureFigure4 shows 4 shows the the assembled assembled single-photon single-photon countingcounting module.module. The The module module consists consists of of a amain main Figure 4 shows the assembled single-photon counting module. The module consists of a main board,board, a cubica cubic black black package, package, and and aa ﬁberfiber adapteradapter that allows coupling coupling of of incident incident light light to tothe the SPAD SPAD board, a cubic black package, and a fiber adapter that allows coupling of incident light to the SPAD throughthrough a ﬁber.a fiber. through a fiber.

Figure 4. The assembled single-photon counting module. FigureFigure 4. 4.The The assembled assembled single-photon single-photon counting module. module. Figure 5a shows the experimental results for the reverse bias voltage setting in the module. By Figure 5a shows the experimental results for the reverse bias voltage setting in the module. By inputting the required voltage in the user graphic interface (GUI), the bias voltage will be adjusted. inputting the required voltage in the user graphic interface (GUI), the bias voltage will be adjusted. Results show that the bias voltage can be accurately set from −14 V to −38 V with standard deviation Results show that the bias voltage can be accurately set from −14 V to −38 V with standard deviation of less than 0.06 V and output voltage ripples of less than 50 mV. The setting range and resolution of less than 0.06 V and output voltage ripples of less than 50 mV. The setting range and resolution can be changed by setting the feedback current flowing to the feedback port of the MAX749 chip in can be changed by setting the feedback current flowing to the feedback port of the MAX749 chip in the programmable negative voltage generator circuit. Figure 5b shows the setting of the hold-off time the programmable negative voltage generator circuit. Figure 5b shows the setting of the hold-off time Electronics 2020, 9, 1131 5 of 11

Figure5a shows the experimental results for the reverse bias voltage setting in the module. By inputting the required voltage in the user graphic interface (GUI), the bias voltage will be adjusted. Results show that the bias voltage can be accurately set from 14 V to 38 V with standard deviation − − of less than 0.06 V and output voltage ripples of less than 50 mV. The setting range and resolution ElectronicscanElectronics be changed 20202020,, 99,, xx FOR byFOR setting PEERPEER REVIEWREVIEW the feedback current flowing to the feedback port of the MAX749 chip55 ofof 11in11 the programmable negative voltage generator circuit. Figure5b shows the setting of the hold-o ff time in SPAD of the module. The hold-off time can also be adjusted using the PC-based GUI. As can be in SPADSPAD ofof thethe module.module. The hold-ohold-offff time can alsoalso bebe adjustedadjusted usingusing the PC-basedPC-based GUI. As can be seen from the figure, by changing the input code from “1” to “255”, the hold-off time in the SPAD seen fromfrom thethe figure,figure, by by changing changing the the input input code code from from “1” “1” to to “255”, “255”, the the hold-o hold-offff time time in the in the SPAD SPAD can can be linearly and precisely adjusted from several nano seconds to 1.6 μs with a setting resolution becan linearly be linearly and and precisely precisely adjusted adjusted from from several several nano nano seconds seconds to 1.6to 1.6µs withμs with a setting a setting resolution resolution of of about 6.5 ns. With the minimum hold-off time, a dead-time of around 28.5 ns can be achieved, aboutof about 6.5 6.5 ns. ns. With With the minimumthe minimum hold-o hold-offff time, time, a dead-time a dead-time of around of around 28.5 ns 28.5 can bens achieved,can be achieved, giving a giving a saturation counting rate of around 35 Mcounts/s. saturationgiving a saturation counting counting rate of around rate of 35 around Mcounts 35 /Mcounts/s.s.

((aa)) ((bb))

FigureFigure 5. 5. (((aa))) TheThe biasbias voltagevoltage boostboost circuit’scircuit’s outputoutput vovo voltageltageltage withwith thethe voltagevoltage setset onon computer; computer; (( bb)) ExternalExternal input input codes codes versus versus hold-off hold-ohold-offﬀ time.time.

ToTo test test the the PDE PDE and and the the dark dark count count characteristics characteristics of of the the module, module, an an experimental experimental setup setup shown shown inin Figure FigureFigure 6 66 was waswas built. built.built. InInIn thethethe setup,setup,setup, aaa broadbroadbroad bandbandband lightlightlight sourcesourcesource (Thorlabs(Thorlabs(Thorlabs SLS201LSLS201L/M)SLS201L/M)/M) is is used used with with its its outputoutput lightlight guidedguidedguided to toto a a filtera filterfilter holder holderholder (Thorlabs (Thorlabs(Thorlabs FOFMF FOFOFMF/M)FMF/M)/M) by abyby fiber aa fiberfiber and theandand output thethe outputoutput of the of filterof thethe holder filterfilter holderisholder connected isis connectedconnected to the SPAD toto thethe using SPADSPAD another usingusing anotheranother fiber. Optical fiber.fiber. Optical band-passOptical band-passband-pass filters and filtersfilters attenuation andand attenuationattenuation filters are filtersfilters used aretoare alter usedused the toto spectrum alteralter thethe andspectrumspectrum power andand intensity powerpower of intensitintensit the lightyy ofof directed thethe lightlight to thedirecteddirected SPAD. toto A thethe counter SPAD.SPAD. (FCA3100) AA countercounter is (FCA3100)used(FCA3100) to record isis usedused the to pulseto recordrecord counting thethe pulsepulse rate councoun at thetingting output raterate at ofat the the output module.output ofof thethe module.module.

Figure 6. Experimental setup for the dark count rate and photon detection efficiency measurement. Figure 6. Experimental setup for the dark count rate an andd photon detection eefficiencyﬃciency measurement.

FigureFigure 77 showsshows thethe DCRDCR measuredmeasured forfor differentdifferent biasbias voltages.voltages. ThisThis measurementmeasurement waswas carriedcarried outout whenwhen thethe lightlight sourcesource isis turnedturned offoff andand thethe hohold-offld-off timestimes waswas setset toto aboutabout 100100 ns,ns, 500500 ns,ns, andand 10001000 ns.ns. ResultsResults showshow thatthat withwith overranoverran excessexcess biasbias voltagevoltage ofof 11 VV (reverse(reverse biasbias voltagevoltage ofof aroundaround 25.625.6 V),V), thethe DCRDCR inin thethe SPADSPAD isis aboutabout 8080 counts/scounts/s andand whenwhen thethe excessexcess biasbias voltagevoltage increasedincreased toto 33 V,V, thethe DCRDCR increasesincreases toto aboutabout 11 kcounts/s.kcounts/s. FigureFigure 88 showsshows thethe PDEPDE measuredmeasured forfor differentdifferent wawavelengthsvelengths andand reversereverse biasbias voltages.voltages. InIn thethe measurement,measurement, aa broad-spectrumbroad-spectrum lightlight sourcesource isis usedused toto provideprovide thethe incidentincident lightlight toto thethe modulemodule andand opticaloptical filtersfilters areare usedused toto selectselect thethe specificspecific wavelengthwavelength andand alteralter thethe powerpower intensityintensity ofof thethe incidentincident Electronics 2020, 9, 1131 6 of 11

Electronics 2020, 9, x FOR PEER REVIEW 6 of 11 Figure7 shows the DCR measured for di fferent bias voltages. This measurement was carried out light.when For the lighteach sourcespecific is wavelength, turned off and the theincident hold-o lightff times power was and set tothe about photon 100 counting ns, 500 ns, rate and of 1000 the modulens. Results are showmeasured that and with the overran PDE of excess the module bias voltage is calculated. of 1 V (reverse Results show bias voltage that the of module around is 25.6 suited V), forthe detecting DCR in the the SPAD short is wavelengths about 80 counts of the/s and incident when light the excessfrom 400 bias nm voltage to 800 increased nm withto a 3peak V, the PDE DCR of aboutincreases 40% to achieved about 1 kcountsat 600 nm./s.

Electronics 2020, 9, x FOR PEER REVIEW 6 of 11 light. For each specific wavelength, the incident light power and the photon counting rate of the module are measured and the PDE of the module is calculated. Results show that the module is suited for detecting the short wavelengths of the incident light from 400 nm to 800 nm with a peak PDE of about 40% achieved at 600 nm.

Figure 7. DarkDark count count rate measured for varies bias voltages.

Figure8 shows the PDE measured for di fferent wavelengths and reverse bias voltages. In the measurement, a broad-spectrum light source is used to provide the incident light to the module and optical filters are used to select the specific wavelength and alter the power intensity of the incident light. For each specific wavelength, the incident light power and the photon counting rate of the module are measured and the PDE of the module is calculated. Results show that the module is suited for detecting the short wavelengths of the incident light from 400 nm to 800 nm with a peak PDE of about 40% achieved at 600Figure nm. 7. Dark count rate measured for varies bias voltages.

Figure 8. Dependence of photon detection efficiency on the incident wavelength for different bias voltages.

Figure 9 shows the experimental setup for measuring the afterpulsing probability. A pulsed pico-second laser source operating at 10 kHz was used to generate the incident light which was coupled to the SPAD of the module. The laser source used is from ALPHALAS GmbH and the part number of the laser driver and the diode laser are “PLDD-50M” and “PICOPOWER-LD-660-50” respectively. The output of the module is connected to the two ports (start and stop) of a Time-to- digitalFigureFigure converter 8. DependenceDependence (TDC). The of of TDC’sphoton photon “start”detection detection channel efficiency eﬃciency is triggered on onthe theincident by incident the wavelength rising wavelength edge for of differentthe for output diﬀerent bias pulse of thebiasvoltages. module voltages. and the “stop” channel is triggered by the falling edge of the next neighboring pulse. In this way, the time interval between every two pulses is recorded. After a large amount of data Figure9 shows the experimental setup for measuring the afterpulsing probability. A pulsed collectionFigure (around 9 shows 10,000 the experimentalsamples), the setup distribution for meas histogramuring the ofafterpulsing the interval probability. times between A pulsed two pico-second laser source operating at 10 kHz was used to generate the incident light which was coupled adjacentpico-second pulses laser can source be built operating and the afterpulat 10 kHzsing was probability used to cangenerate be concluded the incident [23–26]. light which was tocoupled the SPAD to the of theSPAD module. of the Themodule. laser The source laser used source is from used ALPHALAS is from ALPHALAS GmbH and GmbH the partand numberthe part ofnumber the laser of driverthe laser and driver the diode and laserthe diode are “PLDD-50M” laser are “PLDD-50M” and “PICOPOWER-LD-660-50” and “PICOPOWER-LD-660-50” respectively. Therespectively. output of The the moduleoutput of is connectedthe module to is the connected two ports to (start the two and ports stop) of(start a Time-to-digital and stop) of a converter Time-to- (TDC).digital Theconverter TDC’s (TDC). “start” The channel TDC’s is triggered“start” channel by the is rising triggered edge ofby the the output rising edge pulse of of the the output module pulse and of the module and the “stop” channel is triggered by the falling edge of the next neighboring pulse. In this way, the time interval between every two pulses is recorded. After a large amount of data collection (around 10,000 samples), the distribution histogram of the interval times between two adjacent pulses can be built and the afterpulsing probability can be concluded [23–26]. Electronics 2020, 9, 1131 7 of 11 the “stop” channel is triggered by the falling edge of the next neighboring pulse. In this way, the time interval between every two pulses is recorded. After a large amount of data collection (around 10,000 samples), the distribution histogram of the interval times between two adjacent pulses can be built and the afterpulsing probability can be concluded [23–26]. Electronics 2020, 9, x FOR PEER REVIEW 7 of 11 Electronics 2020, 9, x FOR PEER REVIEW 7 of 11

Figure 9. ExperimentalExperimental setup setup for for measuring measuring SPAD’s SPAD’s afterpulsing probability.

Figure 10 shows the measured afterpulsing probability in the module for different different hold-off hold-off times withwith aa biasbias voltagevoltage of of 27.6 27.6 V. V. Results Results show show that that when when the the hold-o hold-offff time time of the of SPADthe SPAD is set is to set more to more than 500than ns, 500 the ns, afterpulsing the afterpulsing probability probability can be can significantly be significantly limited limited to below to below 0.3%. 0.3%. Results Results also also show show that asthat the as bias the voltagebias voltage increases, increases, the afterpulsing the afterpulsing probability probability will alsowill rise.also rise.

Figure 10. AfterpulsingAfterpulsing probability probability versus different diﬀerent hold-ohold-offﬀ times.times.

To measuremeasure thethe timingtiming jitterjitter of the module, the experimental setup shown in Figure Figure 1111 waswas built. built. InIn the setup, a 660 nm pulsed laser is used used to to gene generaterate a pulsed pulsed light light signal signal to the the SPAD SPAD of the module and the time delay between the laser synchronized pulsepulse signal and the pulses of the module’s output isis measuredmeasured usingusing aa TDC.TDC. TheThe timetime delays’delays’ histogramhistogram distributiondistribution isis thenthen builtbuilt andand itsits fullfull widthwidth halfhalf maximum (FWHM)(FWHM) isis takentaken asas thethe system’ssystem’s timingtiming jitterjitter [[27].27]. Figure 12 shows the timing responses of the module for different bias voltages. In the measurement, the hold-off time was set to about 200 ns.ns. AsAs cancan bebe seenseen fromfrom thethe figure,figure, increasingincreasing the bias voltage results in narrower timing jitter and a timing jitter of 145 ps was measured at the bias voltage of 27.6 V. Electronics 2020, 9, 1131 8 of 11 Electronics 2020, 9, x FOR PEER REVIEW 8 of 11

Electronics 2020, 9, x FOR PEER REVIEW 8 of 11

Figure 11. Experimental setup for measuring timing response of the module.

Figure 12 shows the timing responses of the module for different bias voltages. In the measurement, the hold-off time was set to about 200 ns. As can be seen from the figure, increasing the bias voltage results in narrower timing jitter and a timing jitter of 145 ps was measured at the bias voltage of 27.6 V. Figure 11. Experimental setup for measuring timing response of the module.

Figure 12. Timing responses of the single-photon counting module.

To test the heat dissipation of the module, the module was operated continually at room temperature (20 °C) for more than 3 hours and the temperature of the main chips and components on the board (including microcontroller chip, Max749 (boost circuit) chip, USB to TTL chip, DAC chip, Figure 12. Timing responses of the single-photon counting module. 5V–3V stabilizer, chipFigure crystal 12. Timingoscillators, responses SPAD of and the single-photonAQR-IC) was counting measured. module. Results show that most the main components are kept cool with the temperature of less than 30 °C. To testtest thethe heatheat dissipationdissipation ofof thethe module,module, thethe modulemodule waswas operatedoperated continuallycontinually at roomroom The breakdown voltage and DCR temperature dependency (using a temperature chamber) were temperaturetemperature (20(20 ◦°C)C) forfor moremore than than 3 3 hours hours and and the the temperature temperature of of the the main main chips chips and and components components on also tested. As can be seen from Figure 13, the breakdown voltage of the SPAD rises with increasing theon the board board (including (including microcontroller microcontroller chip, chip, Max749 Max749 (boost (boost circuit) circuit) chip, chip, USB USB to to TTL TTL chip, chip, DAC DAC chip,chip, 5temperature5V–3V V–3 V stabilizer, stabilizer, at about chip chip 0.024 crystal crystal V peroscillators, oscillators, °C. Figure SPAD SPAD 14 shows and AQR-IC) the plot wasof th measured.e DCR for differentResults show temperatures, that mostmost in this case the excess bias voltage is kept at 1 V. Results show that the DCR increases with increasing thethe mainmain componentscomponents are are kept kept cool cool with with the the temperature temperature of less of less than than 30 ◦ C.30 °C. temperatureThe breakdown by about voltage 23 counts/s and DCR per ° temperatureC. dependency (using a temperature chamber) were also tested. As can be seen from Figure 13,13, the breakdownbreakdown voltage of the SPAD rises with increasing temperaturetemperature atat aboutabout 0.024 V per ◦°C. Figure 14 shows the plot of th thee DCR for different diﬀerent temperatures, inin thisthis casecase the excess bias voltage is kept at 1 V. Results show that the DCR increases with increasing temperaturetemperature byby aboutabout 2323 countscounts/s/s perper ◦°C.C. Electronics 2020, 9, 1131 9 of 11 ElectronicsElectronics 20202020,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 99 ofof 1111

FigureFigure 13. 13. BreakdownBreakdown voltagevoltage measured measured for for various various temperatures. temperatures.

FigureFigure 14. 14. DarkDark countcount raterate measured measured for for various various temperatures. temperatures.

4.4. ConclusionsConclusions InIn thisthis this paper,paper, paper, thethe the design,design, design, thethe the implementation,implementation, implementation, andand andthethe characterizationcharacterization the characterization ofof newnew of compactcompact new compact single-single- photonsingle-photonphoton countingcounting counting modulemodule module isis presented.presented. is presented. TheThe modulemodule The module enenablesables enables accurateaccurate accurate digitaldigital controlcontrol digital of controlof thethe biasbias of thevoltagevoltage bias andvoltageand hold-offhold-off and hold-o timetime ffofoftime thethe of SPAD.SPAD. the SPAD. ThisThis Thisenablesenables enables thethe theintelligentintelligent intelligent optimizationoptimization optimization ofof of photonphoton photon detectiondetection detection in varyingvarying environments environments toto enhanceenhance thethe module’smodule’s robustness.rorobustness.bustness. In In the the system, system, a a programmable programmable negative negative voltagevoltage generatorgeneratorgenerator was waswas designed designeddesigned to toto provide provideprovide the thethe bias biasbias voltage voltagevoltage for forfor the thethe SPAD SPADSPAD and andand an active anan activeactive quench quenchquench and reset andand resetchipreset was chipchip developed waswas developeddeveloped for the hold-oforfor thetheff timehold-offhold-off control, timetime afterpulsing control,control, afterpulsingafterpulsing reduction, and reduction,reduction, maximum andand counting maximummaximum rate countingextension.counting raterate The extension.extension. new implementation TheThe newnew implementationimplementation includes a silicon includesincludes SPAD aa fabricatedsiliconsilicon SPADSPAD in-house fabricatedfabricated is used in-housein-house as the photonisis usedused asdetectoras thethe photonphoton and a detectordetector microcontroller andand aa microcontrollermicrocontroller system supervises systemsystem the user supervisessupervises interface thethe and useruser the interfaceinterface setting of andand bias thethe voltage settingsetting and ofof hold-off time. The module is 3.8 cm 3.6 cm 2 cm in size and can be powered using a USB connection biasbias voltagevoltage andand hold-offhold-off time.time. TheThe ×modulemodule isis× 3.83.8 cmcm ×× 3.63.6 cmcm ×× 22 cmcm inin sizesize andand cancan bebe poweredpowered usingusing athata USBUSB allows connectionconnection it to be usedthatthat inallowsallows compact itit to single-photonto bebe usedused inin counting compactcompact applications. single-photonsingle-photon Experimental countingcounting applications.applications. results show that the module can provide a controlled bias voltage for the SPAD from 14 V ~ 38 V with a ExperimentalExperimental resultsresults showshow thatthat thethe modulemodule cancan provprovideide aa controlledcontrolled biasbias voltagevoltage− forfor thethe− SPADSPAD fromfrom −setting−1414 VV ~~ resolution −−3838 VV withwith of aa 0.4sesetttt Vinging and resolutionresolution the hold-o ofofff 0.40.4time VV andand (consequently thethe hold-offhold-off the timetime dead (consequently(consequently time) in the the SPADthe deaddead from time)time) a few inin nanoseconds to around 1.6 µs with a setting resolution of 6.5 ns. Results showed the module can thethe SPADSPAD fromfrom aa fewfew nanosecondsnanoseconds toto aroundaround 1.61.6 μμss withwith∼ aa settingsetting resolutionresolution ofof ∼∼6.56.5 ns.ns. ResultsResults showedachieveshowed a thethe minimum modulemodule dead cancan achieveachieve time of 28.5aa minimumminimum ns that leads deaddead to timetime a maximum ofof 28.528.5 nsns counting thatthat leadsleads rate toto of aa around maximummaximum 35 Mcounts countingcounting/s. rateAtrate a of controlledof aroundaround 35 bias35 Mcounts/s.Mcounts/s. voltage of AtAt 26.8 aa V,controlledcontrolled a low DCR biasbias of voltagevoltage 300 counts ofof 26.826.8/s and V,V, a aa low lowlow timing DCRDCR ofof jitter 300300 of counts/scounts/s 158 ps can andand be aa lowachieved.low timingtiming Results jitterjitter ofof also 158158 show psps cancan that bebe when achieved.achieved. thehold-o ResultsResultsff timealsoalso show wasshow set thatthat to when morewhen than thethe hold-offhold-off 500 ns, the timetime afterpulsing waswas setset toto morecanmore be thanthan effectively 500500 ns,ns, reduced thethe afterpulsingafterpulsing to below 0.3%. cancan Inbebe addition, effeeffectivelyctively the reducedreduced photodetection toto belowbelow probability 0.3%.0.3%. InIn measurements addition,addition, thethe photodetectionshowphotodetection that the module probabilityprobability is suited measurementsmeasurements for detection showshow of short thatthat wavelengths thethe modulemodule of isis the suitedsuited light fromforfor detectiondetection 450 nm to ofof 800 shortshort nm wavelengthswithwavelengths a 40% peak ofof thethe PDE lightlight achieved fromfrom 450450 at 600nmnm nm.toto 800800 nmnm withwith aa 40%40% peakpeak PDEPDE achievedachieved atat 600600 nm.nm. Electronics 2020, 9, 1131 10 of 11

Author Contributions: Conceptualization and methodology, M.C., A.P.M., S.D. and X.X.; SPAD fabrication, A.P.M.; validation, C.L. and C.T.; investigation, C.L., H.L. and H.D.; writing—original draft preparation, S.D. and C.L.; writing—review and editing, A.P.M. and X.X.; supervision, L.Y. and X.X.; funding acquisition, M.C. and X.X. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by This work was supported in part by the National Key Research and Development Program of China under Grant 2019YFB2203903, in part by the National Natural Science Foundations of China under Grants, 61827819, 61964005, 61965006, and 61965009, in part by Guangxi Project under Grant AD18281092, AD18281062, AD19245064, 2018GXNSFBA281148, 2019GXNSFBA245057 and 2019GXNSFAA245024, in part by the foundation from Guangxi Key Laboratory of Optoelectronic Information Processing under Grant GD18102, in part by Guangxi Project for ability enhancement of young and middle-aged university teacher under grant 2019KY0211, in part by the foundation from Guangxi Key Laboratory of Automatic Detecting Technology and Instruments under Grant YQ20102, YQ19106 and YQ20107,and in part by Innovation Project of GUET Graduate Education under grant 2020YCXS032. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

1. Tsai, S.-Y.; Chang, Y.-C.; Sang, T.-H. SPAD LiDARs: Modeling and Algorithms. In Proceedings of the 2018 14th IEEE International Conference on Solid-State and Integrated Circuit Technology (ICSICT), Institute of Electrical and Electronics Engineers (IEEE), Qingdao, China, 31 October–3 November 2018; pp. 1–4. 2. Zhang, C.; Lindner, S.; Antolovi´c,I.M.; Pavia, J.M.; Wolf, M.; Charbon, E. A 30-frames/s, 252 144 SPAD Flash × LiDAR With 1728 Dual-Clock 48.8-ps TDCs, and Pixel-Wise Integrated Histogramming. IEEE J. Solid-State Circuits 2019, 54, 1137–1151. [CrossRef] 3. Martelli, P.; Brunero, M.; Fasiello, A.; Rossi, F.; Tosi, A.; Martinelli, M. Quantum Key Distribution Exploiting a Faraday Rotator and a Single SPAD. In Proceedings of the 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Institute of Electrical and Electronics Engineers (IEEE), Munich, Germany, 23–27 June 2019. 4. Gaudin, E.; Arpin, L.; Bouchard, J.; Paille, M.; Bouziri, H.; Bergeron, M.; Pepin, C.M.; Cadorette, J.; Fontaine, R.; LeComte, R. Performance characterization of a dual-threshold time-over-threshold APD-based detector front-end module for PET imaging. In Proceedings of the 2015 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), San Diego, CA, USA, 31 October–7 November 2015; pp. 1–3. 5. Arvani, F.; Carusone, T.C.; Rogers, E.S. TDC Sharing in SPAD-Based Direct Time-of-Flight 3D Imaging Applications. In Proceedings of the 2019 IEEE International Symposium on Circuits and Systems (ISCAS), Sapporo, Japan, 26–29 May 2019; pp. 1–5. [CrossRef] 6. Villa, F.; Lussana, R.; Bronzi, D.; Zappa, F.; Giudice, A. 3D SPAD camera for Advanced Driver Assistance. In Proceedings of the 2017 International Conference of Electrical and Electronic Technologies for Automotive, Torino, Italy, 15–16 June 2017; pp. 1–5. 7. COUNT®. Available online: https://www.lasercomponents.com/us/product/single-photon-counting- modules/ (accessed on 20 May 2020). 8. PDM Series | PicoQuant. Available online: http://www.picoquant.com/products/category/photon-counting- detectors/pdm-series-single-photon-avalanche-diodes (accessed on 20 May 2020). 9. Thorlabs. Available online: http://www.thorlabs.de/thorproduct.cfm?partnumber=SPCM50A (accessed on 20 May 2020). 10. ID100 Visible Single-Photon Detector. Available online: http://www.idquantique.com/photon-counting/ photon-counting-modules/id100/ (accessed on 20 May 2020). 11. Single Photon Counting Modules Excelitas. Available online: http://www.excelitas.com/Pages/Product/ Single-Photon-Counting-Modules-SPCM.aspx (accessed on 20 May 2020). 12. Acconcia, G.; Rech, I.; Labanca, I.; Ghioni, M. 32ps timing jitter with a fully integrated front end circuit and single photon avalanche diodes. Electron. Lett. 2017, 53, 328–329. [CrossRef] 13. Webster, E.A.G.; Grant, L.A.; Henderson, R.K. A High-Performance Single-Photon Avalanche Diode in 130-nm CMOS Imaging Technology. IEEE Electron Device Lett. 2012, 33, 1589–1591. [CrossRef] Electronics 2020, 9, 1131 11 of 11

14. Berdalović,I.; Osreˇcki,Ž.; Šegmanović,F.; Grubišić,D.; Knežević,T.; Suligoj, T. Design of passive-quenching active-reset circuit with adjustable hold-off time for single-photon avalanche diodes. In Proceedings of the 2016 39th International Convention on Information and Communication Technology, Electronics and Microelectronics (MIPRO), Opatija, Croatia, 30 May–3 June 2016; pp. 34–39. 15. Ruggeri, A.; Ciccarella, P.; Villa, F.; Zappa, F.; Tosi, A. Integrated Circuit for Subnanosecond Gating of InGaAs/InP SPAD. IEEE J. Quantum Electron. 2015, 51, 1–7. [CrossRef] 16. Giustolisi, G.; Grasso, A.D.; Palumbo, G. Integrated Quenching-and-Reset Circuit for Single-Photon Avalanche Diodes. IEEE Trans. Instrum. Meas. 2014, 64, 271–277. [CrossRef] 17. Liu, J.; Li, Y.; Ding, L.; Wang, Y.; Zhang, T.; Wang, Q.; Fang, J. Fast Active-Quenching Circuit for Free-Running InGaAs(P)/InP Single-Photon Avalanche Diodes. IEEE J. Quantum Electron. 2016, 52, 1–6. [CrossRef] 18. MAX749 Digitally Adjustable LCD Bias Supply. Available online: https://datasheets.maximintegrated.com/ en/ds/MAX749.pdf (accessed on 20 May 2020). 19. Jackson, J.C.; Phelan, D.; Morrison, A.P.; Redfern, R.M.; Mathewson, A. Toward integrated single-photon-counting microarrays. Opt. Eng. 2003, 42, 112–118. 20. Jackson, J.C.; Donnelly, J.; O’neill, B.; Kelleher, A.M.; Healy, G.; Morrison, A.P.; Mathewson, A. Integrated bulk/SOI APD sensor: Bulk substrate inspection with Geiger-mode avalanche photodiodes. Electron. Lett. 2003, 39, 735–736. [CrossRef] 21. Deng, S.; Morrison, A.P. Active quench and reset integrated circuit with novel hold-off time control logic for Geiger-mode avalanche photodiodes. Opt. Lett. 2012, 37, 3876–3878. [CrossRef][PubMed] 22. Deng, S.; Gordon, D.; Morrison, A.P. A Geiger-Mode APD Photon Counting System With Adjustable Dead-Time and Interchangeable Detector. IEEE Photon-Technol. Lett. 2015, 28, 99–102. [CrossRef] 23. Giudice, A.; Ghioni, M.; Cova, S.; Zappa, F. A process and deep level evaluation tool: Afterpulsing in avalanche junctions. In Proceedings of the Electrical Performance of Electrical Packaging (IEEE Cat. No. 03TH8710), Estoril, Portugal, 16–18 September 2003; pp. 347–350. 24. Humer, G.; Peev, M.; Schaeff, C.; Ramelow, S.; Stipˇcević,M.; Ursin, R. A simple and robust method for estimating afterpulsing in single photon detectors. J. Light. Technol. 2015, 33, 1. [CrossRef] 25. Ziarkash, A.W.; Joshi, S.K.; Stipˇcević,M.; Ursin, R. Comparative study of afterpulsing behavior and models in single photon counting avalanche photo diode detectors. Sci. Rep. 2018, 8, 5076. [CrossRef][PubMed] 26. Palubiak, D.P.; Li, Z.; Deen, J. Afterpulsing Characteristics of Free-Running and Time-Gated Single-Photon Avalanche Diodes in 130-nm CMOS. IEEE Trans. Electron. Devices 2015, 62, 3727–3733. [CrossRef] 27. Villa, F.; Conca, E.; Sesta, V.; Lusardi, N.; Garzetti, F.; Geraci, A.; Zappa, F. SPADs and TDCs for photon-counting, timing and gated-imaging at 30 ps resolution and 60% efficiency. In Proceedings of the 2018 IEEE Nuclear Science Symposium and Medical Imaging Conference Proceedings (NSS/MIC), Sydney, Australia, 10–17 November 2018; pp. 1–5.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).