PicoQuant MultiHarp 150 - Time Tagged Mode Measurements; System Requirements; T2 Mode

PicoQuantGmbH MultiHarpSoftwareV.3.1.0.0
5.3. Time Tagged Mode Measurements
Time–TaggedTime–Resolved(TTTR)modeallowstherecordingofindividualcounteventsdirectlytoharddisk
withoutimmediatelyforminghistograms.
InclassicTTTRmode,inadditiontothestart–stoptimingwithpicosecondresolution,thetimingoftheevents
withrespecttothebeginningofthewholemeasurementisrecordedintheeventrecords.Thisisparticularlyin-
terestingwherethedynamicsinafluorescenceprocessaretobeinvestigated.Theavailabilityofthetime–tags
permitsphotonburstidentification,whichisofgreatvaluee.g.,forSingleMoleculeDetection(SMD)inaliquid
flow.OthertypicalapplicationsareFluorescenceCorrelationSpectroscopy(FCS)andBurstIntegratedFluores-
cenceLifetime(BIFL)measurements.Togetherwithanappropriatescancontroller,TTTRmodeisalsosuitable
forultrafastFluorescenceLifetimeImaging(FLIM).Anothergreatapplicationareaoftimetaggingisquantum
opticswithcoincidencecountingandcorrelation.
TheMultiHarpactuallysupportstwodifferentTime–Taggingmodes,T2andT3mode,whichwillbeexplained
furtherbelow.WhenreferringtobothmodestogetherweusethegeneraltermTTTR.
5.3.1. System Requirements
IncaseswheretheTime–Taggingmodesaretobeusedwithhighcontinuouscountrates(say>5Mcps)the
PCsystemmustmeetsomespecialperformancecriteria.Thereasonforthisistherelativelylargeamountof
databeinggeneratedinTTTRmode.Inordertopreventanoverflowintherecording,thedatamustbetrans-
ferredtothecomputerinreal–time.ThisrequiresamodernPCwithafastI⁄Osubsystem.Arecent,atleast
dualcoreCPUrunningat2GHzormoreisrequired.ForthebestpossibleperformanceinTTTRmodeamod-
ernsolidstatediskwithhighthroughputisrecommended.IfitisintendedtomakeuseofthefullTTTRthrough-
putofaMultiHarp(upto90McpsviaUSB3)thentheharddiskmustbeabletohandlesustainedwriteratesof
360MBytes/s.ThiscanbeachievedwithRAIDarraysormodernsolidstatedisks.Networkstorageisusually
tooslow.
5.3.2. T2 Mode
InT2modealltiminginputsoftheMultiHarpincludingthesyncinputarefunctionallyidentical.Thereisnodedi-
cationofthesyncinputchanneltoasyncsignalfromalaser.Itmaybeleftunconnectedorcanbeusedforan
additionaldetectorsignal.Inthiscasethesyncdividermustbesetto“None”.UsuallytheregularinputsCH1..N
areusedtoconnectphotondetectors.Theeventsfromallchannelsarerecordedindependentlyandtreated
equally.Ineachcaseaneventrecordisgeneratedthatcontainsinformationaboutthechannelitcamefrom
alongwiththearrivaltimeoftheeventwithrespecttotheoverallmeasurementstart.Thetimingisrecordedwith
thehighestresolutionthehardwaresupports(5,10or80psdependentonthemodel).EachT2modeevent
recordconsistsof32bits.Thereare6bitsforthechannelnumberand25bitsforthetime–tag.Ifthetimetag
overflows,aspecialoverflowrecordisinsertedinthedatastream,sothatuponprocessingofthedatastreama
theoreticallyinfinitetimespancanberecoveredatfullresolution.Deadtimesexistonlywithineachchannelbut
notacrossthechannels.Therefore,crosscorrelationscanbecalculateddowntozerolagtime.Thisallowsfor
powerfulapplicationssuchascoincidencecorrelationandFCSwithlagtimesfrompicosecondstohours.Auto-
correlationscanalsobecalculatedatthefullresolutionbutofcourseonlystartingfromlagtimeslargerthanthe
deadtime.
The32-biteventrecordsneedtobecontinuouslystreamedtothehostPC,ideallywithoutlosses.Giventhe
deadtimeoftheTDCsofTd=650psitistheoreticallypossiblethateventrecordsaregeneratedatamaximum
rateof1/Td=1,538GHz.Thetotalrateincreasesevenmorewhenmultiplechannelsareused.Itisobvious
thatsuchdataratescannotbetransferredoverUSBcontinuouslywithoutlosses.Ontheotherhand,giventypi-
calphotonstatisticsonecan(andshould)distinguishbetweenpeakratesandaveragerates.Thelatteraretypi-
callymuchsmallerthanthepeakratesduringbursts.ThesystemisthereforedesignedtouseFIFO(FirstIn
FirstOut)buffersthatcantemporarilyholdacertainnumberofeventsinaburstsothatthebustransfermust
onlydealwithmoderateaveragerates.InordertoallowahighinputburstratetheMultiHarpusesineachchan-
nelafastfront-endFIFObufferthatcanhandleburstsofupto2048eventsatthemaximumrateof1/Td=
1,538GHz.ThisisfollowedbyalargebutslowerFIFObufferforallchannels, capableofholdingupto
134,217,728eventrecords.ThislargesecondaryFIFObufferensuresthatnodataislostduetoinevitabletask
switchesandinterruptionsoftheUSBtransfersonthehostPCside.Eveniftheaveragereadrateofthehost
PCislimited,burstswithmuchhigherratecanberecordedforsometime.Onlyiftheaveragecountrateex-
ceedsthereadoutspeedofthePCoveralongperiodoftime,aFIFObufferoverruncanoccur.Incaseofa
FIFObufferoverrunthemeasurementmustbeabortedbecausedataintegritycannotbemaintained.However,
onamodernandwellconfiguredPCasustainedaveragecountrateofupto90Mcpsispossible.Thistotal
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