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balancemeasurement.doc

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HYPERLINK"http://www.newport.com/store/product.aspx?id=979109&lang=1033"FMSpectroscopyWithTunableDiodeLaserstakesadvantageofthechangeinopticalabsorptionasafunctionofthefrequency(wavelength)oflightpassedthroughthesample. Balancedphotodetectionisacommonlyuseddetectionmethodinthelabwhenanexperimentrequiresincreasedsignal-to-noise.Thisdetectionmethodhasmanyadvantagestosimplyamplifyingasignal,eitherwithanamplifierfollowingthephotodiode,orwithanavalanchephotodiode(APD)forexample.Itisparticularlypowerfulinitsabilitytocancellasernoiseor“commonmodenoise”andinitsabilitytodetectsmallsignalfluctuationsonalargeDCsignal.Inthisapplicationnoteweexploreawidevarietyofdetectionmethodsutilizingbalancedphotodetectorsandbrieflydiscusstheirimplementation.Themostsimplecaseofbalanceddetectionisrealizedwhentwophotodiodesareconnectedsuchthattheirphotocurrentscancel.Inthiscase,itisnecessarytoequalizetheDCopticalpowerimpinginguponeachphotodiodebyusingavariableneutraldensityfilter,forexample.Whenthisisdone,theeffectiveoutputofthebalancedpairofphotodiodesiszerountilthereissomedifferenceintheintensityofoneofthebeams.Whenthisoccurs,itcausesthepairtobecome“unbalanced”andanetsignalappearsontheoutput.Toavoidtheoftentedious,manualtaskofbalancingtheintensitiesofthetwobeams,anauto-balancingcircuitcanbeemployedwhichwasoriginallydevelopedbyHobbsatIBM1-3.Thecircuitusesalow-frequencyfeedbacklooptomaintainautomaticDCbalancebetweenthesignalandreferencearms.AsshowninFigure1,itconsistsoftwophotodiodes(oneeachforthesignalandreferencebeams),acurrentsplitter,acurrentsubtractionnode,atransresistanceamplifier,andafeedbackamplifier.Thisbalancedopticalreceiverallowsdetectionofasmallsignaluponalargebackground.Again,thereceiverworksbysubtractingthephotocurrentsfromtwowell-matchedphotodetectors.Common-modenoisethatispresentonboththereferenceandsignalbeams(suchaslaserintensitynoise)iscancelledoutandthereforedoesnotappearaspartofthesignal.Ontheotherhand,anyimbalancebetweenthephotocurrentsgeneratedbythereferenceandsignald