All‐opticallikelihoodcalculationhas the potentialto improve the performanceofoperatingspeedand power consumptionin futurecommunicationsystems. The authorpreviouslyproposeda novel schemeof likelihoodcalculation,which is capableofapplyingmulti‐value modulation.However, thesestudieshave a limitationthat requireshigherintegrationinto an actualsystem.In this study, an opticaldeviceof likelihoodcalculationfor a 4‐bitquadraturephase‐shiftkeying(QPSK)modulatedsignal(i.e. twosequentialQPSKsymbols)is demonstrated.The integrateddeviceconsistsof twodelayed interferometerswith siliconwaveguide.The deviceis designedaccordingto thetypesof 4‐bitcodestringas follows(00 00), (00 11), (11 00), and (11 11), and the outputlightwaveform fromthe deviceis observed by an oscilloscopefor a pluralityof 4‐bitQPSKsignals. The lightintensityobtainedfromthe deviceaccuratelycorrespondstothe Hammingdistancebetween the codestringand the inputsignal.The resultsindicatethat the proposedschemecorrectlycalculatesa likelihoodfor a 4‐bitQPSKsignalat 10Gbaud.
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