Overthelast20years,siliconphotonicshasrevolutionizedthefieldofintegratedoptics,providinganovelandpowerfulplatformtobuildmass-producibleopticalcircuits.Oneofthemostattractiveaspectsofsiliconphotonicsisitsabilitytoprovideextremelysmallopticalcomponents,whosetypicaldimensionsareanorderofmagnitudesmallerthanthoseofopticalfiberdevices.Thisdimensiondifferencemakesthedesignoffiberto-chipinterfaceschallengingand,overtheyears,hasstimulatedconsiderabletechnicalandresearcheffortsinthefield.Fiber-to-siliconphotonicchipinterfacescanbebroadlypidedintotwoprinciplecategories:in-planeandout-of-planecouplers.Devicesfallingintothefirstcategorytypicallyofferrelativelyhighcouplingefficiency,broadcouplingbandwidth(inwavelength),andlowpolarizationdependencebutrequirerelativelycomplexfabricationandassemblyproceduresthatarenotdirectlycompatiblewithwafer-scaletesting.Conversely,out-of-planecouplingdevicesofferlowerefficiency,narrowerbandwidth,andareusuallypolarizationdependent.However,theyareoftenmorecompatiblewithhigh-volumefabricationandpackagingprocessesandallowforon-waferaccesstoanypartoftheopticalcircuit.Inthispaper,wereviewthecurrentstate-of-the-artofopticalcouplersforphotonicintegratedcircuits,aimingtogivetothereaderacomprehensiveandbroadviewofthefield,identifyingadvantagesanddisadvantagesofeachsolution.Asfiber-to-chipcouplersareinherentlyrelatedtopackagingtechnologiesandtheco-designofopticalpackageshasbecomeessential,wealsoreviewthemainsolutionscurrentlyusedtopackageandassembleopticalfiberswithsilicon-photonicintegratedcircuits.
