MultilayerthinfilmsofalternatelyadsorbedlayersofpolyelectrolytesPDDAandPS-119wereformedonbothplanarsilicasubstratesandopticalfibersthroughtheionicself-assemblytechnique.IntrinsicFabry-Perotcavitieswerefabricatedbystepwiseassemblingthepolyelectrolytesontotheendsofopticalfibersforthepurposesoffiberopticaldeviceandsensordevelopment.Ionicallyassembledpolyelectrolytemultilayerthinfilms,inwhichtherearehydrophilicsidegroupswithstrongaffinitytowardswatermolecules,areacategoryofhumidity-sensitivefunctionalmaterials.ThepolyelectrolytemultilayerthinfilmFabry-Perotcavity-typefiberopticalhumiditysensorcanworkoverawiderangefromabout0%RHtoabout100%RHwitharesponsetimelessthan1s.