A　piezoelectric　ceramic,　based　on　the　converse　piezoelectric　effect,　was　used　as　optical　phase　modulator　to　design　a　fiber　differential　interferometer.　Theoretical　analysis　and　experimental　results　show　that　the　amplitude　and　frequency　of　the　output　light　intensity　are　linearly　related　to　that　of　the　sinusoidal　signal　voltage　applied　on　PZT.　The　system,　with　its　wide　linear　range　and　high　sensitivity,　can　be　applied　in　detection　of　environmental　vibrations　or　dynamic　behavior　of　PZT.