The PVF2film‐based fiber‐optic phase modulator exhibits a resonance around 3.5 kHz. Existing reports of its characterization have considered only the linearity at nonresonance frequencies and the frequency response at a single drive voltage. A large nonlinearity of phase shift with applied voltage at frequencies near the resonance has been measured. The factors that influence the resonance have been identified on the basis of the frequency response for different drive voltages. These measurements were facilitated by the three new methods of phase detection we recently reported.