A fiber optic sensor includes a sensor element responsive to a property or condition of a material, such a resistivity, capacitance, inductance, frequency, etc., for providing an output voltage signal that is dependent upon the property or condition of the material. The sensor further includes an optical-to-electrical conversion element, such as a photo detector or a solar cell, that converts an optical signal (photo detector driving optical signal) carried by an optical fiber into an electrical signal. The electrical signal is applied to the sensor element, and in response, the sensor element provides the output electrical signal. The output electrical signal is applied to a measurement device, such as a piezoelectric element, having at least one dimension that varies with changes in the output electrical signal. An optical strain sensor, such as a Bragg grating or Fabry Perot interferometer, is mounted to the measurement device such that changes in the dimension of the measurement device causes changes in the strain of the optical strain sensor. These changes in strain are detected and related to the property or condition of the material. The fiber optic sensor is particularly useful as a resistivity sensor for measuring the resistivity of a fluid. For example, the resistivity of a fluid mixture of oil and water can be measured for determining the relative water and oil content in the fluid mixture. In this configuration, the sensor element includes a pair of spaced apart conductive elements. The output electrical power from the photo detector is applied to the conductive elements and the fluid is passed between the conductive elements. The output voltage from the conductive elements varies in relation to the resistivity of the fluid sample between the conductive elements.