Measurements are always subject to errors resulting from the interaction of the measurement device with the quantity being measured. Electro-magnetic fields are typically detected with an antenna that must be electrically connected to a recording instrument, and the antenna and its electrical connection can significantly perturb the field being measured. The use of fiber optics in the detection process minimizes the perturbation of the microwave fields since most of the probe and its associated connections are dielectric. This paper presents preliminary results obtained using a commercially available fiber optic thermometer in the measurement of microwave power. A small amount of slightly conductive material is placed in contact with the fiber optic sensor. In the presence of a microwave field, currents are induced in the conductive material, which in turn produces joule heating. It is shown experimentally that under certain conditions the probe temperature is linearly related to the power level present. Experimental results are presented for power measurements at 2.45 and 94 GHz. Probe design criteria and limitations are also discussed.