Sensitivity-Stabilized Fiber-Mounted Electrooptic Probe for Electric Field Mapping

We have developed a sensitivity-stabilized fiber-mounted electrooptic (EO) probe for electric field mapping. In this probe, a polarization-rotated reflection method is used to cancel out the phase difference between the lights transmitted on the fast and slow axes of a polarized maintaining fiber (PMF) in a round trip since a Faraday rotator (FR) on the tip of the PMF rotates the polarization plane of the transmitted light orthogonally. Consequently, the sensitivity of the EO probe is stable against PMF bending and changes in temperature. The sensitivity degradation caused by PMF bending with a radius larger than 4 mm is less than 0.5 dB. The sensitivity fluctuation caused by a temperature change of 50 $^\circ$C to 100 $^\circ$C is suppressed to less than 1 dB. Since the PMF connecting the EO probe to the polarization control module is 5-m long, we can avoid any disturbance from the region around the probe tip. We demonstrate precise near-field measurements for a dipole antenna in a 20 cm $\times$ 10 cm scanning area. This result means that the EO probe can be used for calibrating antennas fixed in practical fields.