Photonic probes for the measurement of electromagnetic fields over broad bandwidths

The characteristics of photonic systems that make them especially well suited for use as broadband electromagnetic (EM) field sensors are discussed. Transfer functions are given for the individual components of such a measurement system, with special emphasis given to those of Pockels-cell and modified-directional-coupler optical modulators. An isotropic electric-field meter having 15-cm resistively tapered dipole elements combined with bulk crystal, Pockels-cell modulators is described. The meter´s frequency response is flat between 30 kHz and 100 MHz, except for resonances in the modulator crystals that occur between 1 and 10 MHz. For a 3-kHz detection bandwidth, the noise floor is equivalent to a field of about 7 V/m, and the calculated linear dynamic range is 70 dB in EM-field power density. The response is within ±2 dB of the ideal isotropic response. A photonic probe that uses a modified directional-coupler modulator is briefly described