Vector Near-Field Measurements Using Optimized Electrical and Photonic Down-Conversion

A microwave vector-measurement technique based on electrooptic probing with a micro-resonator sensor and electrical down-mixing of a demodulated optical sideband is experimentally demonstrated. Detailed near-field distributions of the amplitude and phase of a radio-frequency-identification antenna are extracted and compared with field patterns obtained from an electrooptic measurement system that instead employs photonic down-conversion to create the baseband read-out signal. High-fidelity field scans with a > 35 dB signal-to-noise ratio (SNR) are obtained when using both the electrical and photonic down-conversion techniques in the measurement of a small, planar loop antenna intended for applications in mobile-reader instruments. In addition, the electrooptic-modulation sensitivity to the antenna radiation power is observed to increase by 5 dB when the optical wavelength employed with the resonant probe is shifted from the point of maximum reflected intensity to an optimized low-noise operating regime. Finally, a practical method to double the bandwidth of the electrooptic measurement during photonic down-conversion without any change in system components is introduced, enhancing the SNR to > 40 dB.