Near-Field Mapping System Using Fiber-Based Electro-Optic Probe for Specific Absorption Rate Measurement

We have developed a near-field mapping system with a fiber-based electro-optic (EO) probe for microwave antenna characterization. In this probe, an EO crystal is mounted on the tip of an optical fiber through a collimating lens. Since the lens allows the crystal thickness to be lengthened by reducing the loss of an optical beam coupling back to the optical fiber, sensitivity is improved. Because the tip of the EO probe consists of a 1-mm-cubic EO crystal and contains no metallic components, there is very little disturbance of the mapped electric field. Fixing the optical fiber in a thin glass tube provides stable sensitivity during long-term mapping over a large area. The fabricated EO probe has a dynamic range larger than 45 dB, flat sensitivity from 1.95 to 20 GHz, and directivity with cross-axis sensitivity isolation greater than 30 dB, A comparison of the measured and calculated near fields of a dipole antenna showed negligible static or inductive coupling between the EO probe and the dipole antenna. Using a tissue-equivalent phantom to assess the specific absorption rate (SAR), we demonstrated the potential of the EO probe for mapping the electric field with information of amplitude and phase. The EO probe can detect an electric field of less than 0.6 V/m, which corresponds to a SAR of 0.5 mW/kg. This value satisfies the minimum detection limit defined in the regulations for determining SAR. This result shows the potential of the near-field mapping system with the fiber-based EO probe in practical applications.