Abstract

E-field measurements—which are nessesary, for example, in hyperthermia therapy—are usually performed with pure electrical sensors accepting field distortions from metal wires and the loss of phase information due to the signal detection directly at the receiving antenna ports. We designed an interferometric integrated optical E-field sensor with a novel compact electrode structure yielding small geometrical dimensions and improved spatial resolution of the measured E-field. The EO modulator was based on a Mach-Zehnder interferometer in Ti:LiNbO₃ on an X-cut Y-propagating substrate. An important factor for the performance characteristics of this EO device is the matching between the electrically short dipole antenna and the modulator electrodes. To avoid the mismatch between the high capacitance of a usual modulator (C = 5-20 pF) and the low dipole capacitance a segmentation of the modulator electrodes is chosen even though it increases the switching voltage V_n of the modulator. The chosen electrode structure as given in Fig. 1 can be referred to as a multiloaded dipole antenna. By Harrington's method of moments one can easily obtain an optimum value of segmentation for a given dipole length yielding a modulation enhancement up to one order of magnitude for typical dipole lengths of 1 = 20-50 mm. Several samples with different values of segmentation (m = 8, 12, 16) were built up and examined. The geometrical dimensions of the devices could be kept as small as 30 X 3 X 2 mm³ which provides good spatial resolution of the E-field. A 835-rim wavelength Sharp LT015 fiber pigtailed laser diode providing an optical output power of 1.5 mW was used for the experimental setup (Fig. 2).

© 1992 Optical Society of America