Experimental characterization of external electrooptic probes

The accuracy and invasiveness of various external LiTaO/sub 3/ electrooptic probe (EEP) geometries are investigated experimentally. The EEPs studied are used for microwave coplanar transmission-line measurements. It is shown that commonly employed EEP geometries can cause substantial inaccuracies in their measurements and that these inaccuracies can be related to specific geometric parameters. The major cause of distortions in pulse measurements was found to be attributable to electromagnetic radiation coupling into the LiTaO/sub 3/ crystal and resonating between the top and bottom crystal interfaces. The authors suggest eliminating the deleterious bulk resonance effects due to the EEP material bulk by using a thin LiTaO/sub 3/ crystal that will act as a single lumped element. The thickness that is expected to be optimal should be about the extent of the guided mode confinement near the conductors. A thicker than optimum cryst al will exhibit resonance, and a thinner one will have reduced sensitivity due to a reduced signal integration path. Thinned crystals possess the additional advantages of reduced thermal drift and reduced stray signal pickup from adjacent lines.<>