Coplanar-microstrip transitions for ultra-wideband communications

Nowadays optics is penetrating into the broadband access networks, so the data transmission bit-rate can be guaranteed regardless the distance between the subscriber and the central office. Many laboratories are working on the radio over fiber technology in the home networks. For home network applications, one must use low-cost and broadband modulators to transcribe the electrical signal into optical signal. The electrooptic polymers on which very active research is being carried out have the required properties for realizing this kind of modulators. Their realization needs a certain number of delicate manufacturing steps, therefore, a rigorous study of the component must be made before the realization of the modulators. The optimization of the modulator optical structure must be made first, from the properties of polymers at one’s disposal and the technological constraints, in order to obtain a single mode guide with minimum losses. Then, optimization of the driving electrode, inseparable step from the optical study, should be carried out. In electro-optic modulators based on polymer, the chromophore molecules responsible for the electro-optic effect are oriented perpendicularly to the substrate as they are generally poled by Corona effect or with contact electrodes. Consequently, a microstrip line is suitable to apply the driving signal to induce the electro-optic effect. Before packaging the final component, it is necessary to assess its performances directly on wafer by use of a probe station, usually equipped with probes compatible with coplanar lines GSG (Ground-Signal-Ground) insuring an easy electrical contact. So, a transition between coplanar and microstrip lines (CPW-MS) is indispensable to characterize the components on wafer. This transition must satisfy at least these three criteria: ultra-wideband, easy to realize and low-cost. One solution is to physically connect the coplanar ground planes to the bottom ground plane of the microstrip line through a via-hole, which would make the component more expensive without eliminating all parasitic resonance (Haydl, 2002). It is in this context that we conducted a comprehensive study of via-free transitions between coplanar and microstrip lines, in order to make easy and simple the characterization of components driven by microstrip line with CPW (Coplanar waveguide) probes. These transitions may be also employed in all microwave circuits driven by microstrip lines.