Abstract

Slow-wave (SW) coplanar electrodes can be used in compound semiconductor based electrooptic modulators to provide microwave/lightwave velocity matching and, therefore, make it possible to obtain modulators with wide bandwidths [1]-[3]. The microwave index for conventional, thin, coplanar electrodes on GaAs is ~2.6 whereas the effective index for the lightwave guided by typical AlGaAs/GaAs optical waveguides is in the range 3.2 - 3.4. Higher microwave indices (larger than 2.6) can be achieved by incorporating capacitive fins, and/or capacitive fins with pads on their ends, in the gaps between the coplanar electrodes [1],[3]. Here, we compare SW coplanar strips (SW-CPS) with SW coplanar waveguides (SW-CPW). Measurements on several SW-CPS and SW-CPW show that microwave indices close to, and even larger than, 3.4 can be achieved; i.e., both SW-CPS and SW-CPW are capable of achieving the necessary velocity-match condition. Other factors of interest are the characteristic impedance and the loss. SW-CPS and SW-CPW both can be designed to be 50Ω lines and their losses depend very much on their geometries.

© 1996 IEEE