Conductor Loss of Capacitively Loaded Slow Wave Electrodes for High-Speed Photonic Devices

In this paper, analytical expressions are presented for the microwave attenuation of slow wave electrodes obtained by periodically loading a regular coplanar line with capacitive elements. Such electrodes are commonly used in III-V compound semiconductor electro-optic modulators and other traveling wave devices. These results are obtained by modifying the existing analytical loss expressions for regular coplanar lines based on physical arguments. The predictions of these expressions are compared with experimental results up to 35 GHz and agreement is found to be very good. Validity of this analysis is also discussed. It is found that the approach works very well for lines of practical interest. Hence, the proposed approach enables complete design of capacitively loaded slow wave electrodes by predicting the microwave loss with closed-form equations in addition to velocity and characteristic impedance.