The study of different transmission lines in high speed optical module

In this paper, two kinds of transmission line, microstrip line and coplanar waveguide, with different lengths and widths are designed to study the transmission loss of different geometries. The material between the signal line and reference plane is a kind of organic substrate material which has low permittivity. Firstly, each transmission line is designed with 50 Ohm impedance to ensure little reflected signal when 50 Ohm lump port was used. Then, GSG pads for probe test at the ends of transmission line and transition sections between GSG pads and transmission lines is designed and optimized by electromagnetic simulation software to minimize port reflection. Finally, the optimized structures are simulated and the S parameter is obtained. Additionally, the unit length loss is calculated. The results show that coplanar waveguide (with -0.5dB/cm loss at 25GHz)shows less transmission loss than microstrip (with -1dB/cm at 15GHz). By testing the samples, the property of organic substrate material and the process of organic substrate fabrication can be evaluated. Meanwhile, the influence of process tolerance to different transmission lines can be demonstrated and back to guide the design of highspeed transmission lines on this substrate.