Transmission loss and dispersion in conductor-backed coplanar waveguide. An investigation of waveguide structures for broadband optical modulators

The Conductor-Backed Coplanar Waveguide (CBCPW) as a planar transmission line structure is immune to dielectric loss in the substrate due to its conductor-backed configuration, therefore compatible with many substrate systems. This flexibility is very attractive for silicon-based applications where dielectric loss is a big problem due to its poor intrinsic resistivity. The CBCPW structure is proposed as the electrode structure in a low cost silicon-based electrooptical polymer modulator. However, it is possible that the parallel-plate structure formed by the upper and lower conductor layers of CBCPW gives rise to parallel-plate leaky losses.; This report is focused on the investigation of various loss mechanisms and dispersion effect in CBCPW by matched-through-line and resonance method. A transmission loss of 4.9dB at 20GHz is measured in a 50O CBCPW through line of 3.4617cm long micro-fabricated on silicon wafer, which suggests a modulator built with CBCPW electrodes could achieve half power modulation if velocity match between the electrical and optical signals is achievable at this high bit rates. A very small dispersion effect is observed in CBCPW across a wide frequency range from 50MHz to 20GHz by resonance method, and the attenuation constants obtained by this method agree well with the result from the matched-through-line method. It is proved that conductor loss is the main source of transmission loss in CBCPW, and there is no noticeable parallel-plate leaky loss in the investigated frequency range.