Design and analysis of a poled-polymer electro-optic modulator with a strip-loaded waveguide structure

To fully utilize the advances of electro-optic (EO) polymer research, a detail design and model of a triple stacked poled-polymer, electro-optic waveguide modulator was developed to provide guidance in the selection of cladding layers and demonstrate how the mode conditions affect the overall device performance. Waveguide devices depend not only on the EO materials used in the core, but also on the cladding materials. In contrast to most of the research that has focused on molecular engineering of the electro-optically active materials to improve the core properties of the waveguide, this work studied the effects of cladding material properties to improve overall device performance of an electro-optic waveguide modulator. Various, commercially available polymer materials were identified and investigated for potential cladding layers according to their optical, electrical, process ability and film casting properties. A poled-polymer, strip-loaded waveguide, EO modulator is designed and analyzed in terms of single mode conditions, optical loss due to the metal electrodes, modulation efficiency, and mode size. Two designs were compared: Design 1 optimized the half-wave voltage for a single-arm modulator (Vpi = 2.5 V) with a nearly symmetric waveguide by maximizing modulation efficiency and minimizing the overall thickness of the waveguide; Design 2 optimized the insertion loss (6 dB) with a strongly asymmetric waveguide by maximizing the overall mode size to most efficiently overlap with a single mode fiber. High frequency analysis of a microstrip electrode showed a modulator with a push-pull scheme can be fabricated with half-wave voltage (Vpi) of 4.0 V for Design 1 and 5.1 V for Design 2 at 3 GHz. Some general guidelines in the design of a poled-polymer electro-optic modulator incorporating a strip-loaded waveguide structure are suggested. First, the core layer thickness and ridge width supporting single mode propagation should be fabricated as large as possible by increases the asymmetry of the refractive index between the top and bottom cladding layer. Second, the thicknesses of the top and bottom cladding layers must be optimized through an analysis of the waveguide mode amplitude distribution so that the electrode-associated optical loss is minimized to a required level while simultaneously the total thickness of the waveguide is minimized. The ridge structure greatly simplifies fabrication procedures, reduces fabrication steps and eliminates propagation losses due to roughness of etched sidewalls. The results of the analysis were modeled in beam propagation software to confirm the ideal conditions for single mode propagation