The effects of design parameters on the modulating voltage and optical bandwidth are reported for lithium niobate, GaAs and polymer electro-optic modulators by using rigorous numerical modelling techniques. It is shown that by etching lithium niobate, the switching voltage can be reduced and the bandwidth improved. For a GaAs-based modulator using higher aluminium content in the buffer layer, the device length can be shortened for a given optical loss. It is also observed that the dielectric loss and impedance matching play a key role in velocity-matched high-speed modulators with low conductor loss. It is also indicated in the work that by using tantalium pentoxide coating, velocity matching can be achieved for GaAs modulators. The effects of a non-vertical side wall on the polarisation conversion and single mode operation and the bending loss of polymer rib waveguide for electro-optical modulators are also reported.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.