Hybrid sol-gel electro-optic polymer modulators: Beating the drive voltage/loss tradeoff

There has been great progress in the development of electro-optic (EO) polymers with exceptionally high r₃₃ coefficients, with values ranging from 100-400 pm/V now being reported for single layer electro-optic polymer films. While this enables the fabrication of EO modulators with sub-volt operation, it is also necessary to make devices with acceptably low insertion loss (<6 dB) in order to compete with existing technology. We have developed a solution to the voltage/insertion loss tradeoff in EO polymer modulators by adopting a hybrid geometry that provides for low optical coupling loss, electro-optic polymer limited propagation loss, highly efficient poling, and low cost fabrication. This combination of properties has allowed us to achieve r₃₃ = 170 pm/V in an EO phase modulator. In addition to this proven approach to optimizing the figure of merit, there are several other approaches that can have high impact. The development of low loss EO polymer and cladding materials and waveguides can greatly reduce the insertion loss of EO polymer modulators, through chemical substitution techniques such as selective halogenation, as well as through improved processing to reduce roughness, stress and poling induced losses. Halogenation can be used to reduce the number of C-H bonds, which have well-known stretch and bend vibrational modes whose overtones extend into the optical communications bands at 1550 nm and 1310 nm. While roughness and stress effects are well-understood from work on passive waveguides, the poling process can produce inhomogeneities that lead to increased scattering loss; molecular design can be used to reduce poling induced loss. Another approach is to adopt non-waveguide device formats that are more tolerant of material losses, such as Fabry-Perot etalons. While etalons may not be viable for very high speed applications (i.e., GHz regime), they present entirely new application areas for electro-optic polymers.