The role of photoinitiator and chelating agent in the fabrication of optical waveguides from UV-photocurable organo-mineral sol–gel materials

This work focuses on the role of photoinitiators and chelating agents in waveguide fabrication based on UV sensitive sol–gel materials. The key role that a photoinitiator plays in waveguide formation has been shown through examination of waveguide width and cross-sectional shape. Waveguides fabricated using Irgacure-907 were found to be on average 2.5 times wider at the top surface than those fabricated using Irgacure-819 or -1800. UV–Vis measurements on the photoinitiators confirmed that Irgacure-907 is approximately 35 times more absorbing than the other photoinitiators at the UV exposure wavelength of 325 nm. This has been found to give rise to a highly trapezoidal waveguide shape which, also affects waveguide adhesion. The influence of the chelating agent within the hybrid sol has also been studied with reference to waveguide width. These experiments clearly showed that the Zr(OPr)4 molecule plays an active role in the photopolymerization process, and its activity is controlled by its degree of chelation. Two different chelating agents, methacrylic acid (MAAH) and isobutyric acid (IBA) were compared. The trend for both chelating agents was the same, but IBA chelated sols resulted in narrower waveguides. This has been attributed to the higher steric hindrance of IBA compared to MAAH, which would inhibit the propagation of polymerization in the coating.