Two-photon patterning of optical waveguides in flexible polymers

Abstract

Over the last few years two-photon based photo-processes have become an important method to generate 3D microstructures in organic materials without the use of masks and molds. The present work deals with the fabrication of optical waveguides in a flexible polysiloxane matrix for data transmission on printed circuit boards (PCB). In the developed system the waveguide core is formed by two-photon induced photo polymerization (TPIP) of selected monomers, which are dissolved in a silicone matrix. Through the photo-induced polymerization an interpenetrating network is generated, resulting in a refractive index change between the illuminated waveguide cladding and the illuminated core material. Because of the optical transparency, flexibility, chemical and thermal stability polysiloxanes were chosen as optical matrix material. Different types of phenyl methacrylates with a high refractive index were used as monomers. In order to obtain a high contrast in refractive index, the monomers were removed from non-illuminated regions in a vacuum process after laser exposure. The written optical waveguides were evidenced by phase contrast microscopy, revealing an excellent structuring behaviour of the developed material. Optical techniques e.g. cut-back measurements and light extraction tests were applied to characterize the inscribed waveguide structures and to detect the resulting optical loss. To determine the refractive index change upon UV-irradiation spectroscopic ellipsometry was applied. Thus, a difference of Δn=0.02 between the non-illuminated cladding and the illuminated core material was detected. Further, prototypes of optical interconnects on PCBs were fabricated by inscription of a waveguide bundle between a mounted laser and photo diode, resulting in the desired increase of the transmitted photocurrent after TPA structuring. In conclusion, the obtained results demonstrate that fully flexible optical interconnects are accessible by the developed process.