Index-matching elastomers for fiber optics

Silicone elastomers have been prepared to match the index of refraction of the core of optical fiber. The system is composed of a vinyl-terminated dimethyldiphenylsiloxane copolymer crosslinked with tri- or tetrafunctional silanes in the presence of a chloroplatinic acid catalyst. The refractive index of the copolymer was determined as a function of diphenyl content and temperature. By measuring the reflected power from the elastomer-encapsulated end of a single-mode optical fiber it was found that a copolymer with 15% diphenyl content crosslinked with 1,3-diphenyl-1,1,3,3-tetrakis(dimethylsiloxy)disiloxane with a hydride/vinyl stoichiometry of unity has a perfect index match at about 30°C. Environmental testing under 85/85 conditions has shown the material to be capable of protecting biased positive-intrinsic-negative (p-i-n) devices for more than 5000 h. Some other advantages of the system are that it is relatively nontoxic, environmentally stable, and can be easily cured at temperatures below 120°C