Engineering properties of high refractive index optical gels for photonic device applications

We have investigated a new class of high refractive index, non-yellowing, viscoelastic optical gels. Refractive indices for these materials can be adjusted from that needed to march fused silica fiber (n(D) = 1.46) to above n(D) = 1.6 to match the higher index engineering glasses, plastics, and semiconductors. These materials are designed for permanent optically clear encapsulation in devices where severe mechanical shock or differential thermal expansion, such as occurs during PCB soldering operations (with temperature excursions to > 200 degrees C), may render conventional high strength optical epoxies unusable. These low shear stress gels can also be customized to exhibit a wide range of rheological "stiffness". We have demonstrated quasi-fluid versions with apparent viscosities of 500,000 cP (mayonnaise-like) to hard-rubber-like consistencies registering on the high end of the Shore 00 durometer scale. In this paper, we present measurements of engineering properties on both elastomer-like curing optical gels, and thixotropic non-curing optical gels for: a) optical properties from near UV to near IR (NIR): refractive index over temperature, dispersion, and optical absorption; b) rheological properties: viscosity vs. shear rate, Shore hardness and cone penetration. Validation of ultra-low volatility and high temperature thermooxidative stability required for long-lived photonic devices is discussed. Use of gel technology in fiber splices and photonic devices is described.