Synthesis and evaluation of thermoplastic polyurethanes as thermo-optic waveguide materials

The refractive index and thermo-optic coefficient (dn/dT) of linear polyurethanes and UV-cured polyacrylates were determined over a range of near infrared (NIR) wavelengths. Three different linear polyurethanes were prepared from the polycondensation of isophorone diisocyanate (IPDI) and novel diols containing cyclic substituents, such as 1,3-dithiane, cyclic O,S-acetal and 1,3-dithiolane. The polymers exhibited a glass transition at 71-131 degrees C and were stable up to similar to 250 degrees C. Urethane-diacrylate was prepared via a reaction of IPDI and 2-hydroxyethyl methacrylate, and a urethane-acrylate polymer film was produced under UV-irradiation. The urethane content of the UV-polymers was adjusted using composite films from a diacrylate-derived from 1,3-dithiane-attached diol. The linear polyurethane provided a high thermo-optic coefficient (dn/dT) of -3.63 x 10(-4) degrees C-1 (1550 nm). The coefficient was dependent on the urethane content of the UV-composite films, which exhibited a larger thermo-optic effect than the pure UV-acrylate films because of the urethane group. All the urethane-polymer films exhibited a very low birefringence (<0.0004). The presence of N-H in the urethane backbones rarely resulted in additional absorption loss as waveguide materials at specific wavelengths of 1100, 1310 and 1550 nm. A urethane-acrylate composite film exhibited propagation losses of 0.09 (1100 nm), 0.244 (1310 nm) and 0.288 dB cm(-1) (1550 nm). The losses (1310 and 1550 nm) were reduced to <0.09 dB cm(-1) in the deuterium-exchanged film.