Design and synthesis of graded index plastic optical fibers by copolymeric system

Graded index plastic optical fibers (GI POF) based on the copolymer of 2,3,5,6-tetrafluorophenyl methacrylate (TFPMA) and methyl methacrylate (MMA) were prepared. The copolymers of 10 wt% and 20 wt% of TFPMA with MMA were prepared in bulk using di-tert-butyl peroxide as the radical initiator at 90-120°C for 48 hr. The glass transition temperatures (T_g) of the copolymers were 115 and 118°C, and the refractive indexes were 1.4912 and 1.4901 at 589 nm, respectively. The transmittances of the copolymers were a slightly higher than that of pure PMMA. Copolymer tubes (diameter 18 mm, length 600 mm) were prepared by the copolymerization of TFPMA (10 and 20 wt%) with MMA (90 and 80 wt%) as the cladding of the optical fiber. In order to prepare GI POF based on these copolymers, a high refractive index compound, diphenyl sulfide (DPS) (n_D²⁰ = 1.6330), was added to the mixture of the monomers in the tube (as a core material) and then copolymerized by an interfacial gel polymerization technique. The material dispersion and optimum index exponent g were calculated and found to be very close to that of GI POF prepared from pure PMMA. The obtained copolymer preforms were drawn by heating at 220-250°C with controlling of drawing velocity. The resulting fiber diameter was 0.75 mm and the length was 50-150 m. The attenuation spectra of the fibers were measured and found to be similar to GI POF prepared using pure PMMA. The results indicate that the core material of GI POF can be prepared by a copolymer system in which the refractive indexes of homopolymers are very close to each other and the monomers yield a random copolymer.