Low-loss single-mode plastic waveguide fabricated by photopolymerization

Abstract

Plastic materials used for optical fibers are very attractive as optical waveguides because of the simple fabrication process and their low loss. There have been several reports on plastic waveguides that were fabricated by photopolymerization of the dopant monomer[1], and some had a propagation loss as low as 0.2 dB/cm[2],[3]. However, they were 2-mode or multi-mode waveguides. We now report a single-mode waveguide using polystyrene, PSt-doped poly(methyl methacrylate), PMMA. The combination of PSt/PMMA was chosen because the refractive index of PSt (n=1.59) is considerably larger than that of PMMA (n=1.49), and the solubility of PMMA in styrene is good. The fabrication process is shown in Fig. 1. The clad solution, which consisted of 10% PMMA and 90% MMA, was spin-coated at 3000 rpm on a Si substrate and baked at 90°C for 30 minutes. The core solution was also spin-coated on the PMMA clad film and baked at 60°C for 2 hours. The core solution consisted of 12% PMMA, 72% styrene, 8% aceto-phenone photoinitiater, and 8% 1,4-dioxane. The film thickness was 2 μm for the clad and 3 μm for the core. The styrene monomer for the waveguide region was polymerized by UV-light exposure through a photomask, and the unreacted monomer was removed by postbaking at 90°C for 30 minutes. Figure 2 shows the refractive index change as a function of exposure energy at the 365 nm wavelength. A maximum index change of 0.01 was obtained under this fabrication condition for the 633 nm wavelength. The channel waveguides were fabricated using a photomask which included 1- to 10-μm-wide, 60-mm-long straight waveguide patterns. Single-mode waveguides were obtained for 1 to 3 μm pattern widths with a 0.005 refractive index change. Propagation loss was measured by detecting the scattered light along the waveguide[4] (Fig. 3). The input light from the He-Ne laser was coupled through a microscopic objective lens to the waveguide and scattered light was collected by a 0.9-mm-core plastic fiber. A propagation loss of 0.1 dB/cm was obtained for the single mode waveguide (2- to 3-μm pattern width).