We have observed light coupling between dual-core dye-doped polymer optical fibers. The experimentally observed coupling length is consistent with the standard coupled-mode theory for single-mode–single-mode coupling between step-index waveguides [P. D. McIntyre and A. W. Snyder, J. Opt. Soc. Am. 63, 1518–1527 (1973)]. We also show that a graded refractive-index profile of such dual-core fibers can be made by controlled dye diffusion. The concentration, as measured by optical absorption, shows a graded profile that matches the refractive-index profile, confirming that dye concentration determines the refractive index.
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