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In this paper, we show experimentally that large photosensitivity in dye-doped polymer optical fibers can be obtained with small pump power (about 1 mW) at visible wavelength. The fiber has two identical cores each doped with fluorescein, and the pump beam is launched axially into one of the cores. However, when the pump beam at UV wavelength is launched into the fiber transversely, large photosensitivity is also observed. We also show that gratings can be written in polymer fiber and preform.