AbstractTwo kinds of photosensitive fibers were prepared for on-line writing weak FBG arrays. A Ge/B co-doped fiber was designed for high single-pulse photosensitivity and a Ge highly-doped fiber was applied for low transmission loss. The experimental results show that the weak FBG arrays written on the Ge/B co-doped fiber have more uniform spectral shape and grating reflectivity than those on the Ge highly-doped fiber. The two fibers have stronger single-pulse photosensitive than traditional hydrogen-loaded fibers