40 dB Fiber Bragg Grating Written Through The Fiber Coating at 257 nm

The writing of fiber Bragg gratings through the coating has important implications for the mass production of these devices. For instance, the time-consuming steps of stripping and recoating of the grating (potentially leading to fiber strength degradation), can be completely eliminated resulting in a simpler, more efficient and lower cost fabrication process. In the first demonstration of writing Bragg gratings through the coating, up to ~3 dB Bragg gratings were written through a UV-transparent acrylate-based 25 μm thick coating at 257 nm [1]. Subsequently, other researchers demonstrated a ~1 dB fiber Bragg grating written through a standard coating using 334 nm light [2]. Because the absorption band at 334 nm is much weaker than at 244 nm, these gratings required a very high germanium concentration (~20 mol%) and the addition of boron as a codopant to increase the photosensitivity of the fiber. The high level of germanium makes it difficult to achieve low-loss splices to standard communications grade fiber. Here we report for the first time highly reflective (99.99% Reflectivity, -40 dB transmission) Bragg gratings written through a novel polymer coating using a photosensitive germanium-doped fiber with 5 mole% germania.