Importance of Internal Tensile Stress in Forming Low-Loss Fiber Draw-Tower Gratings

Internal tensile stress inside fiber core can impact significantly on formation of Draw-tower gratings (DTGs). We carry out online dynamic preparation experiments of DTG to analyze the effect of internal stress on grating reflectivity, as well as offline multi-pulse accumulation experiments to further investigate the underlying mechanisms for grating formation. The optimum laser energy density used to grating writing and the variation of reflectivity with exposure extent were obtained, which is instrumental for their optimization and rapid prototyping. A stress modulation model was proposed as the dominant mechanism to describe the formation of the grating structure by a single laser pulse in the low-loss fiber, which is of great significance for the research of the grating photosensitivity mechanism.