The Role of a Kind of DID Defect-Open Tear Defect in the Recording of Gratings Using Excimer Lasers in Low-Loss Optical Fibers

In general, the internal stress in the fiber core is an important factor of the grating formation. However, it is not very clear how the stress affects the grating formation. Based on our experiments, we first proposed the concept of open tear defect on core/cladding interface. The existence of open tear defect and its importance for grating writing are preliminarily verified by single laser pulse writing experiments on four different types of pure silica fibers and high-pressure helium loading experiments on an ordinary G652 fiber. An open tear defect with stress concentration collapses under laser irradiation, leading to relaxation of the stress around the defect, especially the sharp reduction of the concentrated stress at the tip of the defect, so as to realize the grating with periodic stress modulation. The stress state inside the fiber core affects the opening degree of tear defect, thus affecting the stress concentration around the defect, and finally affecting the grating writing reflectivity. The idea provides an important guideline for writing gratings in low-loss fibers by a single laser pulse.