Effects of fiber cladding on UV beams and fringe patterns in side-exposure FBG writing

The so-called side-exposure method has been widely used in the fabrication of fiber Bragg gratings (FBGs). The effects of the fiber cladding on the wavefronts of the UV writing beams and the interference fringes formed inside the fiber have been addressed in this study. The wavefronts of the UV beams have been analyzed and it has been demonstrated that the existence of the fiber cladding significantly alters the wavefronts of the UV beams in one plane and makes them highly astigmatic. Based on the calculated wavefront radii of the UV beams, a model has been established for prediction of the interference fringe pattern formed in the fiber core area.