Effect of irradiation time on refractive index and reflectivity during growth of fiber Bragg grating

Abstract

To measure the induced refractive index and reflectivity of fiber Bragg grating, various samples of FBGs with different experimental conditions were fabricated by phase mask technique using KrF excimer laser operating at 248nm. In this study, only two samples are reported here. These samples were written by varying the UV exposure time and at constant pulse energy of 70 mJ (sample I) and 30 mJ (sample II) respectively. The induced refractive index and reflectivity of two samples were monitored during the growth of fiber gratings at different value of UV exposure time. The maximum refractive index modulations for sample I & II are 5.109 × 10-5 and 1.77 × 10-4 respectively. The reflectivity for sample I is 69% at Bragg wavelength 1551.3nm and sample II is 54% at Bragg wavelength 1552.2nm. The increasing of reflectivity of fiber grating with more UV exposure time results the more microstructural changes occur within the lattice of the photosensitive core and the more the residual axial stress builds up within the cross section of the fiber. It means that the induced index of modulation was increased with the increasing of UV exposure time. It is also inferred that the reflectivity increases gradually with the rise of UV exposure time and then saturates.