Role of UV fluence on the thermal regeneration characteristics of fiber Bragg gratings: experiment and analysis

Abstract

In this paper, the effect of UV exposure time on the thermal regeneration characteristics of fiber Bragg gratings (FBGs) inscribed using 255 nm laser radiation is presented for the first time. This effect is studied in terms of reflectivity, refractive index (RI) modulation and thermal stability of the gratings. Type-I FBGs of RI modulation varying from 2.03 × 10−4 to 3.38 × 10−4 were inscribed in hydrogenated SMF-28 fiber by controlling UV exposure fluence from 0.9 kJ cm−2 to 28.8 kJ cm−2. Thermally regenerated gratings were fabricated from type-I FBGs by applying a step-annealing schedule up to 900 °C. The reflectivity of the regenerated FBGs increases with UV fluence and after reaching a maximum value of 46.05%, it decreases. The different behavior of the thermal regeneration characteristics of FBGs is linked with the stress developed inside the fiber during the writing of FBGs and their relaxation during thermal annealing. The theoretical calculations using the defect center and densification model provided an indirect novel approach to estimate the contribution of stress relaxation in thermal regeneration. The work presented in this paper is significant for optimizing the thermal regeneration of FBGs with the final aim of developing a highly reflective regenerated grating for high temperature sensing applications.