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On the role of Ge-doping concentration in the refractive index rollover and thermal annealing characteristics of type IIa fiber Bragg gratings

Abstract. This paper presents studies on the role of Ge-doping concentration (6 to 18 mol. %) in the refractive index rollover fluence and thermal annealing characteristics of type IIa fiber Bragg gratings (FBGs). A 255 nm UV beam of low-pulse energy density (∼2.2  mJ/cm2), nanosecond (∼30  ns) duration, and high-repetition rate (∼5.5  kHz) was used for FBG inscription. It is observed that the UV fluence needed for refractive index rollover was higher for fiber having low Ge-doping (∼6  mol.%). The temperature sustainability of these gratings has been studied in a multistep thermal annealing process up to 800°C. It was observed that the higher the total UV fluence required for refractive index rollover, the higher the temperature sustainability of a type IIa grating. The temperature rise of the fiber for a single UV pulse and at the maximum cumulative UV fluence was estimated for different Ge-doping concentrations. The thermal stability of the grating is linked to the amount of Ge-doping concentration of the fiber. These observations may be due to the fact that a high cumulative fluence resulted in a larger stress relaxation, leading to enhanced FBG temperature stability.