Photoluminescence mapping of oxygen-defect emission for nanoscale spatial characterization of fiber Bragg gratings

Confocal photoluminescence (PL) microscopy is used to gain insight into the inner structure of Ge-doped Fiber Bragg Gratings (FBGs). These measurements pinpoint room temperature PL emission from oxygen-related defects in the visible range, whose spatial distribution exhibits a periodicity associated with the spatial modulation of the refractive index printed inside the fiber core of the FBG. The period measured by PL mapping performed at submicrometric resolution matches the period of the refractive index variation determined from the optical transmission wavelength using the Bragg condition. Since the PL emission of oxygen-related defects can be used to probe local chemical changes inside fused silica, this novel and non-destructive experimental approach can be implemented for the direct characterization of FBGs, to study the effects of gas conditioning, ageing, and degradation under various environments.