1：Experimental Design and Method Selection:

The study involved conducting experiments with subsequent in-line measurements on different Ge-doped FBGs. The impact of fiber composition and UV-inscription parameters on FBG transmission were quantified to evaluate their contribution to both the spectral width and the wavelength of the Bragg signal.

2：Sample Selection and Data Sources:

The Ge concentration in the core of commercial optical fibers was determined using energy-dispersive X-ray spectroscopy (EDS) associated with Transmission Electron Microscopy (TEM).

3：List of Experimental Equipment and Materials:

A JEOL 2100F TEM equipped with Oxford EDS detector was used at 200 kV. Optical fibers were initially etched using hydrofluoric acid (<48%) at room temperature (RT) for partial removal of the fiber cladding.

4：Experimental Procedures and Operational Workflow:

Both the high Ge-doped fiber and the low Ge-doped fiber were inscribed using the 193 nm line of an MPB-excimer laser and a photomask having an uniform phase mask period of 1062.8 nm. Isothermal annealing and regeneration of Type-I gratings were conducted in steps with a constant heating rate of 25 ?C/min between each step.

5：8 nm. Isothermal annealing and regeneration of Type-I gratings were conducted in steps with a constant heating rate of 25 ?C/min between each step. Data Analysis Methods:

5. Data Analysis Methods: The data were extracted with a dedicated Matlab code. The recovery rates of the transmitted Bragg peak were determined from the transmission losses measured at RT via OSA.