Long-period fiber gratings were inscribed in a commercial silica fiber by a point-by-point arc discharge technique with different discharge conditions. The refractive index (RI) profile change induced by arc discharge was measured using the quantitative phase microscopy for the first time to our knowledge. The causes of the transmission variations induced by different arc discharges and the mechanisms of the RI profile change were investigated based on the measured phase profiles. The RI in the core and the cladding has clearly changed due to arc discharge. The central dip in the core profile diminished very much, and the index gradient became gradual. The resonance wavelengths have fluctuated by discharge current and time owing to variations of the reduction of the core–cladding RI difference and the extent of the RI change region.
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