First, a GeO2-doped silica fiber with a core diameter of 2 mm, clad diameter of 125 mm, and an index difference of 2% was consecutively immersed in buffered HF with volume ratios
of 40%NH4F : 50%HF : H2O ¼ 1:7 : 1 : 1 for 80 min and in 10 : 1 : 1 for 120 min.