INDEX Acid etch

Alcohol sensing. An etched FBG was used [28] to discriminate between methanol (n - 1.326), ethanol (n - 1.359), and isopropyl alcohol (IPA) n =1.378) (Fig. 9). The FBG was written using a KrF excimer laser (248 nm) with a length of 2.5 mm. It was etched down to 15 pm diameter using a 52% solution of hydrofluoric (HF) acid and then to a diameter of 6 pm with a 13% HF solution. The plot of the center wavelengths of the thinned FBG vs ambient refractive index is shown in the next figure. 

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