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Hydroxyl ion absorption is the absorption in optical fibers of electromagnetic radiation, including the near-infrared, due to the presence of trapped hydroxyl ions remaining from water as a contaminant.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
The hydroxyl (OH−) ion can penetrate glass during or after product fabrication, resulting in significant attenuation of discrete optical wavelengths, e.g., centred at 1.383 μm, used for communications via optical fibres.<ref>Template:Cite journal</ref>