Effect of metal coating on the optical losses in heated optical fibers

The effect of heating in air on the optical losses in metal-coated fibers has been studied. Two fibers were drawn from the same silica preform and coated by different metals (copper and aluminum). Dependences of a change in the optical losses on the temperature were measured in a 20–400$^\circ$C range at a 50$^\circ$C step. The optical losses of metal-coated fibers heated to temperatures below 300$^\circ$C change mostly due to the microbending contribution. At temperatures above 300$^\circ$C, the main contribution to increasing optical losses is due to the absorption on OH groups. It is established for the first time that the contribution to optical losses due to the OH groups is much more pronounced in Al-coated fibers than in Cu-coated ones. In addition, the Al-coated fibers exhibit growth in the optical losses above 300$^\circ$C due the absorption on molecular hydrogen.