Infrared optical loss increase in silica fibers due to hydrogen

Hydrogen-related optical loss increases in silica fibers in the infrared wavelength region are discussed. The loss increase phenomena mainly consist of two features: one originated from H
2
absorption loss and the other from OH absorption loss. The origin of these loss increases has been clarified in view of H
2
gas diffusion in fiber and OH bond formation by a chemical reaction via defects in glass network. Countermeasures to suppress the loss increase to a sufficiently low level have been established by preventing the use of P
2
O
5
dopant and by selecting the fiber coating material and the coating condition.