Gain characteristics of erbium-doped single-mode fiber amplifiers operated at liquid-nitrogen temperature

Abstract
Gain characteristics of Er-doped silica single-mode fiber amplifiers are investigated at liquid-nitrogen temperature (LNT). Liquid-crystal-polyester coated Er-doped fibers are used to prevent an increase in microbending loss of LNT. The maximum net gain and the transmission loss for a 1.535 μm wavelength are increased at LNT. This increase is explained by a three-level rate equation taking into account the loss increase. On the other hand, gain characteristics and the transmission loss at 1.552 μm suggest the change in the amplification mechanism at LNT from a three-level system to a quasi-four-level system.