Excited-state absorption and ion pairs as sources of nonlinear losses in heavily doped Erbium silica fiber and Erbium fiber laser

We report an experimental and theoretical investigation of the nonlinear transmission coefficient of a heavily doped (2300 ppm) Erbium silica fiber at continuous-wave pumping at the wavelength 1560 nm. It is shown that the fiber transmission is essentially deteriorated by the nonlinear losses, which are caused by the excited-state absorption (ESA) and Erbium ion pairs (IP) presented in the fiber. These phenomena inevitably result in worsening of the amplifying and lasing potential of the heavily doped Erbium fiber. We demonstrate the latter on the example of an Erbium fiber laser (wavelength, lambda= 1560 nm) under IR (wavelength, lambda= 978 nm) pumping, where the heavily doped Erbium fiber is used as an active medium. The developed theory, addressing both the nonlinear transmission coefficient of the fiber at the 1560-nm pumping and the generation characteristics of the Erbium fiber laser, takes into account the additional losses and non-radiative relaxation factors stemming from the ESA- and IP-effects and allows getting a good agreement between the modeling and experimental results.