N.A.Stathopoulos, S.P.Savaidis, H.Simos, M.Rangoussi, P.Kervalishvili “Simulation and properties of Erbium-doped Distributed Bragg Reflectors (ED-DBR) and Fiber Bragg Gratings (ED-FBG)” Optical Fiber Technology, Vol. 19, Issue 5, 369-377, (2013)