a method of and an apparatus for erbium - doped optical fibers usable as an optical amplifier allowing optical signals to be directly amplified by themselves , which method and apparatus are capable of reducing manufacturing time while increasing productivity . the methods includes the steps of closing one end of a quartz tube deposited with a clad layer and a core layer , injecting a solution containing erbium and other additive elements into the quartz tube , thereby absorbing the solution in the core layer , removing the solution from the quartz tube after a predetermined period of time elapses , clamping the quartz tube to a clamping chuck via a connecting tube which is connected to the quartz tube , opening again the closed end of the quartz tube , injecting a large amount of gas into the quartz tube , heating the connecting tube by the heating source to a predetermined temperature , thereby warming the gas introduced in the quartz tube , rotating the clamping chuck to uniformly heat the quartz tube , thereby slowly drying the solution absorbed in the core layer of the quartz tube , and heating again the quartz tube by the heating source , thereby softening the quartz tube . thereafter , the quartz tube may be collapsed into a condensed erbium - doped optical fiber preform , or both ends of the quartz tube may be completely sealed to form an erbium - doped optical fiber preform having a hollow cylindrical structure .