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Current high-capacity and long-reach optical fiber links would not be possible without optical amplification. Especially the use of erbium-doped fiber amplifiers (EDFAs) has revolutionized optical communication systems during the last two decades. Although the amplification process and various effects occurring in rare earth doped amplifiers have been already well understood and accurately modeled, evolution of thermodynamic entropy and other thermodynamic aspects have not been sufficiently considered in the past. This paper analyzes the amplification process in EDFA from the thermodynamic point of view and proposes a novel modeling approach to evaluate both energy and entropy dynamics. The model is described in detail and some exemplary numerical results are presented.