Fiber Laser Model Considering Excited-State Absorption and Cooperative Upconversion

In this paper, we present the results of a comprehensive study on how excited-state absorption and concentration effects influence fiber laser efficiency and the optimization of the laser cavity’s output coupler reflection. The concentration effects discussed include the cooperative interaction between two closely spaced active ions and the pair-induced quenching typically observed in heavily doped gain fibers. The laser is simulated using a model based on the laser, pump, and spontaneous emission waves propagating along the gain fiber, where the intensities of these waves determine their absorption or amplification. The model considers the radial distributions of optical fields and populations of the energy levels of the active ions, which is crucial to comply with the law of conservation of energy. The results discussed in this paper are essential for applications related to the optimization of heavily doped fiber lasers. The physics behind the reported results is discussed.