Theoretical Analysis of ESA-Enhanced 2.8 μm Lasing in Er-Doped ZBLAN Fiber Lasers

A detailed analysis of theexcited state absorption (ESA) transition I-4(13/2)-> I-4(9/2) which occurs in erbium (Er) doped ZBLAN fiber lasers is performed through numerical modeling. It is found that the ESA plays a significant role in enhancing the 2.8 mu m lasing of Er3+ ions. The ESA promotes the ions residing in I-4(13/2) (the lower state of the 2.8 mu m laser) to I-4(9/2), where the ions subsequently decay to I-4(11/2) (the upper state of the 2.8 mu m laser) rapidly. An energy recycling is consequently realized via the ESA. The ESA can be activated in cascaded 2.8 mu m/1.6 mu m Er-doped ZBLAN fiber lasers by absorbing the 1.6 mu m signal. Through numerical simulation it is found that in cascaded lasers the slope efficiency of the 2.8 mu m laser can be elevated to similar to 48% with the assistance of the ESA, a value that surpasses the Stokes limit (similar to 35%) substantially and agrees well with the experimental results previously reported. It is also found that the ESA-induced energy recycling is exothermic, an effect that is non-ignorable in the high-power circumstance. Moreover, the injection of an additional 1.6 mu m pump for directly exploiting the I-4(13/2)-> I-4(9/2) ESA is simulated using the model. It is found that as the second pump is introduced, the 2.8 mu m lasing almost entirely relies on the ESA-induced energy recycling, and hence, the laser is capable of operating under very low 976 nm pump power. The simulation results indicate that the 2.8 mu m lasing in Er-doped ZBLAN fiber lasers can even be realized using a single 1.6 mu m pump, via a combination of the I-4(15/2)-> I-4(13/2) and I-4(13/2)-> I-4(9/2) transition. The outcomes of this work can inspire the development of novel, high-performance 2.8 mu m Er-doped ZBLAN fiber lasers.