Optimization of Yb3+-doped double-clad fiber lasers using a new approximate analytical solution

We investigated the properties of continuous wave (CW) Yb3+-doped double-clad fiber lasers (DCFLs) with linear-cavities theoretically and numerically using the rate equations. Under steady-state conditions, a new approximate analytical solution for CW Yb3+-doped double-clad fiber lasers (DCFLs) with consideration of the scattering losses were deduced. Good agreement between the proposed solution and the numerical simulation was demonstrated. Compared with the known approximate solutions published in the literature, the proposed solution has a briefer expression, higher accuracy and wider scope of application, which extends the applicable range of the analytical result to low reflective feedback mirror configurations. The solution provides a clear physical understanding of the optimal design of the CW Yb3+-doped DCFLs and can be applied to different pump and output configurations. Using the proposed solution, the optimal design of the CW Yb3+-doped DCFLs was discussed. If cavity reflectivities are given in advance, the optimal fiber length is found to be independent of the pump power. When the pump power and reflectivity of the feedback end are known in advance, the results show that the optimal fiber length increases and the optimal reflectivity of output mirror decreases with increase in pump power. Furthermore, when the feedback mirror is highly reflective, there exists a certain tolerance of the optimal parameters, in which the conversion efficiency decreases only slightly. But the conversion efficiency is sensitive to reflectivity of output mirror if feedback mirror has low reflectivity.