Effect of the surrounding refractive index and fusion-depth on side-pump combiners

A study of the effect on the coupling efficiency due to surrounding refractive index and fusion depth variations in (1+1) side-pump combiners is presented. The external refractive index values used in the simulations are 1.0 and 1.35, which correspond to the refractive indexes of air and Teflon. The results showed that the coupling efficiency is not affected by a Teflon-like surrounding medium along the coupling zone as long as the tapers transition of the pump fiber is not too large to reduce the propagation angle below the critical angle of the receiving double clad fiber. Furthermore, modal simulations showed that the higher the fusion depth the faster the coupling of the pump modes.