Up-converted emission and mode beating in Er(3+)-doped fibers

We demonstrate the differences in the excited state transmission (EST) for different modes in 8 μm core diameter, Er(3+)- doped silica fiber. The S(2) (Spatially and Spectrally resolved) imaging method was used to determine the modal composition of the transmitted beam and to analyze the group delays of the higher order modes. We register the up-converted emission under two beam excitation (980 nm + 850 nm or 790 nm) and propose the numerical model for the anti-Stokes emission analysis. Taking additionally into account the interference of the beating fiber modes, one can expect the inhomogeneous spatial distribution of the excited ions. This was predicted by numerical calculations. The obtained results have been confirmed by taking photo of the up-converted emission as seen from the side of the fiber.