Direct observation of multimode interference in rare-earth doped micro/nanofibers

Modal inspection of optical fibers is important for multimode application but it is challenging to collect in-situ information of propagating modes for evaluation and manipulation. Here we demonstrate direct observation of multimode interference in Er³⁺/Yb³⁺ co-doped micro/nanofibers. Luminescent interference patterns are visualized by visible up-conversion of Er³⁺ ions and are used for establishing the existence of higher order modes co-propagating with fundamental modes. We use fast Fourier transform to analyze the patterns in detail and obtain excellent agreement between experiment and calculation on beat lengths of the interference. Effective index differences among higher order modes and a fundamental mode of a microfiber are also experimentally investigated with the assistance of interference patterns, revealing the characteristic of modal dispersions.