Improvement of refractive index profiling of a small-core single-mode fiber under partially coherent light excitation

The refractive index profile of a non-bending small-core single-mode fiber was first reconstructed from the measured guided mode intensity profile and its spatial derivatives. By combining a rotating diffuser with a coherent laser source, the resulting partially coherent light source overcomes the speckles on the guided mode image of a straight fiber, and the corresponding refractive index profile was successfully reconstructed. Furthermore, guided mode images of a bent fiber with a fiber coil at midway were also measured under different coherency light excitations for comparison. The slight asymmetry of the reconstructed index profile of a bent fiber measured under coherent light excitation was greatly improved when measured under partially coherent light excitation. The generated additional leaky modes propagating partly in both core and cladding induced by the effects of the fundamental core mode deformation at the intermediate fiber bending structure and the mode transition at the bent-to-straight fiber section were averaged out with a low coherence transformed light source.