Determination of the mutual modal degree of coherence for optical fiber modes

The representation of light beams in terms of their eigenfunctions, the so called modes, reduces the description of the full field information into a discrete set of modal coefficients, which enables the complete reconstruction of arbitrary beams by means of mutual modal interference. Particularly for optical fibers this approach is feasible due to the finite number of guided modes. In general the knowledge of the mode resolved power spectrum, intermodal phase differences and mutual degree of coherence is required to describe arbitrary modal superposition. In this work we present an all optical analysis scheme based on holographic correlation filters, which enables the modal resolved determination of all needed parameters. This technique can be applied to describe the general case of partial-coherent superposition in optical fibers compared with common modal decomposition strategies, which are restricted to complete coherent or incoherent superposition. In addition due to the modal dispersion and a resulting loss of coherence during fiber propagation the evaluation of the modal degree of coherence was used to determine the effective refractive index difference of the guided modes.