The Effect of Core Offset on the Mode Converting Characteristics in Twisted Single Mode Fibers

The effect of core offset on the mode converting characteristics in twisted single mode fiber (SMF) is investigated. If the core position of SMF is slightly shifted from its rotation center, multiple spectral dips appear in the transmission spectra due to the coupling between core and cladding modes. Guided mode solver and beam propagation method (BPM) based on finite-element method (FEM) specially formulated for helicoidal waveguides are used for the analysis. It is shown that a submicron offset of the core, which may not be seen in the microscope with 100-μm scale, causes strong modal coupling. Calculated positions of the dips for the submicron offset are in good agreement with the recently reported measured results, showing the slight core displacement may be one of the dominant mechanisms of the mode coupling in twisted SMF. Also, it is confirmed in the BPM analysis that the field in the twisted SMF carry an orbital angular momentum (OAM) at the spectral dip wavelength.