Off-axis core transmission characteristics of helically twisted photonic crystal fibers.

The transmission characteristics of helically twisted photonic crystal fibers (PCFs) having off-axis cores are theoretically investigated for the first time, to the best of our knowledge. Due to the modified equivalent permittivity and permeability tensors induced by helical twisting, the transmission characteristics of helically twisted PCFs depend on the core position. A beam propagation method developed for helically twisted waveguides, which qualitatively and quantitatively explained the reported transmission spectra of helically twisted PCFs, is used for the analysis. The loss is significantly increased for a shorter wavelength side, if the position of the core is far from the rotation axis, due to the coupling to twist-induced cladding modes. On the other hand, for a longer wavelength side, the loss of one of the components of circularly polarized light is increased; namely, input linearly polarized light becomes close to left- or right-circularly polarized light, depending on the rotation direction. These results indicate that helically twisted PCFs with off-axis cores can be used for core-position-dependent polarization controlling devices.