Six high-index cores are embedded around the central solid core of the photonic crystal fiber to form a fiber embedded photonic crystal fiber (FEPCF), which is investigated based on the beam propagation method. In this structure, the Gaussian mode could be transferred to the ring mode. So FEPCF could used as a mode convertor.
Sun, W. M., Liu, X. Q., Chai, Q., Zhang, J. Z., Fu, F. H., Jiang, Y., & Birks, T. A. (2009). Mode field converter based on embedded photonic crystal fiber. Chinese Optics Letters, 7(10), 921-923. https://doi.org/10.3788/col20090710.0921