Numerical analysis of the three co-directional guided modes in optical period dielectric waveguides using inverse Gaussian apodizationL.C. Li, L. Xia, Z.H. Xie and D.M. LiuEur. Phys. J. D, 66 4 (2012) 94DOI: https://doi.org/10.1140/epjd/e2012-20539-1