S. Al-Sowayan,  Selective Excitation of Circular Helical Modes in Graded Index Fibers.   journal   = {International Journal of Electronics and Communication Engineering}, [online]. World Academy of Science, Engineering and Technology.
    November 2013, vol. 84(12). 1711 - 1713
    [viewed 11 August 2020]. Available from: https://publications.waset.org/pdf/9996870.