Power flow in multimode step-index plastic photonic crystal fibers

We investigate the state of mode coupling in a multimode step-index plastic photonic crystal fiber (SI PPCF) with a solid-core by solving the time-independent power flow equation. For various arrangements of air-holes, and therefore a different numerical apertures (NAs), as well as a different widths of launch beam distribution, the length Lc for achieving equilibrium mode distribution (EMD) and length zs at which a steady state distribution (SSD) is established are determined for such fiber. We show that the larger the air holes in the cladding (higher NA), the longer length of the fiber it takes for the modal distribution-transients to reach their equilibrium and steady state. This is as a consequence of the greater participation rate of higher-order modes in higher-aperture photonic-crystal fibers. In contrary, in the case of a wide launch that excites more guiding modes, these lengths shorten. This is because the energy of a wide launch beam is more uniformly distributed among guided modes in the fiber, thus the EMD and SSD are reached at shorter distances than for a narrow launch beam. Such information is of interest for application of multimode photonic crystal fibers in telecommunication and fiber optic sensors.