Anti-resonance, inhibited coupling and mode transition in depressed core fibers

Abstract

The depressed core fiber (DCF), consisting of a low-index solid core, a high-index cladding and air surrounding, is in effect a bridge between the conventional step-index fiber and the tube-type hollow-core fiber from the point of view of the index profile. In this paper the dispersion diagram of a DCF is obtained by solving the full-vector eigenvalue equations and analyzed using the theory of anti-resonant and the inhibited coupling mechanisms. While light propagation in tube-type hollow-core fibers is commonly described by the symmetric planar waveguide model, here we propose an asymmetric planar waveguide for the DCFs in an anti-resonant reflecting optical waveguide (ARROW) model. It is found that the antiresonant core modes in the DCFs have real effective indices, compared to the anti-resonant core modes with complex effective indices in the tube-type hollow-core fibers. The antiresonant core modes in the DCFs exhibit similar qualitative and quantitative behavior as the core modes in the conventional step-index fibers. The full-vector analytical results for the simple-structure DCFs can contribute to a better understanding of the anti-resonant and inhibited coupling guidance mechanisms in other complex inversed index fibers.

Spectral dependence of transmission losses in high-index polymer coated no-core fibers

“…In this paper the calculations for both the straight and bent PC-NCFs were carried out by the full-vector finite element method with COMSOL Multiphysics, where the maximum mesh size is 0.4 and the relative error in the computed eigenvalues is smaller than 10 -12 . The numerical results obtained agree well with the analytical results in [25] for the straight PC-NCF. Furthermore, the waveguiding in the straight PC-NCF was analyzed using the ray optics approach and the transmission loss spectra were simulated analytically based on an asymmetric planar reflection model.…”

“…The step-like dispersion curves consistent with the mode reorganization phenomenon [14][15][16]. The anti-resonant core modes HE1,n+(N-n) exhibit similar qualitative and quantitative behavior as the HE1,n modes of conventional step-index fibers and can be approximated as the linear polarization modes LP0,n in the NCF (without the polymer coating) [25]. As shown in the Fig.…”

“…In our previous work [25] the effective refractive indices and the transverse intensity profiles of modes in a depressed core fiber were calculated by solving the full-vector eigenvalue equations, with the fiber parameters the same as those of the studied PC-NCF. In this paper the calculations for both the straight and bent PC-NCFs were carried out by the full-vector finite element method with COMSOL Multiphysics, where the maximum mesh size is 0.4 and the relative error in the computed eigenvalues is smaller than 10 -12 .…”

“…The cladding modes, which are guided in the cladding region by total internal reflections are depicted within the top part of the graph where neff > n1. On a same dispersion curve, as  increase the cladding modes HE1,N can change into anti-resonant core modes HE1,n+(N-n), n = 1, 2, …, where the subscript 'n' denotes the radial number in the core region while the number 'N-n' denotes the radial number in the cladding region [25]. The total radial number does not change for each of the dispersion curves.…”

“…In our previous paper [25], a dispersion diagram of the depressed core fiber was obtained by solving the full-vector eigenvalue equations. Analysis of the dispersion diagram suggests that the light guiding mechanism in the fiber core is supported by the anti-resonant, inhibited coupling and total C internal reflection effects.…”