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 anti-resonant 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 anti-resonant 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.
Highlights
As opposed to conventional step-index fibers where the light is guided in the high-index core region by total internal reflections, light guidance in the low-index core region of the inversed index fibers can be explained by other mechanisms such as the photonic bandgap effect, inhibited coupling effect and the anti-resonant effect
The eigenvalue equations for the vector modes in the depressed core fiber (DCF) are shown in Appendix, which were solved by a graphical method
It has been shown that the DCFs support annular-like cladding modes in the tube cladding region and disc-like anti-resonant core modes in the rod core region and both of them were obtained by solving the same group of full-vector eigenvalue equations using the graphical method
Summary
Inversed index fibers with a low-index core and a high-index cladding such as the tube-type hollow-core fibers and hollow-core photonic crystal fibers have attracted a lot of interest in the fields of high-capacity telecommunication networks [1,2,3,4,5], high power/supercontinuum/ultrafast lasers [6,7,8,9,10,11], terahertz waveguiding [12,13] and high sensitivity optical sensing [14,15]. The second type is inhibited coupling fibers, for which the neff - λ of the core modes lies inside the cladding mode continuum, but the coupling between them is minimized due to the high degree of transversefield mismatch [6]. Both the photonic bandgap fibers and the inhibited coupling fibers have the same anti-resonant nature, which can be described by an anti-resonant reflecting optical waveguide (ARROW) model [2,3,6]. The planar waveguide acts as a FabryPerot resonator, which allows the anti-resonant light to be reflected back while allowing forward transmission of the resonant light [16,17,18,19]
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