A theoretical analysis of the propagation characteristics of an annular circular waveguide with a helical winding as the inner cladding

In this paper, we present a fairly rigorous analysis of a helically inner-cladded annular step-index waveguide. Using the sheath-helix boundary conditions on the inner core-cladding boundary and simple step-index boundary conditions on the outer core-cladding boundary, the modal characteristic equation of the waveguide is derived by taking a vector approach and dispersion curves are drawn. We observe the effect of the pitch angle of the helical windings on the modal dispersion characteristics of the proposed waveguide. The main feature of the waveguide is the occurrence of negative dispersion and band gap. This property may be attributed to the periodicity of the inner helical cladding. The width of the forbidden region increases with the pitch angle and thereafter at the pitch angle ≈90° the region is not found. It is also observed that the lowest cutoff value for the usual curves slightly decreases with the increase in the pitch angle. Thus the pitch angle acts as the controlling factor for the negative dispersion and photonic band gap. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 37: 69–74, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10828