Full-scatter vector field analysis of an overmoded and periodically-loaded cylindrical structure for the transportation of THz radiation

TL;DR

This paper develops a comprehensive vector field scattering theory for overmoded, periodically-loaded cylindrical structures, enabling precise analysis of THz pulse transport, diffraction, polarization, and losses over long distances.

Contribution

It introduces a generalized full-scatter field theory using vector fields and Lorentz reciprocity, applicable to thick screens and nonuniform structures, advancing previous scalar and thin-screen models.

Findings

Analytical spectral coefficients derived for iris-line waveguides.

Theory accurately models diffraction, polarization, and ohmic losses.

Visualization of transient to steady-state regime achieved.

Abstract

Highly overmoded and periodically loaded structures, such as the iris-line waveguide, offer an attractive solution for the efficient transportation of diffraction-prone THz pulses over long distances (hundreds of meters). This paper presents the full-scatter field theory that allows us to analytically derive all the spectral (modal) coefficients on the discontuities of the iris line. The spectral analysis uses vector fields, superseding scalar field descriptions, to account for diffraction loss as well as polarization effects and ohmic loss on practical conductive surfaces. An advanced application of Lorentz's reciprocity theory, using a generalized guided-field configuration, is developed to reduce complexity of the mode-matching problem over nonuniform sections. The used technique is quite general and applies to a wide class of structures, as it only assumes a paraxial incidence (i.e. a parabolic wave equation) along the axis of the structure. It removes the traditional assumption of very thin screens, allowing for the study of thicker screens in the high-frequency limit, while formulating the problem efficiently by scattering matrices whose coefficients are found analytically. The theory agrees with and expands previously established techniques, including Vainstein's asymptotic limit and the forward-scatter approximation. The used formulation also facilitates accurate visualization of the transient regime at the entrance of the structure and how it evolves to reach steady state.