Simple Correctors for Elimination of High-Order Modes in Corrugated Waveguide Transmission Lines

When using overmoded corrugated waveguide transmission lines for high power applications, it is necessary to control the mode content of the system. Ideally, overmoded corrugated transmission lines operate in the fundamental HE₁₁ mode and provide low losses for long distances. Unwanted higher order modes (HOMs), particularly LP₁₁ and HE₁₂, are often excited in the experimental systems due to practical misalignments in the transmission line system. This paper discusses how the unwanted modes propagate along with the fundamental mode in the transmission line system by formulating an equation that relates the center of power offset and angle of propagation of a beam (for the HE₁₁ and LP₁₁ modes) or the waist size and phase front radius of curvature of a beam (for the HE₁₁ and HE₁₂ modes). By introducing two miter bend correctors into the transmission system-miter bends that have slightly angled or ellipsoidal mirrors-the HOMs can be precisely manipulated in the system. This technique can be used to eliminate small quantities of unwanted modes, thereby creating a nearly pure fundamental mode beam with minimal losses. Examples of these applications are calculated and show the theoretical conversion of up to 10% HOM content into the fundamental HE₁₁ mode with minimal losses.