Mode Content Determination of Terahertz Corrugated Waveguides Using Experimentally Measured Radiated Field Patterns

Abstract

This work focuses on the accuracy of the mode content measurements in an overmoded corrugated waveguide using measured radiated field patterns. Experimental results were obtained at 250 GHz using a vector network analyzer with over 70 dB of dynamic range. The intensity and phase profiles of the fields radiated from the end of the 19 mm diameter helically tapped brass waveguide were measured on planes at 7, 10, and 13 cm from the waveguide end. The measured fields were back propagated to the waveguide aperture to provide three independent estimates of the field at the waveguide exit aperture. Projecting that field onto the modes of the guide determined the waveguide mode content. The three independent mode content estimates were found to agree with one another to an accuracy of better than ±0.3%. These direct determinations of the mode content were compared with indirect measurements using the experimentally measured amplitude in three planes, with the phase determined by a phase retrieval algorithm. The phase retrieval technique using the planes at 7, 10, and 13 cm yielded a mode content estimate in excellent agreement, within 0.3%, of the direct measurements. Phase retrieval results using planes at 10, 20, and 30 cm were less accurate due to truncation of the measurement in the transverse plane. The reported measurements benefited greatly from a precise mechanical alignment of the scanner with respect to the waveguide axis. These results will help to understand the accuracy of mode content measurements made directly in cold test and indirectly in hot test using the phase retrieval technique.

THz signal transmission in a compact modular waveguide system

“…20 x 20 mm scans with 0.5 mm resolution are taken at a distance of 20 mm from the aperture. The measured complex field is then numerically back-propagated by 20 mm [6]. Fig.…”

“…The projection of the obtained complex field distribution onto an ideal HE mn distribution yields the fraction of the total power carried by the respective mode [6]. Fig.…”

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

“…The radiation pattern from a transmission line powered by a gyrotron has been measured in hot test using an IR camera [4], [9] where the mode content was retrieved from intensity data using the phase retrieval code. In addition, experimental mode content has been determined from cold test measurements by scanning the pattern of radiation from the end of an oversized waveguide [10], [11]. Therefore, this paper will focus on a correction based on the assumption that these two modes are the most significant HOMs in the transmission line system.…”

Corrugated Waveguide Mode Content Analysis Using Irradiance Moments

“…The accuracy of these techniques for high-average-power gyrotrons is limited by the signal-to-noise ratio and spatial resolution of the infrared measurement technique. In cold test, the mode contents are evaluated to higher accuracy using higher resolution data from a scanner [18], [19], [21]–[23], where intensity profiles are measured down to −50 dB or less. Another approach is to measure the mode contents using a monitor installed with the miter bend assembly on the transmission line [24], [25].…”

“…Section IV numerically benchmarks the method using combinations of the known modal powers of the dominant mode. In Sections V and VI, we use the experimentally measured data from two different corrugated waveguide setups and compare the results obtained using the proposed method in this paper to the phase retrieval method [23]. …”