Theoretical Investigation of Iterative Phase Retrieval Algorithm for Quasi-Optical Millimeter-Wave RF Beams

Abstract

Abstract-In this paper, we present a detailed analysis of the iterative phase retrieval approach (IPRA) for determining the phase profile of the output microwave beam of a gyrotron from known intensity patterns emphasizing the field propagation techniques which are used to propagate the RF field of the microwave beam between known intensity planes. The propagation method, based on first Rayleigh-Sommerfeld diffraction integral (RSDI), is solved using fast Fourier transform (FFT) technique and zero padding. It is observed that the use of FFT and, therefore, the discretization of the RSDI propagation kernel introduce aberrations in the propagated field due to the superposition of the original field with its replicated versions. This problem is solved by approximations leading to the Huygens-Fresnel propagation method which further imposes the restrictions on the distances of propagation depending on the size of the transverse plane used to discretize the intensity pattern. This constraint of the distance of propagation causes problem in the iterative phase retrieval approach (IPRA) when more than two intensity planes are used. A method based on interpolation is proposed to overcome this restriction. IPRA is then further discussed to optimize several parameters, such as plane separation, plane dimension, mesh size, and measurement accuracies, which become more of an issue during the measurements of infrared intensity thermograms of the output microwave beam.Index Terms-Fourier transform, gyrotron, infrared intensity thermogram, millimeter wave, phase retrieval, zero padding.

Infrared Measurements of the RF Output of 170-GHz/2-MW Coaxial Cavity Gyrotron and Its Phase Retrieval Analysis

“…Therefore, we can make a direct comparison of the measured intensity profiles and retrieved intensity profiles with the simulated profiles. This paper, with the experimental data, provides a verification of the analysis given in [8].…”

“…The alignment of the target material was checked, with respect to the window, using a laser. The measurements should be done within the Rayleigh range and as explained in [8] and [9]; the reconstruction algorithm will result in an accurate phase profile if the measurements are performed as close as possible to the beam-waist location (also explained in Section IV-B). We therefore tried to measure the beam profile as close as possible (at 441 mm) to the gyrotron window.…”

Mode content analysis of the RF output of a gyrotron based on the astigmatic Gaussian beam of higher order

“…The above said modal decomposition technique is applied on the theoretical data of the first prototype of "EU 170GHz/2MW coaxial cavity gyrotron for ITER" [1]. A preliminary set of the optimized results are plotted in figure 1, 2 and 3.…”

“…After the necessary data processing, Iterative Phase Retrieval Algorithm, which uses an accurate diffraction integral propagation technique [1], is applied to access the phase information. Once the complete RF field information is obtained, we propagate this field at several locations along the beam path.…”

Note: Three-dimensional stereolithography for millimeter wave and terahertz applications

“…3. Theoretical predictions are based on an ideal HE 11 mode generated at the output of the GOLA antenna which is numerically propagated 16 and reflected onto the phase correction mirror.…”