Terahertz Spiral Spatial Filtering Imaging

Hui Liu,Guangyou Fang,Shiyou Wu,Meng Zhao,Xiaojun Liu,Chao Li

doi:10.3390/app11062526

Abstract

In this paper, we propose a terahertz (THz) spiral spatial filtering (SSF) imaging method that can enable image contrast enhancement. The related theory includes three main steps: (1) the THz image of the target is Fourier transformed to the spatial spectrum distribution; (2) the spatial spectrum is modulated by a spiral phase at the Fourier plane; (3) the filtered spatial spectrum is inverse Fourier transformed to the desired THz image. Meanwhile, analytic expression of the final THz image is derived. Due to the unique nature of the spiral phase, THz image contrast enhancement can be achieved and verified by various simulated target images with different contrasts. In our designed THz SSF imaging system, Fourier transform is carried out by the lens, and the spiral phase is acquired by the spiral phase plate (SPP). Proof-of-principle experiments with three different types of targets (carved metal letters, a high-density polyethylene (HDPE) piece with a scratch, and a leaf) were carried out, and the effectiveness of contrast enhancement and edge extraction on the THz reconstruction images was validated.

Highlights

For low-contrast amplitude and phase objects, traditional THz imaging methods cannot obtain terahertz (THz) images with ideal image contrast, which makes the subsequent edge-detection and feature-recognition stages of image processing difficult [1,2,3,4,5]
Where ρ is the polar radius in the Fourier plane, circ(ρ⁄R) is the circular function deThe THz electric field can be obtained at the THz detector, which reads scribing an spiral phase plate (SPP) with a radius of R, and θ is the azimuth in the Fourier plane
We studied the capability of enhancing the imaging contrast of the tested object with low intensity and phase jumps of the THz spiral spatial filtering (SSF) imaging system

Summary

Introduction

For low-contrast amplitude and phase objects, traditional THz imaging methods cannot obtain terahertz (THz) images with ideal image contrast, which makes the subsequent edge-detection and feature-recognition stages of image processing difficult [1,2,3,4,5]. THz imaging techniques can be applied in various fields, such as security screening [16,17], non-destructive testing [18,19,20], biological detection [1,21], and data-rata communications [22,23]. To improve the image contrast, Linas et al proposed the Bessel zone plate design based on silicon multi-phase diffractive optics, which was demonstrated in continuous wave mode at 0.6 THz [35]. Edge extraction and contrast enhancement were demonstrated perfectly, which is in good agreement with the theory and simulation results We believe that this THz imaging technique could pave the way for edge detection and contrast enhancement in THz imaging

Theory of SSF Imaging

Simulations

Results realize

Conclusions