Amplitude and phase imaging using incoherent free-running self-oscillating device in millimeter- and terahertz-wave bands

Eito Miyake,Shintaro Hisatake

doi:10.1063/5.0046521

Abstract

To improve the accuracy of material identification in millimeter- and terahertz-wave nondestructive imaging during security and other inspections, it is important to perform both amplitude and phase imaging. Highly coherent sources generated based on the frequency multiplication of stable microwave sources are promising for these applications. However, this method reduces the fidelity of the image due to the interference phenomenon with reflected waves on the sample surface. In this paper, we propose and demonstrate amplitude and phase imaging using an incoherent free-running self-oscillating device. We used a Gunn oscillator operating at 122.5 GHz as a millimeter-wave source and a photoconductive antenna as a detector. The frequency and phase noise of the source are canceled by the low-frequency electronics, which realize phase imaging using an incoherent source. We performed amplitude and phase imaging on low-loss polypropylene objects and obtained ∼22 times higher imaging contrast in the latter than in the former. Furthermore, we were able to obtain high-fidelity imaging results with less interference in both amplitude and phase imaging. The proposed system is reliable and inexpensive because the source operates with a DC power source only, and the frequency and phase noise-cancellation electronics consist of low-frequency circuits. Moreover, because the noise-cancellation electronics are independent of the RF frequency, the system is easily extendible to the higher frequency region to improve its spatial resolution. This paves the way for the realization of a diode-based, sensitive, and low-cost integrated system that allows both amplitude and phase imaging for material identification in security inspections.

Highlights

When phase and amplitude imaging are used in combination, objects with low-loss and minute changes in amplitude, such as dangerous objects made of thin dielectric glass or acrylic, can be detected as the phase changes with high sensitivity
We imaged characters written in ink on a 90-μm paper surface using a photonics-based continuous wave (CW) system and reported that terahertz amplitude imaging failed to identify the characters due to the lack of contrast while terahertz phase imaging succeeded in the task
We propose and demonstrate a novel method to realize both amplitude and phase imaging using an incoherent free-running self-oscillating device

Summary

Introduction

Terahertz amplitude imaging has been employed to demonstrate the detection of prohibited drugs and explosives in envelopes and weapons, such as guns and knives, hidden in clothes and bags. When phase and amplitude imaging are used in combination, objects with low-loss and minute changes in amplitude, such as dangerous objects made of thin dielectric glass or acrylic, can be detected as the phase changes with high sensitivity. When phase and amplitude imaging are used in combination, objects with low-loss and minute changes in amplitude, such as dangerous objects made of thin dielectric glass or acrylic, can be detected as the phase changes with high sensitivity These have been typically difficult to image at high contrast using amplitude imaging alone due to large transmissivity. Increasing the spectral power density, and the signal-to-noise ratio (SNR) of measurements in the linear regime, is difficult because the pulse energy spreads over a wide spectral range. This problem is mitigated by a system based on continuous wave (CW) technology, which provides a higher SNR and more linear measurements than a TDS-based system.. This problem is mitigated by a system based on continuous wave (CW) technology, which provides a higher SNR and more linear measurements than a TDS-based system. Previously, we imaged characters written in ink on a 90-μm paper surface using a photonics-based CW system and reported that terahertz amplitude imaging failed to identify the characters due to the lack of contrast while terahertz phase imaging succeeded in the task.

Methods

Results

Conclusion