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

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.