S.Zappia THz imaging activities at IREA CNR

THz imaging activities at IREA CNR

I.Catapano, G. Ludeno, and F. Soldovieri Institute for Electromagnetic Sensing of the Environment National Research Council of Italy, I-80124, Italy

Abstract— This communication aims at summarizing the THz diagnostic imaging activities carried out at the Institute for Electromagnetic Sensing of the Environment (IREA) - National Research Council of Italy (CNR). As far as THz device is concerned, the Z-Omega Fiber-Coupled Terahertz Time Domain (FiCO) system equipped with an imaging module is deployed and enhanced thanks to an ad-hoc data processing strategy. Several examples concerning different kind of both known and unknown samples as well as unknown ones will be presented at the conference in order to assess the capabilities offered by the adopted hardware and software technology in different applicative contexts.

I. INTRODUCTION

THE exploitation of electromagnetic waves in the frequency range from 0.1 THz to 10 THz (free-space wavelength ranging from 30 µm to 3 mm) as diagnostic tool is attracting an increasing interest in many applicative fields, among which security, cultural heritage, material characterization, agri-food and biomedical diagnostics [1-3]. The widespread use of THz waves is due to their non-ionizing nature, their capability of penetrating into dry, nonpolar, nonmetallic materials, and has been improved thanks to recent technological advances that have allowed the commercialization of compact, flexible and portable systems.

Specifically, Time Domain Terahertz (TD-THz) systems are considered among the new frontier sensing methodologies, because they allow a sub-millimetre cross-sectional representation and give information about position and thickness of possibly inner layers as well as hidden features. Accordingly, they allow to analyse texture and stratigraphy of materials as well as to detect hidden defects or anomalies without implying long term risks to the molecular stability of the exposed object and humans.

However, the effectiveness of THz imaging is crucially dependent not only on peculiar features of the surveyed materials, but also on the adopted hardware devices and data processing approaches.

This communication aims at summarizing the THz imaging activities carried out at the Institute for Electromagnetic Sensing of the Environment (IREA) - National Research Council of Italy (CNR) in different applicative fields.

II. IMAGING ACTIVITIES

IREA owns the Z-Omega Fiber-Coupled Terahertz Time Domain (FiCO) system [4] equipped with an imaging module. The FiCO system collects signals into a 100 ps observation time window, which can be moved along a time scan range of 1 ns according to the path length between transmitter and receiver. The waveform acquisition speed can be up to 500 Hz and the maximum dynamic range (DNR) is 30dB, while the typical DNR is 20dB. The working frequency range is from about 50 GHz to 3 THz. The system is equipped with an automatic positioning system enabling to scan a 150 mm x 150 mm area, with the smallest spatial offset in both directions describing the measurement surface equal to 0.012 mm. It is worth to pointing out that the positioning system allows normal reflection measurements by keeping fixed emitter and receiver and moving the sample under test.

With respect to data processing, an ad-hoc strategy has been designed at IREA to filter data and visualize THz images [5]. This strategy involve three different steps in time-domain aimed at reducing noise, filtering out undesired signal introduced by the adopted THz system and performing (when required) surface topography correction [5]. The first step regards noise filtering and exploits a procedure based on the Singular Value Decomposition (SVD) of the time-domain raw data matrix, which does not require knowledge of noise level and it does not involve the use of a reference signal. The second step aims at removing the undesired signal that we have experienced to be introduced by the FICO system. Indeed, when the system works in a high-speed mode, an undesired low amplitude peak occurs always at the same time instant from the beginning of the observation time window (trace) and needs to be removed from the useful data matrix in order to avoid a wrong interpretation of the imaging results. The third step of the considered data processing chain is a topographic correction, which is necessary in order to image properly the samples surface and its inner structure. Such a procedure performs an automatic alignment of the first peak of the measured waveforms by exploiting the a-priori information on the focus distance at which the specimen under test is located during the measurement phase.

III. SUMMARY

Several representative examples of the IREA THz imaging activities will be presented at the conference in order to assess the capabilities offered by the adopted hardware and software technology in different applicative contexts, such as cultural heritage and quality control of materials, among which food and composite sample.

REFERENCES

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