Wideband Imaging Systems in the mm-Wave and THz range for Security and Nondestructive Testing

Martin Nezadal*1,2, Julian Adametz1,2, and Lorenz-Peter Schmidt1

1Lehrstuhl für Hochfrequenztechnik, Friedrich-Alexander Universität Erlangen-Nürnberg, Germany,

martin.nezadal@fau.de, julian.adametz@fau.de, lorenz-peter.schmidt@fau.de

2Erlangen Graduate School in Advanced Optical Technologies (SAOT), Friedrich-Alexander Universität Erlangen-Nürnberg

Abstract

This paper shows the application of planar antenna arrays employing large bandwidths in the millimeter-wave and low terahertz frequency region. Considerations on the design of such array systems, i.e. the arrangement of the antenna elements and the image reconstruction process are presented. Measurement scenarios for security screening and non-destructive testing (NDT) are investigated.

1. Introduction Imaging systems operating in the mm-Wave (mmW) and THz region have been in the focus of thorough research in the last years. In these frequency regions, electromagnetic waves have the ability to penetrate through non-metallic materials which are typically opaque in the visible part of the spectrum, like clothing or packaging materials. Furthermore, good resolution values can be achieved because of the relatively small wavelengths. These properties make millimeter and terahertz waves a suitable choice for various applications. In security-sensitive fields, they can be employed for the detection of concealed weapons or other dangerous objects and substances. In industrial production, they enable the detection of discontinuities, i.e. defects, in homogeneous materials or compounds. Imaging systems operating in the named frequency regions usually employ one, a few or many antenna elements to sample a two-dimensional array. Therefore, mechanical [1], fully electronic [2] or hybrid [3] sampling approaches can be pursued. Multistatic setups allow for moderate hardware complexity and costs. For image generation, the measured data are focused digitally according to wave propagation phenomena [4]. State-of-the-art fully electronic devices with several thousands of transmit (Tx) and receive (Rx) antenna elements and near-real-time data processing have been demonstrated lately [5].

2. Antenna Array Design and Image Reconstruction The main objective/goal of the considered imaging systems is to measure and map the distribution of electrical properties of devices under test. These properties can for example be the reflectivity or the dielectric constant, which in general depend on position. The mapping of these properties is usually desired to be performed at a good resolution in both lateral and range directions to allow for an identification of small details or defects of the object under test. From physics it is known, that the lateral resolution of imaging systems operating with electromagnetic waves is limited by diffraction. In free space, the lateral resolution depends on the ratio between the target distance z0 and the aperture size L, as well as on the operating frequency [6]. The range resolution mainly depends on the available bandwidth. However, since the target distance is chosen to be approximately as large as the aperture in practical close range imaging setups, the range resolution also shows dependencies of z0 and L [7]. A one-dimensional multistatic antenna constellation is presented in Figure 1. The setup is based on a sparse periodic array approach optimized for short range applications [3]. It consists of two identical Tx arrays which flank one Rx array. The three subarrays are designed and positioned such that the point spread function (PSF) along the array-axis has minimum residual grating lobes. Adaptive weighting during reconstruction of each sampled frequency point and coherent averaging of the reconstructions over a broadband frequency interval further decreases the grating lobe level. A suppression of the grating lobes of approximately 40 dB compared to the main lobe has been achieved in literature [3]. The array can be moved along the y-axis to span a two dimensional array. In this y-direction, short-range synthetic aperture radar (SAR) operation is performed. For the image generation from acquired data the reconstruction kernels in array and azimuth direction can be separated, which offers a time-efficient reconstruction process.

978-1-4673-5225-3/14/$31.00 ©2014 IEEE

3. Security Screening To demonstrate the capability of the multistatic broadband approach a simplified measurement system was assembled. The system consists of two antennas which are mounted on two separate vertical positioning units. Since one antenna acts as transmitter and the other as receiver, any array configuration in y-direction as shown in Fig. 1 can be implemented. Furthermore, both antennas are mounted on one horizontal positioning unit to perform azimuth SAR imaging. The antennas are connected to a VNA with extenders which make the W-band (75 – 110 GHz) accessible. The first measurement example originates from security parcel inspection. In the chosen antenna array each Tx array consists of 16 antenna elements, which are equally spaced over the Tx array lengths LTx,A = LTx,B = 30 mm. The Rx array has 14 antenna elements evenly distributed over LRx = 312 mm. The multistatic group antenna is moved along the x-axis with a step size of 2 mm to sample a two dimensional array. The contents of the parcel, both metallic and dielectric objects, can be seen in Fig. 2 a). The parcel was closed with the cover as shown in Fig. 2 b) and positioned in a distance of z0 = 350 mm from the planar array. An mmW image was generated between 75 and 90 GHz at 201 equally spaced frequency sampling points. After the reconstruction at each measured frequency, the broadband data were coherently averaged. The result of the imaging process is shown in Fig 2 c) for the range gate at the bottom of the parcel. As can be seen, the depiction is very detailed and all objects can clearly be recognized. No ambiguities, even at strong metallic reflectors like the Siemens star and the pair of scissors, are present. This is a result of the large employed bandwidth.

a) b) c) Figure 2: a) Measurement example with a parcel filled with various objects: a ceramic knife, a bottle containing liquid, a small bag filled with salt to simulate dangerous powders, a pair of scissors, a dielectric plate with metal applications and a Siemens star. b) Parcel closed with cover before imaging process. c) Modulus of the reconstructed millimeter-wave image. Another scenario concerning personnel security screening is depicted in Figure 3 a). To span a larger imaging array, the length of the receive array was enlarged to be LRx = 600 mm and consists of 26 evenly spaced antenna elements. The Tx arrays and the step size in azimuth direction remain as in the first example. The mannequin was positioned in a distance of approximately 600 mm from the planar array. Here, a fully polarimetric dataset was acquired [8]. Therefore, four individual measurements were performed yielding both co-polarized (HH, VV) and cross-polarized (HV, VH) scattering properties of the scenario. Different antennas were employed correspondingly in all four measurements. The HH component can be seen in Fig. 3 b). Here, as in the parcel inspection example, all concealed objects are clearly visible and could be identified by an operator. The cross-polarized component HV is depicted in Fig. 3 c). As can be seen, it yields especially large values at edges (wax sheet), regions with thin or pointy geometry

Figure 1: Multistatic planar array configuration.

(screwdriver) and in depolarizing regions (salt bag). Hence, all hidden man-made objects show strong responses here. This feature makes the evaluation of the cross-polarized scattering data promising for automated detection approaches. Further information about the objects can be gained from the fully polarimetric measurement data with ellipsometric [9] or decomposition algorithms [8]. a) b) c)

4. NDT Measurements in the WR03-Band For NDT measurements the high depth resolution makes it easier to localize defects inside of samples. To demonstrate the NDT capabilities a monostatic arrangement in the WR-03 Band from 220 to 325 GHz with SAR reconstruction was tested. The theoretical depth resolution is around 1.4 mm and the lateral resolution at around 0.6 mm. Fig. 4 a) shows a time-domain image of an Al2O3 ceramic sample. The sample has several hidden defects like drilling holes and trenches inside. The front and the backside of the ceramic have a strong signal at around 75 ps and 400 ps. The layer of the defects appears at around 230 ps. To examine the defects closer Figure 4 b) shows an image of the inside of the ceramic sample at a time of 229 ps. Since the depth resolution is quite high and the refractive index of 3.1 is also rather high we can see all the drilling holes in the second column from the right, even the shallowest hole at the bottom with a depth of only 25 µm. The trench with the width of 1.0 mm at around 80 mm in x-direction is also fully visible over the whole depth range from 25 µm on the bottom to 1000 µm at top. The trench with 0.5 mm width at x = 65 mm can just be seen. The two more narrow trenches to the left are not visible, since their width of 50 and 100 µm is below the lateral resolution of the system.

Figure 3: a) Measurement example with a mannequin covered with copper paint to emulate the human body surface. The mannequin is equipped with a salt bag to simulate dangerous powders, a wax sheet to simulate plastic explosives, and a screwdriver as a potentially dangerous thrust weapon. The dummy was covered with a thick pullover before measurement. b) Modulus of HH component. c) Modulus of HV component.

Figure 4 a) Normalized magnitude in time-domain: At y = 33 mm defects appear at around 230 ps.b) Magnitude of time slice at 229 ps. From right to left: Holes with depths of 500 µm; holes with varying depths from 25 µm to 1.0 mm and constant diameter of 2.0 mm; trench with 1.0 mm width and trench with 0.5 mm width; a third trench with 100 µm width is barely visible.

a) b)

5. Conclusion Large bandwidths show great potential for security and nondestructive testing applications. Due to the high

depth resolution it is possible to locate defects or hidden objects. With flexible array design the measurement time and complexity of the test setup can be dramatically reduced. Measurements for security screening in the W-Band and defect detection in the 300 GHz band demonstrate the flexibility and reliability of the utilized reconstruction algorithms.

6. Acknowledgments

The authors gratefully acknowledge funding of the Erlangen Graduate School in Advanced Optical Technologies (SAOT) by the German Research Foundation (DFG) in the framework of the German excellence initiative.

7. References

1. F. Gumbmann, H.P. Tran, J. Weinzierl, and L.-P. Schmidt, “Optimization of a fast scanning millimetre-wave short range SAR imaging system,” European Radar Conference, 2007. 2. A. Schiessl, S.S. Ahmed, A. Genghammer, and L.-P. Schmidt, “A technology demonstrator for a 0.5 m x 0.5 m fully electronic digital beamforming mm-Wave imaging system,” Proceedings of the 5th European Conference on Antennas and Propagation (EUCAP), 2011. 3. F. Gumbmann, L.-P. Schmidt, “Millimeter-Wave Imaging With Optimized Sparse Periodic Array for Short-Range Applications,” IEEE Transactions on Geoscience and Remote Sensing, vol. 49, no. 10, pp. 3629-3638, 2011. 4. M. Soumekh, Fourier Array Imaging, Englewood Cliffs, NJ, PTR Prentice Hall, 1994 5. S.S. Ahmed, A. Genghammer, A. Schiessl, L.-P. Schmidt, “Fully electronic active E-band personnel imager with 2 m2 aperture,” IEEE MTT-S International Microwave Symposium Digest (MTT), 2012. 6. M. I. Skolnik, Radar Handbook, New York a.o., McGraw-Hill, 1970. 7. S.S. Ahmed, A. Schiessl, L.-P. Schmidt, “A novel active real-time digital-beamforming imager for personnel screening,” 9th European Conference on Synthetic Aperture Radar, 2012. 8. J. Adametz and L.-P. Schmidt, “Threat object classification with a close range polarimetric imaging system by means of H-alpha decomposition,” European Radar Conference, 2013. 9. A. Cenanovic, F. Gumbmann, L.-P. Schmidt, “Automated Threat Detection and Characterization with a Polarimetric Multistatic Imaging System,” 9th European Conference on Synthetic Aperture Radar, 2012.