2156-342X (c) 2020 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TTHZ.2020.3008330, IEEE Transactions on Terahertz Science and Technology IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. XX, NO. XX, APRIL 2020 1 Guided Reflectometry Imaging Unit using Millimeter Wave FMCW Radars M. Pan, A. Chopard, F. Fauquet, P. Mounaix*, J.-P. Guillet* Abstract—Frequency Modulated Continuous Wave (FMCW) radar systems in the millimeter and sub-millimeter range are technologically mature for many applicative fields such as au- tomotive and aerospace industries for imaging and non de- structive testing. This work reports on a new implementation of a guided FMCW radar reflectometry unit for sensing and imaging applications. Only a terahertz dielectric waveguide is used for signal transmission between the transceiver module and the sample, thus drastically simplifying the experimental setup. Compared to continuous wave guided systems, one of the main advantages granted by the use of FMCW radars in combination with waveguides, is the differentiation capability between the reflected signals generated along the wave guide as parasitic signals or at its probing end as sensing information and therefore improving the expected signal-to-noise ratio. This innovative approach is demonstrated by using a dielectric hollow- core waveguide integrated with two different radar transceivers; the high-performance, III-V based 100 GHz SynView unit as a reference system and a compact, low-cost, PCB-Integrated, 122 GHz transceiver developed by Silicon-Radar GmbH. Both 3D electromagnetic simulations and raster scans are performed to investigate quantitatively the propagation behaviors including the coupling capabilities, dynamic range limitations, beam profile and induced artefacts of the guided FMCW reflectometry system. The feasibility of a simplified guided terahertz FMCW reflectometry probing unit is proven. The integration of a solid immersion lens at the end of the waveguide is also demonstrated for imaging resolution improvement. Index Terms—Guided waves, FMCW Radar, Imaging, Tera- hertz I. INTRODUCTION THE scaling down of Si-based transistors and the in- vestigations into III-V technologies further push up the operating frequency of solid-state devices towards the terahertz regime [1]. On this basis, high-speed electronic devices with low power consumption and better compactness have been developed in the 100 - 300 GHz range. Being cost-effective systems supported by continuously improved fabrication tech- niques, solid-state terahertz devices play an increasingly im- portant role in academic researches and industrial applications [2]. Compared to conventional continuous wave systems com- bined with 3D imaging reconstruction approaches, such as Shape From Focus [3] or Computed Tomography [4], terahertz and/or millimeter wave Frequency-Modulated Continuous- Wave (FMCW) radar systems can natively provide additional Manuscript received April 09, 2020. The first two authors contributed equally to this work. Asterisk indicates corresponding authors. M.Pan, A. Chopard, F. Fauquet, P. Mounaix and J.-P. Guillet are with the IMS Laboratory, UMR CNRS 3218, University of Bordeaux, 351 Cours de la Libération 33405 Talence Cedex, FRANCE A. Chopard is also with Lytid SAS, 8 rue la Fontaine, 92120 Montrouge. E-mail :patrick.mounaix@u-bordeaux.fr; jean-paul.guillet@u-bordeaux.fr phase information for further result analysis and simplified 3D reconstructions with suitable resolution, while allowing further processing for improvement [5]. In particular, exploit- ing a quasi-optical coupling method (using lenses or parabolic mirror) with FMCW systems allows in-depth measurements for non-destructive testing purposes. This technique combines the high sensitivity of FMCW methods with the penetration capabilities of millimeter waves. Based on those benefits, wideband FMCW radars have found suitable application fields in the automotive and aerospace industries [6], [7] and art- painting analysis [8] amongst others for their non-destructive testing capabilities. However, the use of optical components involves tedious alignment and imposes mechanical restric- tions along the propagation path. The use of such optical coupling methods limits the development of compact, portable and easily-implementable terahertz measurement systems to a broader scope of applications. To address this issue, a terahertz waveguide [9] is proposed as an alternative solution. The guided reflectometry concept has already been investigated with some continuous wave sources or pulsed sources [10] in conjunction with different waveguide conceptions. A variety of geometries have been assessed, from a rectangular waveguide coupled to a vector network analyzer at low frequencies for burn damage detec- tion [11], to higher frequencies tests with low-cost Teflon waveguides for remote chemical detection [12] or metallic waveguide for remote endoscopic measurements [13], [14]. Those studies demonstrated the feasibility and the potential of THz guided reflectometry systems. However, the complexity induced by the optical coupling setups (beam splitter, lenses, parabolic mirrors) remains and limits the progress towards compact guided sensing units. In our work, the guided reflectometry configuration is highly simplified by testing compact FMCW transceivers with a dielectric thin-wall hollow-core waveguide of suitable dimensions. An optic-free transmission channel between the transceiver and the sample is then ensured with all the benefits of the FMCW radar sensing technique. In the following sections, two guided reflectometry configurations consisting of different compact FMCW radar units are introduced in order to demonstrate adequacy and universality of this approach regard- ing the employed technology and the front-end of the system. Investigations into the propagation behaviors in the waveguide and final system performances are given, demonstrated and further improved with the integration of an extra termination hemispherical lens. Authorized licensed use limited to: City, University of London. Downloaded on July 10,2020 at 09:36:55 UTC from IEEE Xplore. Restrictions apply. 2156-342X (c) 2020 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TTHZ.2020.3008330, IEEE Transactions on Terahertz Science and Technology IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. XX, NO. XX, APRIL 2020 2 II. GUIDED FMCW RADAR REFLECTOMETRY SYSTEM A. FMCW Radar: basic principle A simplified architecture diagram depicts, in Fig. 1a, the typical implementation of a monostatic radar transceiver op- erating in reflection mode. Driven by a cyclic command signal, a linear FMCW reference signal is generated at low frequency by a Voltage Control Oscillator (VCO) or a Phase Locked Loop (PLL) oscillator, which is then fed into the frequency multiplication chain, for signal up-conversion, to reach the desired operating frequency band. A 3-ports coupler leads this probing signal towards the emission antenna while redirecting the reflected signal towards the mixing unit for down-conversion and sampling. Similar bi-static architectures are also practicable but require a partition between the emit- ting chain and receiving unit, complicating the system and impacting its compactness. As showed in Fig. 1b, the cyclic signal reflected from the target is delayed with respect to the reference emission sweep and thus gives rise to a beating frequency fb at the mixer’s output proportional to the propagation length. Hence the object’s distance can be expressed as Eq. 1 d = c0 ∆t 2 n = c0 fb 2 n . Ts B , (1) δres = c0 2 n B . (2) where ∆t is the propagation-induced time delay, n represents the optical refractive index of the propagation media, fb donates the mixer’s output beating frequency, Ts is the period of a sweep cycle, and B is the sweep frequency bandwidth. When multiple targets are involved, each object contributes as a given distance-related beating frequency fbi , allowing for differentiation and remote sensing capability through data processing steps. Nevertheless, derived from Eq. 1, the lon- gitudinal resolution, δres, is directly correlated to the finite bandwidth (see Eq. 2). Coupler Antenna TX/RX Mixer VCO/PLL Mixer’s output Beating signal Multiplication Chain Frequency sweep DAQ Voltage sweep V(t) Reference input f(t) (a) 0 0,2 0,4 0,6 0,8 1 -0,5 0 0,5 1 1,5 Reduced frequencyReduced time B Ts