A 3D close range Synthetic Aperture Radar system based on the Range Migration Algorithm and working with affordable and compact Si-Ge Frequency Modulated Continuous Wave radar units is presented. The two radar sensors operates at 120 GHz with 19.5 GHz bandwidth and at 240 GHz with 34 GHz bandwidth. A new method for enhancing lateral resolution combined with a normalization procedure is proposed, leading to 0.33 central wavelength resolution for the 120 GHz radar transceiver and 0.47 for the 240 GHz radar unit. Lateral resolution is compared to a conventional focused beam imaging system. Objects of interest are displayed to reveal Synthetic Aperture Radar advantages in term of lateral resolution and depth of field for Non Destructive Testing and inspection purposes.

A 3D close range Synthetic Aperture Radar system based on the Range Migration Algorithm and working with an affordable and compact Si-Ge Frequency Modulated Continuous Wave radar unit operating at 120 GHz with 19.5 GHz bandwidth is presented. A new method for enhancing lateral resolution combined with a normalization procedure is proposed leading to 0.33 central wavelength resolution. Lateral resolution is compared to a focusing lens system. An object of interest is displayed to reveal Synthetic Aperture Radar advantages in term of lateral resolution and depth of field for Non Destructive Testing and inspection.

This paper presents the development, performance, integration, and implementation of a 150 GHz FMCW radar based on a homodyne harmonic mixing scheme for noncontact, nondestructive testing. This system offers high-dynamic-range measurement capabilities up to 100 dB and measurement rates up to 7.62 kHz. Such interesting characteristics make this system attractive for imaging applications or contactless sensing. Numerous samples of different materials and geometries were imaged by taking advantage of the radar's performance. By taking into account the nonionizing capability of the system, new applicative fields such as food industry and pharmaceutical packaging were explored.

Ce papier détaille des avancées applicatives de contrôle térahertz liées à deux technologies de radars FMCW à visée de contrôle volumique non destructif et de métrologie d’épaisseur. D’un côté, les développements d’un radar FMCW a base de multiplicateurs Schottky GaAs à 150 GHz possédant des performances à l’état de l’art sont détaillées, avec, en particulier, un SNR de 100 dB, une cadence de 7.7 kHz et une stabilité à long terme, optimales pour des moyens de détection en ligne. Et de l’autre, la présentation d’une approche de détection pour le contrôle de caractéristiques matériaux et la métrologie d’épaisseur, faisant usage d’un radar SiGe, plus abordable et très compact. Grâce aux développements d’algorithmes nouveaux, une extraction d’épaisseur, d’indices de réfraction et de coefficients d’absorption est réalisable sur des épaisseurs de matériaux inférieures au millimètre, bien en-deçà des performances intrinsèques du transceiver limité par la bande passante.

Iterative phase retrieval algorithms from multiple diffraction patterns in the terahertz (THz) frequency range are a promising tool of computational imaging capable of providing high spatial resolution of reconstructed phase images. One of the commonly used algorithms is SBMIR, which employs multiple intensity distributions of the diffraction object wavefield as input data. Com -pared with single-frame methods, the multi-plane approach allows for a faster convergence, but requires time-consuming data acquisition from a receiver positioned at a variety of distances from the object. Previously, we proposed a method for THz data acquisition in a single scan mode, which allows one to quickly obtain an exhaustive set of diffraction distributions. In this paper we evaluate an up-to-date phase retrieval algorithm based on the SBMIR/R-SBMIR method (which utilizes stochastic wavefront propagation) on the experimental data captured by a single-scan technique. Unlike a number of conventional phase retrieval algorithms, which may require a series of numerical experiments for determining optimal intensity distributions from a large dataset, disordered propagation of the estimation wavefront guarantees the high-contrast and high-resolution image reconstruction without pre-setting the parameters. It is shown that the package use of the single-scan technique with the subsequent data processing using the R-SBMIR algorithm has ap-plication potential for automation of the multi-plane phase retrieval in the THz range.

Multiplane iterative phase retrieval is a promising approach to diffraction imaging, which accurately determines the topographic and internal characteristics of various objects. Nevertheless, the detection systems used often have a limited dynamic range, resulting in overexposure of recorded intensity distributions. In this Letter, we present a novel, to the best of our knowledge, reconstruction algorithm that inpaints saturated areas on the measured intensity datasets and reliably retrieves wave complex amplitude. The proposed technique can be used in various spectral ranges, while we have tested it in the terahertz frequency range, where the problem of sources and detectors is most acute. We show that retrieved amplitude and phase distributions have a quality comparable to that of the images reconstructed from the reference high dynamic range technique. Herewith, the proposed approach seriously simplifies the process of data acquisition, what expands the possibilities in the design of measurement tools and studies of dynamic scenes.

Terahertz technology (spanning between 0.1 and 10 THz) is now a well-established tool to achieve contactless sensing and non-destructive testing (NDT). Among the advanced approaches, THz computed tomography (THz CT) is an emerging technique for 3D reconstruction and has been extensively investigated over the last decade. This work focuses on those capabilities for 3D volumetric reconstructions of complex objects through the use of a real-time THz imaging system operating at 2.5 THz. Further work demonstrates that the resulting data are compatible with automated processing for (i) an ad-hoc segmentation, extracting the sample from the background and reconstruction surrounding noise, (ii) a component labelling, and (iii) a skeletonization, providing crucial additional metadata about the sample morphology.

We introduced several approaches of terahertz wavefront phase retrieval from intensity measured in a volumetric grid. Our developments include several experimental solutions for the registration of multiple intensity distributions spaced along the optical axis for two types of terahertz sources, namely Gunn diode with frequency multiplication chain and quantum cascade laser. We implemented several measurement modes: (i) sequential raster scanning by single Schottky diode with two lock-in amplifiers, complimentary tuned to different sensitivities for high dynamic range recording; (ii) step by step registration on matrix photodetectors, with averaging over several images for every measurement plane; (iii) continuous measurement during the displacement of the motorized translation stage. The high dynamic range data acquisition allowed us to successfully implement single-beam terahertz surface profilometry in the reflection, while the on-the-go recording ensures the shortest measurement times. In addition, we experimentally appraised two matrix detectors (INO and I2S) and applied several phase retrieval algorithms which proved their effectiveness in various experimental conditions, namely for the intensity registration in various diffraction zones and axial measurement plane allocations.

Terahertz phase retrieval from a set of axially separated diffractive intensity distributions is a promising single-beam computational imaging technique that ensures the obtention of high spatial resolutions and phase wavefronts, but remains restricted by time-consuming data acquisition processes. In this work, we have adopted an approach, relying on the radiation of a quantum cascade laser and the implementation of an express single-scan measurement of intensity distributions through the continuous on-the-go displacement of a high-sensitivity antenna-coupled microbolometer sensor array. In addition to the simplicity of this practical implementation and the minimization of measurement times, such an approach overcomes the problem of preliminary optimal selections of transverse intensity distributions used in the iterative phase retrieval algorithm and guarantees the required data diversity for high-quality wavefront reconstruction.

Terahertz and millimeter waves technologies have followed to a decade of strong development, thanks to leading promising applications fields such as hyper-spectral imaging, non-destructive testing and spectroscopy. However, the generally high cost of such systems limits their use to academic research laboratories or high added value industries. In this article, we introduce the early stage results of a several tens times lower cost , fully integrated imaging system, operating above 100 GHz, available as an open source project.

In order to be able to use a handled portable terahertz sensor, a solution combining augmented reality and a frequency modulated continuous wave (FMCW) radar is proposed. The achieved architecture ensures simple handling during the acquisition process while the result visualization can be performed in live directly with a smartphone, with a 3D live superimposed augmented reality view.

The usability of terahertz systems for specific inspection tasks and imaging in the aeronautics industry is assessed. Especially, we demonstrate the suitability of Frequency-modulated continuous-wave (FMCW) radars for health monitoring and see-through imaging. Additionally, terahertz time-domain data processing is performed for multi-layered paint structure characterization. FMCW radar principles are introduced. Available systems are described along with their benefits and limitations. Defect detection capabilities and progresses towards airplane covering see-through imaging are illustrated through FMCW experimental results on real samples. The suitability of FMCW radars as an advanced contactless non-destructive testing (NDT) tool for the aeronautics industry and maintenance services is demonstrated. A second application, based on terahertz time domain techniques, targets the characterisation of multi-layered painting structures through the assessment of dielectric properties and individual thickness of each layer deposited. Beside the review of extraction methods, the introduced new algorithm allows to derive a parametric transfer function, thus denoting the main contributions which give rise to the recorded terahertz electromagnetic field. Such a development pushes further the understanding and characterization of stratified structures by means of terahertz radiations and represents an indispensable tool to efficiently localise any deviation to the nominal painting stack in terms of thickness or dielectric properties.

THE implementation of a guided FMCW reflectometry transceiver is proposed and investigated through two millimeter-wave radar units. Electromagnetic simulations, corroborated by imaging tests, assessed the capabilities of such sensing systems. The achieved simplified architecture ensures suitable performances while the distance sensing capability of the FMCW radar grants significant improvement in regard to guided CW system.

In order to overcome the coherence-induced artifacts, the illumination heterogeneity as well as a limited dynamic range when using full-field technique, a solution employing a galvanometric beam-steering is proposed, demonstrating the possibility for real-time imaging in terahertz domain. Working toward industrial applications, different imaging illumination process, including Lissajous pattern for fast beam steering, are evaluated thanks to the versatility of the lighting method.

Terahertz diffractive phase imaging is demonstrated by an iterative phase retrieval processing algorithm to assess the complex diffracted wave properties. We implemented this reflection geometry using a continuous wave system working at 0.3 THz. Targeting signal to noise ratio improvement, we take advantage of high dynamic range data fusion. We obtained and reconstructed diffraction images achieving better resolution compared to a conventional diffraction limited raster scanning technique. Submillimeter lateral resolution in reflection is demonstrated together with phase reconstruction.

Terahertz technologies are attracting strong interest from high-end industrial fields, and particularly for non-destructive-testing purposes. Currently lacking compactness, integrability as well as adaptability for those implementations, the development and commercialisation of more efficient sources and detectors progressively ensure the transition toward applicative implementations, especially for real-time full-field imaging. In this work, a flexible illumination system, based on fast beam steering has been developed and characterized. Its primary goal is to suppress interferences induced by the coherence length of certain terahertz sources, spoiling terahertz images. The second goal is to ensure an enhanced signal-to-noise ratio on the detector side by the full use and optimized distribution of the available power. This system provides a homogeneous and adjustable illumination through a simplified setup to guarantee optimum real-time imaging capabilities, tailored to the sample under inspection. Working toward industrial implementations, different illumination process are conveniently assessed as a result of the versatility of this method.

Frequency-modulated continuous wave (FMCW) radar systems in the millimeter and submillimeter range are technologically mature for many applicative fields such as automotive and aerospace industries for imaging and nondestructive 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, printed circuit board (PCB)-integrated, 122 GHz transceiver developed by silicon-radar GmbH. Both three-dimensional 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.