terahertz imaging: applications and...

Terahertz imaging: applications and perspectives

Christian Jansen,1,2,* Steffen Wietzke,1,2 Ole Peters,1,2 Maik Scheller,1,2 Nico Vieweg,1,2

Mohammed Salhi,1 Norman Krumbholz,1 Christian Jördens,1 Thomas Hochrein,3

and Martin Koch2

1Institut für Hochfrequenztechnik, Technische Universität Braunschweig,Schleinitzstrasse 22, 38106 Braunschweig, Germany2Fachbereich Physik, Philipps-Universität Marburg,

Renthof 5, 35032 Marburg, Germany3Süddeutsche Kunststoff-Zentrum, SKZ-KFE GmbH, Friedrich-Bergius-Ring 22,

97076 Würzburg, Germany

*Corresponding author: [email protected]‑bs.de

Received 22 December 2009; accepted 4 March 2010;posted 6 April 2010 (Doc. ID 121759); published 3 May 2010

Terahertz (THz) spectroscopy, and especially THz imaging, holds large potential in the field of non-destructive, contact-free testing. The ongoing advances in the development of THz systems, as wellas the appearance of the first related commercial products, indicate that large-scale market introductionof THz systems is rapidly approaching. We review selected industrial applications for THz systems, com-prising inline monitoring of compounding processes, plastic weld joint inspection, birefringence analysisof fiber-reinforced components, water distribution monitoring in polymers and plants, as well as qualityinspection of food products employing both continuous wave and pulsed THz systems. © 2010 OpticalSociety of America

OCIS codes: 110.6795, 300.6495, 120.4290, 160.5470.

1. Introduction

Located between microwaves and the infrared, theterahertz (THz) region remained a nearly untappedpart of the spectroscopic portfolio for a long time, asneither electronic nor optical sources could illuminatethis shadowy region. The advent of femtosecond (fs)lasers paved the way for numerous pulsed THz emis-sion and detection schemes [1–6]. Furthermore, con-tinuous wave (cw) THz systems, e.g., those based onphotomixing two diode laser beams, were developedin the mid-1990s. Besides these optical approachesto THz generation, electronic devices, e.g., the Gunnoscillator-driven Schottky multiplier chains, also ad-vanced in this field [7–11], enabling low-cost systemarchitectures.A plethora of applications for THz systems has

been introduced in the literature [12], ranging from

biological imaging, nondestructive testing (NDT),security scanning, and process control to next-generation wireless communication systems [13,14].The enormous inherent potential of THz technologyled to a rapid development of THz systems, and,today, the first commercial products are available.While measurement speeds still require improve-ment and system costs are relatively high, THzsystems are on the brink of large-scale marketintroduction.

The remainder of the paper is structured as follows:First,wewill discuss a representative example of botha pulsed (Subsection 2.A) and a cw THz system(Subsection 2.B) followed by a review of selectedindustrial applications for THz systems (Section 3),especially focusing on the prospects of THz imag-ing. Among the investigated subjects are the iden-tification of the glass transition temperature ofsemicrystalline polymers, inline monitoring of com-pounding processes, plastic weld joint inspection, bi-refringence analysis of fiber-reinforced components,

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water distribution monitoring in polymers andplants, aswell as the safety-critical quality inspectionof food products. Our conclusions are presented inSection 4.

2. Terahertz Spectroscopy Systems

Among the many realizations of THz systems, twobasic principles can be differentiated: either electro-magnetic picosecond pulses, which have spectralcomponents ranging from approximately 100GHzto several THz, are generated, or cw radiation is em-ployed. While the former method offers the advan-tage of broad spectral information from a singlescan, cw techniques are the first choice when sharpspectral features are of interest and a frequency re-solution down to a few MHz is desired [15].Pulsed THz systems have recently overcome a

major drawback, namely, high system costs, as theadvent of Erþ-doped fs fiber lasers has broughtan inexpensive replacement for titanium:sapphiresources. Furthermore, the emission wavelength ofsuch fiber lasers is 1550nm, so that standard telecomcomponents can be employed. Unfortunately, GaAs-based photoconductive antennas (PCAs), which arefrequently employed as emitters and detectors in800nm systems (also see Subsection 2.A), are trans-parent to the 1550nm wavelength. Therefore, analternative substrate material had to be found.This obstacle was overcome by the introduction of1550nm compatible semiconductor materials (e.g.,Fe-implanted InGaAs or InGaAs/InAlAs quantumfilm stacks on InP wafers) [16,17]. The Erþ-doped fi-ber lasers, the 1550nm telecom components, and thenew compatible PCAs have led to the development ofless expensive, robust, all-fiber THz systems that areattractive for many industrial applications [18]. Inthe following two sections, we briefly discuss the ba-sic principle of both pulsed and cw THz generationand detection. For a more complete overview ofTHz sources and detectors, the interested reader isreferred to the excellent reviews published elsewhere[1,3,19–21].

A. Broadband Pulsed Terahertz Systems

Optical gating of PCAs is one of the most popularTHz generation and detection schemes [22–24]. Asillustrated in the upper part of Fig. 1(a), a fs lasersource (e.g., a titanium:sapphire laser or Erþ-dopedfiber laser) emits ultrashort, sub-100 fs pulses, whichare split into an emitter and a detector arm. In theemitter arm, the optical pulse is focused onto abiased PCA. The optically excited electron hole pairsare accelerated in the externally applied field, whichinduces the emission of a THz pulse with spectralcomponents exceeding 3THz. The THz pulse isguided by polymeric lenses or by off-axis parabolicmirrors and is finally focused onto the detectorPCA. Here, the inverse principle of the generationprocess is employed for the detection of the THzpulse: again, the optical pulse gates the photoconduc-tive detector by generating free carriers. Instead of

applying an external field, the incoming THz field ac-celerates the carriers and drives them into the elec-trodes. The temporal length of the optical gatingpulse is of the order of tens of fs, while the THz pulseis of the order of picoseconds. Thus, the resultingphotocurrent, which is detected by a low-noise multi-stage transimpedance amplifier, corresponds to asingle point of the THz pulse shape. By introducinga varying time delay in one of the arms, which can beeither a mechanical delay line or, for video rate ap-plications, a fiber stretcher, the complete THz pulsecan be sampled step by step.

It is worth mentioning that such a generation anddetection scheme enables the monitoring of highlydynamic processes, as it offers a typical signal-to-noise ratio of up to 70dB. Furthermore, due to thecoherent nature of this technique, both phase andamplitude information are available. Measuring afirst pulse without a sample in the THz beam pathand a second pulse with the sample present allowsfor the simultaneous extraction of the refractive in-dex, the absorption coefficient, and the thickness ofthe sample, providing detailed information about thesample under investigation [25].

In Fig. 1(b), a photograph of a fiber-coupled THztime domain spectroscopy (TDS) system is depicted.The metallic enclosure contains a free space fs lasersystem with chirp precompensation for the attachedglass fibers and a delay line. The fiber-coupled out-puts are connected with polarization maintainingfibers to the emitter and the receiver measuringheads. Later, we will discuss inline measurementsthat were performed using such a rugged, mobileindustrial THz TDS system.

B. Continuous Wave Systems

If spectral resolution is the primary concern, e.g., tostudy sharp absorption lines of gases, cw THz sys-tems deliver outstanding performance. In contrastto pulsed THz systems, no fs lasers are required.As shown in the lower section of Fig. 1(a), the outputof two frequency stabilized laser diodes is spatiallyoverlapped in a beam combiner and focused onto aPCA with an optimized electrode geometry [26]. In-stead of the ultrafast carrier dynamics employed inthe pulsed case, mixing of the two incident waves isexploited to generate a continuous THz wave, whichoscillates with the difference frequency of the two in-coming waves [27–29]. By detuning one of the laserdiodes, the emission frequency can be swept in a widespectral range. A second PCA is employed for coher-ent sampling of the incoming THz radiation [30].Even though pulsed systems are decreasing in price,cw systems are still less expensive and feature a fre-quency resolution down to 2MHz.

C. Approaches Toward Fast Terahertz Imaging

Many applications in NDT require imaging techni-ques. Methods such as x-ray or ultrasonic testingemploying detector arrays generate images in orclose to real time. Conventional THz TDS systems,

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as described above, are designed to generate highlyprecise spectroscopic information at a single point ofa sample. Thus, to generate full images at highspeeds, several considerations have to be taken intoaccount.Among the concepts for terahertz imaging, several

noncoherent techniques, including mircrobolometerarrays [31], have been presented. While increasingthe measurement speeds, these approaches giveonly limited information due to the lack of phaseinformation.The most basic coherent imaging can be achieved

by raster scanning a sample through the THz focusand generating full spectroscopic information at eachpixel. Considering a small image of 60 by 60 pixels,3600 single measurements are required. Dependingon the delay line concept, the desired lock-in timeconstants, and other factors, to record a high-qualityTHz pulse can take longer than 30 s and can result ina total measurement time of 30h for a full image.This example shows the necessity of parallel or accel-erated measurements.A straightforward approach to parallelization is a

multiantenna setup enabling a linear downscalingof the measurement time with the number of em-ployed emitter and detector pairs. However, arrayshave to be developed to counter the drastically in-creasing costs, e.g., as demonstrated in [32]. Eventhough multipixel approaches yield a clear accelera-tion of THz measurements and could lead to attrac-tive system architectures in the future, severeproblems in the alignment process are still majordrawbacks thatmust be overcome. So far amaximum

of 16 parallel channels has been experimentallydemonstrated [33].

A parallel approach to coherent two-dimensionalelectro-optic imaging is presented in [34]. These sys-tems use a ZnTe crystal to convert a THz signal into atwo-dimensional optical image,which canbe recordedby a CCD camera. Recently, real-time measurementspeeds have been achieved [35]. However, the re-quired laser power scales linearly with the numberof pixels, limiting the applicability of systems suchas expensive laser systems with an output power ofseveral hundred milliwatts to be employed.

Compressed sensing is an approach to generateTHz images without time-consumingly moving thesample or detector [36]. The sample is illuminatedby a collimated THz beam, together with a knownrandom pattern of opaque pixels. When changingthe pixel pattern often enough, an image of the sam-ple can be reconstructed, for example, where 300 dif-ferent patterns result in a resolution of 1024 pixels.

Alternatively, conventional single-pixel imagingsystems can be used if steps toward a shorter singlepoint scan time are taken. Especially, new delayline concepts can drastically shorten the requiredpulse acquisition time. Conventional delay linesare based on mechanical linear stages with a retro-reflector mounted on top. Moving these delay lineswith a high precision leads to relatively slow scanrates. An alternative concept uses fiber stretchers,which create a delay by elastically stretching a glassfiber, inducing a relative change in the fiber length ofless than 1%. Combined with interferometric lengthmeasurements, this device can precisely scan THz

Fig. 1. (Color online) (a) Schematic of a pulsed (upper figure) and a cw (lower figure) THz system. (b) Photograph of a fiber-coupled THzTDS system for industrial applications [21].


pulses with several Hz, so that, if no further aver-aging is applied, the 3600 pixel image mentionedabove can be acquired in less than 10 min—ordersof magnitude faster than with a conventional system.Furthermore, these devices neatly integrate inrugged all-fiber setups. In the future, improvementsin this field could lead to fiber stretchers with kilo-hertz acquisition rates, enabling real-time THzimaging.Visualization techniques for coherent THz image

information include peak-to-peak measurement,time-of-flight measurement, amplitude measure-ment at single frequencies, integrated signal ampli-tude over a specified frequency range [37], andevaluation of the dielectric material properties [re-fractive index n and absorption coefficient α; seeFigs. 2(a) and 2(b)]. Thus, multiple images revealingdifferent kinds of information can be created from asingle data set, e.g., variations in thickness, density,material composition, or humidity, as well as inclu-sions of air or other foreign bodies in the sample[see Figs. 3(a) and 3(b)].

3. Industrial Applications for Terahertz Systems

In the previous sections we discussed the basics ofTHz generation and detection as well as approachesto THz imaging. Now we shall investigate industrialapplications for THz systems, focusing on imagingbut also considering some single point measurementscenarios. The illuminated scenarios comprise exam-ples from the polymer processing industry, the foodindustry, and plant breeding. Because of the mani-fold activities in this field and the limited lengthof this article, we can only discuss a small excerptof the emerging THz applications and refer the inter-ested reader to other excellent reviews for additionalinformation [20,21,38–40].

A. Terahertz Spectroscopy of Polymers

In past decades, the production volume of plasticshas overhauled that of steel and is continually in-creasing. To refine production techniques and opennew markets for plastics, the polymer processing in-

dustry is continuously investigating new processcontrol and NDT techniques, as many challengingmeasurement tasks of the production lines remainunanswered.

Polymers and THz waves form a fruitful symbiosis,as many plastic materials are transparent to THz ra-diation and serve as a base material for passive THzsystem components [41–43]. In return, THz systemscan reveal valuable information of the composition ofpolymers and the inner structure of polymericcomponents. In the next three sections, we will suc-cessively discuss applications of THz technologythroughout the value creation chain, starting withoffline measurements for the characterization of mo-lecular properties, considering the following inlineprocess monitoring with real-time feedback, and re-ferring finally to quality inspection of the polymericcomponents.

B. Polymer Characterization on a Molecular Level

To fully understand the potential of THz sensors forpolymer testing applications, a closer look at the un-derlying physics is mandatory. The most prominentinteractions between THz waves and polymers arecollective motions of large molecules, which appearas broad absorption bands. Some far-infrared absorp-tion spectroscopy studies have already investigatedskeletal vibrations, liquid lattice modes, hydrogenbonds, and intermolecular vibrations in the crystal-line phase [44]. However, THz TDS systems have aclear advantage here, as the simultaneous extractionof the refractive index, the absorption coefficient, andthe sample thickness [25] offer information that com-plements and exceeds the far-infrared absorptiondata.

Exemplarily, we will discuss the contribution thatTHz TDS can provide to the identification of the glasstransition temperature. Glass transition in amor-phous or semicrystalline polymers often limits theiroperating temperature range. Elastomers, for exam-ple, become brittle below the glass transition tem-perature Tg, as segmented chain motion is frozen.

Fig. 2. (Color online) Different parameters for the visualization of THz images based on (a) THz pulses and (b) the corresponding spectra.


Conventional methods for determining Tg are dif-ferential scanning calorimetry (DSC) [45] or dynamicmechanical analysis [46]. However, as the glass tran-sition is restricted to the amorphous domains of apolymer, the conventional methods can fail when ahigh crystallinity is present. Thus, a number of poly-mers exist in which the glass transition temperatureis not clearly identified.Scientifically speaking, the glass transition is not a

classical thermodynamic phase transition but,rather, a relaxation process that can be understoodby the concept of the free volume [47]. In this model,the unoccupied space between the macromolecules isreferred to as the free volume Vf . Beneath Tg, thefree volume does not change with the temperature.However, above Tg, a linear increase ofVf is observed[see Fig. 4(a)].In [48], the authors demonstrated that the

temperature-dependent THz refractive index can re-veal evenminute changes in the free volume, which isrelated to the specific volume and the density of thesample. Thus, THz TDS is able to indicate the glasstransition temperature with high precision and alsoin the case of highly crystalline polymers, where othertechniques have failed so far. In Fig. 4(b), we showthe temperature-dependent refractive index of poly

(oxymethylene) (POM) at 1:5THz. The gradient ofthe refractive index changes at the glass transitionidentified as Tg ¼ 199K. Above Tg, the free volumestarts to increase inducing a change in the density,which directly correlates with the refractive proper-ties of the sample. In comparison, we also show theglass transition temperature range identified byDSC measurements (gray area), which is in perfectagreement with the THz TDS results. Hence, we con-clude that THz TDS can reveal the glass transitiontemperature of polymers.

C. Terahertz Systems for Inline Process Control

The majority of technical plastics consist of a basepolymer compounded with additives that enhanceits functionality and allow for the design of a custom-tailored product. Commonly employed additives in-clude flame retardants, colorants, and low-cost fillingmaterials. Inline or online monitoring of the additiveconcentration inside the polymeric melt is a challen-ging task. Recently, we demonstrated that ruggedTHz TDS systems (as shown in Fig. 1) can givevaluable information about the compounding process[49–51]. Figure 5(a) shows inline measurements onpolypropylene with a varying content of CaCO3 per-formed at the end of an extruder [Fig. 5(b)]. The

Fig. 3. (Color online) Images of drilled holes in a plastic sample: (a) photography, (b) peak-to-peak THz-image, and (c) integrated intensityin the range of 0.2 to 0:3THz.

Fig. 4. (Color online) (a) Volume-temperature diagram of a polymer and (b) temperature-dependent refractive index of POM at 1:5THzobserved with THz time-domain spectroscopy [the gray area in (b) denotes the glass transition temperature range identified by DSCmeasurements].


dashed curve indicates the to-be weight percentageof CaCO3 in the melt determined by the dosing unitsat the extruder input on the basis of gravimetricmeasurements. The solid curve corresponds to THzamplitude data measured at the extruder outlet. Toincrease the THz TDSmeasuring speed, the THz am-plitude is determined only at a single point of thedelay line. When the concentration of the additivechanges, the THz pulse shifts, which induces a sensi-tive response in the detected THz amplitude at thefixed time delay. The THz data (dashed curve) clearlyreveal the residence and washout time of the additiveinside the extruder. For a detailed discussion of theseinitial THz inlinemeasurements, the interested read-er is referred to Ref. [49].

D. Quality Inspection of Plastic Components

Besides basic material characterization and processmonitoring, THz systems could play a major role asinstruments for the nondestructive, contact-freeinspection of plastic components. THz imaging candetermine, for example, the spatially resolved fiberorientation in reinforced polymers or reveal delami-nation and inclusions in plastic weld joints.

E. Fiber Orientation in Reinforced Plastic Components

An increasing number of components, especially inthe automotive and aircraft sectors, consist of fiber-reinforced materials, which offer high mechanical

strength at low component weight. Especially,safety-critical parts should pass a contact-free, non-destructive 100% testing procedure to ensure correctoperation. Conventional techniques still struggle tofulfill this demand, so that THz technology couldprovide a valuable contribution to this field asdemonstrated in [52,53].

The preferential orientation of fibers and fibrils in-side a polymeric host medium induces a birefringentdielectric behavior at THz frequencies [54,55]. Gener-ally speaking, the refractive indices noa and nea of theordinary and the extraordinary axes of a birefringentmedium span an ellipsoid [Fig. 6(a)]. By determiningthe THz refractive index of the composite at three dif-ferent angles between the THz polarization and thesample, this index ellipsoid can be reconstructed, re-vealing the preferential orientation direction as wellas the overall orientation degree of the fibers at asingle spatial position of the plastic component.

In Fig. 6(b), we show the simulated fiber orienta-tion tensor (false shaded image) according to Mold-flow Plastic Insight software of an injection moldedpolyamide reinforced by 30wt:% short glass fibers.The black arrows mark the average fiber orientationdetermined by the THz measurements at the corre-sponding points, applying the index ellipsoid modelto the refractive index data at 0:6THz. The lengthof the arrow corresponds to the fraction of preferen-tially oriented fibers due to the magnitude of the

Fig. 5. (Color online) (a) Inline terahertz measurements on polypropylene melt with a varying content of CaCO3 performed at the end ofan extruder and (b) compact THz probe for inline process control.

Fig. 6. (Color online) (a) Index ellipsoid of a birefringent medium and (b) simulated fiber orientation tensor (false shaded image)according to the software Moldflow Plastic Insight of an injection molded polyamide specimen reinforced by 30wt:% short glass fibers.


birefringence. Comparing both the simulations andmeasurements reveals deviations of the preferentialorientation from the mold flow direction, which canbe explained by deflection at the borders of the speci-men, the potential influence of a middle layer, andthe presence of dwell pressure during the injectionmolding process. For further information on this in-novative application of THz technology, we refer thereader to Ref. [56].

F. Quality Inspection of Plastic Weld Joints

Plastic components are commonly employed as con-struction materials, replacing more expensive mate-rials, such as metals or ceramics. Hence considerableinterest is also drawn to related joining technologies,such as plastic welding, which enables this constantprogress. For example, nearly all the newly con-structed pipelines for gas transportation are madeof polyethylene. Hence, the quality of weld joints isof paramount importance, as their structural integ-rity might be severely affected by contaminations, airbubbles or delamination. Unfortunately, x rays andultrasonic waves are not able to satisfactorily identi-fy such defects. Today, due to the lack of a properNDT technology, only the parameters of the weldingprocess are monitored at the risk that the joint mightfail—in consequence, possibly inflicting considerabledamage.In [57], we demonstrated that THz imaging could

improve this unsatisfying condition. As proof of thisprinciple, we investigated welded high-density poly-ethylene samples with dielectric sand contamina-tions and with a delamination. Backlit photographsand corresponding THz images of the contaminatedand delaminated sample are shown in Figs. 7(a) and7(b), respectively. Both the sand inclusions and thedelaminations are clearly resolved. Figure 7(c) showsa future application scenario of a THzNDTweld jointinspection system.

G. Determination of Water Content in Plastics

The physical and mechanical properties of a polymerare severely influenced by its water content [58,59].THz waves are strongly absorbed even by minuteamounts of water, so that changes in water concen-

tration can be reliably detected. Hence, THz sensinghas an inherent potential in this field. For instance,we demonstrated that the water content in hygro-scopic polymers and compounds such as polyamideand wood plastic composite (WPC) can be accuratelydetermined [60]. Furthermore, an effective mediumtheory-based model of the dielectric material charac-teristics independent of the water content has beenintroduced. Figures 8(a) and 8(b) show a photographand a THz-image of the water content inside a WPCplate (60wt:% wood fibers). Higher water contentsresult in a stronger attenuation of the transmittedTHz pulse. The sample was immersed in distilledwater from the right-hand side. As expected, we findthe highest concentration with approximately2:5 vol:% of water at the right sample edge. To theleft, a gradual transition from higher to lower waterconcentrations is revealed by an increase in the THztransmission. For more details, the reader is referredto [58].

H. Hydration Monitoring for Optimized Plant Breeding

In recent years, water efficient irrigation strategiesfor economic plants have gained importance to coun-teract desertification and water shortages. A centralquestion in the optimization of irrigation schemes is

Fig. 7. (Color online) (a) Backlit photograph (left) and THz image (right) of a sample contaminated with sand, (b) backlit photograph(upper) and THz image (lower) of a delaminated sample, and (c) future application scenario of a THz NDT plastic weld joint inspectionsystem.

Fig. 8. (Color online) (a) Photograph and (b) spatially resolvedTHz image of the water content inside a WPC plate (60wt:% woodfibers).


the in situ identification of drought stress in plants.Because of the high sensitivity of THz radiation towater, even low water contents inside a leaf are re-liably detectable. To demonstrate these capabilities,Fig. 9(a) shows a THz image of a leaf obtained in acw system using a confocal configuration [61]. Darkerareas in the picture indicate a higher water con-centration in the leaf, revealing the leaf veins.Figure 9(b) shows the THz transmission through asingle point of the leaf as a function of the volumetricwater content. Based on these findings, plant physio-logical studies of the hydration state of leaves, andthe underlying water transport mechanisms insidethe plant appear as another innovative prospectfor THz technology [62]. Figure 9(c) depicts our visionfor a future mobile hydration monitoring THz TDSsystem for optimized plant breeding, which is cur-rently subject to active research.

I. Quality Inspection of Food Products

Aside from the discussed applications in the polymerprocessing industry and plant hydration monitoring,THz imaging systems might also find their way intothe quality control of food products. While state-of-the-art quality control systems can easily detect me-tallic contaminations, nonmetallic contaminationsare often hard to find. Existing approaches, such asultrasonic or x-ray scans, fail when the density differ-

ence or the dielectric contrast between the materialand the contamination is low. These methods, for ex-ample, cannot reliably detect stones, plastic pieces, orglass splinters inside chocolate, even though suchcontaminations could impose serious health risksfor consumers. As THz scans consider not only the ab-sorption spectrum but also the phase information dueto the underlying coherent emission and detectionscheme, many parameters can be employed for theidentification of undesired inclusions. Figure 10(a)shows a THz scan of a chocolate bar that is contami-nated by a buried glass splinter. TheTHz image basedon the phase delay clearly reveals the presence of thecontamination. In the same manner, we have demon-strated the detection of plastic inclusions and smallstones. THz TDS can even differentiate between in-tended ingredients, such as nuts, and dangerous con-taminations, so that it holds the potential to becomean established method in the quality inspection offood products [Fig. 10(b)]. However, before that visioncan become reality, the imaging speed has to beincreased, so that close to real time images can beobtained.

4. Conclusion and Outlook

Many applications, ranging from nondestructive pro-cess monitoring and component testing in the poly-mer industry to quality inspection of food productsand even optimized plant breeding, can benefit fromthe valuable insight THz imaging systems offer. Tolive up to its enormous inherent potential, THz tech-nology has to become fast and affordable, which re-quires new approaches to many details of the THzsystem architecture. A first step has been made in[63] where a compact quasi-THz TDS system withoutthe need for a femtosecond laser source was intro-duced, enabling mobile and cost-effective system de-signs in the near future. Such research progressencourages the vision that in less than a decade fromnow, THz systems will have made the move out of la-boratories to real-world applications.

Fig. 9. (Color online) (a) THz image of a leaf obtained in a cw system with a confocal configuration, (b) THz transmission through a singlepoint of the leaf as a function of the volumetric water content, and (c) a future vision of a mobile plant hydration monitoring THz TDSsystem.

Fig. 10. (Color online) (a) THz image of a chocolate bar contami-nated with a buried glass splinter and (b) vision of a future THzquality inspection system for food products [21].


The authors thank the Federal Ministry of Educa-tion and Research (BMBF) for funding projects13N8572, 13N9406, and 13N9466. Furthermore, weare grateful for the support of the Federal Ministryof Economics and Technology (BMWi), grants 182ZNand 269ZN, administered by the German Federationof Industrial Research Associations (AiF) and theFederal Ministry of Food, Agriculture and ConsumerProtection (BMELV) from grant 2815305707. NicoVieweg thanks the Studienstiftung des deutschenVolkes for financial support. In addition, the authorsappreciate the fruitful cooperation of the SouthernGerman Plastic Center (SKZ), especially that ofBenjamin Baudrit and Dietmar Kraft, as well asthe valuable contributions of Khaled Shakfa, RafalWilk, Kai Baaske, Saranyu Lüders, Dirk Romeike,Marco Reuter, and Tilmann Jung.

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