Terahertz quasi-time-domain spectroscopy imaging

Maik Scheller,* Stefan F. Dürrschmidt, Matthias Stecher, and Martin KochFachbereich Physik, Philipps Universität Marburg, Renthof 5, 35032 Marburg, Germany

*Corresponding author: maik.scheller@physik.uni‐marburg.de

Received 22 December 2010; accepted 2 February 2011;posted 10 February 2011 (Doc. ID 139829); published 28 April 2011

We demonstrate terahertz (THz) imaging with a quasi-time-domain spectrometer. This type of THz sys-tem is inexpensive, compact, and relatively easy to set up. Beating the simultaneously emitted equidi-stant modes of a compact diode laser allows for analysis of samples at multiple frequencies with a singlemeasurement. Thus, this technique merges the potential of terahertz time-domain spectrometers withthe simplicity of continuous wave lasers. Multiple imaging applications and stability issues arediscussed. © 2011 Optical Society of AmericaOCIS codes: 110.6795, 300.6495, 150.3045.

1. Introduction

The advent of terahertz (THz) time-domain spectro-scopy (TDS) based on femtosecond laser sources in-itiated the proliferation of THz research startingin the early 1990s [1]. Employing shorter and shorterlaser pulses and optimized emitters extended the ac-cessible frequency window steadily toward higherfrequencies. One of the most powerful techniques,THz generation in air, recently reached signalbandwidths of several tens of terahertz [2]. How-ever, for this generation scheme, scientific scalehigh-intensity femtosecond regenerative amplifiersystems are required. Thus, telecom-laser-based,all-fiber coupled spectrometers that offer reliabilityand acceptable price levels [3] can be used for thefirst out-of-laboratory applications for the THz tech-nology. These applications include nondestructivetesting of industrial goods [4–7], chemicals [8], andplants [9]. However, broad commercial use will re-quire even lower prices that are hard to attain forsystems driven by sophisticated femtosecond lasersources.

Less expensive continuous wave (CW) photomix-ing THz systems [10–12], on the other hand, only pro-vide information in a narrowband frequency windowand thus restrict the information value of the mea-

sured data. Absolute thickness determination, inparticular, requires phase information at multiplefrequencies [13], making it necessary to measure asample several times at different THz frequenciesusing a conventional photomixing setup. Thus, theapplicability of this system architecture is restricted.

A kind of hybrid approach, the quasi-TDS (QTDS)technology, merges the two worlds—the simplicity ofCW systems and the broadband frequency informa-tion of TDS. This is accomplished by utilizing multi-ple equidistantly spaced laser lines, e.g., emitted byconventional multimode diode lasers [14]. The sig-nals detected in a QTDS system have a pulselikeshape and contain spectral information over severalhundreds of gigahertz. The energy of the signals iscondensed in a discrete frequency in combinationwith a spacing defined by the distance of the longi-tudinal laser modes in the range of some tens of giga-hertz [14] or only several hundreds of megahertz[15]. As most challenges of the conventional TDStechnology, like the need for dispersion compensationand reliable mode-locking mechanisms, are avoideddue to the CW nature of the laser light used, theQTDS technology bears the potential to be employedfor a plethora of real-world applications.

In this paper, we discuss the potential of QTDSsystems for THz imaging purposes by presenting dif-ferent application scenarios. Because of the flexibil-ity of analyzing the data in both time domain andfrequency domain, all of the sophisticated signal

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processing methods known from TDS [16–19] andCW [13,20] can be applied. First, we show the poten-tial for determining spatial varying thicknesses, ne-cessary for quality inspection. For that, we measureda polymeric airbag cover comprising a predeterminedbreak line with reduced thickness. By analyzing theresulting double pulse feature, we are able to deter-mine the thickness of the break line as well as thethickness of the remaining polymer cover.

As a second example, a piece of glassfiber-reinforced polymer was measured in cross-polarization geometry. Here, the polarization of theemitter antenna is rotated by 90° against the detec-tor’s acceptance polarization. This geometry allowsfor detecting the orientation and degree of birefrin-gence and thus, for detection the spatial varyingorientation of the glass fibers.

2. Setup

We set up a QTDS spectrometer as illustrated inFig. 1. A laser module with a size of 0:5 in: × 2 in: in-cluding collimating optics and the driver electronic,emitting around 100mW at 662nm is used as lasersource. The mode spacing of the longitudinal lasermodes is 25GHz with a spectral emission bandwidthof about 3nm. The laser beam is split into two parts.One is directed to a photoconductive antenna to gen-erate THz radiation. The other one is guided over amechanical linear stage to the detector antenna.Both antennas have a 100 μm dipole shape and aremade of low-temperature grown gallium arsenide.This material provides a short carrier lifetime, a ne-cessity for efficient photomixing as discussed in [21].Off-axis parabolic mirrors are used to guide the THzbeam from the emitter through the sample to thedetector antenna.

The THz waveforms are recorded by moving thelinear delay stage while measuring the photocurrentin the detector antenna utilizing a lock-in amplifier.This coherent signal detection scheme is furtherdescribed in [14]. The time constant of the lock-in

was set to 20ms. Because of the limited speed ofthe employed mechanical stage, the acquisition timefor a single waveform was 9 s. To record individualimages, i.e., to measure the sample transmissionpixel-by-pixel, the samples were mounted onto a two-dimensional linear stage.

3. Measurement Results

The first sample under investigation is a polymericairbag cover. This cover features a predeterminedbreak line with reduced thickness. A photograph ofthe cover is shown in Fig. 2(a). In the correspondingTHz image, shown in Fig. 2(b), the break line (i) canclearly be identified. In this image, the peak-to-peakvalue of the QTDS signal is shown. This figureof merit, as illustrated in Fig. 3(a), allows for aneffective description of the transmission through asample.

The reason for the reduced peak-to-peak value atthe break line is the double pulse features induced bythe different material thickness of the cover and thebreak line. One fraction of the THz wave passesthe thin material of the break line, the other one,

Fig. 1. Scheme of the QTDS system. The emission of a laser mod-ule is divided by a beam splitter into two parts. One fraction of thelight is directed to the emitter antenna by mirror M1. The otherfraction is guided over a delay stage and then focuses onto the de-tector by mirrors M2 and M3. Four parabolic mirrors (P1 to P4)collimate and focus the THz waves. The samples were mea-sured at the position of the THz focus and translated in a two-dimensional plane ðx; yÞ.

Fig. 2. (Color online) (a) Photograph of investigated airbag cover.The positions are (i) break line, (ii) adhesive label, (iii) stampingswithin the polymer, and (iv) retainer bars. (b) THz image of thecover showing the peak-to-peak value of the THz signal.

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the thicker polymer of the remaining cover. As con-sequence, one part of the signal arrives before theother, as shown in Fig. 3(b). Highlighting the highsensitivity of the QTDS technology, the image alsoreveals other minor features of the sample: (ii) an ad-hesive label, (iii) stampings within the polymer, and(iv) retainer bars at the edge of the cover.

While the position of the break line can be clearlydetected by this peak-to-peak evaluation, the deter-mination of the actual thickness is additionally of

high interest for quality inspection purposes. Usingthe information on the time-domain waveform allowsfor extracting the thickness of the break line aswell asthat of the general cover. For this, we use a time-of-flight approach where the time differences betweena reference measurement through air and the samplemeasurement are determined. As the signal at the po-sition of the break line features a double pulse shape[shown in Fig. 3(b)], it is possible to extract simulta-neously the two time differencesΔt1 andΔt2 betweenan air reference and the wave traveling through thebreak lineand the cover, respectively. The correspond-ing material thicknesses (0:9mm for the break lineand 2:4mm for the cover) agree very well withmechanical measurements indicating the applicabil-ity of the time-of-flightmethod to theQTDSmeasure-ments to characterize sample thicknesses.

The second investigated sample was a glass fiber-reinforced polymer sample. This sample was pro-duced by injection molding of glass fibers into apolymer host material. Because of this production

Fig. 3. (a) THz signal for a reference measurement through airand the airbag cover. The peak-to-peak value (PP) as figure of mer-it is illustrated. (b) Measured signal at the break line exhibiting adouble pulse shape, allowing determination of the two time differ-ences Δt1 and Δt2.

Fig. 4. Scheme of the cross-polarization geometry. The emittedTHz wave is polarized perpendicularly (along x axis) to the detec-tor’s accepted polarization (along y axis). If a birefringent sample isplaced within the THz beam path, the THz wave is rotated (byΔφ)and a signal can be detected.

Fig. 5. (Color online) (a) Photograph of the fiber-reinforced poly-mer. At the edges of the sample, the fibers are expected to beoriented parallel to the edges, as at positions I and II. (b) Corre-sponding THz image showing the peak-to-peak value of the THzsignal. The pronounced orientation of the fibers at the edgescan be identified by the increased THz signal.

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process, the fibers are expected to exhibit a pro-nounced orientation at the edges of the sample. Thisorientation induces a pronounced THz form birefrin-gence [20] that can be utilized to determine the fiberorientation [22,23].

To efficiently detect the fiber orientation, a cross-polarization geometry illustrated in Fig. 4 was cho-sen. similar to [24]. Here, the emitted THz wavesare polarized perpendicularly to the accepted polar-ization of the detector.

Only in the case of birefringent samples, a signalcan be detected due to the resulting rotation of theTHz wave. Figure 5 shows the peak-to-peak valueof the QTDS measurement together with a photo-graph of the sample. Only at the edges [e.g., pixelsI and II marked in Fig. 5(a)] can a noticeable THz

signal be detected as expected from the geometryof the samples.

In addition, the THz wave is rotated clockwise orcounterclockwise, determined by the direction of thefibers with respect to the orientation of the THzwave. This results in a change of the sign of the THzsignal. Two measured time traces for two differentpixels are shown in Fig. 6. As can be seen in thefigure, the pulse at pixel II is flipped compared tothe one at pixel I, as is expected from the fiberorientation.

Using the amplitude value of the QTDS pulse atthe time position of the maximal amplitude (Tmaxin Fig. 6) allows for visualization of both the degreeof orientation and the direction of the fibers. This isshown in Fig. 7. The fibers at the upper edge are or-iented parallel to the edge, which results in a max-imum of the rotated THz signal at the time Tmax.In contrast, the perpendicular orientation of the fi-bers at the lower edge rotates the THz wave in theother direction, and the THz amplitude takes itsminimum at Tmax.

The last point addressed in this paper concerns thestability of the system.To extract reliable informationabout the investigated samples from the imagingdata, it is essential that the point-per-point measure-ments yield comparable results. Instabilities of thesystem would result in misleading artifacts. Whilethe quality of the presented images already indicatesthat the reproducibility of the measurement is suffi-cient for imaging purposes, we additionally comparedindividual waveforms. Figure 8 shows three wave-forms recorded one after another. The observeddeviation of less than 1% is due to a signal-to-noiseratio in the range of 50dB, and could be further re-duced by increasing the time constant of the lock-inamplifier.

4. Conclusion

We have demonstrated the applicability of the QTDStechnique for THz imaging purposes. Because of thepulsed time-domain waveform, different analyzing

Fig. 6. THz signal for two positions of the fiber reinforced poly-mer (pixels I and II; cf. Fig 5). Depending on the orientation of thefibers the THz wave is rotated clockwise or counterclockwise. Atthe time position Tmax, the THz signal exhibits its maximum atpixel I and its minimum at pixel II.

Fig. 7. (Color online) THz image of the fiber reinforced sampleshowing the amplitude value at the time Tmax. At the upper edge,the value is positive, since the THz wave is rotated counterclock-wise due to the fiber orientation, while the wave is rotated clock-wise and the amplitude is negative.

Fig. 8. Three waveforms measured one after another. The devia-tion between the individual time traces is below 1%.

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methods such as time of flight, peak-to-peak, andspectral-based figures of merit, known from conven-tional TDS, can be applied. We also presented animaging approach based on cross-polarization toeffectively measure the orientation and degree ofbirefringence.

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