Fast three-dimensional terahertz computed
tomography using real-time line projection of
intense terahertz pulse
Mukesh Jewariya,1 Emmanuel Abraham,2,3 Takayuki Kitaguchi,1 Yoshiyuki Ohgi,4Masa-aki Minami,4 Tsutomu Araki,4 and Takeshi Yasui1,4,*1Institute of Technology and Science, University of Tokushima, 2-1, Minami-Josanjima-cho, Tokushima, Tokushima
770-8506, Japan
2
LOMA, University of Bordeaux - CNRS, UMR 5798, 351 Cours de la Libération, 33405 Talence, France
3
em.abraham@loma.u-bordeaux1.fr
4
Graduate School of Engineering Science, Osaka University, 1-3, Machikaneyama-cho, Toyonaka, Osaka 560-8531,
Japan*yasui.takeshi@tokushima-u.ac.jp

Abstract: We demonstrated fast three-dimensional transmission terahertz
computed tomography by using real-time line projection of intense terahertz
beam generated by optical rectification in lithium niobate crystal. Afteremphasizing the advantage of intense terahertz pulse generation for twodimensional spatio-temporal terahertz imaging, peak-to-peak amplitudes ofpulsed terahertz electric field have been used to obtain a series of projection
images at different rotation angles. Then a standard reconstruction
algorithm has been employed to perform final three-dimensional
reconstruction. Test samples including a medicine capsule have been
investigated with a total acquisition time to only 6 minutes.
©2013 Optical Society of America
OCIS codes: (110.6795) Terahertz imaging; (110.6955) Tomographic imaging; (100.6890)
Three-dimensional image processing; (320.7090) Ultrafast lasers.

References and links
1.

G. T. Herman, Image Reconstruction from Projections: The Fundamentals of Computerized Tomography(Academic, 1980).
2. M. J. Aitken, Thermoluminescence Dating (Academic, 1985).
3. W. L. Chan, J. Deibel, and D. M. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys. 70(8), 1325–
1379 (2007).
4. B. Ferguson, S. Wang, D. Gray, D. Abbot, and X.-C. Zhang, “T-ray computed tomography,” Opt. Lett. 27(15),1312–1314 (2002).
5. X. Yin, B. W. H. Ng, and D. Abbott, Terahertz Imaging for Biomedical Applications: Pattern Recognition and
Tomographic Reconstruction (Springer, 2012).
6. A. Brahm, M. Kunz, S. Riehemann, G. Notni, and A. Tünnermann, “Volumetric spectral analysis of materials
using terahertz-tomography techniques,” Appl. Phys. B 100(1), 151–158 (2010).
7. E. Abraham, A. Younus, C. Aguerre, P. Desbarats, and P. Mounaix, “Refraction losses in terahertz computed
tomography,” Opt. Commun. 283(10), 2050–2055 (2010).
8. K. L. Nguyen, M. L. Johns, L. F. Gladden, C. H. Worrall, P. Alexander, H. E. Beere, M. Pepper, D. A. Ritchie, J.
Alton, S. Barbieri, and E. H. Linfield, “Three-dimensional imaging with a terahertz quantum cascade laser,” Opt.
Express 14(6), 2123–2129 (2006).
9. J. P. Caumes, A. Younus, S. Salort, B. Chassagne, B. Recur, A. Ziéglé, A. Dautant, and E. Abraham, “Terahertz
tomographic imaging of XVIIIth dynasty Egyptian sealed pottery,” Appl. Opt. 50(20), 3604–3608 (2011).
10. M. Bessou, H. Duday, J. P. Caumes, S. Salort, B. Chassagne, A. Dautant, A. Ziéglé, and E. Abraham,“Advantages of terahertz radiation versus X-ray to detect hidden organic materials in sealed vessels,” Opt.
Commun. 285(21–22), 4175–4179 (2012).
11. M. Bessou, B. Chassagne, J. P. Caumes, C. Pradère, P. Maire, M. Tondusson, and E. Abraham, “Threedimensional terahertz computed tomography of human bones,” Appl. Opt. 51(28), 6738–6744 (2012).
12. N. Sunaguchi, Y. Sasaki, N. Maikusa, M. Kawai, T. Yuasa, and C. Otani, “Depth-resolving THz imaging withtomosynthesis,” Opt. Express 17(12), 9558–9570 (2009).
13. T. Yasui, E. Saneyoshi, and T. Araki, “Asynchronous optical sampling terahertz time-domain spectroscopy for
ultrahigh spectral resolution and rapid data acquisition,” Appl. Phys. Lett. 87(6), 061101 (2005).
14. Q. Wu, T. D. Hewitt, and X.-C. Zhang, “Two-dimensional electro-optic imaging of THz beams,” Appl. Phys.
Lett. 69(8), 1026–1028 (1996).

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15. T. Yasuda, T. Yasui, T. Araki, and E. Abraham, “Real-time two-dimensional terahertz tomography of moving
objects,” Opt. Commun. 267(1), 128–136 (2006).
16. T. Yasui, K. Sawanaka, A. Ihara, E. Abraham, M. Hashimoto, and T. Araki, “Real-time terahertz color scanner
for moving objects,” Opt. Express 16(2), 1208–1221 (2008).
17. M. Schirmer, M. Fujio, M. Minami, J. Miura, T. Araki, and T. Yasui, “Biomedical applications of a real-timeterahertz color scanner,” Biomed. Opt. Express 1(2), 354–366 (2010).
18. J. Shan, A. S. Weling, E. Knoesel, L. Bartels, M. Bonn, A. Nahata, G. A. Reider, and T. F. Heinz, “Single-shotmeasurement of terahertz electromagnetic pulses by use of electro-optic sampling,” Opt. Lett. 25(6), 426–428(2000).
19. J. Hebling, K.-L. Yeh, M. C. Hoffmann, B. Bartal, and K. A. Nelson, “Generation of high-power terahertz pulses
by tilted-pulse-front excitation and their application possibilities,” J. Opt. Soc. Am. B 25(7), B6–B19 (2008).
20. M. Jewariya, M. Nagai, and K. Tanaka, “Enhancement of terahertz wave generation by cascaded χ(2) processes in
LiNbO3,” J. Opt. Soc. Am. B 26(9), A101–A106 (2009).
21. J. A. Fülöp, L. Pálfalvi, G. Almási, and J. Hebling, “Design of high-energy terahertz sources based on opticalrectification,” Opt. Express 18(12), 12311–12327 (2010).
22. J. A. Fülöp, L. Pálfalvi, M. C. Hoffmann, and J. Hebling, “Towards generation of mJ-level ultrashort THz pulses
by optical rectification,” Opt. Express 19(16), 15090–15097 (2011).
23. H. Hirori, A. Doi, F. Blanchard, and K. Tanaka, “Single-cycle terahertz pulses with amplitudes exceeding 1
MV/cm generated by optical rectification in LiNbO3,” Appl. Phys. Lett. 98(9), 091106 (2011).
24. M. Nagai, E. Matsubara, and M. Ashida, “High-efficiency terahertz pulse generation via optical rectification by
suppressing stimulated Raman scattering process,” Opt. Express 20(6), 6509–6514 (2012).
25. M. Jewariya, M. Nagai, and K. Tanaka, “Ladder climbing on the anharmonic intermolecular potential in anamino acid microcrystal via an intense monocycle terahertz pulse,” Phys. Rev. Lett. 105(20), 203003 (2010).
26. K. Tanaka, H. Hirori, and M. Nagai, “THz nonlinear spectroscopy of solids,” IEEE Trans. Terahertz Sci.
Technol. 1(1), 301–312 (2011).
27. A. Doi, F. Blanchard, H. Hirori, and K. Tanaka, “Near-field THz imaging of free induction decay from atyrosine crystal,” Opt. Express 18(17), 18419–18424 (2010).
28. F. Blanchard, A. Doi, T. Tanaka, H. Hirori, H. Tanaka, Y. Kadoya, and K. Tanaka, “Real-time terahertz nearfield microscope,” Opt. Express 19(9), 8277–8284 (2011).
29. Z. Jiang, X. G. Xu, and X.-C. Zhang, “Improvement of terahertz imaging with a dynamic subtraction technique,”Appl. Opt. 39(17), 2982–2987 (2000).
30. E. Abraham, Y. Ohgi, M. A. Minami, M. Jewariya, M. Nagai, T. Araki, and T. Yasui, “Real-time line projection
for fast terahertz spectral computed tomography,” Opt. Lett. 36(11), 2119–2121 (2011).
31. T. Yasui, M. Jewariya, T. Yasuda, M. Schirmer, T. Araki, and E. Abraham, “Real- time two-dimensional spatiotemporal terahertz imaging based non-collinear free-space electro-optic sampling application to functional
terahertz imaging of moving object,” IEEE J. Sel. Top. Quantum Electron. 19(1), (2013),
doi:10.1109/JSTQE.2012.2210393.
32. F. Miyamaru, T. Yonera, M. Tani, and M. Hangyo, “Terahertz two-dimensional electrooptic sampling using highspeed complementary metal-oxide semiconductor camera,” Jpn. J. Appl. Phys. 43(4A 4A), L489–L491 (2004).
33. W. S. Rasband, ImageJ (U.S. National Institutes of Health, Bethesda, Maryland, USA) http://imagej.nih.gov/ij/,
1997–2012.
34. B. Recur, A. Younus, S. Salort, P. Mounaix, B. Chassagne, P. Desbarats, J.-P. Caumes, and E. Abraham,“Investigation on reconstruction methods applied to 3D terahertz computed tomography,” Opt. Express 19(6),5105–5117 (2011).
35. M. D. Thomson, M. Kreß, T. Löffler, and H. G. Roskos, “Broadband THz emission from gas plasmas inducedby femtosecond optical pulses: From fundamentals to applications,” Laser & Photon. Rev. 1(4), 349–368 (2007).
36. T. T. L. Kristensen, W. Withayachumnankul, P. U. Jepsen, and D. Abbott, “Modeling terahertz heating effects
on water,” Opt. Express 18(5), 4727–4739 (2010).

1. Introduction
Computed tomography (CT) is an effective imaging method to visualize the internal structureof a three-dimensional (3D) object as cross-sectional images. In this method, a series oftransmitted images are measured at different projection angles and the internal structure of theobject is reconstructed by analyzing these images with specific reconstruction algorithms,
such as the filtered back-projection (FBP) algorithm using Radon inverse transform [1].
Conventionally, X-ray CT has been widely used in non-destructive testing, quality control,material characterization and biomedical imaging [2]. However, the hazardous ionizing
effects of X-ray often limit its utility. Also, high penetration power of X-ray reduces image
contrast in composite or soft materials, such as plastics or ceramics. Furthermore, thisimaging modality only produces monochrome pictures of the sample, making it difficult to
identify its chemical components.
Recently, terahertz (THz) radiation has emerged as a new mode for CT because of freespace propagation, low photon energy, moderate penetration through soft materials and broad#182051 - $15.00 USD(C) 2013 OSA

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spectral range [3]. Since the first demonstration of THz CT in 2002 [4], it has beensuccessively used to visualize internal structures of various objects from industrial,
pharmaceutical to biological applications [5-12]. Moreover, THz spectral CT can provide
colored pictures in THz range as compared to monochrome X-ray CT, associated to thepossibility to tentatively analyze the chemical composition of the sample based on THz
spectral fingerprints. However, considering practical applications of THz CT, we have to facea strong limitation which is the long acquisition time arising from use of THz time-domain
spectroscopy (THz-TDS). This is because THz-CT based on THz-TDS needs serial scanningof four mechanical stages for time delay, two-dimensional (2D) sample position and sample
rotation to record the series of THz projection images at multiple angles. In particular, themechanical stage for time-delay scanning is the most time-consuming due to its reciprocating
motion.
The simplest way to reduce time-delay scanning is to use a fast scanning mechanical delay
stage. Asynchronous optical sampling (ASOPS) is another promising method without any
mechanical delay [13]. However, to accomplish adequate signal-to-noise ratio and highdynamic range, multiple scans have to be integrated together associated with the problem oftiming jitter between both lasers. An alternative method to achieve faster THz CT is thedevelopment of depth-resolving THz imaging with tomosynthesis which is similar to CT
except that the number of projections is much smaller [12]. However, the efficiency of thissystem is mainly limited to thin samples. Furthermore, those methods are still based on pointby-point scanning measurements. If the pixel data could be measured in parallel
simultaneously, the data acquisition time would be greatly reduced. 2D free-space electrooptics sampling (2D-FSEOS) enables us to acquire a 2D image of a sample in real-time [14].
Although 2D-FSEOS eliminates 2D scanning of the sample for THz CT, the mechanical timedelay scanning is still required. Finally, even if all those techniques can be used for THz-CT,
they are still limited in practical use by the slow speed of final image acquisition.
Recently, we have developed a stage-free configuration combining single shotmeasurement of THz temporal waveform and one-dimensional (1D) transverse imaging,leading to real-time 2D spatio-temporal (2D-ST) THz imaging [15-17]. For this, we employed
a combination of non-collinear electro-optic time-to-space conversion [18] with line focusingof a THz beam. Thanks to the elimination of both mechanical time-delay scanning due totime-to-space conversion and 1D sample scanning due to line focusing, THz reflective
tomography [15] and spectral imaging of moving objects [16, 17] were successively
demonstrated. Although this 2D-ST THz imaging is available for THz CT, the image
acquisition time (typically a few tens to several tens seconds for one image) is stillinsufficient to achieve fast THz CT available for practical applications. Further decrease of
the image acquisition time is indispensable to achieve fast THz CT.
Recent advance of intense THz source based on optical rectification in LiNbO3 crystal
associated with the tilted pulse front technique [19-24] revolutionizes THz imaging as well as
THz nonlinear spectroscopy [25, 26]. For example, real-time 2D THz near-field imaging [27,28] has been achieved by combining such an intense THz source with 2D near-field imaging
technique based on 2D-FSEOS [29]. Furthermore, we recently demonstrated fast 2D THz
spectral CT based on combination of this intense THz source with 2D-ST THz imaging andsucceeded to measure cross-sectional images of continuously rotating samples in only a few
seconds [30, 31]. If such fast THz CT can be extended to visualize 3D internal structure of thesample, the application field of THz-CT will be largely increased. In this article, we propose
to demonstrate, for the first time to the best of our knowledge, the possibility of 3D THz CT
by using an intense THz source and only two mechanical stages without reciprocating motion(one for continuous translation of sample and another one for incremental rotation of sample)
instead of four, as pointed out previously. The system has been used to investigate test
samples such as four metallic bars, a toothpick into a plastic case and a more realistic one
consisting in a medicine capsule.

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2. Experimental setup and 3D data processing
The 3D THz CT system is based on an amplified Ti:Sapphire laser with 1 kHz repetition rate,
0.8 mJ/pulse, 800 nm central wavelength and 150 fs pulse duration. The laser beam is splitted
into pump and probe beams for THz generation and detection, respectively. The pump beam
is sent onto an Au-coated 2000 mm/lines holographic grating to introduce the desired pulse
front tilt and satisfy the phase matching condition for optical rectification in a stoichiometric
LiNbO3 crystal (1.5% Mg concentration), as shown in Fig. 1. Appropriate tilt angle andgrating imaging into the crystal are controlled by two lenses (L1, f = 200 mm; L2, f = 100mm). The pump beam is finally focused onto the crystal with a spot size of 2 × 2 mm2. One
surface of the crystal is cut with the phase matching angle of 62°. The generated THz beam isguided with an off-axis parabolic mirror (f = 190 mm), as shown in Fig. 1 where top and sideviews of the THz beam are presented for a better understanding of the line focusing andimaging optics. The THz beam is line-focused onto the sample using a THz cylindrical lens(THz-CL1, f = 50 mm, Tsurupica), resulting in a line of illumination (length = 25 mm, width= 1 mm) across the sample. Next, the line is imaged along the vertical Y-axis of a (110)oriented ZnTe crystal (thickness = 1 mm, size = 25 × 25 mm2) by a combination of a THz
plano-convex lens (THz-L, f = 50 mm, Tsurupica) and a THz cylindrical lens (THz-CL2, f =50 mm, Tsurupica). For detection, the probe beam is non-collinearly incident on the ZnTe
crystal at a crossed angle of 25° with the THz beam. Since the temporal width of the probe
pulse is much shorter than that of the THz pulse, wave fronts of the THz beam at different
times overlap with that of the probe beam at different transverse positions. This non-collinear
2D-FSEOS provides a time-to-space conversion of the pulsed THz electric field, such as asingle-shot autocorrelator [18]. After passing through two crossed polarizers (P and A), thespatial intensity distribution of the probe beam induced by the spatial birefringence
distribution in the EO crystal is imaged with a plano-convex lens (L3, f = 80 mm) onto ahigh-speed CMOS camera (232 × 232 pixels, 1000 frames per second) synchronized with the
1 kHz laser pulse.
The resulting image, called “2D-ST image”, is composed of the time profile of the THz
pulse (temporal window = 37 ps) and the line image (spatial range = 25 mm) of the sample,
which are developing along horizontal and vertical coordinates of the camera, respectively.
This THz image was acquired at a frame rate of 500 Hz by the CMOS camera working in the
dynamic subtraction mode [32]. Finally, owing to the 10 ms integration time of the CMOS
camera, the 2D-ST image is acquired with a 100 Hz acquisition rate. The transverse
resolution along the line projection attains the diffraction limit of the imaging system [16].
The pixel number in the vertical direction of the 2D-ST image is 232 pixels, which provides a
pixel rate of 2320 pixels/s. This is much faster than conventional THz imaging system where
pixel rate is usually from 1 to 100 pixels/s. However, as for 3D CT it is important to limit theamount of data storage, we arbitrarily reduced the size of the 2D-ST image to 116 × 116
pixels. Data storage and total acquisition time of our 3D THz CT system will be presented
into more details in Table 1.

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Fig. 1. Experimental setup and schematic representation of final 3D reconstruction (in the caseof 36 projections). THz-CL1 and THz-CL2: Tsurupica THz cylindrical lenses, THz-L:
Tsurupica THz plano-convex lens, P: polarizer, A: analyzer, L: plano-convex lens.

To perform fast 3D THz CT, the sample is horizontally scanned along the X-axis with atranslation stage at a constant speed of 2 mm/s whereas the THz beam is vertically linefocused along Y-axis of the sample so that a series of 2D-ST images can be acquired for agiven projection angle. Unfortunately, the acquisition time to get the series of 2D-ST images
is limited by the maximum scan speed ( = 2 mm/s) of the translation stage although the
integration time of a 2D-ST image is only 10 ms. For instance, for a 20 mm scan range(acquisition time of 10 s), we usually record 100 2D-ST images corresponding to 100
horizontal pixels. We believe that it should be possible to further reduced the acquisition time
down to only 1 s if a translation stage with 10-times faster scan speed (20 mm/s) can be used.
Next, the sample is rotated (angle step of 5°) and a second 2D-ST image set is recorded. The
operation is repeated 36 times from 0° to 175°. Finally, the total acquisition timecorresponding to 36 projections is only 6 minutes. This shows a significant reduction in themeasurement time compared with standard 3D THz CT systems using point-to-point scanningmeasurement (typically from 1 to 10 hours).
Finally, for each projection angle and at every horizontal and vertical sample positions,the pixel value is extracted by measuring the peak-to-peak value of the THz electric field in
temporal waveform. Similarly, after FFT of temporal data, the spectral amplitude at a given
frequency could also be extracted for spectral imaging [30]. Then, the application of the wellknown FBP algorithm implemented in the ImageJ software will numerically provide themultiplanar slices of the sample in order to finally reconstruct the 3D volume [33]. It is wellknown that FBP suffers from several drawbacks such as beam hardening, which can induce#182051 - $15.00 USD(C) 2013 OSA

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cupping, streaks and blurring because rays from some projection angles are hardened to adiffering extent than rays from other angles, confusing the reconstruction algorithm. To
reduce this phenomenon, it is important to record at least 36 projections. Of course,
depending on maximum data storage and acquisition time fixed by the operator, it is also
possible to record 72 projections with a 2.5° angle step. However, we considered that the bestcompromise between reconstruction quality (contrast, intensity and geometric preservation)
and acquisition time consists in recording only 36 projections [34]. To summarize theexperimental conditions and data processing, Table 1 provides the total acquisition time andlaptop data processing of the 3D THz CT for 36 projections, 20 mm horizontal scan with 100
pixels (continuous speed of 2 mm/s) recording and 116 vertical pixels. Main remarkablefeature is the short total acquisition time of only 6 minutes for this experiment associated withreasonable data storage of 295 MB.
Table 1. Acquisition time and laptop data processing of the fast 3D THz CT
Data acquisition
2D-ST
image 116× 116
pixelsTime (s)Data storage(MB)

Sampletranslation
2 mm/s100 pixels

Data processing
Samplerotation to get
36 projections

Peak-to-peak analysisof 2D-ST images to
get the 36 frontal
views

FBP algorithm
to get the 116
axial views

0.01

10

360

240

60

0.082

8.2

295

1.58

4.35

3. Experimental results and discussion
3.1 Comparison between optical rectification in LiNbO3 and ZnTe crystals for 2D-ST THz
imaging
In this sub-section, we want to evaluate the basic performances (i.e. maximum THz electricfield and dynamic range) of 2D-ST THz imaging using optical rectification in LiNbO3 crystal
as compared to ZnTe crystals.
First, we estimated the maximum THz electric field in the 2D-ST THz image.
Unfortunately, since this 2D-ST THz image was obtained by FSEOS near the zero opticaltransmission point (crossed Nichol configuration for the probe beam, Fig. 1), it is difficult tocalibrate the absolute value of the THz electric field directly from the measurement of theinduced phase modulation [20, 23]. Alternatively, we modified the optical setup for THz
detection in Fig. 1 to collinear FSEOS detection composed by a (110)-oriented GaP crystal(thickness = 0.3 mm) and two balanced photodiodes (not shown in Fig. 1) while maintaining
the same setup for generation of intense THz pulse. Figure 2 shows the temporal evolution of
the corresponding electric field ETHz evaluated from the EO signal, in which the electric field
amplitude in the left axis has been calibrated according to the procedure of previous papers[20, 23]. The maximum amplitude of the THz electric field achieves 170 kV/cm, which ismore than one order of magnitude higher than standard THz source using optical rectificationin ZnTe (typically, a few tens kV/cm).
In the above experiment, the intense THz beam was point-focused onto the GaP crystal
with a spot diameter of about 1 mm, resulting in the maximum electric field of 170 kV/cm.
On the other hand, in the 2D-ST THz imaging, this intense THz beam is sent onto the ZnTe
crystal with a beam diameter of about 25 mm. Taking into account this expansion factor of 25
for the THz beam, we can roughly estimate that the maximum electric field of the intense
THz beam at the ZnTe crystal position in the case of 2D-ST THz imaging is about 7 kV/cm.
For that, we simply argue that the electric field is inversely proportional to the beam radius
for a given energy of the THz beam [35]. Of course, for further practical applications, itwould be useful to get a THz line illumination of 50 mm onto the sample. If we want to keep
the same dynamic range for the THz detection, we can estimate that this would require a THz
electric field of 340 kV/cm, which may be achievable with an increase of the incident laser#182051 - $15.00 USD(C) 2013 OSA

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pulse energy onto the LiNbO3 crystal. Figure 3(a) shows a 2D-ST THz image in the absenceof a sample after integrating 5 images (corresponding measurement time = 10 ms). The green
and red areas in the image indicate positive and negative electric field, respectively. The main
THz pulse is visible around 6 ps (additional THz pulse around 28 ps is a replica generated by
the ZnTe crystal). Figure 3(b) shows a temporal waveform of the THz electric field extracted
along the dashed white line in the 2D-ST THz image. The color scale in Fig. 3(a) and verticalaxis in Fig. 3(b) have been calibrated based on the expansion factor of the THz beam, as
explained previously. Here, it is worth to emphasize that this intense THz source maintains amaximum electric field similar to that of a point-focused THz beam from usual ZnTe-based
THz source. This is due to the large enhancement of THz electric field even though the THz
beam is not focused onto the crystal in our experimental setup. It is worth noting that,
according to Kristensen et al. [36], such an intense THz pulsed electric field will give anegligible temperature increase of a water content sample. These authors mentioned that if theTHz beam is focused down to a spot size of 0.5 mm, the steady-state temperature increase permW of transmitted power is 1.8 °C/mW. In our case, the maximum THz beam power sent
onto the sample is in the order of a few tens of to one hundred microwatts associated to a line
focusing of 1 × 25 mm2. This indicates that heating effects on water can be neglected in our
measurements which can be essential for further biological applications.

Fig. 2. Temporal profile of THz electric field generated by optical rectification from LiNbO3.
Collinear FSEOS detection with a (110)-oriented GaP crystal (thickness = 0.3 mm) andbalanced photodiodes are used for this measurement.

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(a)

(b)

Fig. 3. (a) 2D-ST THz image and (b) temporal waveform of the pulsed THz electric field alongwhite line in panel (a). Non-collinear FSEOS detection with a (110)-oriented ZnTe crystal(thickness = 1 mm) and CMOS camera were used for this measurement.

Next, we investigated the dynamic range in the temporal waveform of THz electric field
extracted from a 2D-ST image. Here, the dynamic range is defined as a ratio of a peak-topeak value of pulsed electric field in the presence of THz beam to a standard deviation ofnoise signal in the absence of THz beam. Figure 4 shows the result for the present THz-CT
system based on optical rectification in LiNbO3 crystal. For comparison, we also plotted thesimilar dynamic range obtained with our previous system using optical rectification in ZnTe
crystal [16, 17]. One can confirm that the dynamic range is largely enhanced in LiNbO3
crystal combined with tilted pulse front technique compared to collinear phase-matching in
ZnTe crystal. For example, at an integration time of 10 ms (100 Hz frame rate), the dynamic
range is 619 with LiNbO3 and only 49 with ZnTe. This confirms the potential of opticalrectification in LiNbO3 crystal using tilted pulse front for fast THz imaging applications, as
shown in the next sub-sections.

Fig. 4. Dynamic range of the temporal waveform extracted from 2D-ST THz image as afunction of the integration time in the case of optical rectification in LiNbO3 (tilted pulse front)
and ZnTe (collinear phase-matching).

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3.2 Fast 3D THz CT
To demonstrate the ability of fast 3D THz CT, we first investigated a simple test sample
consisting of four vertical asymmetric metallic bars (2 mm diameter), as shown in an inset of
Fig. 5(a), illuminated by the vertical THz focal line. As explained previously, 3D CT is
performed in only 6 minutes after continuous horizontal translation (2 mm/s, 100 pixels) of
the sample and 36 rotations with a 5° angle step in order to collect the projection data. Fromthese raw data, after the peak-to-peak extraction of the THz electric field, we can visualize thefrontal views (XY-plane) of the sample corresponding to the different projections. One ofthese views (100 × 116 pixels) is presented in Fig. 5(a), showing the position of the four
asymmetric bars, white and black colors indicating weak and strong transmitted THz signal,
respectively. From this image, we can clearly see the arrangement of the sample consisting in
two long bars at the extremities and two shorter ones in the middle. Diffraction effects canalso be observed near the edges of the two central bars. The video (36 projections) isavailable in Media 1 and makes it possible to clearly see the position of the bars from eachother. Using the standard FBP algorithm [1], we can reconstruct the 116 cross-sectional
images (also called “axial view”) corresponding to the portion of the object illuminated by theTHz line. Figure 5(b1) shows the axial view corresponding to the position of the upper red
horizontal line in Fig. 5(a), whereas Fig. 5(b2) is for the lower red horizontal line. From thesetwo cross sections, it is obvious to identify the respective position of all the bars from eachother, with a good contrast (no THz signal through the metallic bars) and a sub-millimeterspatial resolution. Finally, using the ImageJ software, the 3D reconstruction of the sample canbe obtained, as shown in Fig. 5(c) which corresponds to one still picture from the 3D movie(available in Media 2). We have to notice that inhomogeneities in thickness over the complete
length of the bars can be visible in Figs. 5(a) and 5(c), attributed to peak-to-peak extraction
errors and reconstruction artifacts inherent to the FBP algorithm.

Fig. 5. Four metallic bars. (a) Frontal view (Media 1) and picture of the sample (inset). (b1)
Axial view from the upper red line in (a). (b2) Axial view from the lower red line in (a). (c) 3D
reconstruction (Media 2).

The second test sample is a plastic box (section 10 × 10 mm2) with a wooden toothpick
inside. Similarly, Fig. 6(a) shows one of the 36 frontal views (video in Media 3), Figs. 6(b1)and 6(b2) give the two axial views at the position of the red horizontal lines, and Fig. 6(c) is
the final 3D reconstruction including a picture of the sample as inset (video in Media 4). First,
we can notice that we are able to determine the position of the toothpick inside the plastic box
in all figures. Then, especially from the axial views, we observe that the shape of therectangular plastic box is not perfectly reconstructed. This comes for the relative transparency#182051 - $15.00 USD(C) 2013 OSA

Received 19 Dec 2012; revised 17 Jan 2013; accepted 17 Jan 2013; published 24 Jan 201328 January 2013 / Vol. 21, No. 2 / OPTICS EXPRESS 2431

of the plastic material for THz radiation giving a weak attenuation of the transmitted THz
pulse except around the four corners, which are much clearly visible in the figure. These firsttwo examples demonstrate the ability of the system to properly reconstruct in a relatively
short time the 3D shape of simple samples.
(a)

(b1)

(c)

(b2)

4 mm

4 mm

Fig. 6. A wooden toothpick into a plastic case. (a) Frontal view (Media 3). (b1) Axial view
from the upper red line in (a). (b2) Axial view from the lower red line in (a). (c) 3D
reconstruction (Media 4). Inset: picture of the sample.

The last object is a gelatin soluble capsule enclosing a dose of medicine powder (inset in
Fig. 7(a), object size: 7 × 20 mm). This sample represents a more realistic object in order totest the potential of the system for pharmaceutical applications. From the frontal view [Fig.
7(a)], it is possible to clearly see the capsule with the localization of its internal content. We
note that the extreme lower part of the capsule is not imaged because it is attached on ametallic rod for precise rotation. Of course, from the peak-to-peak analysis, it is not possible
to get any information from the chemical structure of the medicine powder. This should
require a spectroscopic analysis of the 2D-ST image as in ref 29. Here, we can observe thatTHz radiation is totally absorbed by the medicine powder. Consequently, it is impossible toget any information on the powder homogeneity from this measurement. From this view, it isalso not possible to see the separation between the two semi-cylindrical gelatin parts
constituting the capsule because the amount of medicine powder is too high. However, theaxial views make it possible to see the empty part of the capsule (Fig. 7(b1)) and the
localization of the powder [Fig. 7(b2)]. Finally, Fig. 7(c) and Media 5 show the 3D
reconstruction of the capsule which is useful to get a global view of the object even if, for aquantitative visualization of 3D objects using CT, mainly axial and front views are used.
From this 3D view, even if the powder is clearly identified in the lower part of the capsule, its
outer shape is not perfectly reconstructed. Especially, the top of the capsule presents somereconstruction artifacts, probably due to diffraction effects and THz beam deviationoccasioned by the spherical shape of the sample in this region.

#182051 - $15.00 USD(C) 2013 OSA

Received 19 Dec 2012; revised 17 Jan 2013; accepted 17 Jan 2013; published 24 Jan 201328 January 2013 / Vol. 21, No. 2 / OPTICS EXPRESS 2432

(a)

(b1)

(c)

(b2)

5 mm

5 mm

Fig. 7. Medicine capsule. (a) Frontal view (inset: photograph). (b1) Axial view from the upper
red line in (b). (b2) Axial view from the lower red line in (b). (c) 3D reconstruction (Media 5).

4. Conclusion
In this paper, we demonstrated the potential of intense THz pulse generated by opticalrectification in LiNbO3 crystal for 3D THz CT. We estimated that the maximum amplitude ofthe electric field of the 25 mm diameter THz beam attained 7 kV/cm. We also measured a
dynamic range of 619 for the THz detection at the integration time of 10 ms, which is 13times higher than that of our previous system based on collinear optical rectification in ZnTe
crystal [16, 17]. The main advantage of the proposed method relies on real-time THz line
projection providing 10 ms acquisition time of 2D-ST THz image. Therefore, 3D THz CT has
been performed in only 6 minutes, representing a significant improvement compared with
common systems. The use of another translation stage with 10-times faster speed will further
decrease the total measurement time for 3D THz CT down to only 36 s. Finally,
demonstration of 3D reconstructions of selected samples clearly indicated a high potential forsensing, non-destructive inspection and material characterization in real world applications.
Acknowledgments
E. Abraham is grateful to the BRIDGE Fellowship Program, Japan Society for the Promotionof Science, Japan. This work was supported by Grants-in-Aid for Scientific Research21650111 and 23656265 from the Ministry of Education, Culture, Sports, Science, andTechnology of Japan. We also gratefully acknowledge financial support from the Renovation
Center of Instruments for Science Education and Technology at Osaka University, Japan.

#182051 - $15.00 USD(C) 2013 OSA

Received 19 Dec 2012; revised 17 Jan 2013; accepted 17 Jan 2013; published 24 Jan 201328 January 2013 / Vol. 21, No. 2 / OPTICS EXPRESS 2433