Abstract: In this work, we develop a pulsed terahertz imaging system inreflection geometry, where due to scanning of the terahertz beam neitherthe sample nor the emitter and detector have to be moved. We use a twomirror galvanoscanner for deflecting the beam, in combination with asingle rotationally symmetric focusing lens. In order to efficiently imageplanar structures, we develop an advanced scanning routine that resolves allbending effects of the imaging plane already during measurement. Thus,the measurement time is reduced, and efficient imaging of surfaces andinterfaces becomes possible. We demonstrate the potential of this method inparticular for a plastic-metal composite sample, for which non-destructiveevaluation of an interface is performed.
References and links1. D. H. Auston, “Picosecond optoelectronic switching and gating in silicon,” Appl. Phys. Lett. 26, 101–103 (1975).2. B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett. 20, 1716–1718 (1995).3. P. U. Jepsen, R. H. Jacobsen, and S. R. Keiding, “Generation and detection of terahertz pulses from biased
semiconductor antennas,” J. Opt. Soc. Am. B 13, 2424–2436 (1996).4. D. M. Mittleman, J. Cunningham, M. C. Nuss, and M. Geva, “Noncontact semiconductor wafer characterization
with the terahertz Hall effect,” Appl. Phys. Lett. 71, 16–18 (1997).5. M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1, 97–105 (2007).6. C. Jansen, S. Wietzke, O. Peters, M. Scheller, N. Vieweg, M. Salhi, N. Krumbholz, C. Jordens, T. Hochrein, and
M. Koch, “Terahertz imaging: applications and perspectives,” Appl. Optics 49, E48–E57 (2010).7. N. Karpowicz, H. Zhong, C. Zhang, K.-I. Lin, J.-S. Hwang, J. Xu, and X.-C. Zhang, “Compact continuous-wave
subterahertz system for inspection applications,” Appl. Phys. Lett. 86, 054105–1–3 (2005).8. S. Wietzke, C. Jordens, N. Krumbholz, B. Baudrit, M. Bastian, and M. Koch, “Terahertz imaging: a new non-
destructive technique for the quality control of plastic weld joints,” J. Eur. Opt. Soc. - Rapid 2, 07013–1–5 (2007).9. M. A. Salhi, I. Pupeza, and M. Koch, “Confocal thz laser microscope,” J. Infrared Millim. Te. 31, 358–366
(2010).10. D. Zimdars, G. Fichter, C. Megdanoff, M. Murdock, I. Duling, J. White, and S. L. Williamson, “Portable video
rate time domain terahertz line imager for security and aerospace nondestructive examination,” in TerahertzPhysics, Devices, and Systems IV: Advanced Applications in Industry and Defense, Proc. SPIE Vol. 7671, (2010),pp. 76710K–1–8.
11. C. Jansen, S. Wietzke, H. Wang, M. Koch, and G. Zhao, “Terahertz spectroscopy on adhesive bonds,” Polym.Test. 30, 150–154 (2011).
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23042
12. C. Jordens and M. Koch, “Detection of foreign bodies in chocolate with pulsed terahertz spectroscopy,” Opt.Eng. 47, 037003 (2008).
13. R. M. Woodward, B. E. Cole, V. P. Wallace, R. J. Pye, D. D. Arnone, E. H. Linfield, and M. Pepper, “Terahertzpulse imaging in reflection geometry of human skin cancer and skin tissue,” Phys. Med. Biol. 47, 3853–3863(2002).
14. Y.-C. Shen, “Terahertz pulsed spectroscopy and imaging for pharmaceutical applications: A review,” Int. J.Pharm. p. in press (2011).
15. M. C. Kemp, P. F. Taday, B. E. Cole, J. A. Cluff, A. J. Fitzgerald, and W. R. Tribe, “Security applications ofterahertz technology,” in Terahertz for Military and Security Applications, Edited by R. J. Hwu, D. L. Woolard,Proc. SPIE, Volume 5070, (2003), pp. 44–52.
16. Y. C. Shen, T. Lo, P. F. Taday, B. E. Cole, W. R. Tribe, and M. C. Kemp, “Detection and identification ofexplosives using terahertz pulsed spectroscopic imaging,” Appl. Phys. Lett. 86, 241116–1–3 (2005).
17. K. Kawase, Y. Ogawa, and Y. Watanabe, “Non-destructive terahertz imaging of illicit drugs using spectral finger-prints,” Opt. Express 11, 2549–2554 (2003).
18. M. Lu, J. Shen, N. Li, Y. Zhang, C. Zhang, L. Liang, and X. Xu, “Detection and identification of illicit drugsusing terahertz imaging,” J. Appl. Phys. 100, 103104–1–5 (2006).
19. P. Jepsen, D. Cooke, and M. Koch, “Terahertz spectroscopy and imaging - modern techniques and applications,”Laser Photon. Rev. 5, 124–166 (2011).
20. A. W. Lee and Q. Hu, “Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-planearray,” Opt. Lett. 30, 2563–2565 (2005).
21. Q. Li, S.-H. Ding, R. Yao, and Q. Wang, “Real-time terahertz scanning imaging by use of a pyroelectric arraycamera and image denoising,” J. Opt. Soc. Am. A 27, 2381–2386 (2010).
22. S. Ariyoshi, C. Otani, A. Dobroiu, H. Sato, K. Kawase, H. M. Shimizu, T. Taino, and H. Matsuo, “Terahertzimaging with a direct detector based on superconducting tunnel junctions,” Appl. Phys. Lett. 88, 203503–1–3(2006).
23. D. Grischkowsky, S. Keiding, M. van Exter, and C. Fattinger, “Far-infrared time-domain spectroscopy with tera-hertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B 7, 2006–2015 (1990).
24. L. Xu, X.-C. Zhang, and D. H. Auston, “Terahertz beam generation by femtosecond optical pulses in electro-opticmaterials,” Appl. Phys. Lett. 61, 1784–1786 (1992).
25. A. Rice, Y. Jin, X. F. Ma1, X.-C. Zhang, D. Bliss, J. Larkin, and M. Alexander, “Terahertz optical rectificationfrom < 110 > zinc-blende crystals,” Appl. Phys. Lett. 64, 1324–1326 (1994).
26. Q. Wu, M. Litz, and X.-C. Zhang, “Broadband detection capability of ZnTe electro-optic field detectors,” Appl.Phys. Lett. 68, 2924–2926 (1996).
27. W. L. Chan, J. Deibel, and D. M. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys. 70, 1325–1379(2007).
28. B. Ferguson, S. Wang, D. Gray, D. Abbot, and X.-C. Zhang, “T-ray computed tomography,” Opt. Lett. 27, 1312–1314 (2002).
29. V. P. Wallace, E. MacPherson, A. J. Zeitler, and C. Reid, “Three-dimensional imaging of optically opaque mate-rials using nonionizing terahertz radiation,” J. Opt. Soc. Am. A 25, 3120–3133 (2008).
30. J. Xu and G. Cho, “A real-time terahertz wave imager,” in Conference on Lasers and Electro-Optics/QuantumElectronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA TechnicalDigest (CD) (Optical Society of America, 2008), paper CThN2, (2008).
31. K. Serita, S. Mizuno, H. Murakami, I. Kawayama, M. Tonouchi, Y. Takahashi, M. Yoshimura, Y. Kitaoka, andY. Mori, “Development of laser scanning terahertz imaging system using organic nonlinear optical crystal,” in35th International Conference on Infrared Millimeter and Terahertz Waves, (IRMMW-THz), (2010).
32. B. Pradarutti, R. Muller, W. Freese, G. Matthaus, S. Riehemann, G. Notni, S. Nolte, and A. Tunnermann, “Tera-hertz line detection by a microlens array coupled photoconductive antenna array,” Opt. Express 16, 18443–18450(2008).
33. C. Wiegand, M. Herrmann, S. Bachtler, J. Klier, D. Molter, J. Jonuscheit, and R. Beigang, “A pulsed THz imagingsystem with a line focus and a balanced 1-D detection scheme with two industrial ccd line-scan cameras,” Opt.Express 18, 5595–5601 (2010).
34. T. Yasui, K.-I. Sawanaka, A. Ihara, E. Abraham, M. Hashimoto, and T. Araki, “Real-time terahertz color scannerfor moving objects,” Opt. Express 16, 1208–1221 (2008).
35. Q. Wu, T. D. Hewitt, and X.-C. Zhang, “Two-dimensional electro-optic imaging of THz beams,” Appl. Phys.Lett. 69, 1026–1028 (1996).
36. Z. Jiang and X.-C. Zhang, “2D measurement and spatio-temporal coupling of few-cycle THz pulses,” Opt. Ex-press 5, 243–248 (1999).
37. H. Zhong, A. Redo-Sanchez, and X.-C. Zhang, “Identification and classification of chemicals using terahertzreflective spectroscopic focal-plane imaging system,” Opt. Express 14, 9130–9141 (2006).
38. X. Wang, Y. Cui, D. Hu, W. Sun, J. Ye, and Y. Zhang, “Terahertz quasi-near-field real-time imaging,” Opt.Commun. 282, 4683–4687 (2009).
39. W. Bohler, M. Bordas Vicent, and A. Marbs, “Investigating laser scanner accuracy,” in Proc. of XIXth CIPA
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23043
International Symposium (2003).40. K. Ezdi, B. Heinen, C. Jordens, N. Vieweg, N. Krumbholz, R. Wilk, M. Mikulics, K. Wiesauer, S. Katletz, and
M. Koch, “Pulsed THz antennas – a time-domain approach,” in Proc. IRMMW-THz 2009 in Busan, South Korea,(2009).
41. I. N. Duling, J. White, and S. Williamson, “High speed imaging with time domain terahertz,” in 35th InternationalConference on Infrared Millimeter and Terahertz Waves (IRMMW-THz), (2010), p. 1.
42. P. U. Jepsen and B. M. Fischer, “Dynamic range in terahertz time-domain transmission and reflection spec-troscopy,” Opt. Lett. 30, 29–31 (2005).
43. M. Naftaly and R. Dudley, “Methodologies for determining the dynamic ranges and signal-to-noise ratios ofterahertz time-domain spectrometers,” Opt. Lett. 34, 1213–1215 (2009).
1. Introduction
Three-dimensional scanning can be a waste of time, if one wants to image a planar surfaceor interface only. However, sometimes accumulation of non-relevant data cannot be avoided,either because the position of the target is not known, or the scanning scheme is not adjustedto the surface to be inspected. We present a method for reducing the number of scans neededto cover a planar surface in pulsed terahertz (THz) imaging, thus making this technology moreviable for non-destructive material testing.
THz technology deals with the generation and detection of electromagnetic radiation withwavelengths roughly between 3 mm and 30 μm (0.1–10 THz). The science behind it has comea long way from its early beginnings [1–4] to modern day imaging techniques (see, e.g., [5,6]).Applications range from defect detection in polymers and foams [7–11], food inspection [12],biomedical and pharmaceutical applications [13, 14] to the detecion of hazardous [15, 16] orillicit [17, 18] substances. For a comprehensive review, refer to, e.g., [19].
A multitude of detection schemes have been established so far. On the one hand, these aresystems using intensity dependent THz detectors such as bolometers [20], pyroelectric de-tectors [21] or superconducting tunnel junctions [22]. However, they have their drawbacks bynot recording the phase of the THz waves and by being either slow, very sensitive to their sur-roundings or requiring expensive cooling. On the other hand, phase sensitive detection schemes,which are also referred to as pump and probe techiques, have the advantage of providing directinformation on the THz refractive index and absorption coefficient, as well as on the three-dimensional (3d) structure of the sample. Well established pump and probe techniques eitheruse photo-conductive antennas (PCAs) [1] made from semiconductor materials [23], or non-linear optical crystals (e.g., LiNbO3 [24], CdTe [25], ZnTe [26]) through optical rectificationand electro-optic sampling (EOS). The full potential of phase sensitive measurement is ex-ploited only in reflection geometry, when the echoes from interfaces at different depths arerecorded separately [27]. Hence, this measurement method is inherently sensitive to the depthposition of the sample or interface. In contrast, measurement in transmission geometry onlygives an integral information of the sample, i.e., for 3d imaging of irregular objects or inhomo-geneities, the sample has to be rotated and computed tomography has to be performed [28].
In order to obtain lateral information on the specimen, one can move the focused THz beamrelative to the sample by translating the sample with a gantry [13, 29], drag the THz emit-ter/detector across it [10], or scan the THz beam over a fixed object [30, 31]. Alternatively,single line scanners [32, 33], line scanners with a temporal axis [34] and full field [35–38]imaging systems have been realized. They provide much faster acquistion times than a singlepixel imaging system by removing one or even two scanning axes. However, the THz sourcehas to illuminate a line or the whole object at once, and the scan area is restricted mainly bythe size of the non-linear optical crystal. Hence, single pixel scanning systems are still the bet-ter choice if high signal-to-noise ratios (SNR), dynamic ranges (DR) or large scan regions areneeded.
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23044
In this work, we have realized a phase sensitive THz scanner in reflection geometry, whichallows for scanning of the THz beam over a fixed object. The setup will be explained in detailin section 2. For scanning of the THz beam, a two mirror galvanoscanner in combination witha single scanning lens is used. One of the scanning mirrors is necessarily positioned out of thefocal plane of the lens, which thus leads to a distortion of the image of the scanned object, i.e.,a planar surface or interface of an object appears to be curved along one direction. The effectof bending of the scanning plane is demonstrated in Fig. 1. The imaged sample consists of sixsectors cut from a 2 mm thick aluminum sheet, and a second sheet used as back panel. Thispattern, also known as a Bohler star [39], is the 3d equivalent of the Siemens star for determin-ing the resolution of optical imaging systems. A 3d scanner cannot reproduce the correct heightof the two planes once the spacing between the sectors becomes smaller than the beam diame-ter. Hence, the resolution limit can be determined from the diameter of the central plateau andthe number of sectors. In Fig. 1(a), a photo of the sample and in Fig. 1(b), the correspondingsurface reconstruction obtained from a THz 3d scan are depicted. In one direction (in our casethe vertical), the flat surface is not correctly reproduced, but appears curved. In order to correctthe image, usually a large number of scans have to be performed, covering a scanning volumethat contains the curvature of the surface, followed by data post-processing. This is a severedownside of such scanning systems, as it unnecessarily increases the measurement time.
(a)
vertical horizontal
(b)
Fig. 1. (a) Photo of a Bohler star, which is used as a resolution target. (b) Reconstructionof the sample surface, obtained from the time delay of the reflected THz pulse. The flatsurface appears curved along one axis, which is an artefact due to scanning.
In this work, we present an improved measurement routine for our scanning THz system,which takes care of the problem of the deformed scanning plane already during measurement.By aligning the scanning plane with the surface of the sample, the necessary scanning time isdecreased by one order of magnitude and therefore, efficient THz imaging of planar surfacesand interfaces becomes possible.
2. Materials & methods
2.1. Experimental setup
In Fig. 2, the schematic measurement setup of the scanning THz system is shown, which allowsfor phase sensitive measurements in reflection geometry. The source for the femtosecond gatingpulses is a Mai Tai laser (Newport Spectra-Physics), tuned to 790 nm and delivering 1.15 Waverage power with <100 fs short pulses. We use PCAs as THz emitter and detector, whichare made from low temperature grown gallium arsenide (LT-GaAs). For the emitter, a strip line
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23045
antenna is used. The separation of the electrodes is 10 μm, and the applied voltage is modulatedat 5 kHz. In order to measure the THz beam reflected by the sample, we use a THz beamsplittermade from a 2 mm thick wafer of high resistive float zone silicon (HRFZ-Si) (Tydex). Thepart of the emitted THz pulse that is reflected by the beam splitter is guided towards the target,where it is reflected, passes through the beam splitter and is finally recorded by the detector.The part of the emitted THz pulse that directly passes the beam splitter (dashed lines in Fig. 2)is received by a second detector. This second signal can be measured simultaneously with thebeam from the sample and provides a reference for the emitter. We later refer to these two partsas the reflected and the transmitted THz beam, respectively. The two detectors include a 20 μmlong and 5 μm wide dipole structure [40]. Their signal is amplified with two current amplifiers(DLPCA-200 from Femto) before being fed into a lock-in amplifier (eLockin 203 from An-fatec). Data acquistion is realized by a DAQ card (PCI-6281 from National Instruments) and apersonal computer (Dell), which also controls the translation stage (M-403.8PD from PhysikInstrumente) and the two axes galvanoscanner (model 6900 from Cambridge Technologies).The THz beam is collimated and focused by hyperhemispherical lenses made from HRZF-Si(attached to the PCAs) and off-axis parabolic mirrors.
horizontal
vertical
Fem
tose
cond
lase
r
delay line
Mechanical
Scanning Lens
BS
DetectorsEmitter
THzBS
Target
OAPM
Galvanoscanner
Fig. 2. Schematic drawing of the measurement setup. BS = non polarizing infrared beamsplitter, OAPM = off-axis parabolic mirror, THzBS = THz beam splitter
Special attention is given to the aspheric, rotationally symmetric f-θ scanning lens, which iscustom made from a 20 mm thick slab of polytetrafluoroethylene (PTFE, Teflon). The front andback surfaces are not symmetric and are optimized for a beam diameter of 50 mm, correspond-ing to the clear aperture of the galvanoscanner. The focal length of around 145 mm results fromgeometric requirements (maximum scan angle of ±10◦ and lens diameter of 101.6 mm or 4”).The shape of the lens is the result of a raytracing simulation of the optic system consisting ofthe scanning mirror, the lens and the imaging plane. The merit function, which is minimizedfor optimization, includes 1.) the spot size on a flat focal plane, 2.) the linear dependence of theimage height on the deflection angle, and 3.) the incidence angle of the chief ray of the THzbeam on the focal plane. The realization of the scanning optics with a single lens not only sim-plifies the adjustment of the system, but also maintains a high DR by reducing Fresnel lossesand the optical path length through absorbing material.
For our THz scanning system, the principal mode of operation is transversal or so-calleden-face scanning. There, the two mirrors of the galvanoscanner deflect the THz pulses alongthe horizontal and the vertical direction, respectively, while the delay time is held fixed. Thesescans provide information from a distinct scan depth or about the phase of the THz pulse.As a second measurement mode, cross-sectional scanning along one lateral direction and thedepth axis can be performed. Our system permits a maximum lateral scanning area of about
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23046
100 mm x 100 mm. The scan speed is limited to a few Hertz by the speed of the galvanoscanner,and is typically set to 1 line/s. Depending on the number of lines per image, one en-face scantherefore requires 1–2 minutes. Much faster THz imaging systems and scanners with up to10 lines/s have been have been demonstrated [41]. With a lighter mirror this scan rate would bepossible with our galvanoscanner as well, but at the same time the integration time of the lock-inamplifier needs to be decreased, which leads to a lower dynamic range and noisier images.
As the mirrors of the galvanoscanner act as virtual apertures, one mirror can be positioned inthe back focal point of the scanning lens. This has the advantage that one obtains a telecentricsystem for this direction, i.e., the THz beam always stays parallel to the optical axis. However,the second mirror is necessarily positioned out of the focal plane, and therefore the scanningplane becomes curved along the second scanning axis, since the optical path length of the THzbeam is now dependent on the deflection angle. An estimation of the additional measurementtime due to the bending of the scanning plane can be made, based on the following roughnumbers: the jitter (i.e., the standard deviation of the position of the maximum of the THzpulse), was found to be around 0.01 ps, equivalent to an uncertainty in the depth position ofabout 3 μm. The curvature of the scanning plane amounts to about 2 mm, as was determinedfrom images of the Bohler star. In order to accurately measure the pulse amplitude, the en-face scans are performed every 20 μm, corresponding to a temporal resolution of 133 fs ora Nyquist frequency of 3.75 THz. Therefore, about 100 scans are needed to cover a singlesurface. Without the bending of the scanning plane only 5 scans would suffice to cover a scandepth of 100 μm, a surely adequate number for the flat surface of this specimen. The bendingwill therefore increase the imaging time of a single interface by a factor of 20. In section 2.3.,we will introduce a scanning scheme based on vertical cross-sectional scans, that compensatesthese bending effects already during en-face scanning.
2.2. Dynamic range and quality of the reflection arm
As the mechanical stage delays the laser pulse for the emitting PCA, it is possible to recordboth, the transmitted and the reflected THz beam simultaneously by adjusting the optical pathlengths of the gating pulses for the two receiver antennas (see Fig. 2). A mirror with a diam-eter of 50.8 mm (2”) is used as object for the reflected signal. In Fig. 3(a), the average of 16time-domain traces for the reflection and the transmission arm, respectively, is shown. Everytrace is resampled with equidistant supporting points and a straight line is subtracted by linearregression to eliminate an offset or linear drift of the signal. For calculating the THz spectra, afast Fourier transform (FFT) is applied to the single traces prior to any averaging.
In Fig. 3(b), the spectra of both, the reflected and the transmitted THz signals are shown,normalized by their respective noise levels, which corresponds to the DR of the measurements[42]. It can be seen that the much longer optical pathlength in ambient air, Fresnel losses fromthe THz beam splitter and lens, as well as the double transmission through the PTFE lens entaila smaller DR and useable bandwidth for the reflected beam.
For characterization of the scanning THz system, the maximum DR is determined alonga horizontal and a vertical scan line through the center of the mirror. The DR is calculatedfollowing the procedure described in [43]. The noise level is determined from a measurementwith blocked THz beam as the standard deviation (root-mean-square, RMS) of the THz tracein time-domain (TD). In frequency-domain (FD), the noise level can be calculated as the RMSvalue of the magnitude of the amplitude spectrum of the blocked beam, however, both values(TD and FD) of the noise level correspond to each other. The maximum DR is obtained fromthe ratio DR = mean peak value
noise level in FD at the maximum of the spectrum.The result of these measurements is shown in Fig. 4. The DR stays nearly constant (around
1000 or 60 dB) across the horizontal scan line, up to the edge of the mirror with 2” diameter.
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23047
0 5 10 15 20 25−4
−2
0
2
4
6
delay time [ps]
sign
al [
V]
transmitted pulsereflected pulse
x0.5
(a)
0 1 2 3 4 5 6
100
101
102
103
frequency [THz]
dyna
mic
ran
ge
transmitted beamreflected beamnoise level
(b)
Fig. 3. (a) Time-domain traces for transmitted and reflected THz beam, averaged over 16measurements. The transmitted signal is scaled by a factor 0.5. (b) Average of 16 specta,obtained by Fourier transform for the transmitted and reflected THz beams and scaled bytheir respective noise levels.
The slight sagging in the center can be attributed to higher absorption by the scanning lens,which has its maximum thickness in the center as well. For the vertical scan, the signal drops toaround 40% at the edge of the mirror, which indicates that the THz beam does not stay parallelto the optical axis and therefore is not perpendicularly incident on the target mirror. Thus, thereflected THz pulse does not completely pass through the scanning lens anymore, and somesignal is lost. This is a consequence of the fact that only one mirror can be at the back focalpoint of the scanning lens, i.e., fulfills the requirements for a telecentric system.
-30 -20 -10 0 10 20deflection [mm]
250
500
750
1000
1250
dyna
mic
ran
ge
verticalhorizontal
Fig. 4. Maximum dynamic range as a function of vertical and horizontal beam deflectioncalculated in frequency-domain.
2.3. Phase correction
In our THz imaging setup, the horizontal scanning mirror is located at the back focal point ofthe scanning lens. The out-of-focus position of the vertical scanning mirror generates not onlya loss of amplitude, but also a dependence of the optical path length on the vertical deflection.Because of the phase sensitivity of the THz detection method this prevents an easy en-face scanof a planar surface, i.e., if an xy-scan at fixed delay line position is performed, one would cutthrough different phases of the THz pulse (much like a sagittal cut of an onion).
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23048
In order to overcome this limitation, a cross-sectional scan perpendicular to the sample sur-face is made prior to en-face imaging. Figure 5 shows the THz signal amplitude for (a) verticaland (b) horizontal deflection (abscissa) and position of the delay line (ordinate). The midpointsbetween the maximum and minimum of the THz pulse lie on a straight line for a horizontaltrace (dashed line in Fig. 5(b)). However, a phase shift of the echoes can be observed for thevertical scan. This additional optical path length has a quadratic dependence on the deflectionand can be nicely fitted by a second order polynomial (dashed line in Fig. 5(a)).
−5 −4 −3 −2 −1 0 1 2 3 4 5 6 7122
123
124
125
126
vertical scanner voltage [V]
posi
tion
of d
elay
line
[m
m]
(a)
−10 −8 −6 −4 −2 0 2 4 6 8 10120
120.5
121
121.5
122
horizontal scanner voltage [V]po
sitio
n of
del
ay li
ne [
mm
](b)
Fig. 5. (a) Vertical and (b) horizontal cross-sectional depth scans of a planar surface. Thevertical phase shift can be fitted by a second order polynomial (dashed line).
Hence, it is possible to compensate the non-planarity of the isophase surface already duringscannning if the following scheme is used:
1. The fastest axis of the scan is chosen to be horizontal (typically 1 line per second). Inorder to obtain good results, the surface of interest of the sample needs to be alignedperpendicularly to the optical axis.
2. The second scan direction is vertical. It is advanced one step further once a horizontalscan-line has been finished. With each step, the delay line is moved simultaneously by anamount given by the coefficients determined before by the curve fit. The linear term ofthe polynomial also takes care of a vertical tilt of the specimen (though a large tilt woulddecrease the signal amplitude).
3. The depth scan with the delay line finally makes up the slowest scan direction. It is movedto the next position when a complete en-face scan has been finished.
3. Results
The benefit of this procedure is first demonstrated using the sample (Bohler star) from Fig. 1.From this object 512 en-face scans, each with a resolution of 128x128 pixels, are taken, cov-ering a total scan depth of 8 mm. For each pixel, the position of the absolute maximum of theamplitude is determined. The result is shown in Fig. 6(a), which can be interpreted as the topol-ogy of the sample. Figure 6(b) shows the height profile of the two diagonal cutlines in Fig. 6(a).Note that without additional post-processing the planar surfaces are accurately reproduced, andthe curvature of the image is neglible.
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23049
horizontal deflection [mm]
vert
ical
def
lect
ion
[mm
]
0 10 20 30 40 500
10
20
30
40
50
0 500
1000
1500
2000
2500
height [μm]
(a)
0 10 20 30 40 500
500
1000
1500
2000
2500
distance [mm]
heig
ht [
μm]
(b)
Fig. 6. (a) Position of the absolute maximum of the time signal from the sample in Fig. 1.The dashed circle indicates the limiting gray ring for determining the resolution. (b) Heightprofile from the solid and dashed cutlines in (a).
The diameter of the outer circle of the Bohler star, which amounts to 40 mm, is used to deter-mine the conversion factor between the voltage applied to the two mirrors of the galvanoscannerand the corresponding deflection after the scanning lens by correctly scaling the image (hori-zontally 5.1 mm/V, vertically 5.4 mm/V). The resolution l of the scanning system is given byl = πd
2N , where d is the diameter of the inner ’gray ring’ (dashed circle in Fig. 6(a)), and N = 6is the number of rays of the star. d is determined by calculating the Fourier coefficient of the6th harmonic by inscribing circles of variable radius. The limiting radius is taken at 50% of themaximum value and is found to be d = 10.5 mm. Thus, the resolution of the imaging systemis roughly l = 2.7 mm. With a beam radius of w0 = l/2 ≈ 1.5 mm, the Rayleigh length of the
THz pulse amounts to about zR =πw2
0λ = 23.5 mm (at 1 THz). Thus, the depth of focus is about
b = 2zR = 47 mm (in air) and much larger than typical scan depths. Therefore, the spot sizefor a depth scan can be assumed constant for all practical purposes and a dynamic focus is notneeded.
3.1. Application
The potential of the scanning THz system in combination with the improved scanning routineis demonstrated for a sample consisting of a metal cylinder attached to a 3 mm thick and 59 mmwide plastic plate by an around 0.6 mm thick adhesive layer (see Fig. 7). This is a componentas used in the automobile industry for attaching a plastic part to the metal car body. SinceTHz waves easily penetrate the plastic, it is possible to examine the adhesive layer by scanningthrough the opaque plastic plate, which would not be possible with visible light.
For the subsequent measurements, the parameters for phase correction are obtained froman initial cross-sectional scan of the front surface of the plastic plate. In addition, the pulseamplitude originating from the front surface is used to normalize the images along the verticaldirection, according to the solid curve displayed in Fig. 4.
In the first 3d measurement, a scan volume including the front surface is chosen, giving anoverall impression of the sample. Thus, a total of 512 frames with a scan depth of 10 mm areacquired. The lateral resolution is 128x128 pixels with a scan area of about 65 mm x 80 mm.For the further data analysis, the part of the scanning volume containing the front surface isdiscarded. In Fig. 8(a), the maximum of the pulse amplitude originating from the intact adhesivelayer is shown. The little circle on the right hand side is a nose on the front surface, produced
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23050
Fig. 7. Photo of the disassembled composite material sample. The white plastic plate isshown on the left hand side and is usually attached on top of the black ring of glue. Theholes drilled into the adhesive layer are indicated by white arrows on the right hand side.
from injection molding of the plastic plate. In Fig. 8(b), a single en-face scan is shown, whichis taken at an optical depth corresponding to the plastic/glue interface.
The well pronounced boundaries of the adhesive layer are interference effects between theTHz pulses reflected from the plastic/glue and the plastic/air interfaces. In Fig. 8(a), the highcontrast is due to the fact that the plastic/air interface represents a large change of the refrac-tive index compared to the plastic/glue interface, and hence the amplitude of the reflected THzbeam is larger. In Fig. 8(b), the two regions are even more pronounced, but here the enhancedcontrast can be attributed to the difference between internal and external reflection, i.e., thereflection at a boundary from higher to lower or from lower to higher refractive index, respec-tively. At the plastic/air interface, the refractive index changes from high to low, and hence thereflected light does not undergo a phase change. On the other hand, at the plastic/adhesive layerthe refractive index increases, and therefore the reflected pulse amplitude changes sign. Thisresults in an increased contrast between the two regions in the en-face image. As it is visiblefrom Fig. 8(b), the polynomial coefficients obtained from scanning the front surface cannotcompletely remove the cutting through different phases of the THz beam, suggesting that usingthe plastic/air interface at the back for phase compensation would be more favorable.
After this initial scan, the plastic plate is removed and two holes with diameters of 2 mmand 4 mm are drilled into the adhesive (indicated by the arrows in Fig. 7). The two parts arereattached and the measurement is repeated. The lateral resolution is set again to 128x128pixels with a scan area of about 65 mm x 70 mm, but this time only 64 en-face scans with atotal depth of 1 mm containing the adhesive layer are collected. Although the glue is not asstrong as it was initially, it is still strong enough to hold the metal cylinder. Figure 8(c) showsthe corresponding amplitude image. The two voids from drilling are visible, but also a regionof delamination of the adhesive can be observed left to the nose. Figure 8(d) represents a singleen-face scan. Again, the contrast is higher due to the phase shift when external reflection occurs,and the edges are enhanced such that the drillings are visible even more than in the amplitudeimage.
It should be pointed out that for the last measurement, the covered depth is ten times smallercompared to the previous measurement, and the adhesive layer is only tightly contained in thescanning volume. Nevertheless, the whole information is obtained with the same quality, butwith a reduced measurement time by a factor of about ten. In addition, it is shown that evensingle en-face scans can be used for the investigation of planar structures or interfaces. These
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23051
results clearly underline the effectiveness of our scanning routine for phase correction.
10 mm
(a)
10 mm
(b)
10 mm
(c)
10 mm
(d)
Fig. 8. (a) and (c): THz amplitude images of a ring of adhesive layer between a plastic plateand a metal cylinder. Total acquisition time for (a) was about 18 hours, and 2 1
4 hours for(c). (b) and (d): Single en-face scans, each taking about 2 minutes. (a) and (b) are takenwith intact glue, (c) and (d) (Media 1) after two holes were drilled into the glue and thecomponents reattached. All images are amplitude corrected along the vertical direction. Ananimation through the sequence of enface scans is available online.
4. Conclusion
We have presented a scanning THz system for measurements in reflection geometry. Our designhas the advantage that neither the THz emitter and source, nor the object has to be moved.Instead, an xy-galvanoscanner and a single scanning lens made from PTFE allow performing3d scans of partially transparent objects in the THz wave range.
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23052
For planar structures, we have developed an advanced scanning routine to compensate thebending of the scanning plane. This has the benefit of not only making geometric compensationduring post-processing superfluous, but also of significantly reducing the scanning time becausenon-relevant scanning volume is spared. Therefore, efficient THz imaging becomes feasible.We want to emphasize that this method is not restricted to our system, but could be implementedin other phase sensitive line scanning THz imaging setups.
Furthermore, we have demonstrated that areas of interest can be identified in single en-facescans with our phase compensation enabled. This means that preparation and execution ofmeasurements becomes much more simple and effective. Therefore, as we have shown for aplastic-metal composite sample, this advanced THz scanning system is highly promising forthe investigation of layered parts and components, hopefully bringing THz technology a stepfurther on its route to application.
Acknowledgements
This work has been supported by the Austrian Science Fund FWF (Project L507-N20), by theEuropean Regional Development Fund (EFRE) in the framework of the EU-programme Regio13, and the federal state Upper Austria. Nico Vieweg wants to thank the Studienstiftung desDeutschen Volkes and the Braunschweig International School of Metrology. Benedikt Schergeracknowledges financial support from the Friedrich Ebert Stiftung. S. K. wants to thank INPROInnovationsgesellschaft fur fortgeschrittene Produktionssysteme in der Fahrzeugindustrie mbHfor providing samples and Armin Hochreiner for assisting with sample preparation.
#152275 - $15.00 USD Received 8 Aug 2011; revised 28 Sep 2011; accepted 28 Sep 2011; published 28 Oct 2011(C) 2011 OSA 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23053
![[DE] CDIA+ Certified Document Imaging Architech | Einführung zum Zertifizierungskurs | PROJECT CONSULT](https://static.unterlagen.site/doc/80x56/54b3996f4a79590a308b4582/de-cdia-certified-document-imaging-architech-einfuehrung-zum-zertifizierungskurs-project-consult.jpg)