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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 18 — Aug. 29, 2011
  • pp: 17738–17749
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High-speed terahertz spectroscopic imaging using noncollinear electro-optic sampling and a multistep mirror

Kazunori Maruyama, Norihiko Itani, Shin-ya Hasegawa, and Shinichi Wakana  »View Author Affiliations


Optics Express, Vol. 19, Issue 18, pp. 17738-17749 (2011)
http://dx.doi.org/10.1364/OE.19.017738


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Abstract

We propose a method for high-speed terahertz spectroscopic imaging that is based on electro-optic sampling with a noncollinear geometry of the THz beam and probe laser beam and has a multistep mirror in the path of the probe beam. We made an imaging system that operates in the over 2.0-THz range and enables the sample region corresponding to a (28 × 28)-pixel area on the sensor to be imaged with a spatial resolution of 1.07 mm and a frequency resolution of 0.079 THz. We also show how the proposed method might be extended for faster THz spectroscopic imaging.

© 2011 OSA

1. Introduction

Terahertz waves with frequencies from 0.1 to 10 THz have recently been attracting attention because they penetrate nonconducting materials (e.g., paper, cloth, and plastic), enabling us to identify many materials that have spectral fingerprints in the THz range [1

1. D. M. Mittleman, M. Gupta, R. Neelamani, R. G. Baraniuk, J. V. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B 68(6), 1085–1094 (1999). [CrossRef]

]. Terahertz time-domain spectroscopy (THz-TDS) instrumentation can be used for frequency-resolved imaging [2

2. B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett. 20(16), 1716–1718 (1995). [CrossRef] [PubMed]

], which is useful in security [3

3. D. A. Zimdars, “Fiber-pigtailed terahertz time domain spectroscopy instrumentation for package inspection and security imaging,” Proc. SPIE 5070, 108–116 (2003). [CrossRef]

], medical [4

4. V. P. Wallace, A. J. Fitzgerald, B. C. Cole, R. J. Pye, and D. D. Arnone, “Biomedical applications of THz imaging,” Microwave Symposium Digest, 2004 IEEE MTT-S International 3, 1579–1581 (2004).

], and pharmaceutical [5

5. A. J. Fitzgerald, B. E. Cole, and P. F. Taday, “Nondestructive analysis of tablet coating thicknesses using terahertz pulsed imaging,” J. Pharm. Sci. 94(1), 177–183 (2005). [CrossRef] [PubMed]

] fields as well as in the nondestructive analysis of historical cultural assets [6

6. K. Fukunaga, I. Hosako, Y. Kohdzuma, T. Koezuka, M.-J. Kim, T. Ikari, and X. Du, “Terahertz analysis of an East Asian historical mural painting,” J. Europ. Opt. Soc. Rap. Public. 5, 10024 (2010). [CrossRef]

]. In THz-TDS the frequency spectrum at any point in the sample is obtained by taking the Fourier transform of the time-domain data gathered when the electric field strength is measured while an optical delay line varies the delay between the THz pulse and the optical probe pulse. Because this time-domain data is the result of hundreds of measurements and to get a two-dimensional (2D) spectroscopic image we need to move the sample or measurement system and repeat the measurements at hundreds if not thousands of points in a plane, the data acquisition time is long. It of course depends on the size of the imaged object, the total number of the pixels in the image, and the frequency resolution required but is typically measured in hours. Various methods for reducing the measurement time in THz-TDS imaging have been proposed. A rotary optical delay mechanism with six curved mirror plates, for example, enables data to be generated faster than it can be generated when using a linear stage optical delay line [7

7. G. J. Kim, S. G. Jeon, J. I. Kim, and Y. S. Jin, “High speed scanning of terahertz pulse by a rotary optical delay line,” Rev. Sci. Instrum. 79(10), 106102 (2008). [CrossRef] [PubMed]

]. Also faster than using an optical delay line is asynchronous optical sampling in which two femtosecond lasers each generate and detect THz pulses at slightly different repeat frequencies [8

8. 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). [CrossRef]

]. Neither of these methods, however, eliminates the need to make a time-consuming 2D scan of the object.

The advantage of electro-optic (EO) sampling is that all the information needed to make an amplitude image of an object can be obtained instantly without moving the object [9

9. 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). [CrossRef]

]. In conventional EO sampling, the polarization of an optical probe pulse propagating collinearly with the THz pulse changes by an amount that depends on the strength of the THz electric field in the EO sampling medium. When 2D EO sampling is performed, the 2D field distribution in the medium is converted into a 2D optical intensity distribution after the readout beam passes through a crossed polarizer, and the optical image is captured by a camera. Obtaining a frequency spectrum, however, requires this 2D imaging to be performed a few hundred times using a mechanical stage for the optical delay line [10

10. M. Usami, T. Iwamoto, R. Fukasawa, M. Tani, M. Watanabe, and K. Sakai, “Development of a THz spectroscopic imaging system,” Phys. Med. Biol. 47(21), 3749–3753 (2002). [CrossRef] [PubMed]

], so the use of conventional EO sampling is still very time consuming. The optical delay line can be eliminated and the temporal waveforms captured in a single shot by making the THz beam and the probe laser beam being incident on the EO crystal in a noncollinear geometry [11

11. J. Shan, A. S. Weling, E. Knoesel, L. Bartels, M. Bonn, A. Nahata, G. A. Reider, and T. F. Heinz, “Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling,” Opt. Lett. 25(6), 426–428 (2000). [CrossRef] [PubMed]

]. The THz color scanner [12

12. 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). [CrossRef] [PubMed]

] uses this noncollinear EO sampling method for THz-TDS imaging but still requires the object to be scanned in one direction.

We have therefore developed an optical system that makes it unnecessary to move the object. The method this system uses is based on noncollinear EO sampling and uses a multistep mirror (MSM) for parallel measurements of temporal waveforms at regions on the surface of an EO crystal. In Section 2 of this paper we describe an MSM that ensures that the probe laser pulse front is the same at each of the regions on an EO crystal on which probe laser beam and THz beam are incident. In Section 3 we show an experimental system for spectroscopic imaging, and in Section 4 we explain the results of experiments with that system.

2. Multistep mirror device

2.1 Basic concept

We first describe the noncollinear EO sampling method that can measure temporal waveforms of THz beams without using optical delay lines [11

11. J. Shan, A. S. Weling, E. Knoesel, L. Bartels, M. Bonn, A. Nahata, G. A. Reider, and T. F. Heinz, “Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling,” Opt. Lett. 25(6), 426–428 (2000). [CrossRef] [PubMed]

, 12

12. 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). [CrossRef] [PubMed]

]. As shown in Fig. 1
Fig. 1 Conceptual diagram of temporal waveform measurement by noncollinear EO sampling (CL: cylindrical lens, P: polarizer, A: analyzer).
, in this method the probe laser beam pulse is incident on the EO crystal at an angle of incidence θ and the THz beam focused on the object and passing through it is collimated and is incident on the EO crystal perpendicularly. At the distance x from a point at which the THz pulse front and the probe laser pulse front coincide on the EO crystal, the optical path difference (xtanθ) is proportional to x. Thus the probe laser beam that passes through the EO crystal forms an image on a camera and the temporal waveform f(t) corresponding to a pixel position in the x direction can be obtained without using an optical delay line. The Fourier transform of f(t) is the mathematical representation of the frequency spectrum. Spatial information in the y' direction can be obtained directly from the pixel data in the y direction on the camera. Therefore, a spectroscopic image can be obtained by scanning the object in only the x' direction.

The method we have devised, in contrast, requires no scanning at all. As shown in Fig. 2
Fig. 2 Parallel temporal waveform measurement using an MSM.
,

  • 1) A collimated THz beam passes through the object and
  • 2) The relation between the pulse front of the probe laser beam and the THz beam is kept the same in each of the regions in the x' direction.

To keep the same relation between the pulse front of the probe laser beam and the THz beam in number k of regions in the x' direction, we added a multistep mirror to the noncollinear EO sampling system to create a pulse front that has k steps of equal width. Thus, points at which the THz beam and the probe laser pulse fronts coincide are spaced equidistantly on the EO crystal surface, and from each of the k regions the temporal waveform in the x direction is obtained. Spatial information in the y' direction is also obtained from this image. Since the time window of the temporal waveform is reduced to 1/k, we considered three methods for broadening the time window (see Fig. 3
Fig. 3 Conceptual diagrams of three methods of expanding the time windows of temporal waveforms: (a) using an optical delay line, (b) using a rotary plate with glass plates of various thicknesses, (c) using MSMs arrayed in the y' direction.
). In the first method, shown in Fig. 3(a), an optical delay line is used to obtain an image at every time window T and the total temporal waveform is synthesized from these images. With this method, it is possible to freely expand the total time window of the temporal waveforms by changing the number of times that the optical delay line is moved. In the second method, shown Fig. 3(b), the femtosecond laser beam sequentially passes through glass plates with different thicknesses on a rotary plate, and optical delays are produced by the difference between the refractive indexes of glass and air. Images are acquired faster this way because the rotary movement in method 2 is faster than the linear movement in method 1. The third method, shown in Fig. 3(c), involves repeated measurements of temporal waveforms within each region (“cell”) by using MSMs arrayed in the step-height direction so that the probe pulse front also has steps in the y' direction. This method enables THz spectroscopic imaging without an optical delay line. Several variations of these methods are possible, but in this paper, we focus on confirmation of the feasibility of using an MSM to produce pulse front steps in the x' direction and use the first method to extend the time window of the temporal waveform. Although time delays could also be produced by using a transparent optical element such as a multistep glass block, a reflective optical element has the advantage of not dispersing the femtosecond laser pulse.

2.2 Design of multistep mirror

To design a mirror that has multiple steps in the x' direction, the first thing we did was to calculate the angle θ needed for obtaining the desired resolution Δt of the temporal waveform. If the probe laser beam passing through an EO crystal of width D forms an image on N pixels of a CMOS camera with sensor pitch p as shown in Fig. 4
Fig. 4 Relationships between THz beam, probe laser, EO crystal, and CMOS camera.
, the following relation holds:
DcosθN=pM,
(1)
where M is the image-formation magnification. The relation between the resolution Δt of the temporal waveform and the noncollinear angle is given by:
DsinθN=cΔt,
(2)
where c is the speed of light.

The temporal waveform resolution Δt from the angle θ and the magnification M can be determined by Eqs. (1) and (2). The frequency range of the THz spectrum determined from the Fourier transform of the temporal waveforms is 1/(2Δt) and the waveform time resolution Δt is set to 0.10 ps to enable measurement of the 5-THz frequency range. If D is 30 mm, p is 20 µm, and N is 448, noncollinear angle θ is 27 degrees and image-formation magnification M is 0.33. Note that the number of pixels in the y' direction differs from those in the x' direction, because the EO crystal surface is not perpendicular to the probe laser beam. The number of pixels in the y' direction is DM/p and corresponds to 504 pixels. We will explain the reason why N is 448 in the end of this section.

w'+d'=Dcosθk.
(4)

Since the step height d and the step width w are given by d'=2d and w'=(wd)/2, we can determine d and w of the MSM from Eqs. (3) and (4). The time window T is w'/(w' + d') times narrower than it would be if the probe laser beam was incident perpendicularly to the MSM steps, and we can use Eqs. (2) - (4) to obtain the following equation:

T=w'w'+d'NΔtk={1NcΔtD2(NcΔt)2}NΔtk.
(5)

If we assume the number of cells k is 14, step height d is calculated to be 0.68 mm and step width w is calculated to be 2.03 mm. Therefore, both width w' and the unused width (d') are calculated to be 0.96 mm, and the utilized duty of the probe laser beam, w'/(w' + d'), is 50%. Because resolution Δt is 0.10 ps, we can also determine time window T is 1.6 ps from Eq. (5). In other words, temporal waveforms are measured on each cell by using 16 camera pixels ( = 448 pixels/14 × 50%). Here, we assume that the number of pixels of the CMOS camera is 512 × 512 pixels. The maximum value of N is 448 because N should be divisible by 14 (the number of cells) and the number of pixels in the y' direction should be less than 512 pixels.

2.3 Lens system correcting the effects of diffraction by the multistep mirror

Calculated and measured intensity distributions of an 800-nm laser beam diffracted by a slit 0.96 mm wide are shown in Fig. 6(a)
Fig. 6 Distributions of diffracted beam intensity obtained (a) without and (b) with correction lenses.
. The full width at half maximum of the beam diameter decreases with distance from the slit, is smallest at 300 mm, and then again increases with distance. The distance R between the MSM surface and the EO crystal is limited to 400 mm by our optical table. These results therefore indicated that the MSM should be placed as close as possible to the EO crystal in order to ensure that the width of the beam on the EO crystal is close to the width of the MSM step. Because in our noncollinear EO sampling system the MSM cannot be closer than 100 mm from the crystal, we use a telecentric lens system to correct the effect of diffraction. The experimental result shows that if a distance from the slit to the diffraction-correction lens system is the focal length (100 mm), the width of the plateau in the intensity distribution at R' (100 mm) from the exit of the diffraction-correction lens system is close to the width of the slit shape [see Fig. 6(b)]. This result indicates that a beam profile on the EO crystal almost the same as that on the MSM surface can be obtained by using the diffraction-correction lens system. Nevertheless, taking account of problems such as chipping at the edges of the fabricated MSM, we neglect the single pixels at each end and use only the center 14 pixels in a cell for measurement of the temporal waveforms.

3. Experimental setup

The experimental system is shown schematically in Fig. 7
Fig. 7 Conceptual diagram of experimental system (PCA: photoconductive antenna, TL: THz lens, BS: beam splitter, P: polarizer, A: analyzer, FG: function generator).
. A THz beam is generated by a photoconductive antenna (BATOP Gmbh, PCA-40-05-10-800-h) driven by pulses emitted by a Ti-sapphire femtosecond laser (Mai Tai by Spectra Physics, pulse energy = 13 nJ, pulse duration = 100 fs, central wavelength = 800 nm, repetition rate = 80 MHz). A bias voltage of ± 40 V is applied to the photoconductive antenna with a modulation frequency of 125 Hz. The THz beam is collimated to a beam diameter of 60 mm by an aspherical plano-convex lens (Tsurupica, by Pax Co., focal length = 112 mm) and forms an image (at 1:1) on the EO crystal after passing through two THz aspherical plano-convex lenses (focal length = 120 mm). The imaging lens system is designed in such a way that the image spot at maximum image height on the EO crystal is not larger than the diffraction-limit spot size at 1 THz (diameter = 1.97 mm). The EO crystal is a (110) ZnTe single crystal (30 mm × 30 mm, 1.5 mm thick). The probe laser beam passes through a polarizer and a Berek compensator, is expanded to a diameter of 60 mm, is reflected by the MSM, passes through the diffraction-correction lens system, and is incident on the EO crystal. The diffraction-correction lens system is two aspherical plano-convex lenses having a focal length of 100 mm. The probe laser beam that passes through the EO crystal is incident on a CMOS camera (Intelligent Vision System, by Hamamatsu Photonics K.K., number of pixels = 512 × 512 pixels, pixel size = 20 µm × 20 µm, frame rate = 250 fps, digital output = 12 bits) with macro lens (the image-formation magnification = 0.33). The CMOS camera is synchronized with the bias voltage of the photoconductive antenna, and an image is acquired at 125 fps by a dynamic subtraction technique [14

14. 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). [CrossRef] [PubMed]

]. The Berek compensator retards the phase of the probe laser by about 7 degrees from zero bias in order to ensure that the sampling sensitivity increases within a range in which the CMOS sensor is not saturated. The diameters of the THz beam and the probe laser beam are expanded larger than the EO crystal size to reduce the intensity distribution as possible.

4. Results and discussion

The main peaks of the temporal waveforms measured with no object in the THz beam are shown in Fig. 9
Fig. 9 Electrical field image acquired by dynamic subtraction method. 500 frames were integrated.
, where green represents positive and red represents negative amplitudes of the electrical field. It is confirmed that the temporal waveforms for each of 14 cells (equivalent to the number of steps of the MSM in the horizontal direction of the image) are displayed. The dark areas in the four corners are due to blocking of the THz beam and probe laser beam by the metal plate holding the EO crystal. The retardation distribution due to both residual birefringence of the EO crystal and oblique incidence of the probe laser beam to lens surfaces [15

15. H. Kubota and S. Inoue, “Diffraction images in the polarizing microscope,” J. Opt. Soc. Am. 49(2), 191–198 (1959). [CrossRef] [PubMed]

] should be corrected in the captured image. Hence we correct the image using the method in Ref. 16

16. T. Hattori and M. Sakamoto, “Deformation corrected real-time terahertz imaging,” Appl. Phys. Lett. 90(26), 261106 (2007). [CrossRef]

, and the corrected image is shown in Fig. 9.

To evaluate the effects of the diffraction-correction lens system, we measured the distribution of sampling sensitivity in a cell. We compared the relative sampling sensitivity of each cell, using the maximum value of the temporal waveform, while moving the optical delay line in 0.10-ps steps. The shift of the maximum values of the temporal waveforms at the 7th line from the left in Fig. 9, which corresponds to the optical axis of the diffraction-correction lens system, and the lines at the edges furthest from the axis (the 1st and 14th lines from the left in Fig. 9) are shown in Fig. 10
Fig. 10 Sampling sensitivity distribution within cell.
. The lowest sampling sensitivity in the 16-pixel region illuminated by the probe laser was 43%, but the lowest sampling sensitivity in the central 14-pixel region was 70%, confirming the effectiveness of the diffraction-correction lens system. Since the probe laser is incident on the EO crystal obliquely, at the two edges away from the optical axis the image-formation position of the diffraction-correction lens system varies by a as much as ± 14 mm. The profiles there are almost the same as the profile at the optical axis, so this system is suitable for practical use. Because of this position variability at the edges, though, in the experiments we set the time window to 1.4 ps (14 pixels).

We then evaluated (28 × 28)-cell spectroscopic imaging with no object in the THz beam. The optical delay line is moved 9 times in steps of 210 µm. With this condition, the time window of the temporal waveform is 12.6 ps, with a resolution of 0.10 ps. As a result, we can obtain a frequency spectrum of over 2.0-THz range with a resolution of 0.079 THz. Figure 11(a)
Fig. 11 (a) Temporal waveforms and (b) THz Spectrum at the center cell in absence of the object (fast Fourier transform with zero-filling up to 1024 data points).
and 11(b) show the temporal waveform and the THz spectrum at the center cell by integrating 500 frames. The signal to noise (SN) ratio of temporal waveform, which is the ratio of the peak-to-peak amplitude to the standard deviation of the data before main peak, is 35. The absorption peaks indicated by arrows in the THz spectrum in Fig. 11(b) are at 1.10, 1.20, 1.40, and 1.66 THz, and these values correspond to the water-vapor absorption peak pattern in the NASA database [17

17. M. Exter, Ch. Fattinger, and D. Grischkowsky, “Terahertz time-domain spectroscopy of water vapor,” Opt. Lett. 14(20), 1128–1130 (1989). [CrossRef] [PubMed]

, 18

18. W. Withayachumnankul, B. M. Fischer, S. P. Mickan, and D. Abbott, “Numerical removal of water-vapor effects from THz-TDS measurements,” Proc. R. Soc. A 464, 2435–2456 (2008). [CrossRef]

]. Thus we confirmed the accuracy of the measured THz spectrum.

With 764 cells (excluding the 5 cells in each of the four corners), the in-plane dispersion of the peak-to-peak amplitude of the temporal waveforms, calculated by dividing the standard deviation by the mean, was 29%. This was caused by the power distribution of the THz beam generated from a photoconductive antenna. Due to the interface reflection between the GaAs substrate and Si lens, the power at the center of the Si lens is lower than the area around the center [19

19. G. D. Boreman, A. Dogariu, C. Christodoulou, and D. Kotter, “Dipole-on-dielectric model for infrared lithographic spiral antennas,” Opt. Lett. 21(5), 309–311 (1996). [CrossRef] [PubMed]

]. The standard deviation of the times at the maximum of the temporal waveforms, on the contrary, was 0.11 ps. We assume that this variation is caused by the dimension error of the MSM, the wavefront aberration of the THz and/or probe laser beam, and quantization error due to the measurement of camera pixels. When the least significant bit is q, quantization error is normally-distributed in a range of ± q/2. Quantization error has a mean of zero and the standard deviation of quantization error σ QE, is given by:
σQE=(εε¯)p(ε)dε=q/2q/2ε21qdε=q/12,
(8)
where ε¯ is the mean of quantization error and p(ε) is the probability function of the normal distribution. Since the least significant bit q is 0.10 ps, the peak time variation caused by quantization error σ QE is 0.029 ps. The ratio of quantization error to the peak time variation is comparatively small. Therefore, the variation is largely caused by the first two factors.

The results obtained when measuring a test sample by integrating 500 frames are shown in Fig. 12
Fig. 12 (a) Test sample used for THz imaging, (b) Amplitude image of temporal waveforms measured by this method, (c) Temporal waveforms at A and B in Fig. 12(b), (d) Spectrum at A in Fig. 12(b).
. The sample, shown in Fig. 12(a), is an aluminum plate into which 2-mm-wide lines forming letters were cut. Figure 12(b) shows an amplitude image that is the peak-to-peak amplitudes of the temporal waveforms converted to 8-bit grayscale. The letters can be clearly recognized. The temporal waveforms of two cells indicated by rectangular frames in Fig. 12(b) are shown in Fig. 12(c), and the THz spectrum at cell A in Fig. 12(b) is shown in 12(d). The THz spectrum has the absorption peaks as is the case in absence of the object.

The pure image-acquisition time was 72 seconds: 4 seconds × 9 images × 2 (due to one movement of the sample), when the CMOS camera operated at 125 fps. Because moving the sample took one second and moving the delay-line stage for each image took 0.5 seconds, the overall measurement time was 81 seconds. It should be noted that it was necessary to move the object once because we used an MSM that made the utilized duty of the beam 50%. If an MSM of 28 steps with reflective surfaces perpendicular to the optical axis was used, the object would not have to be moved and the measurement time would be only 40 seconds.

A THz beam generated by a low-peak-power Ti-sapphire femtosecond laser and a photoconductive antenna was used in our experiments, but the measurement time could be shortened by instead using a high-peak-power laser and a nonlinear optical crystal. As an example we estimate what the measurement time would be when using the high-intensity THz beam generated by a regenerative amplified femtosecond laser (Spectra-Physics Hurricane, pulse energy = 600 µJ, pulse duration = 150 fs, central wavelength = 800 nm, repetition rate = 1 kHz, beam diameter = 10 mm) and a nonlinear optical crystal, ZnTe, in an experimental setup similar to that described in Ref. 12

12. 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). [CrossRef] [PubMed]

. We assume in this case that the MSM has reflective surfaces perpendicular to the optical axis (see Fig. 2) and it is thus not necessary to move the object. In Ref. 12

12. 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). [CrossRef] [PubMed]

the same CMOS camera that was used in our experimental system was used and the image-acquisition time with the camera driven at a frame rate of 125 fps is 8 ms, so the measurement time for synthesizing the temporal waveforms from 9 images would be 72 ms. If a usual stepping motor stage for the optical delay line was used, however, 500 ms would be necessary for each delay. So the total time would be 4.072 seconds and the measurement time would depend largely on the mechanical stage movement time. If the same range of delay times was obtained by using the method shown in Fig. 3(b), however, the measurement time would be only about 72 ms.

Finally, we show the result of THz spectroscopic imaging of metal hole arrays (MHAs, Mutsumi Corporation). A MHA is a metal plate with circular holes arranged in an equilateral-triangle lattice and acts as a bandpass filter in THz range. We imaged MHAs whose center frequencies were 0.4 THz (pitch = 0.75 mm, hole diameter = 0.50 mm) and 1.0 THz (pitch = 0.30 mm, hole diameter = 0.13 mm), and the other areas in the imaging field were covered by aluminum plates with no holes [see Fig. 13(a)
Fig. 13 (a) a photograph of metal hole arrays, (b) spectroscopic images at 0.40 THz (left) and 1.00 THz (right), (c) spectra at A and B in Fig. 13(b).
]. Figure 13(b) shows spectroscopic imaging results obtained by integrating 2000 frames. Each MHA is bright at the center frequency and distinguished from the other area. Figure 13(c) shows the spectra at cells A and B in Fig. 13(b). The number of images integrated affects image quality, such as uniformity and contrast. Figure 14
Fig. 14 Results obtained integrating various numbers of frames in the spectroscopic imaging of 0.4-THz and 1.0-THz MHAs. Contrast ratio is calculated from contrast values in the MHA area (solid line) and aluminum-plate area (dashed line).
shows the spectroscopic imaging results obtained by integrating different numbers of frames. For 500, 2000, and 5000 frames the contrast ratios of the 0.4-THz MHA area to the aluminum plate area were respectively 2.0, 3.1, and 6.0. We infer from this that the integration of a larger number of frames results in an image with higher contrast.

5. Conclusion

References and links

1.

D. M. Mittleman, M. Gupta, R. Neelamani, R. G. Baraniuk, J. V. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B 68(6), 1085–1094 (1999). [CrossRef]

2.

B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett. 20(16), 1716–1718 (1995). [CrossRef] [PubMed]

3.

D. A. Zimdars, “Fiber-pigtailed terahertz time domain spectroscopy instrumentation for package inspection and security imaging,” Proc. SPIE 5070, 108–116 (2003). [CrossRef]

4.

V. P. Wallace, A. J. Fitzgerald, B. C. Cole, R. J. Pye, and D. D. Arnone, “Biomedical applications of THz imaging,” Microwave Symposium Digest, 2004 IEEE MTT-S International 3, 1579–1581 (2004).

5.

A. J. Fitzgerald, B. E. Cole, and P. F. Taday, “Nondestructive analysis of tablet coating thicknesses using terahertz pulsed imaging,” J. Pharm. Sci. 94(1), 177–183 (2005). [CrossRef] [PubMed]

6.

K. Fukunaga, I. Hosako, Y. Kohdzuma, T. Koezuka, M.-J. Kim, T. Ikari, and X. Du, “Terahertz analysis of an East Asian historical mural painting,” J. Europ. Opt. Soc. Rap. Public. 5, 10024 (2010). [CrossRef]

7.

G. J. Kim, S. G. Jeon, J. I. Kim, and Y. S. Jin, “High speed scanning of terahertz pulse by a rotary optical delay line,” Rev. Sci. Instrum. 79(10), 106102 (2008). [CrossRef] [PubMed]

8.

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). [CrossRef]

9.

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). [CrossRef]

10.

M. Usami, T. Iwamoto, R. Fukasawa, M. Tani, M. Watanabe, and K. Sakai, “Development of a THz spectroscopic imaging system,” Phys. Med. Biol. 47(21), 3749–3753 (2002). [CrossRef] [PubMed]

11.

J. Shan, A. S. Weling, E. Knoesel, L. Bartels, M. Bonn, A. Nahata, G. A. Reider, and T. F. Heinz, “Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling,” Opt. Lett. 25(6), 426–428 (2000). [CrossRef] [PubMed]

12.

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). [CrossRef] [PubMed]

13.

M. Born and E. Wolf, Principles of Optics (Cambridge University Press, 1959), Chap. VII.

14.

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). [CrossRef] [PubMed]

15.

H. Kubota and S. Inoue, “Diffraction images in the polarizing microscope,” J. Opt. Soc. Am. 49(2), 191–198 (1959). [CrossRef] [PubMed]

16.

T. Hattori and M. Sakamoto, “Deformation corrected real-time terahertz imaging,” Appl. Phys. Lett. 90(26), 261106 (2007). [CrossRef]

17.

M. Exter, Ch. Fattinger, and D. Grischkowsky, “Terahertz time-domain spectroscopy of water vapor,” Opt. Lett. 14(20), 1128–1130 (1989). [CrossRef] [PubMed]

18.

W. Withayachumnankul, B. M. Fischer, S. P. Mickan, and D. Abbott, “Numerical removal of water-vapor effects from THz-TDS measurements,” Proc. R. Soc. A 464, 2435–2456 (2008). [CrossRef]

19.

G. D. Boreman, A. Dogariu, C. Christodoulou, and D. Kotter, “Dipole-on-dielectric model for infrared lithographic spiral antennas,” Opt. Lett. 21(5), 309–311 (1996). [CrossRef] [PubMed]

OCIS Codes
(120.6200) Instrumentation, measurement, and metrology : Spectrometers and spectroscopic instrumentation
(300.6495) Spectroscopy : Spectroscopy, teraherz
(110.6795) Imaging systems : Terahertz imaging

ToC Category:
Imaging Systems

History
Original Manuscript: May 31, 2011
Revised Manuscript: August 4, 2011
Manuscript Accepted: August 4, 2011
Published: August 25, 2011

Citation
Kazunori Maruyama, Norihiko Itani, Shin-ya Hasegawa, and Shinichi Wakana, "High-speed terahertz spectroscopic imaging using noncollinear electro-optic sampling and a multistep mirror," Opt. Express 19, 17738-17749 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-18-17738


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References

  1. D. M. Mittleman, M. Gupta, R. Neelamani, R. G. Baraniuk, J. V. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B68(6), 1085–1094 (1999). [CrossRef]
  2. B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett.20(16), 1716–1718 (1995). [CrossRef] [PubMed]
  3. D. A. Zimdars, “Fiber-pigtailed terahertz time domain spectroscopy instrumentation for package inspection and security imaging,” Proc. SPIE5070, 108–116 (2003). [CrossRef]
  4. V. P. Wallace, A. J. Fitzgerald, B. C. Cole, R. J. Pye, and D. D. Arnone, “Biomedical applications of THz imaging,” Microwave Symposium Digest, 2004 IEEE MTT-S International 3, 1579–1581 (2004).
  5. A. J. Fitzgerald, B. E. Cole, and P. F. Taday, “Nondestructive analysis of tablet coating thicknesses using terahertz pulsed imaging,” J. Pharm. Sci.94(1), 177–183 (2005). [CrossRef] [PubMed]
  6. K. Fukunaga, I. Hosako, Y. Kohdzuma, T. Koezuka, M.-J. Kim, T. Ikari, and X. Du, “Terahertz analysis of an East Asian historical mural painting,” J. Europ. Opt. Soc. Rap. Public.5, 10024 (2010). [CrossRef]
  7. G. J. Kim, S. G. Jeon, J. I. Kim, and Y. S. Jin, “High speed scanning of terahertz pulse by a rotary optical delay line,” Rev. Sci. Instrum.79(10), 106102 (2008). [CrossRef] [PubMed]
  8. 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). [CrossRef]
  9. 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). [CrossRef]
  10. M. Usami, T. Iwamoto, R. Fukasawa, M. Tani, M. Watanabe, and K. Sakai, “Development of a THz spectroscopic imaging system,” Phys. Med. Biol.47(21), 3749–3753 (2002). [CrossRef] [PubMed]
  11. J. Shan, A. S. Weling, E. Knoesel, L. Bartels, M. Bonn, A. Nahata, G. A. Reider, and T. F. Heinz, “Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling,” Opt. Lett.25(6), 426–428 (2000). [CrossRef] [PubMed]
  12. T. Yasui, K. Sawanaka, A. Ihara, E. Abraham, M. Hashimoto, and T. Araki, “Real-time terahertz color scanner for moving objects,” Opt. Express16(2), 1208–1221 (2008). [CrossRef] [PubMed]
  13. M. Born and E. Wolf, Principles of Optics (Cambridge University Press, 1959), Chap. VII.
  14. 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). [CrossRef] [PubMed]
  15. H. Kubota and S. Inoue, “Diffraction images in the polarizing microscope,” J. Opt. Soc. Am.49(2), 191–198 (1959). [CrossRef] [PubMed]
  16. T. Hattori and M. Sakamoto, “Deformation corrected real-time terahertz imaging,” Appl. Phys. Lett.90(26), 261106 (2007). [CrossRef]
  17. M. Exter, Ch. Fattinger, and D. Grischkowsky, “Terahertz time-domain spectroscopy of water vapor,” Opt. Lett.14(20), 1128–1130 (1989). [CrossRef] [PubMed]
  18. W. Withayachumnankul, B. M. Fischer, S. P. Mickan, and D. Abbott, “Numerical removal of water-vapor effects from THz-TDS measurements,” Proc. R. Soc. A464, 2435–2456 (2008). [CrossRef]
  19. G. D. Boreman, A. Dogariu, C. Christodoulou, and D. Kotter, “Dipole-on-dielectric model for infrared lithographic spiral antennas,” Opt. Lett.21(5), 309–311 (1996). [CrossRef] [PubMed]

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