Ex) Article Title, Author, Keywords
Current Optics
and Photonics
Ex) Article Title, Author, Keywords
Curr. Opt. Photon. 2022; 6(6): 619-626
Published online December 25, 2022 https://doi.org/10.3807/COPP.2022.6.6.619
Copyright © Optical Society of Korea.
Hwan Sik Kim1, Jangsun Kim2, Yeong Hwan Ahn1
Corresponding author: *ahny@ajou.ac.kr, ORCID 0000-0002-8563-076X
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
In this study, we develop a three-dimensional (3D) terahertz time-of-flight (THz-TOF) imaging technique with a large depth range, based on asynchronous optical sampling (ASOPS) methods. THz-TOF imaging with the ASOPS technique enables rapid scanning with a time-delay span of 10 ns. This means that a depth range of 1.5 m is possible in principle, whereas in practice it is limited by the focus depth determined by the optical geometry, such as the focal length of the scan lens. We characterize the spatial resolution of objects at different vertical positions with a focal length of 5 cm. The lateral resolution varies from 0.8–1.8 mm within the vertical range of 50 mm. We obtain THz-TOF images for samples with multiple reflection layers; the horizontal and vertical locations of the objects are successfully determined from the 2D cross-sectional images, or from reconstructed 3D images. For instance, we can identify metallic objects embedded in insulating enclosures having a vertical depth range greater than 30 mm. For feasible practical use, we employ the proposed technique to locate a metallic object within a thick chocolate bar, which is not accessible via conventional transmission geometry.
Keywords: Terahertz spectroscopy and imaging, Three-dimensional imaging
OCIS codes: (110.6795) Terahertz imaging; (110.6880) Three-dimensional image acquisition; (300.6495) Spectroscopy, terahertz
Terahertz (THz) spectroscopy has emerged as a powerful tool for nondestructive inspection applications, such as biomedical diagnosis, device inspection, and characterization of materials [1–12]. In particular, THz imaging allows for the characterization of the internal structures of objects, which is possible because nonconducting materials are transparent to THz waves [13–23]. Imaging using continuous-wave (CW) sources has increased in popularity, owing to recent technological advances in the development of sources and detectors [24–32]. Although coherent THz tomography based on frequency-modulated CW (FMCW) methods has been developed using tunable or ensemble sources, it is limited in terms of the general depth and longitudinal resolution [33–37]. In contrast, the use of time-domain spectroscopy (TDS) enables the collection of information such as the depth and range of objects with superior resolution, without the need to sweep the frequency [6, 38, 39]. In other words, THz pulsed imaging provides direct 3D mapping of an object by using the time of flight (TOF) of the reflected THz pulses. Importantly, the longitudinal resolution of a THz-TOF imaging system is determined by the temporal width of the pulsed THz source, which yields resolution as good as 150 μm.
Recently we introduced a high-speed nondestructive THz-TOF imaging technique for the inspection of packaged integrated circuits and metal corrosion beneath paint layers [38, 39]. An imaging rate of 100 Hz/pixel has been achieved based on the THz–TDS technique using the optical sampling by cavity tuning (OSCAT) system [40, 41], which delivers a rapid time delay with a full range of 50 ps. The OSCAT system uses a single pulsed fiber source split into two pulses, used for the THz antenna and receiver respectively, and the rapid time delay between the two pulses is generated by varying the repetition rate of the fiber laser. Although the OSCAT system delivers stable THz–TDS signals with relatively low jitter between the two pulses, the range of the time delay is limited, especially for fast scanning (>100 Hz), because it is based on a single laser source. In addition, because the time delay varies sinusoidally, the linear-response range reaches only approximately 20 ps; this corresponds to an optical path length of 3 mm. Therefore, a THz-TOF system based on OSCAT methods is restricted in nondestructive imaging applications that require large depth profiling.
In contrast, THz-TDS systems with asynchronous optical sampling (ASOPS) use two fiber lasers, with repetition rates that can be adjusted independently [42, 43]. In this case, the range of the time delay is limited by the time window between consecutive pulses; thus the time delay reaches 10 ns for a laser repetition rate of 100 MHz. Conversely, the A-scan rate (100–500 Hz) is determined by the repetition-rate difference between the two fiber lasers. This has numerous advantages, because 3D TOF images with virtually unlimited depth range can be obtained. However, it is still restricted by the optical geometry of the imaging system, such as the depth of focus of the region of interest. THz-TOF imaging with the electronically controlled optical sampling (ECOPS) technique has also been introduced, using two independent femtosecond lasers; however, it allows a finite range of time delay, as in the case of the OSCAT system [44, 45].
In this study, we report rapid THz-TOF imaging results based on the ASOPS technique, which delivers a large vertical scan range of more than 30 mm when a focused lens of 50 mm is used. The lateral resolution is addressed as a function of the longitudinal location of the object. Further, we obtain nondestructive TOF images of metallic objects embedded in insulating enclosures, such as polyethylene foam and chocolate.
A schematic of the THz-TOF imaging equipment is illustrated in Fig. 1. For the rapid THz–TDS system we use a commercial ASOPS instrument (TERA-ASOPS; Menlo Systems GmbH, Planegg, Germany) consisting of two independent femtosecond fiber lasers with a wavelength of 1,560 nm, operating with a repetition rate of 100 MHz. The ASOPS system allows the full time-delay range of 10 ns with a rate of 100–500 Hz; we remained fixed at 100 Hz throughout the experiments. In contrast, in the OSCAT system the time-delay scan range is limited to 50 ps, and in particular the region of linear response is restricted to 20 ps, as mentioned previously. Therefore, THz-TOF with ASOPS has a great advantage in terms of large depth profiling.
We incorporate a galvano scanner (Scanlab GmbH, Puchheim, Germany), which enables us to obtain images with a speed that is limited by the ASOPS THz-TDS system, whereas the combined galvano-scanning (
We first characterize the focused beam, to address the vertical extent available for THz-TOF imaging. As shown in Fig. 2(a), we use a metal plate with a knife edge to establish the lateral resolution of the system. We obtain THz images for different heights
Figure 2(c) illustrates the line profile of the reflected amplitude signal as a function of lateral position
Figure 3 shows THz-TOF results to prove the usefulness of the ASOPS system. The test sample is a metal block with three rectangular holes of lateral size 10 mm × 10 mm and depth 11.5 mm, as shown in Fig. 3(a). In Fig. 3(b), the reflected THz signals are shown as a function of the lateral position (C-scan), with a scan range of 15 × 45 mm2 and a pixel number of 50 × 150. Here we show the time-integrated THz amplitudes; in other words, the THz envelope signal (obtained via Hilbert transformation) has been time-integrated for each pixel over the entire time-delay range of 120 ps. This corresponds to an optical path length of 18 mm. We clearly identify in Fig. 3(b) the bright regions for both the top (green) and bottom (red) metal surfaces, whereas the dark region (blue) appears in the middle because of the presence of a circular hole. The bottom surface is brighter than the top surface because the focal plane is closer to the bottom layer. We also note that reflection is suppressed when the reflecting surface is inclined, as shown at the periphery of the circular hole [the region enclosed by the dashed orange line in Fig. 3(a)].
THz–TDS allows us to address the depth profiling directly from the B-scan images, as shown in Fig. 3(c). The THz signal is plotted as a function of position
Figure 4 shows the THz-TOF imaging result for one of the test samples, in which metallic objects are embedded in a polyethylene-foam enclosure. As shown in the photograph in Fig. 4(a), the test sample consists of a series of five metal blades (size 10 × 4.5 mm2) stacked vertically and displaced by 5 mm with respect to one another. They are placed between sheets of low-density-polyethylene foam (thickness 5 mm). The size of the enclosure surrounding the metal blades is 25 mm × 45 mm × 30 mm. We record THz imaging with the reflected signals as a function of the position (
Recently there has been increasing demand for finding conducting and nonconducting foreign substances in food via THz imaging with CW THz sources [29–32]. This has been mostly performed in transmission geometry; therefore, locating these substances in terms of the distance from the surface has not been accomplished. We first show the transmission imaging results as a C-scan in Fig. 5(a) (top) for thick chocolate (4.5 mm) in which a metal blade has been intentionally embedded in the middle, by melting the chocolate. The THz transmission image is measured using a focusing lens of 5 cm focal length, by moving the motorized stage containing the chocolate sample in both
Conversely, in reflection geometry, which allows the 3D mapping of the object, we can identify the metal structure in the B-scan image [bottom of Fig. 5(b)], whereas it is not identified in the time-integrated C-scan image [top of Fig. 5(b)]. This is partly because the reflected signal from the metal blade interferes with the engraved letters on the top surface. The metal blade is found approximately 30 ps from the top surface of the chocolate, which indicates that it is located 2.5 mm beneath the surface (considering that the refractive index of the chocolate is 1.8). This result is consistent with the actual location of the blade in the chocolate, confirmed by the photograph in Fig. 5(c), taken after the chocolate has been cracked. The 3D plot shown in Fig. 5(d) clearly demonstrates the location of the metal blade in both horizontal and vertical directions; this means that the TOF image allows one to locate a foreign object in terms of both horizontal and vertical positions. We note that the reflection image can be obtained for a host material with relatively low attenuation, such as chocolate, which has low water content [48, 49]. In addition, the detection of a metallic object is restricted when the object is inclined with respect to the surface, as in the case of reflection geometry.
We have developed a 3D THz time-of-flight imaging technique based on the ASOPS method, which allows us to address large vertical ranges. For a focal length of 5 cm, we measured the spatial resolution for different vertical locations; the resolution reached 0.8 mm at the focal plane, and increased up to 1.8 mm when the reflecting surface was 25 mm away from the focal plane. We obtained THz-TOF images for samples with multiple reflection layers; the horizontal and vertical locations of the objects could be successfully determined from 2D cross-sectional images or from reconstructed 3D images. We successfully identified metallic objects embedded in insulating enclosures with a vertical depth range greater than 30 mm. In addition, we used our technique to locate a metallic object in a thick chocolate bar, which is not accessible via the conventional transmission geometry. Furthermore, the THz-TOF system has proven to be an effective candidate for finding foreign substances in foods such as thick chocolate bars; in other words, we can successfully locate a metallic object in terms of both horizontal and vertical positions. Therefore, THz-TOF imaging based on the ASOPS system will work as a powerful means of nondestructive testing with a large depth range, for applications in the fields of security, foods, packaged microchips, and infrastructure.
The authors declare no conflict of interest.
Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.
Midcareer Researcher Program (2020R1A2C1005735); and Basic Science Research Program (2021R1A6A1A1 0044950) through a National Research Foundation grant of Korea Government; GRRC Program (GRRCAJOU2022B01, Photonics-Medical Convergence Technology Research Center) of Gyeonggi province.
Curr. Opt. Photon. 2022; 6(6): 619-626
Published online December 25, 2022 https://doi.org/10.3807/COPP.2022.6.6.619
Copyright © Optical Society of Korea.
Hwan Sik Kim1, Jangsun Kim2, Yeong Hwan Ahn1
1Department of Physics and Department of Energy Systems Research, Ajou University, Suwon 16499, Korea
2Panoptics Corp., Seongnam 13516, Korea
Correspondence to:*ahny@ajou.ac.kr, ORCID 0000-0002-8563-076X
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
In this study, we develop a three-dimensional (3D) terahertz time-of-flight (THz-TOF) imaging technique with a large depth range, based on asynchronous optical sampling (ASOPS) methods. THz-TOF imaging with the ASOPS technique enables rapid scanning with a time-delay span of 10 ns. This means that a depth range of 1.5 m is possible in principle, whereas in practice it is limited by the focus depth determined by the optical geometry, such as the focal length of the scan lens. We characterize the spatial resolution of objects at different vertical positions with a focal length of 5 cm. The lateral resolution varies from 0.8–1.8 mm within the vertical range of 50 mm. We obtain THz-TOF images for samples with multiple reflection layers; the horizontal and vertical locations of the objects are successfully determined from the 2D cross-sectional images, or from reconstructed 3D images. For instance, we can identify metallic objects embedded in insulating enclosures having a vertical depth range greater than 30 mm. For feasible practical use, we employ the proposed technique to locate a metallic object within a thick chocolate bar, which is not accessible via conventional transmission geometry.
Keywords: Terahertz spectroscopy and imaging, Three-dimensional imaging
Terahertz (THz) spectroscopy has emerged as a powerful tool for nondestructive inspection applications, such as biomedical diagnosis, device inspection, and characterization of materials [1–12]. In particular, THz imaging allows for the characterization of the internal structures of objects, which is possible because nonconducting materials are transparent to THz waves [13–23]. Imaging using continuous-wave (CW) sources has increased in popularity, owing to recent technological advances in the development of sources and detectors [24–32]. Although coherent THz tomography based on frequency-modulated CW (FMCW) methods has been developed using tunable or ensemble sources, it is limited in terms of the general depth and longitudinal resolution [33–37]. In contrast, the use of time-domain spectroscopy (TDS) enables the collection of information such as the depth and range of objects with superior resolution, without the need to sweep the frequency [6, 38, 39]. In other words, THz pulsed imaging provides direct 3D mapping of an object by using the time of flight (TOF) of the reflected THz pulses. Importantly, the longitudinal resolution of a THz-TOF imaging system is determined by the temporal width of the pulsed THz source, which yields resolution as good as 150 μm.
Recently we introduced a high-speed nondestructive THz-TOF imaging technique for the inspection of packaged integrated circuits and metal corrosion beneath paint layers [38, 39]. An imaging rate of 100 Hz/pixel has been achieved based on the THz–TDS technique using the optical sampling by cavity tuning (OSCAT) system [40, 41], which delivers a rapid time delay with a full range of 50 ps. The OSCAT system uses a single pulsed fiber source split into two pulses, used for the THz antenna and receiver respectively, and the rapid time delay between the two pulses is generated by varying the repetition rate of the fiber laser. Although the OSCAT system delivers stable THz–TDS signals with relatively low jitter between the two pulses, the range of the time delay is limited, especially for fast scanning (>100 Hz), because it is based on a single laser source. In addition, because the time delay varies sinusoidally, the linear-response range reaches only approximately 20 ps; this corresponds to an optical path length of 3 mm. Therefore, a THz-TOF system based on OSCAT methods is restricted in nondestructive imaging applications that require large depth profiling.
In contrast, THz-TDS systems with asynchronous optical sampling (ASOPS) use two fiber lasers, with repetition rates that can be adjusted independently [42, 43]. In this case, the range of the time delay is limited by the time window between consecutive pulses; thus the time delay reaches 10 ns for a laser repetition rate of 100 MHz. Conversely, the A-scan rate (100–500 Hz) is determined by the repetition-rate difference between the two fiber lasers. This has numerous advantages, because 3D TOF images with virtually unlimited depth range can be obtained. However, it is still restricted by the optical geometry of the imaging system, such as the depth of focus of the region of interest. THz-TOF imaging with the electronically controlled optical sampling (ECOPS) technique has also been introduced, using two independent femtosecond lasers; however, it allows a finite range of time delay, as in the case of the OSCAT system [44, 45].
In this study, we report rapid THz-TOF imaging results based on the ASOPS technique, which delivers a large vertical scan range of more than 30 mm when a focused lens of 50 mm is used. The lateral resolution is addressed as a function of the longitudinal location of the object. Further, we obtain nondestructive TOF images of metallic objects embedded in insulating enclosures, such as polyethylene foam and chocolate.
A schematic of the THz-TOF imaging equipment is illustrated in Fig. 1. For the rapid THz–TDS system we use a commercial ASOPS instrument (TERA-ASOPS; Menlo Systems GmbH, Planegg, Germany) consisting of two independent femtosecond fiber lasers with a wavelength of 1,560 nm, operating with a repetition rate of 100 MHz. The ASOPS system allows the full time-delay range of 10 ns with a rate of 100–500 Hz; we remained fixed at 100 Hz throughout the experiments. In contrast, in the OSCAT system the time-delay scan range is limited to 50 ps, and in particular the region of linear response is restricted to 20 ps, as mentioned previously. Therefore, THz-TOF with ASOPS has a great advantage in terms of large depth profiling.
We incorporate a galvano scanner (Scanlab GmbH, Puchheim, Germany), which enables us to obtain images with a speed that is limited by the ASOPS THz-TDS system, whereas the combined galvano-scanning (
We first characterize the focused beam, to address the vertical extent available for THz-TOF imaging. As shown in Fig. 2(a), we use a metal plate with a knife edge to establish the lateral resolution of the system. We obtain THz images for different heights
Figure 2(c) illustrates the line profile of the reflected amplitude signal as a function of lateral position
Figure 3 shows THz-TOF results to prove the usefulness of the ASOPS system. The test sample is a metal block with three rectangular holes of lateral size 10 mm × 10 mm and depth 11.5 mm, as shown in Fig. 3(a). In Fig. 3(b), the reflected THz signals are shown as a function of the lateral position (C-scan), with a scan range of 15 × 45 mm2 and a pixel number of 50 × 150. Here we show the time-integrated THz amplitudes; in other words, the THz envelope signal (obtained via Hilbert transformation) has been time-integrated for each pixel over the entire time-delay range of 120 ps. This corresponds to an optical path length of 18 mm. We clearly identify in Fig. 3(b) the bright regions for both the top (green) and bottom (red) metal surfaces, whereas the dark region (blue) appears in the middle because of the presence of a circular hole. The bottom surface is brighter than the top surface because the focal plane is closer to the bottom layer. We also note that reflection is suppressed when the reflecting surface is inclined, as shown at the periphery of the circular hole [the region enclosed by the dashed orange line in Fig. 3(a)].
THz–TDS allows us to address the depth profiling directly from the B-scan images, as shown in Fig. 3(c). The THz signal is plotted as a function of position
Figure 4 shows the THz-TOF imaging result for one of the test samples, in which metallic objects are embedded in a polyethylene-foam enclosure. As shown in the photograph in Fig. 4(a), the test sample consists of a series of five metal blades (size 10 × 4.5 mm2) stacked vertically and displaced by 5 mm with respect to one another. They are placed between sheets of low-density-polyethylene foam (thickness 5 mm). The size of the enclosure surrounding the metal blades is 25 mm × 45 mm × 30 mm. We record THz imaging with the reflected signals as a function of the position (
Recently there has been increasing demand for finding conducting and nonconducting foreign substances in food via THz imaging with CW THz sources [29–32]. This has been mostly performed in transmission geometry; therefore, locating these substances in terms of the distance from the surface has not been accomplished. We first show the transmission imaging results as a C-scan in Fig. 5(a) (top) for thick chocolate (4.5 mm) in which a metal blade has been intentionally embedded in the middle, by melting the chocolate. The THz transmission image is measured using a focusing lens of 5 cm focal length, by moving the motorized stage containing the chocolate sample in both
Conversely, in reflection geometry, which allows the 3D mapping of the object, we can identify the metal structure in the B-scan image [bottom of Fig. 5(b)], whereas it is not identified in the time-integrated C-scan image [top of Fig. 5(b)]. This is partly because the reflected signal from the metal blade interferes with the engraved letters on the top surface. The metal blade is found approximately 30 ps from the top surface of the chocolate, which indicates that it is located 2.5 mm beneath the surface (considering that the refractive index of the chocolate is 1.8). This result is consistent with the actual location of the blade in the chocolate, confirmed by the photograph in Fig. 5(c), taken after the chocolate has been cracked. The 3D plot shown in Fig. 5(d) clearly demonstrates the location of the metal blade in both horizontal and vertical directions; this means that the TOF image allows one to locate a foreign object in terms of both horizontal and vertical positions. We note that the reflection image can be obtained for a host material with relatively low attenuation, such as chocolate, which has low water content [48, 49]. In addition, the detection of a metallic object is restricted when the object is inclined with respect to the surface, as in the case of reflection geometry.
We have developed a 3D THz time-of-flight imaging technique based on the ASOPS method, which allows us to address large vertical ranges. For a focal length of 5 cm, we measured the spatial resolution for different vertical locations; the resolution reached 0.8 mm at the focal plane, and increased up to 1.8 mm when the reflecting surface was 25 mm away from the focal plane. We obtained THz-TOF images for samples with multiple reflection layers; the horizontal and vertical locations of the objects could be successfully determined from 2D cross-sectional images or from reconstructed 3D images. We successfully identified metallic objects embedded in insulating enclosures with a vertical depth range greater than 30 mm. In addition, we used our technique to locate a metallic object in a thick chocolate bar, which is not accessible via the conventional transmission geometry. Furthermore, the THz-TOF system has proven to be an effective candidate for finding foreign substances in foods such as thick chocolate bars; in other words, we can successfully locate a metallic object in terms of both horizontal and vertical positions. Therefore, THz-TOF imaging based on the ASOPS system will work as a powerful means of nondestructive testing with a large depth range, for applications in the fields of security, foods, packaged microchips, and infrastructure.
The authors declare no conflict of interest.
Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.
Midcareer Researcher Program (2020R1A2C1005735); and Basic Science Research Program (2021R1A6A1A1 0044950) through a National Research Foundation grant of Korea Government; GRRC Program (GRRCAJOU2022B01, Photonics-Medical Convergence Technology Research Center) of Gyeonggi province.