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Abstract
We develop a high-speed two-dimensional (2D) terahertz (THz) color imaging system for practical applications. This system performs THz time-domain spectroscopy (THz-TDS) measurements in one-dimensional (1D) space simultaneously to reduce the spatial scan from 2D to 1D and obtains the 2D THz color image in which the spectral data is possessed in each pixel. We realize measurements on the image with 750 × 1000 pixels (13 mm × 25 mm) with the spatial resolution of 1.5 mm within 10 seconds. This is two orders of magnitude faster than conventional THz color imaging methods. High-speed 2D THz color imaging will be used in non-destructive and non-invasive inspections of industrial products and biological tissues in the future.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Electromagnetic waves in the terahertz (THz) frequency region strongly interact with free carriers in semiconductors and intermolecular vibrations of hydrogen bonds in aqueous solutions. THz light possesses very small photon energy, in the region of a few meV, and therefore, cannot induce ionization and dissociation of molecules. These properties of THz light make THz imaging a useful tool for sensitive and non-destructive probe in security, biomedical and food inspections, cultural heritage investigations, agricultural sensing, and so on [1–9].
THz imaging can be extended to the THz “color” imaging (hyperspectral imaging), by employing THz time-domain spectroscopy (THz-TDS) in which the spectral data is possessed in each pixel [4,10–15]. In THz-TDS, the waveform of the THz light pulse is measured in the time domain and the complex THz spectrum is obtained by Fourier transform of the waveform [16,17]. The color images of the reflected and transmitted THz light from the sample provide information on the spatial distributions of chemical composition and physical properties. The THz color imaging with THz-TDS has been further extended to the three-dimensional tomography techniques [4,13,18,19]. When the THz pulse is reflected on the surface with multi-thin layers, the difference of time-of-flight (TOF) from different interfaces in multilayers reveals depth information of the material.
To use THz color imaging in industrial and biomedical applications, a high capture rate of images is required for real-time inspection and sensing. However, the conventional THz imaging system with THz-TDS is too slow for in-line inspection and sensing [6], and therefore, only a few applications, for instance, non-destructive investigation of ancient artefacts [20–23], have been demonstrated. To establish THz imaging as a practical tool, the acquisition speed must be increased by two to three orders of magnitude, as suggested in the road map of the THz science and technology given in Ref. [24]. The slow acquisition speed of THz color imaging is mainly because both the time-domain scan and the spatial two-dimensional (2D) scan are required. Recently, the THz color imaging systems have been commercially released with efforts of increasing the sampling speed of spectra. But the achieved sampling speed is ∼100 pixels/second, in which the system requires the measurement time of ∼2 hours for the megapixel 2D image.
To overcome the slow acquisition speed, several approaches have been investigated in this decade. The high-speed continuous wave THz spectrometer which quickly scans the frequency instead of the time is one of the candidates for high-speed THz color imaging. The compact spectrometer with the high scan speed of 24 spectra/second has been reported [25]. The real-time THz color imaging has been also developed with the broadband THz light [26]. The line focused THz light was used for a one-dimensional (1D) imaging to reduce the spatial scan from 2D to 1D, and the spectra were measured with the diffraction grating and the monochromatic THz camera. This system achieved the scan speed of 15 lines/second. Although these two systems are promising for the THz color imaging, the phase information for the TOF measurements cannot be provided. The high-speed imaging with THz-TDS has been also proposed. The two mirror galvanoscanner has been introduced for high-speed spatial scan with the image capture time of 1∼2 minutes for the image with 128 × 128 pixels [27]. The simultaneous measurement of the 1D line image has been introduced with the multi-channel line THz light source [28] and the line detector [29]. The small number of pixels in these systems has still limited the image capture rate. The two strategies have been proposed for the simultaneous 2D spatial imaging with the compressed sensing [7,30,31] and the direct mapping [32,33], respectively. Both methods are also promising for the real-time THz color imaging. However, for the former method, the number of pixels of the spatial phase modulator limits the spatial resolution of the image. For both methods, a long accumulation time is required since in the simultaneous 2D measurement the irradiation intensity of the THz light per unit area becomes much lower than that for the point and line focused THz light. Another approach for the simultaneous 2D measurement has been demonstrated with the line focusing of the THz light and the single shot measurement of the THz-TDS, and achieved the scan rate of 23 200 pixels/second [34].
In this study, we demonstrate high-speed THz color transmission imaging by employing the high-speed THz waveform measurement system reported in our previous paper [35] and the simultaneous measurement of a 1D spatial profile of THz waveforms with the high scan speed, 100 lines/second. The achieved image capturing speed is 10 seconds for the megapixel image which is two orders of magnitude faster than that of the commercial products. This paper is organized as follows: First, we describe experimental details of high-speed THz color imaging in Section 2. The system consists of efficient THz generation, high-speed waveform measurement, and 1D color imaging. Next, we demonstrate and evaluate our system in Section 3. We conclude with future perspectives in Section 4.
2. High-speed THz color imaging system
2.1 THz light source
We require high-power THz light with a high-repetition rate to realize high-speed simultaneous THz waveform measurements in 1D space. The intensity and spectral profiles have to be uniform in space for high-quality imaging. A small size is required for industrial and medical applications. To meet these requirements, we employ the “contact grating setup” [36–38] pumped by a Yb-doped fiber laser operated at a repetition rate of 100 kHz [35,39].
Currently, optical rectification in LiNbO3 (LN) crystal pumped by near-infrared (NIR) short pulse laser with a tilted pulse front is widely used to generate intense THz light [40–42]. In the conventional method, the pulse front is tilted by a diffraction grating and transferred to the LN prism by an image-relay system which requires a large space. Furthermore, the prism shape device generates an inhomogeneous THz output in space. These are large disadvantages for practical imaging applications. In the contact grating setup used in this study, the diffraction grating was placed in contact with the input surface of the LN substrate, which resulted in a drastic downsizing of the THz generation system and uniform THz output compared with the conventional method.
The NIR pump light at a center wavelength of 1039 nm produced by the Yb-doped fiber laser was delivered to the contact grating setup for THz light generation. The pump pulse energy and temporal width were 13 µJ and 320 fs, respectively. The contact grating device proposed in our previous studies [37,38] was fabricated on a 1.3 mol% Mg-doped stoichiometric LN substrate with dimensions 16 mm (z) × 20 mm (y) × 2.2 mm (x), where z axis is the crystal axis. The grooves were fabricated parallel to the z-axis with an effective area of 10 mm (z) × 14 mm (y).
2.2 High-speed THz waveform measurement
Details are described in our previous paper [35], so here we will outline the method for 1D profile measurement of THz waveform. Figure 1 shows the schematic diagram of the THz generation and detection system which was constructed on the breadboard with a size of 600 mm × 900 mm. The output NIR pulse from the Yb-doped fiber laser was split into two parts for THz light generation and electro-optic (EO) sampling by the first polarizing beam splitter (PBS). The splitting ratio between two beams was adjusted by the half-wave plate (λ/2) in front of the PBS. The former part was passed through the second PBS and was reflected by the mirror installed on the membrane of the speaker vibrating at a frequency of 50 Hz. To pick up the reflected light by PBS, a quarter wave plate (λ/4) was placed between the PBS and the speaker. The reflected light pumps the contact grating device (CGD) to generate THz light. Both the pump and THz light were s-polarized to the device. The THz light was separated from the pump light by a NIR light cut filter (NIR filter) consisted of a plastic plate with a high-reflection coating for the NIR light and a black polypropylene film. The output THz light was line-focused to a sample by a cylindrical plastic lens (CPL). The line-focused THz light at the sample was imaged by an aspheric plastic lens (PL) onto a 3-mm-thick CdTe crystal with a surface orientation (110) and an aperture of 10 mm × 18 mm. The THz electric field was measured by EO sampling in the CdTe crystal. The EO sampling light was also line-focused on the CdTe crystal. The THz and sampling light were merged by an indium-tin-oxide (ITO)-coated glass plate that reflects the THz light and transmits the NIR probe light. The birefringence induced by the EO effect that linearly depends on the amplitude of the THz electric field changes the polarization state of the NIR probe light. We obtained the amplitude of the THz electric field through the polarization change of the NIR probe light measured by a balanced detection system.
Fig. 1. Experimental setup for 2D THz color imaging. The figures show the top view. The side views are inserted at the positions of the sample and the EO crystal. (a) THz generation and detection system. PBS: polarizing beam splitter that reflects and transmits s and p polarized light, respectively; BS: beam splitter; PD: photodiode for the trigger synchronizing between the THz light generation and the line scan of the camera; λ/2 and λ/4: half- and quarter-wave plates, respectively; CL: BK7 cylindrical lens with focal length of 300 mm; CGD: contact grating device; NIR filter: plastic plate with a high-reflection coating for the NIR light; CPL and PL: cylindrical and aspheric plastic lenses with focal length of 50 mm, respectively; ITO: indium-tin-oxide coated glass plate. x, y, and z are the crystal axes of the LN crystal. X and Y indicate the axes of the 2D spatial images shown in the following figures. (b) Balanced line imaging detection for the 1D spatial profile of the THz amplitude imaging. Lens: BK7 spherical lens with focal length of 100 mm; BPD: balanced photodiode.
The optical delay between the THz and EO sampling light was first adjusted by the stepping motor translational stage and then scanned by the speaker’s vibration at 50 Hz. The THz waveforms were obtained at a repetition rate of 100 Hz, because the two waveforms were measured for a single round trip of the speaker. The displacement of the speaker membrane from the equilibrium was precisely controlled by the function generator during vibration [35]. The THz waveform consisted of 1000 data points measured with the laser repetition rate of 100 kHz. The speaker scanned the optical path length of 2.25 mm which corresponded to the time window of 15 ps at a time interval of 15 fs. The maximum path length was limited where the speaker vibration maintains the linearity of the displacement with respect to the applied voltage.
2.3 One- and two-dimensional THz color imaging
We performed simultaneous measurements of the 1D profile of the THz waveforms with a high-speed CMOS line scan camera. The 1D spatial distribution of the polarization state of the NIR probe light modulated by the EO effect maps the 1D spatial profile of the THz electric field amplitude on the CdTe crystal. This 1D polarization image was again transferred onto the CMOS line sensor. As shown in Fig. 1(b), the parallel and perpendicularly polarized NIR EO probe lights were separately imaged on different positions of the line sensor. The balanced line image was obtained by subtracting one from the other. The line images were measured while scanning the time delay between the THz and the EO probe light to obtain the 1D spatial profile of the THz waveform. The waveforms were Fourier transformed to calculate the 1D spatial profile of the THz spectrum that is the 1D THz color image.
The high-speed CMOS line scan camera (ELiiXA+ 8K Mono, e2v) was used to detect the line focused EO probe image. This camera has an array of 8192 CMOS sensors for the NIR light. The number of bits in each pixel was 10. The maximum scan rate of the camera is 100 kHz, which can be synchronized to the repetition rate of the THz light generation. Because the THz waveform can be measured at the scan rate of 100 Hz as described in Section 2.2, we can also obtain the 1D THz color image with the same scan rate, that is, 100 lines/second. Subsequently, the 1D THz color images were measured while scanning the sample orthogonally to the line focus of the THz light to obtain the 2D THz color image. The sample was placed on the focusing position between CPL and PL, and linearly moved by the motorized translation stage. Throughout this study, the transmission geometry was employed.
3. Results and discussion
3.1 Real-time THz waveform measurement and line focusing of the THz light
Figures 2(a) and (b) show the THz waveform and its Fourier transformed spectrum, respectively. In this measurement, all the EO sampling light was collected into the single balanced photodiode. The single waveform with a time window of 15 ps was measured in a measurement time of 10 ms. By employing pump light with a pulse energy of 13 μJ, the maximum THz electric field of 0.96 kV/cm was obtained. The peak frequency and the bandwidth were 0.6 THz and 1.5 THz, respectively. The signal-to-noise ratio (SNR) of the spectrum was evaluated from the ratio between the maximum intensity and the noise floor level to be 104 even without averaging, which drastically reduced the data acquisition time for practical applications of the THz-TDS with intense THz light.
Fig. 2. Characteristics of THz light. (a) Waveform and (b) Fourier-transformed spectrum obtained by a single scan of the speaker vibration. The real-time THz waveform and spectrum measurements are demonstrated in Visualization 1. (c) Beam pattern of the line-focused THz light at the sample.
Real-time THz waveform measurement is demonstrated in Visualization 1. The software was built by LabVIEW (National Instruments). The red and yellow waveforms and spectra were measured for the outward and return paths of the speaker vibration, respectively. The waveforms were measured every 10 ms and were not averaged. The THz waveform was very stable against the speaker vibration. During the measurement, the 0.5-mm-thick Si plate and the paper stacks were inserted at the sample point shown in Fig, 1(a). As shown in the movie (Visualization 1), we can recognize the changes in THz waveforms and spectra due to insertion of the samples very quickly.
Figure 2(c) shows the spatial profile of the line-focused THz beam at the sample. This image was taken by the THz monochromatic imaging camera (Tera-256, Terasense). The THz light was well focused in a vertical direction (Y), and the width was approximately 9 mm along the horizontal (X) axis.
3.2 Real-time 1D THz color imaging
Figure 3 describes data analysis performed during measurements on the 1D profiles of the THz waveforms. Each horizontal line of the image is the 1D line scan image of the EO probe light measured by each single laser shot at a repetition rate of 100 kHz. Then, the successively measured line images are stacked to form the 1D spatial profile of the THz waveform. The horizontal axis shows the X-axis position as the pixel, and the vertical axis is the scan number of the line scan camera indicating the time delay of the EO probe light. Figures 3(a) and (b) are the raw images of the EO probe light with and without THz light irradiation to the CdTe crystal, respectively. The neighboring 4 pixels along the X-axis were averaged, and then, the size of the line image was downsized from 8192 to 2000. The left and right components in each figure show the images of the horizontally and perpendicularly polarized EO probe light, respectively. As shown in Fig. 3(a), the EO effect induced by the THz light can be hardly seen in the raw image. To clearly see the THz waveform, the difference images between the left and right components are calculated and shown in Figs. 3(c) and (d) obtained from Figs. 3(a) and (b), respectively. Around line number 570 in Fig. 3(c), the signature of the THz light irradiation can be found. Finally, we subtract the background image (d) from the image with THz light (c) and obtain the 1D spatial profile of the THz waveforms as presented in Fig. 3(e). The monocycle THz pulse was clearly measured without any averaging by repeat scans. Figure 3(f) shows the THz waveform at the X position of 400 in the image. The digitizing noise was not striking as compared to the other noise source.
Fig. 3. Procedure of the data analysis during measurements on the 1D spatial profile of the THz waveform. (a) and (b) Raw images of the EO probe light with and without THz light irradiation to the CdTe crystal, respectively. The left and right components in each figure show the images of the horizontally and perpendicularly polarized EO probe light, respectively. (c) and (d) Difference images between the left and right components in (a) and (b), respectively. (e) Final output of the 1D spatial profile of the THz waveform. (f) THz waveform at the X position of 400 in the image (e).
Figure 4 shows the 1D spatial profile of the THz spectra calculated by the Fourier transform of the waveforms measured in each X position. In our imaging system, the 1D profile of the THz spectra with 750 pixels (13 mm length) can be obtained in a single scan with 10 ms. Visualization 2 demonstrates the real-time measurement on the 1D THz waveforms and spectra. The repetition rate of the 1D imaging is 100 Hz. When the sample is inserted in the THz optical path, we can recognize changes in the waveforms and the spectra very quickly.
Fig. 4. 1D THz color image calculated by the Fourier transform of the 1D spatial profile of the THz waveforms shown in Fig. 3(e). Real-time measurements of 1D THz color imaging is demonstrated in Visualization 2.
3.3 High-speed 2D THz color imaging
The 2D THz color image was obtained by scanning the sample orthogonally to the axis of the line image while measuring the 1D line images. To fully display spectral data across the 2D object, we require two spatial, one frequency, and one intensity axes. For evaluation of our 2D imaging system, we show the frequency-resolved 2D intensity maps divided in five frequency ranges because of the difficulty of displaying the full color image on the 2D paper.
Figure 5 demonstrates the frequency-resolved 2D intensity maps of the transmitted THz light through the aluminum mask as illustrated in the lower-right of the figure. The transmittance images are shown in the figures. The line image was captured during 10 ms in each Y position while scanning the sample along the Y direction with a velocity of 2.5 mm/s. Then, we obtained the 2D images with 750(X) × 1000(Y) pixels (13 mm × 25 mm) for 10 seconds. The shadow images of the aluminum mask are seen in the figures. Figure 5(a) shows the mask image measured with a frequency lower than 0.4 THz (the wavelength longer than 0.75 mm). The mask cannot be recognized because of the blurring of the image caused by the diffraction of the THz light owing to its large wavelength. By contrast, in the higher frequency region (>0.4 THz), the shadow of the mask can be seen clearly in Figs. 5(b)–(d). The broadband nature of the THz light used in THz-TDS causes a chromatic aberration and a frequency dependence of the diffraction effect. Therefore, it is found that we have to analyze the 2D THz color images by considering the frequency dependence of the spatial resolution carefully. At the region from 0.67 to 0.93 THz, the spatial resolution was evaluated to 1.5 mm as shown in the Fig. 5(c).
Fig. 5. Frequency-resolved 2D intensity maps of the transmitted THz light through the aluminum mask illustrated in the lower right of the figure. The transmittance images are shown in the figures. The frequency ranges are (a) 0–0.40 THz, (b) 0.40–0.67 THz, (c) 0.67–0.93 THz, (d) 0.93–1.20 THz, and (e) 1.20–1.47 THz, respectively. The inserted black curve in (c) indicates the cross-section of the transmittance image at Y = 23 mm.
Figure 6 shows the frequency-resolved 2D THz intensity maps transmitted through paper of thickness 0.3 mm as illustrated in the lower-right of the figure. The transmittance of the paper with this thickness is more than 50% at the present frequency region. In Figs. 6(b)-(d), the edge of the paper is clearly recognized in the transmitted images in spite of high transmittance of the paper. By considering the long wavelength of the THz light (∼ 0.3 mm), the scattering effect at the edge is not significant. Therefore, the edge enhancement is mainly due to the consequence of destructive interference in the analogy of the phase-contrast microscopy. At the edge of the object, the irradiating THz light beam is split into two parts: one half passes through the object, and the other does not. The former light is delayed by transmission with respect to the latter one. At the final focus point on the EO crystal, the destructive interference occurs between two beams, and enhances the edge of the object. The width of the outline was approximately 1 mm, which may indicate the spatial resolution in our THz phase-contrast microscopy. In Fig. 6(b), the left-right asymmetry of the edge enhancement is found. This may be due to the misalignment of the image-relay from the sample to the EO crystal. Because the low frequency THz light is easily diffracted, it is not easy to achieve the uniform interference completely.
Fig. 6. Frequency-resolved 2D intensity maps of the transmitted THz light through the 0.3-mm-thick paper illustrated in the lower right of the figure. The transmittance images are shown in the figures. The frequency ranges are (a) 0–0.40 THz, (b) 0.40–0.67 THz, (c) 0.67–0.93 THz, (d) 0.93–1.20 THz, and (e) 1.20–1.47 THz, respectively.
4. Summary and perspectives
We developed a high-speed 2D THz color imaging system for practical applications. This system is based on the high-speed measurement of THz waveforms at a repetition rate of 100 Hz [35] and the simultaneous measurements on the 1D image of the EO probe light at a scan rate of 100 kHz by a line scan camera. First, we obtained the 1D spatial profiles of the THz waveform and the Fourier-transform spectra with 750 pixels for 10 ms. Then, we scanned the sample in a direction orthogonal to the 1D array of the CMOS sensors while measuring the 1D images, and obtained the 2D THz color image with 750 × 1000 pixels (13 mm × 25 mm) for 10 seconds. We can make the imaging measurement two orders of magnitude faster as compared to commercial THz color imaging system with THz-TDS.
There are several points of issues in our THz color imaging system as the practical instruments. First, the time window of the waveform measurement is small for the tomographic techniques. The time window of 15 ps in our system corresponds to the round-trip time in the layer with the refractive index of 1.5 and the thickness of 1.5 mm which is too short for the measurement of the multilayer systems. One of the candidates which overcomes this problem is to use the quickly oscillating optical delay stage demonstrated in Ref. [43]. This system scans the length of 22.5 mm at the repetition rate of 100 Hz, and easily measures the layer thicknesses of the 1 cm thick sample. Second, the maximum bandwidth of 1.5 THz will narrow the range of applications for analysis of the chemical compound. The bandwidth was limited by the pump pulse width for the THz light generation, 320 fs. This is the common problem for the high-speed THz imaging with THz-TDS. One of the methods to extend the bandwidth is to use the organic crystal for THz generation instead of the LiNbO3 crystal [44]. The organic crystal extends the bandwidth to 5 THz, but the commonly used intense ultrafast lasers (Yb and Ti:Saphhire lasers) cannot be employed as the pump laser due to phase-mismatching. Recently, a Cr:Mg2SiO4 laser has been developed as a pump laser for the THz light generation with organic crystals [45]. The bandwidth will be improved with this system. Third, the practical applications prefer the reflection geometry as compared to the transmission one used in this study, especially for the inspection of objects on the material which do not transmit the THz light. Recently, the high-quality broadband THz beam splitter has been available to decouple the reflected THz light from the input light and construct the system with the reflection geometry.
THz spectroscopy and imaging will be applied for non-destructive and non-invasive inspection of industrial products and biological tissues. When THz color (hyperspectral) imaging is fully utilized for image diagnosis, we have to develop the numerical analysis of the images by including big data. Chemometric techniques such as principal component analysis [46] and multivariate curve resolution [47] have been widely used to analyze multivariable data to extract the desired information, e.g., the spatial distribution of the specific chemical compounds.
Funding
Ministry of Education, Culture, Sports, Science and Technology of Japan; Japan Science and Technology Agency.
Acknowledgments
We are grateful for the financial support of the Japan Science and Technology Agency under Collaborative Research Based on Industrial Demand “Terahertz-wave: Towards Innovative Development of Terahertz-wave Technologies, and the CPhoST program funded by the Special Coordination Funds for Promoting Science and Technology commissioned by the Ministry of Education, Science, Culture, and Sports of Japan. The authors acknowledge Dr. Yoshihiro Ochi and Dr. Momoko Maruyama for their technical support to the laser system.
Disclosures
The authors declare no conflict of interest.
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