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Abstract
Tunable terahertz (THz)-wave absorption spectroscopy is a promising technique to detect trace gases suspended in ambient air owing to their strong absorption fingerprints in the THz-wave spectral region. Here, we present a THz-wave spectroscopic gas detection platform based on a frequency-tunable injection-seeded THz-wave parametric generator and compact multipass gas absorption cells. Using a 1.8-m-path-length multipass cell, we detected gas-phase methanol (CH3OH) down to a trace concentration of 0.2 ppm at the 1.48-THz transparent atmospheric window. We also developed a transportable walk-through screening prototype using a 6-m-path-length multipass cell to identify suspicious subjects. Our results demonstrate the potential of the proposed system for security screening applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Security screening technologies are gaining considerable importance worldwide because of violence and terrorism incidents at airports, stadiums, and transportation facilities. Optical imaging and sensing have already been an essential part of such security inspections, including X-ray imaging and active millimeter-wave imaging. Tunable terahertz (THz)-wave gas-phase absorption spectroscopy using a frequency-tunable source is a promising technique to detect hazardous chemicals and their by-products suspended in ambient air. This is because various polar gaseous molecules have fingerprint absorption spectra in the THz-wave region, and several molecules, such as H2S and NH3, show greater absorption in the THz-wave region than in other infrared regions [1]. The THz-wave gas sensing systems can be operated in real-time without pre-processing, which refers to the removal of dust and small particles suspended in the air, because the THz wave has a lower scattering rate than those of other infrared regions [2]. This is a unique advantage over other gas sensing techniques such as mass spectrometry and infrared spectroscopy [3,4]. For the real-world implementation, the detection sensitivity of less than 1 ppm is required to detect trace gaseous species and their mixtures suspended in atmospheric air. Consequently, THz-wave-based gas sensing technology with sub-ppm sensitivity can enhance the screening quality while smoothing the inspection throughput and minimizing a physical contact at the security inspection point.
Tunable THz-wave absorption spectroscopy can be performed using frequency-tunable, narrow-linewidth, and high-power THz-wave sources that are classified into two categories: electrically driven semiconductor sources such as quantum cascade lasers [5] and multiplier diodes [6]; laser-driven nonlinear optical sources such as THz-wave parametric generators/oscillators [7] and photomixers [8]. The latter category can provide wider frequency tunability than the former by tuning one of the excitation laser wavelengths. Specifically, a subnanosecond laser-driven injection-seeded THz-wave parametric generator (is-TPG) using a MgO-doped LiNbO3 (MgO:LiNbO3) crystal covers a wide and continuous tunability in the range from 0.4 up to 5 THz with a transform-limited linewidth of ∼4 GHz [9–12]. The is-TPG linewidth is equivalent to or narrower than the pressure-broadened absorption linewidth of gas-phase molecules at ambient pressure. Furthermore, the THz-wave output from the is-TPG source is easily measured with a commercially available, room-temperature THz-wave detector because the THz-wave peak power is greater than 10 kW [11]. These unique advantages over other THz-wave measurement systems make is-TPG sources promising candidates for detecting and measuring various gaseous species and their mixtures suspended in atmospheric air for security screening applications.
In this paper, we present a THz-wave spectroscopic gas detection system based on tunable THz-wave absorption spectroscopy with an is-TPG source. We employed two designs of compact multipass gas absorption cells to enhance the sensitivity of trace gas detection. We measured gas-phase methanol (CH3OH), as a phantom chemical, down to a trace concentration of 0.2 ppm at the 1.48-THz transparent atmospheric window using a 1.8-m-path-length multipass cell. We also developed a prototype of a transportable THz-wave gas detection system using a 6-m-path-length multipass cell to demonstrate a proof-of-concept walk-through screening to identify a suspicious subject.
2. Method and characterization
2.1 Experimental setup
Figure 1(a) shows a schematic of the experimental configuration for the THz-wave spectroscopic gas detection. Our method is based on the is-TPG using a MgO-doped lithium niobate (MgO:LiNbO3) crystal [7,9–12]. The is-TPG source was driven by a laboratory-built subnanosecond Nd:YAG master oscillator power amplifier (MOPA) operated at a repetition rate of 100 Hz. The output energy of a passively Q-switched Nd:YAG microchip laser was amplified by two-stage Nd:YAG amplifiers up to 25 mJ/pulse. Figure 1(b) shows the measured energy stability of 25-mJ output pulses from the Nd:YAG MOPA system. The energy fluctuations for 60 s were measured to be 0.11%, indicating stable pump pulses with a Gaussian-like spatial beam profile, as shown in the inset of Fig. 1(b). Figures 1(c) and 1(d) show the temporal and spectral profiles of the 25-mJ output pulses measured using a high-speed photodetector and an optical spectrum analyzer, respectively. The pulse duration at full width at half maximum (FWHM) was observed to be 0.37 ns with a single longitudinal mode of 1064.4 nm, enabling the efficient and stable operation of the is-TPG source [13].
Fig. 1. (a) Schematic experimental setup. (b) Measured energy stability of 25-mJ output pulses from Nd:YAG MOPA system. The inset shows its spatial beam profile. (c) Measured temporal profile and (d) longitudinal mode of 25-mJ output pulses. (e) Temporal profile and (f) stability of THz-wave output pulses measured using a pyroelectric detector.
Continuous-wave and wavelength-tunable outputs from a fiber-coupled external cavity diode laser (ECDL) were used as an injection seed source for idler in the is-TPG process, in combination with a Yb-doped fiber amplifier (YDFA). It provided a maximum average power of 1 W with a tuning range of 1064–1080 nm. An achromatic phase-matching geometry consisting of a grating and lens pair was implemented to satisfy the noncollinear phase-matching condition in the MgO:LiNbO3 crystal, leading to the agility and continuous tunability of THz-wave frequency without any mechanical adjustment of the optical arrangement [14]. A single silicon (Si) prism coupler was attached on the side surface of the MgO:LiNbO3 crystal to extract the THz wave from the crystal into free space [15,16].
Subsequently, the THz-wave output from the is-TPG source was introduced into a gas absorption cell. In this study, we used a single-pass gas absorption cell and two types of compact multipass gas absorption cells to enhance the sensitivity: (1) toroidal-type multipass cell and (2) Herriott-type multipass cell. The main difference between these three cells is a difference in optical pass length for absorption measurement. The details of each cell design are described hereinafter. Then, the THz wave transmitted through the absorption cell was measured using a commercially available room-temperature THz-wave detector, such as a pyroelectric detector and Schottky-barrier-diode (SBD) detector. With such a THz-wave detector, a measurement dynamic range was typically 40 dB. Figure 1(e) shows the temporal profile of the THz-wave pulse train measured using a pyroelectric detector (PHLUXi, Inc., PYD-2). The THz-wave pulses from the is-TPG source were observed every 10 ms because of the 100-Hz repetition rate of the Nd:YAG MOPA system. The stability of THz-wave output for 60 s was measured to be 1.62%, as shown in Fig. 1(f).
2.2 Tunable THz-wave absorption spectroscopy of gas-phase CH3OH
First, we performed the tunable THz-wave absorption spectroscopy with a 0.5-m-long single-pass gas absorption cell to characterize the absorption features of a target gas-phase sample in the THz-wave frequency. The total optical path length from the is-TPG source, through the gas cell, to the detector was approximately 1 m. The THz-wave frequency of the is-TPG source was swept in the range from 0.8 to 3.0 THz with a 1-GHz frequency step by tuning the injection seed wavelength from 1067.4 to 1075.9 nm. The measurements were performed in atmospheric air at 1-atm ambient pressure, 24℃ room temperature, and 45% relative humidity.
Figure 2 shows the measured frequency-domain spectra with and without a gas-phase CH3OH sample in the single-pass cell. The CH3OH concentration was approximately 100 ppm. The spectral dips observed without CH3OH are attributed to the attenuation by water vapor in the atmospheric air [1,17,18]. The signature absorption lines of CH3OH were clearly visible within transparent windows of the atmospheric air, especially around 1.3 THz and 1.5 THz bands. These absorption features are useful for the spectroscopic detection of CH3OH trace gas in the atmospheric air. In addition to the gas-phase CH3OH, other gas mixtures can be detected simultaneously within the same transparent atmospheric windows by applying a multicomponent analysis of the measured frequency-domain absorption spectra.
Fig. 2. Frequency-domain spectra of atmospheric air without (gray) and with ∼100 ppm gas-phase CH3OH sample (blue) in single-pass gas absorption cell. The solid arrows indicate CH3OH absorption lines observed within transparent atmospheric windows.
2.3 Detection sensitivity enhanced by multipass gas absorption cell
We designed a multipass gas absorption cell to increase the optical interaction length of the cell and enhance the sensitivity of trace gas detection. First, a toroidal-type multipass cell [19,20] was developed to perform a closed-path experiment to evaluate the detection sensitivity. The schematic configuration is shown in the inset of Fig. 3(a). The toroidal multipass cell was made of a monolithic stainless steel cylinder with an inner diameter of 200 mm. Its inner surface was polished to be highly reflective at the THz-wave frequency. A plastic wrap was attached to its aperture as an optical window for THz-wave coupling. A cylindrical Tsurupica lens with a focal length of 150 mm was used to focus the input THz-wave beam, from the is-TPG source onto the center of the cell, and to collimate the output beam directed to the detector. The angle between the input and output beams was selected to be 20° to construct the 9-star polygon-shaped beam path in the toroidal cell, corresponding to an effective total path length of approximately 1.8 m. The THz-wave transmittance of this toroidal cell was higher than 90%.
Fig. 3. (a) Measured CH3OH absorption spectra for various concentrations with closed-path, 1.8-m-path-length toroidal-type multipass cell. Atmospheric attenuation bands are shaded in gray. The inset shows the schematic experimental configuration. (b) Measured absorbance at 1.48 THz as a function of CH3OH concentration, showing the detection limit of 0.2 ppm.
Figure 3(a) shows the frequency-domain absorption spectra of gas-phase CH3OH for various concentrations measured between 1.2 and 1.7 THz. The CH3OH concentration in the closed-path toroidal cell was precisely controlled by the volume of pure CH3OH liquid droplets, which were naturally vaporized inside the cell filled with atmospheric air. As a result, a strong absorption line was observed at 1.48 THz among a series of absorption lines within the transparent atmospheric windows, indicated by a solid arrow in Fig. 3(a). Figure 3(b) shows the absorbance at 1.48 THz as a function of the CH3OH concentration. The absorbance was decreased linearly with decreasing concentration, resulting in a minimum detectability of 0.2 ppm. This result suggests that sensitive detection of CH3OH trace gas in the atmosphere is feasible using a multipass gas absorption cell. The minimum detectability was limited by the intensity fluctuations of the is-TPG source and the optical interaction length. Therefore, the detection sensitivity can be improved by further increasing the effective path length in the cell.
3. Proof-of-concept demonstration of walk-through screening
3.1 Prototype of the transportable system
To demonstrate proof-of-concept walk-through security screening, we developed a prototype transportable THz-wave spectroscopic gas detection system with dimensions of 100 cm (W) × 90 cm (H) × 36 cm (D), as shown in Fig. 4(a). Its schematic configuration is illustrated in Fig. 4(b). The optical setups of the is-TPG and gas analysis units were constructed on the front and back sides, respectively, of a vertically mounted, 50-mm-thick honeycomb optical breadboard [60 cm (W) × 80 cm (H)]. To make the gas analysis unit more compact and lightweight compared with the toroidal-type multipass cell, we used a 6-m-path-length Herriott-type multipass flow cell. The THz-wave output from the front-side is-TPG unit was introduced into the back-side gas analysis unit. A fiber-coupled distributed feedback laser diode (DFB-LD), instead of ECDL, was used as a seed source for the prototype. The wavelength of DFB-LD was tuned to 1070.01 nm to generate the THz-wave frequency of 1.48 THz to detect CH3OH. An injection seed beam was delivered into the system via an optical fiber after amplification by the YDFA. The entire system was installed on a compact optical table, whereas the power supply and control electronics were installed under the table. While operating outside the laboratory, the entire system was covered by electromagnetic wave absorbers and a metal frame to prevent THz-wave and laser radiation from leaking out of the system.
Fig. 4. (a) Transportable THz-wave spectroscopic gas detection system installed on a compact optical table. The system was covered by electromagnetic wave absorbers during operation outside the laboratory. (b) Schematic illustration for walk-through security screening demonstration. (c) Typical trace of measured absorbance at 1.48 THz as a function of time when a naturally vaporized CH3OH sample was injected through the air intake port once for 10 s, indicating a fast response of less than 1 s.
The developed Herriott-type multipass cell consisted of a pair of metal-coated concave mirrors [21,22]. Two concave mirrors had the same radius of curvature of 400 mm, while one of the mirrors had an off-axis hole with a diameter of 4 mm for THz-wave coupling. The distance between the mirrors was kept at 330 mm to realize 28 multiple-pass inside the cell, corresponding to an effective total path length of approximately 6 m. Although the THz-wave transmittance of this Herriott cell was approximately 10% due to a low coupling efficiency of the off-axis hole aperture, such a Herriott cell design with a long interaction length enables us to simultaneously achieve high sensitivity for trace gas detection and large throughput of gas flow. The atmospheric air was taken into the Herriott cell via an air intake port with the assistance of an air blower placed in front of the system. A pump was connected to the Herriott cell to blow the airflow in a singular direction. No filter was used in the airflow path to remove the dust. The estimated flow rate was 800 L/min. The THz-wave output from the cell was detected using a high-speed SBD detector; an analog-to-digital converter (ADC) was used for signal processing with a help of the PC program. The analyzed results were displayed on a signal monitoring screen.
Figure 4(c) shows the typical trace of measured absorbance at 1.48 THz as a function of time. The signal intensity of each THz-wave pulse with the 100-Hz repetition rate was monitored continuously without averaging, and an atmospheric air containing a naturally vaporized CH3OH sample was injected through the air intake port once for 10 s. Consequently, the THz-wave absorbance rapidly increased and recovered within 0.5 s, which is fast enough for walk-through screening applications. When the measured THz-wave absorbance exceeds a certain level (e.g., the absorbance of 0.05, which corresponds to the CH3OH concentration of higher than 0.5 ppm), the program generates a warning sign; the signal color changes from green to red, and the alarm beeps simultaneously, indicating the detection of target gas molecules.
3.2 Walk-through screening tests
As a part of proof-of-concept demonstrations, we conducted a walk-through screening test using the prototype system to identify a suspicious subject holding or concealing the target material–gas-phase CH3OH. Three subjects walked along a lane between the air intake port and air blower, as shown in Fig. 5. One of them (subject #2) was wearing a yellow jacket with a tissue paper saturated with CH3OH liquid. As a result, the system displayed a red signal on the signal monitoring screen shortly after subject #2 passed by, while it remained green for the other two subjects (subjects #1 and #3). A movie of the test is attached as supplementary material (see Visualization 1).
Fig. 5. Walk-through screening test with the prototype system. Three subjects (#1, #2, and #3) walked between the air intake port and air blower. The system detected the CH3OH target and generated the warning sign on the signal monitoring screen shortly after the subject #2 passed by, wearing a yellow jacket with CH3OH-saturated tissue paper (see Visualization 1).
We also demonstrated the same screening test in an exhibition hall in Tokyo located approximately 300 km from our laboratory. Although the system was exposed to a peak acceleration force of 2.8 G during truck transportation, we could install the system in Tokyo without any issue; this indicates the stability and robustness of the transportable system. Consequently, we successfully detected and identified a suspicious subject in the screening test. The 5-second-long video of this demonstration can be found in [23].
4. Conclusion
We demonstrated spectroscopic gas detection using the is-TPG source and multipass gas absorption cells. We used gas-phase CH3OH in the atmospheric air as a target sample and achieved a 0.2 ppm detection limit with a 1.8-m-path-length toroidal-type multipass cell. We finally demonstrated a walk-through screening to detect and identify a suspicious subject by constructing the prototype transportable system with a 6-m-path-length Herriott-type multipass cell. With an expected sensitivity of ppm level, our system is widely applicable to detect various gaseous species, such as H2S, CO, NH3, and N2O, owing to the frequency tunability of the is-TPG source. The measurement rate can be enhanced by a factor of 1000 to exploit its real-time capability, by using a high-repetition-rate pump laser in the is-TPG [24,25]. Differential absorption measurements with multifurcated THz-wave pulse trains are also useful for accurate and stable operation of the proposed system [26]. Furthermore, by performing the real-time measurement of multiple absorption lines of target gas molecules, the detectability could be improved, and therefore, the screening result could become accurate and robust even with the existence of a mixture of multiple gaseous species in an open field where the surrounding temperature fluctuates. Although the present demonstrations were performed with a commercially available THz-wave detector, a measurement dynamic range of higher than 100 dB could be achieved with sensitive THz-wave up-conversion detection in combination with a mature optical detector [11,27]. Our results demonstrate that such THz-wave spectroscopic detection techniques will be one of the important technologies in security screening applications in the future and will also be relevant for environmental monitoring of air pollution, and stand-off detection of hazardous chemicals.
Funding
Cabinet Office, Government of Japan (ImPACT Program).
Acknowledgments
The authors thank Prof. H. Ito, Prof. M. Kumano, and Dr. T. Notake for fruitful discussions.
Disclosures
The authors declare no conflicts of interest.
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