Naofumi Shimizu,1* Ken Matsuyama2
1NTT Device Technology Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198 Japan
2Center for Fire Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510 Japan
Proceedings of the Nature Research Society, 2017, 1, 01002
Published Online: 23 September 2017 (Article)
This paper describes the capability of active imaging with sub-terahertz (THz) illumination in a smoky environment. The developed illuminator consists of nine uni-traveling-carrier photodiode modules and an optical circuit that generate incoherent sub-THz waves with a center frequency of 833 GHz. Tests on a target at a distance of 115 cm showed that imaging with incoherent sub-THz illumination provides a clear view in black, dense, and high-temperature smoke, for which the visibility was 30 cm for visible light. These results indicate that sub-THz active imaging is an effective way to ensure visibility in fire environments such as a space filled with heavy smoke.
Stringent building codes have improved fire protection and resistance. However, it is impossible to completely prevent outbreaks of fire in buildings. This is mainly because of careless human activities and appliance/equipment malfunctions and because it is very difficult for us to live without flammable daily commodities such as paper, fabrics, and plastics. Therefore, we must always on guard against outbreaks of fires.
One of the dangerous elements at the scene of a fire is smoke. Smoke is a mixture of air, airborne particles, and gases emitted during combustion or pyrolysis. Most of the particles range in size from 0.1 to 1 µm . Since the size is comparable to the wavelength of light, smoke particles strongly scatter light. As a result, dense smoke can reduce the visibility to zero. The lack of visibility makes it difficult for people to evacuate safely not only from unfamiliar places but also from familiar ones. It also obstructs the activities of even well-trained fire fighters with complete equipment.
Terahertz (THz) waves have a much longer wavelength than the diameter of smoke particles and can penetrate smoke [2-5]. If they can be used to observations, a clear view can be ensured even in severe smoky environments with poor visibility. THz imaging can be done actively or passively, and both methods are currently under intensive study. However, active THz imaging, which uses a high-brightness THz illuminator, is much better for observing objects in smoke. This is because smoke that has been heated to high temperature by fire becomes a strong source of thermal radiation—one that is much stronger than the thermal radiation from the targeted room-temperature objects and completely masks them. The masking makes it impossible to detect objects by passive methods.
Taking the need for fire safety and the transmission and radiation characteristics of electromagnetic waves in smoke into consideration, we carried out feasibility studies on active imaging with sub-THz waves in smoky environments and produced certain results that demonstrated its effectiveness in ensuring a clear view in smoke [6-8]. In this paper, we present the experimental results obtained in an environment that simulates fires most faithfully among those studies.
The primary source of the sub-THz waves used as illumination was a broadband noise signal in the sub-THz region generated by a photomixer . The signal’s low coherency enables the simultaneous use of multiple photomixers to increase the brightness of the illumination. Figure 1 shows the optical circuit of the sub-THz illuminator, which consists of an incoherent light source, a tunable bandpass filter (BPF) with pass band width of 13 GHz, a single-mode laser, an optical coupler, and an erbium-doped fiber amplifier (EDFA), an intensity modulator, a 1:9 optical splitter, nine variable optical attenuators (VOAs), and nine uni-traveling-carrier photodiode (UTCPD) modules with an integrated antenna [7, 10]. The BPF was used to tune the center frequency of the incoherent light, which was then coupled to the light from the single-mode laser. The resultant light was amplified by the EDFA and fed into the intensity modulator for on-off modulation. Finally, the signal was split into nine channels, each of which was sent through a VOA to a UTCPD module. The center frequency of the incoherent sub-THz waves was 833 GHz, which falls within one of four frequency bands between 500 and 1000 GHz suitable for imaging in a space filled with smoke, gases, and other combustion products .
Figure 1. Schematic diagram of the optical circuit for generating incoherent sub-THz waves.
Figure 2 shows the front view of the sub-THz illuminator . Nine plano-convex lenses are attached to a 16-cm-diameter white polyethylene disk. One UTCPD module is positioned behind each lens. A DC bias is applied to each module in parallel. The axes of the sub-THz beam emitted from the surrounding eight modules incline two degrees to the central beam axis. Therefore, the nine beam axes cross at approximately one meter in front of the disk. The power of the incoherent sub-THz waves emitted from the illuminator was estimated to be 5 µW.
Figure 2. Photograph of the sub-THz illuminator.
THz camera and setup for imaging experiment
The imaging experiments employed a THz camera (Model T0832, NEC Corp.) equipped with a 2-inch-diameter aspherical plano-convex Tsurupica lens and a low-pass filter (LPF) . The focal length of the lens and the cutoff frequency of the LPF are 36 mm and 1 THz. The THz camera was operated in the lock-in mode at a rate of 30 frames per second. A synchronous signal produced by the camera was sent to the intensity modulator of the optical circuit shown in Figure 1. As a result, the intensity of the sub-THz waves from the illuminator was on-off modulated in synchronization with half of the frame rate of the camera. The camera continuously captured images with and without sub-THz illumination in an alternating sequence. The output image of the THz camera was a differential image between the illuminated image and the non-illuminated image. This process suppressed background noise in the output image.
For the imaging capability test in a smoky environment, a box-shaped structure consisting of an upper and a lower compartment with inside dimensions of 170 cm × 90 cm × 120 cm (LWH) was prepared . The two compartments were separated by metal mesh, and the upper one contained a window on one of two walls facing the longitudinal direction. In the lower compartment, the incomplete combustion of thin latex film produced black smoke that filled the whole structure. A heating device that uses charcoal fire was also set in the lower compartment to raise the temperature of the smoke. A thermocouple attached to the wall of the structure measured the temperature of the smoke. During the experiment, the window was sealed with clear polyvinyl chloride film to keep the smoke inside the structure. The extinction coefficient of visible light, Cs, is a typical measure of the smoke density . In this study, the intensity of the light emitted from an incandescent lamp set in the structure was monitored from the outside the window, and Cs in the structure was calculated .
The sub-THz illuminator and THz camera were placed 20 cm apart facing the window in the upper compartment at a distance of 20 cm from it. The target consisted of the letter T made of an aluminum plate and ceramic fiber board. Its width and height were 6 and 5.5 cm, respectively, as shown in Figure 3. It was placed inside the upper compartment at a distance of 95 cm from the window. Thus, the distance from the illuminator to the target and the distance from the target to the camera were both 115 cm. The part of the aluminum plate exposed to the illuminator reflects back the irradiated sub-THz waves at a reflectance of nearly 100%. Therefore, an image of the T can be captured by the THz camera if a clear view to the target is achieved.
Figure 3. Imaging test target.
Results and discussion
Figure 4 shows Cs and the smoke temperature in the structure. Here, t = 0 s is when the smoke source and the heating device were set on fire. Both Cs and the smoke temperature rose over time. Since maximum Cs of more than 8 m-1 and smoke temperature near 150 °C were realized, a fire environment, such as a space filled with black, dense, and high-temperature smoke, was faithfully simulated in the structure.
Figure 4. Cs and smoke temperature inside the structure.
Figure 5 shows the images taken at t = 0, 360, 1020, and 1230 s. As shown in the images, the sub-THz imaging constantly provides a clear view of the target. Furthermore, the signal-to-noise ratio of the sub-THz image scarcely changes with Cs and smoke temperature. A Cs of 8 m1 means a visibility of 30 cm for visible light . Near-infrared light imaging done under the illumination by near-infrared light almost completely lost the view of the target at Cs of 4.1 m-1 in the control experiment, as shown in Figure 6. These results indicate that sub-THz active imaging is an effective way to ensure visibility in fire environments such as a space filled with black, dense, and high-temperature smoke.
Figure 5. Sub-THz image of target at t = 0, 360, 1020, and 1410 s.
In this study, the distance between the target and camera was only around one meter. This distance limit will be improved to a level suitable for practical imaging in smoky environments when an advanced illuminator and THz camera are developed.
Figure 6. Near infrared image of target at Cs = 0.0, 2.5, 4.1, and 8.6 m-1.
The capability of active imaging with sub-THz illumination in a smoky environment was investigated. The source of the illumination was incoherent sub-THz waves generated by photomixing. Imaging tests performed under a condition that faithfully simulated a fire site showed that sub-THz active imaging provides a clear view in the space filled with black, dense, and high-temperature smoke with a visibility of 30 cm. The result indicates that active imaging with incoherent sub-terahertz radiation is an effective way to see through smoke at the scene of a fire.
This work was supported in part by the Japan Science and Technology Agency.
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