10 Sep 2018 |
Research article |
Innovative Materials and Advanced Manufacturing
Nondestructive Inspection Using a THz Parametric Source
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An Improved is-TPG Spectroscopic System
Terahertz (THz) wavelengths lie between the microwave and infrared regions of the electromagnetic spectrum, and share the characteristics of both. As with radio waves, a THz-wave can be partially transmitted through a wide variety of materials, including plastics, ceramics, and papers and, as with infrared waves, a THz-wave can be guided using mirrors and lenses. Besides this, a large number of materials have unique absorption characteristics (also termed fingerprints spectrum) at THz frequencies, which allow us to identify materials like illicit drugs, explosives, and so on, by measuring the absorption spectrum of a sample. For these reasons, the THz-wave has a number of potential applications in biomedical, communication, non-destructive sensing, and imaging fields.
For years, we have worked on the development of a high-power THz-wave source, based on parametric processes in a MgO:LiNbO3 crystal in order to develop the THz-wave spectroscopic system for non-destructive testing. [1-3] Recently, the peak output power of an injection-seeded terahertz parametric generator (is-TPG) approached >100 kW  by introducing a new pump laser: a microchip Nd:YAG laser with a shorter pulse width. Moreover, highly sensitive THz-wave detection was possible using the principle of is-TPG. Therefore, we could develop a new THz-wave spectroscopic system with dynamic range orders of magnitude of ten by combining such a source and detector. . It means that our system can detect very weak energy of one millionth of maximum input. Using this system, we can detect chemicals through much thicker obstacles than before. This study reports recent improvements of the is-TPG spectroscopic system for nondestructive inspection.
Figure 1 shows the THz spectroscopic system using an is-TPG. When an intense laser beam (pump beam in Fig.1) is input into a nonlinear optical crystal (MgO:LiNbO3), near-infrared light (idler wave) and a THz-wave (signal wave) are generated; this is caused by Stimulated Raman Scattering due to polaritons, which is the result of coupling between phonons and the THz-wave. The emitted THz-wave is focused on a sample and then input into another nonlinear optical crystal (MgO:LiNbO3) for detection. Finally, the THz-wave is converted to the infrared detection beam based on nonlinear optical wavelength conversion in the crystal, and we measured it to get the THz intensity.
Practical Applications: Seeing through Packaging Materials
Using this system, we demonstrated nondestructive sensing of mail (Fig. 2) and THz three-dimensional (3D) computed tomography of plastics (Fig. 3). The result of spectroscopic imaging through thick shielding material is shown in Fig. 2,  which illustrates three saccharide samples identified separately despite the 23 mm-thick wrapping materials consisting of cardboard, corrugated board, and bubble wrap. Moreover, the density of the white color captures the spatial distribution of the powders.Figure 3 shows the result of 3D computed tomography.  We measured (a) an outlet plug cover, (b) cigar socket plug, and (c) nylon connector, which attenuated the THz-wave more than 70 dB. We obtained this 3D image by utilizing the very high dynamic range of our system. In addition to the outer shape, the inner three holes and center partition are also distinguishable. We have achieved these practical applications using an is-TPG. However, these applications require long measurement times because the is-TPG is a tunable source that emits only one wavelength per pulse. To overcome these restrictions, we have been developing a real-time measurement system that uses simultaneous multi-wavelength THz-wave generation from an is-TPG. In this system, the wavelength does not need to be scanned, thus the multi-wavelength is-TPG enables real-time spectroscopy with a repetition rate equal to that of the excitation laser. 
The is-TPG System Expanded for Real-Time Measurement
In order to demonstrate the real-time measurement, we placed three pellets, each containing maltose, glucose, and lactose respectively, on the optical path of the multi-wavelength THz and recorded the spectroscopic information. According to the reference spectrums accessed from the THz database, the absorption lines occur at approximately 1.61 and 2.04 THz for maltose, 1.45 and 2.10 THz for glucose, and 1.36 and 1.81 THz for lactose. These reference spectrums are displayed in Fig. 4, represented by the light blue lines.
We used five frequencies of 1.36, 1.47, 1.64, 1.82 and 2.00 THz in the multi-wavelength is-TPG to match with the reference absorption line frequencies. The resulting spectroscopic measurements are also shown in Fig. 4. We confirmed that attenuation of the THz-waves matched the absorption spectra of the corresponding reagent. It means that we achieved qualitative identification of the saccharides in real-time.
In this study, we reported on our recent progress of the is-TPG and its applications. The is-TPG achieved a peak output power of more than 100 kW, and the dynamic range of the spectroscopic system reached orders of magnitude of almost 10. This system was able to identify saccharide samples through thick envelopes and reconstruct a 3D image of plastic samples with an absorbance greater than 70 dB. Moreover, we developed a real-time measurement system using multi-wavelength generation from an is-TPG. Our system will become one of the leading tools for nondestructive inspection of mail, drugs, and plastic products in the near future.
Kosuke Murate is a professor in the Electronics Department at Nagoya University, in Japan. He completed a six-month internship for the ETS Research Chair on Terahertz (THz) Optoelectronics.
Program : Electrical Engineering