Special Issue on Development and Applications of Microwave/Millimeter Wave Diagnostics in Industry

Summary

Microwave/millimeter wave devices and systems have been developed as core technologies in the fields of communication and measurement. The ever-accelerating progress in devices and circuits, leading to integrated and arrayed sensing systems, and data processing, including computer technology, has contributed to the rapid advancement in diagnostic technology. One of the most important contributions is the improvement of spatial resolution, which is one of the drawbacks of microwave/millimeter wave diagnostics. The importance of visualization (imaging) has also been widely recognized for the rapid and effective elucidation of physical mechanisms. By coordinating these improvements, microwave/millimeter wave diagnostics have been employed in various industrial applications. The purpose of this Special Issue is to introduce recent advancements in microwave/millimeter wave diagnostic technologies and the current status of their applications.

A total of eight manuscripts are presented in this Special Issue. The manuscripts can be categorized into three subjects, considering the above-mentioned issues: (i) the study of monolithic circuit fabrication and array design for sensing systems, (ii) attempts to improve measurement resolution and accuracy, and (iii) various applications including visualization technology. Summaries of each manuscript are described according to the above-mentioned tasks.

(i)

Zhang et al. [1] reported on a novel synthetization approach for filter-integrated multi-section wideband impedance transformers (ITs). The measurement value of the fabricated transformer, the scattering parameter (S₁₁), was found to be in good agreement with the circuit simulation and the electromagnetic simulation. This IT will become effective through integration with various wideband circuits and antennas, thereby expanding the potential fields of application. Lee et al. [2] reported on the design of a W-band (75–110 GHz) modular antenna/detector array for application in an electron cyclotron emission system of fusion-oriented plasma in order to obtain a three-dimensional profile of the electron temperature fluctuations in plasma. The proposed array system was found to offer a good signal-to-noise ratio compared with conventional systems, and is now installed and being tested in the Korea superconducting tokamak (named KSTAR) imaging system.

(ii)

Tokuzawa et al. [3] described the development of a new Doppler radar using the Ka-band frequency (26.5–40 GHz). This radar, called a dual-comb Doppler reflectometer, enables the simultaneous measurement of turbulent intensity and velocity in high-temperature plasmas at multiple spatial points, with 20–40 radial points and a sampling rate of 80 GS/s. The system has the potential to significantly reduce the heterodyne IF frequency compared with conventional comb systems. Kogi et al. [4] reported on the development of a synthetic aperture imaging laser radar combining a laser radar and synthetic aperture radar (SAR). A laser light (1.55 μm) was amplitude-modulated at a wideband chirp frequency (1–4 GHz at the moment, 1–18 GHz in the future). Since the spatial resolution of a SAR is generally half the wavelength of the chirp frequency width in the range direction, this system was found to offer good spatial resolution in both two and three dimensions. Iba et al. [5] reported on a metamaterial-based super-oscillatory lens designed to operate at sub-terahertz frequencies. This lens was found to have a focal length of 2.5λ (λ: wavelength) and a hot-spot size of 0.67 λ at λ = 3 mm. The lens’ characteristics of a long focal depth and sub-diffraction focusing capability were found to be effective for the nondestructive detection of small objects, which is currently needed in industry.

(iii)

Osaki et al. [6] applied a radar reflectometer to actual non-destructive inspection. The principle of this application involves utilizing the Fabry–Perot effect between two dielectric layers. This system enables one to obtain imaging from inside a building wall, that is, the inspection of tile materials attached to a concrete wall. Quantitative evaluation of the adhesion rate was conducted, and an accuracy of 4.1% was obtained. This result was better than that obtained via the hammering method of a skilled worker. Qian et al. [7] proposed a microwave direction-sensitive sensor for metal crack detection. This sensor adopted two feeding ports to obtain perpendicular polarization currents, thus realizing directional detection. The performance of the sensor was verified via simulations and experiments. The width sensitivities of the two feeding ports were found to be 100 and 63.3 MHz/mm. Zhou et al. [8] presented a live-standing wood defect detection model. The authors analyzed the contrast source inversion and used a neural network inversion algorithm to conduct simulation experiments on the detection of internal defects in trees. The results show that the model-driven deep learning network detection accuracy can reach 91.6% for single defects within trees, and can reach 86.3% and 78.3% for heterogeneous double defects and complex multi-media defects, respectively.

This Special Issue has already been closed. However, this topic is expected to continue to progress in the future.