The design, operation, and characterization of a continuous-wave (CW) tunable second-harmonic 460-GHz gyrotron are reported. The gyrotron is intended to be used as a submillimeter-wave source for 700-MHz nuclear magnetic resonance experiments with sensitivity enhanced by dynamic nuclear polarization. The gyrotron operates in the whispering-gallery mode TE _{11, 2} and has generated 16 W of output power with a 13-kV 100-mA electron beam. The start oscillation current measured over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high-order axial modes TE _{11, 2,} q . The minimum start current is 27 mA. Power and frequency tuning measurements as a function of the electron cyclotron frequency have also been carried out. A smooth frequency tuning range of 1 GHz was obtained for the operating second-harmonic mode either by magnetic field tuning or beam voltage tuning. Long-term CW operation was evaluated during an uninterrupted period of 48 h, where the gyrotron output power and frequency were kept stable to within ±0.7% and ± 6 ppm, respectively, by a computerized control system. Proper operation of an internal quasi-optical mode converter implemented to transform the operating whispering-gallery mode to a Gaussian-like beam was also verified. Based on the images of the gyrotron output beam taken with a pyroelectric camera, the Gaussian-like mode content of the output beam was computed to be 92% with an ellipticity of 12%.

We report a high-performance plasmonic terahertz (THz) detector based on an antenna-coupled asymmetric FET by using the 65-nm CMOS technology. By designing an asymmetric FET on a self-aligned poly-Si gate structure, more enhanced channel charge asymmetry between the source and the drain has been obtained in comparison with the nonself-aligned metal gate structure of our previous paper. In addition, using a vertically integrated patch antenna, which is designed for a 0.2-THz resonance frequency, we demonstrated the highly enhanced detection performance with a responsivity of 1.5 kV/W and a noise-equivalent power of 15 pW/Hz ^0.5 at 0.2 THz.

In this work we describe the operation of a continuous-wave 330/460 GHz gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency. The oscillator is intended to be used in 700 MHz ¹H nuclear magnetic resonance (NMR) experiments enhanced by dynamic nuclear polarization (DNP). At 460 GHz, the gyrotron operates in the TE_11,2 mode and is able to generate 16 W of power continuously and stably for a beam current of 100 mA and beam voltage of 13 kV. Power and frequency tuning measurements as a function of beam voltage and magnetic field were carried out. The results show continuous frequency tunability over a range of 700 MHz with a minimum power level of 5 W for a fixed beam current of 100 mA. Extended frequency tuning due to the interaction of the electron beam with the backward-wave component of successive high order axial modes is observed. Power and frequency tuning measurements are also reported for the TE_4,3 mode at 330 GHz, a frequency suitable for 500 MHz DNP/NMR spectrometers.

A method of measuring the amplitude ratios and phase differences among the same RF output signals from multiple ports at the same frequency is proposed. A power combining module is used to couple multiple signals vectorially from the ports. In the proposed method, the amplitude ratios among RF signals from multiple ports are calculated by using the output power of the module to which only one port is connected, and by using the S-parameters of the module itself. However, the phase differences among multiple ports cannot be simply represented by the differences between the output power of the module and the ideally combined power, because the impedances of all ports except measuring ports should be perfectly matched. The proposed method uses a parameter that compensates for the impedance mismatches generated at the other ports to achieve precise measurement of the phase differences among multiple ports. Measurement results using the passive component with three ports reveal that the mean errors of the proposed method are 1.272.15: in the amplitude and phase, respectively, as compared with measurements using the vector network analyzer. The standard deviations of the errors show 1.69% and 1.68° in the amplitude and phase, respectively.

A signal conditioning block of a 1 × 200 Complementary Metal-Oxide-Semiconductor (CMOS) detector array is proposed to be employed with a real-time 0.2 THz imaging system for inspecting large areas. The plasmonic CMOS detector array whose pixel size including an integrated antenna is comparable to the wavelength of the THz wave for the imaging system, inevitably carries wide pixel-to-pixel variation. To make the variant outputs from the array uniform, the proposed signal conditioning block calibrates the responsivity of each pixel by controlling the gate bias of each detector and the voltage gain of the lock-in amplifiers in the block. The gate bias of each detector is modulated to 1 MHz to improve the signal-to-noise ratio of the imaging system via the electrical modulation by the conditioning block. In addition, direct current (DC) offsets of the detectors in the array are cancelled by initializing the output voltage level from the block. Real-time imaging using the proposed signal conditioning block is demonstrated by obtaining images at the rate of 19.2 frame-per-sec of an object moving on the conveyor belt with a scan width of 20 cm and a scan speed of 25 cm/s.

Having the capability of generating superior output power with good spatial pattern at sub-terahertz (THz) region compared to other sources, gyrotron is well suited for nondestructive inspections. The author developed a compact (0.5 × 0.84 × 1.6 m ³ ) gyrotron capable of producing 0.2 THz (TE _4,2 mode) and 0.4 THz (TE _9,3 mode). With reasonable spatial property of the 0.2-THz beam and its long-term stability at high power, nondestructive inspections are successfully demonstrated, especially for soft foreign bodies hidden in food which is hardly detectable with X-ray, in real-time.