A tunable G-band frequency-modulated continuous-wave radar system has been developed and used to perform differential absorption atmospheric humidity measurements for the first time. The radar's transmitter uses high- power-handling GaAs Schottky diodes to generate between 15-23 dBm over a 10-GHz bandwidth near 183 GHz. By virtue of a high-isolation circular polarization duplexer, the monostatic radar's receiver maintains a noise figure of about 7 dB even while the transmitter is on. With an antenna gain of 40 dB, high-SNR detection of light rain is achieved out to several hundred meters distance. Owing to the strong spectral dependence of the atmospheric absorption over the upper flank of the 183-GHz water absorption line, range-resolved measurements of absolute humidity can be obtained by ratioing the rain echoes over both range and frequency. Absorption measurements obtained are consistent with models of atmospheric millimeter-wave attenuation, and they demonstrate a new method for improving the accuracy of humidity measurements inside of clouds.

In[1, Fig. 5], the assumed atmospheric conditions were incorrectly stated for the curve showing the millimeter-wave propagation model of excess attenuation at frequencies with respect to 193 GHz. Rather than being for 91% relative humidity (RH) and 289 K, as was stated in[1], the model curve in[1, Fig. 5]was for 50% RH and 289 K. (The experimental measurements are unaffected.) The figure is reproduced here asFig. 1(a), now with correct labeling.

Micromachined packaging is emerging as the best choice for development of terahertz multipixel heterodyne array instruments and other advanced terahertz systems. Traditional computer numerically controlled (CNC) metal machining can still be used for component and simple single-pixel receiver fabrication but falls short when highly compact and integrated systems are needed. Several micromachining methods have shown potential at these frequencies with permanent thick-resists, thick-resist electroforming and deep reactive ion etching (DRIE) of silicon micromachining are discussed in detail. These techniques use photolithographic techniques to produce features accurate to 3 μm or less. Silicon micromachining and electroforming offers the additional advantage of vertical stacking of components, enabling higher circuit densities for these waveguide-based components. To demonstrate these capabilities several integrated systems are discussed.

Found in many terahertz (THz) and submillimeter-wave systems, GaAs Schottky diodes continue to be one of the most useful THz devices. As a low-parasitic device that operates well into the THz range, Schottky diodes provide useful detection and power generation for a number of practical applications. Mixers and multipliers, working as high as ~3 THz, have already been demonstrated. This paper reviews the current status of diode technology, detailing some of the different ways for fabricating THz chips. An overview regarding the current state of technology and performance for THz frequency multipliers and mixers is presented, along with applications enabled by these diodes.