these performance levels. The Defense Advanced Research Projects Agency (DARPA) has recently funded a program developing new sources in the region of 0.5 THz to 2.0 THz and a second, which began in 2006, to produce useable monolithic microwave integrated circuits (MMICs) up to 340 GHz.

FIGURE 3-1 Available solid-state sources. NOTE: CW, continuous wave; TUNNETT, Tunnel Injection Transit Time; SLED, Super Luminescent Diode light source; RTD, Resonant Tunneling Diode; HG, harmonic generation; QC laser, Quantum Cascade laser; III-Vs, compounds from the periodic chart columns III and V (e.g., GaAs); IMPATT, Impact Ionization Avalanche Transit Time; and Gunn, electron drift velocity is decreasing as electric field in semiconductor is increasing above certain critical value. SOURCE: Courtesy of Heribert Eisele, University of Leeds, Leeds, UK.

A second point, somewhat less obvious, is that the power levels that are currently available (with some individual vacuum electronic device [VED] exceptions) above 100 GHz are below the power levels actually represented in Figure 3-2.

Backward wave oscillators have been a staple in the region of 100 GHz to 1,200 GHz as tunable sources for instrumentation. BWOs have been built by numerous concerns over the years but are now primarily available from the former Soviet Union. These sources have power ranging from tens of milliwatts at 100 GHz to a few milliwatts above 1 THz. While they operate at high voltages (above 2 kilovolts [kV]) and suffer from some tuning problems when using wideband cavities, they continue to be a useful tool for spectroscopy and diagnostics. The DARPA Terahertz Imaging Focal Plane Array Technology (TIFT) program is currently investing in improved designs for VEDs at 650 GHz that offer higher efficiencies and power levels that could reach 50 milliwatts. These designs are making use of micromachining techniques to improve the tolerances of the resonant structures. These structures that are being micromachined also have the potential of being used for traveling wave tubes. This development would result in potential coherent operation that should enhance resolution, both spatially and spectrally. By using



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