these devices with multipliers, it should be possible to realize several milliwatts of power above 1 THz.

FIGURE 3-2 State-of-the-art radio-frequency sources (circa 2001). NOTE: IHVS and LMDS: Intelligent Highway Vehicle System and Local Multipoint Distribution System, respectively. SOURCE:R.H. Abrams, B. Levush, A. A. Mondelli, and R.K. Parker. 2001. Vacuum Electronics for the 21st Century. IEEE Microwave Magazine 2(3):61-72.

While numerous fundamental sources, such as Gunn,2 Impact Ionization Avalanche Transit Time (IMPATT) diodes, or amplified phase-locked oscillators, can be found below 200 GHz, the most practical technique for generating coherent energy above 200 GHz is through multiplication. This is the primary technique for providing LO power to heterodyne receivers for radio astronomy or high-resolution spectroscopy. While in the past it was common to use whiskered Schottky diodes as multipliers, planar diodes have become readily available and offer suitable performance. Their output power levels can be limited by the maximum amount of input power that they can handle or by even the existence of a suitable source of RF that can be multiplied efficiently. Multipliers have been used successfully above 1 THz by chaining together a series to obtain the desired frequency. Power levels from various sources available from Virginia Diodes, a leading producer of the device to the community, is shown in Figure 3-3.

2

A form of diode that relies on the Gunn effect. Unlike other diodes, it consists only of N-doped semiconductor material.



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