techniques for the generation and detection of radiation above 100 GHz. Although there are a few exceptions, little has changed in the state of the art since that review was written. The present report provides a more basic review of the technology, and the referenced paper can be consulted where additional details are required.
The three regions of the electromagnetic (EM) spectrum being examined in this chapter have different levels of component maturity but are not divided along the lines previously defined (millimeter wave, submillimeter wave, and terahertz). The component development has been led by military applications and then followed by commercial applications. Starting from the radio-frequency (RF) side of the electromagnetic spectrum, numerous commercial foundries exist to provide components below 50 GHz, and limited capability is available up to about 100 GHz. Above 100 GHz, choices are more limited. Some amplifiers have been developed to date that operate up to 200 GHz, primarily as low-noise amplifiers (LNAs) for receivers. Above 200 GHz, high-sensitivity receivers and associated low-power sources have been primarily developed for the radio astronomy and remote sensing community.
Sources are probably the first component requirement for active spectroscopy or imaging systems operating in the millimeter-wavelength/terahertz spectra. They serve as a means to calibrate systems, as illuminators for sensors measuring reflections from or transmissions though materials, or as the local oscillator (LO) for heterodyne receivers. Heterodyne receivers convert a received signal into a lower frequency through multiplication with a reference source. The heterodyne receiver is generally more sensitive than a direct detecting receiver, as this down-conversion process allows the use of lower frequency and thus more sensitive detection schemes. The use of stable narrowband sources with a heterodyne receiver also provides an ability to measure spectral features rapidly with high resolution.
Figure 3-1 briefly summarizes the challenge with achieving a system concept between 0.1 THz and 10 THz. Generally, the approach to achieving source power has been either to use multipliers to generate radiation from RF sources or to translate down in frequency from optical regions using laser and various forms of nonlinear mechanism. There are exceptions to this trend in that backward wave oscillators (BWOs), a vacuum electronic device, and carbon dioxide (CO2) pumped gas lasers have been available for many years and have provided power adequate for the applications of interest, namely, spectroscopy. The various techniques currently being investigated for generating power for security are discussed below with respect to the frequency range over which they function most efficiently.
Figure 3-2 shows the state of the art in RF sources. A first point is that obviously there appears to be a “cliff” that technology falls off above 100 GHz. Most source advances in the past 30 to 40 years have been due either to high-energy physics research or to military applications, with most of the investment in affordable and readily manufacturable sources below 100 GHz for the military. While commercial interest has further improved the affordability of sources, primarily below 30 GHz, it has done little to improve power levels. Little has happened since Figure 3-2 was developed to change