ABSTRACT
For many years it has been a goal of optical researchers to produce coherent radiation at wavelengths where existing sources are weak. Free-electron lasers (FELs) are rapidly filling voids in the electromagnetic spectrum left by more conventional sources. FELs are attractive because of their potential to produce both high-peak and high-average power and because of their wavelength flexibility. When compared to conventional lasers, FELs are neither simple nor inexpensive. FELs can, however, produce intense coherent light at wavelengths where other sources are either weak or non-existent. This makes them the ideal choice when one moves away from the visible region of the spectrum to the far IR, or to the vacuum UV, x-ray and beyond. At present FELs operate over a broad spectrum from millimetre waves to the vacuum ultraviolet and at average power levels from watts to several kilowatts. No single device spans this entire range of wavelength or power. In spite of having the same underlying physical principles, the technology behind mm-wave FELs is quite different from that of UV devices.
