Traveling wave tubes (TWT) and klystrons that generate and amplify microwaves use rare-earth magnets in their waveguides. In defense applications, TWT's and klystrons are used in satellite communications, troposcatter communications, pulsed or continuous wave radar amplifiers, and communication links. To focus the electron beam, periodic permanent magnets of rare earths are used in wide bandwidth helix TWT, while nonperiodic permanent magnets are applied in higher-energy, narrow bandwidth klystrons.
 
Rare-earth lasers are used in line-of-sight communication links in satellite- and ground-based systems. Communication lasers have had limited application in satellites that are in geosynchronous or geostationary orbit (GEOs), but expanded use is planned in low Earth-orbit (LEOs) satellites. Laser light’s higher frequencies allows a greater bandwidth and faster data throughput than conventional microwave transfer.

Erbium-doped fiber and fiber amplifiers are applied in high-capacity fiber optic systems. In field scenarios, distribution of a fiber optic communications grid can be quickly accomplished by ground troops, vehicles, or fast line distribution from helicopters. Advantages of the erbiumdoped fiber are its capacity to carry large amounts of digital data, its ability to cover large distances without a repeater or signal amplifier, a wide range of temperature stability, immunity to interchannel crosstalk, and security from outside interference and electronic interception.

Source: RARE EARTHS IN SELECTED U.S. DEFENSE APPLICATIONS, James B. Hedrick, U.S. Geological Survey