Free-electron lasers are multipurpose tools.

Above: The principle of free-electron laser (https://newscenter.lbl.gov/2009/09/09/electron-gun/)

Free-electron lasers are promising tools for many solutions like quantum computers, UV- and gamma-ray sources, weapons, and interstellar rockets. The principle of the free-electron lasers is simple when electrons are traveling in the particle accelerators and change the direction they are sending photons. 

So the free-electron laser is the particle accelerator, where the electrons are traveling by following the waving trajectories. The thing that makes that laser so impressive is that the wavelength of the radiation that system sends can adjust, by adjusting the length and height of the wave. 

The free-electron lasers can use to produce radiation, which has been difficult to produce earlier. So the high energetic UV-, coherent X- and gamma rays can produce by using the free-electron lasers. And that thing makes gamma- and X-ray lasers true. The X-ray lasers were planned to position in orbital trajectory in the early 1980s during the "Star Wars" program.

Those lasers would use the neutron bombs for making the laser ray. Those single-use lasers where the laser element is in the middle of a small neutron warhead are so-called "surgical weapons". But the ability for creating coherent X-rays by using the free-electron lasers is giving that system the ability to make many other things like transmit data. 

The free-electron laser-based engine can be used as an ion, electron, and photon rocket. But the same system can use also as a communication tool for long distances. The problem with photon rockets is that the thrust of photons is so low. That means that at the beginning of the journey rocket can use ions. Which are giving more thrust. Then it can turn to use electrons, and in the final stage, it can use photons. 

The free-electron lasers can use to transmit data, it can use as a weapon in killer satellites and one of the most fascinating solutions for that kind of system is that it can use as the engine in the interplanetary and interstellar spacecraft. The same tube, that accelerates electrons can be used to accelerate ions. 

So the futuristic spacecraft can use this kind of accelerator engine in stages. At the first, the system can drive the ions through the tube. When the speed of the craft is rising, it can turn to use electrons for making thrust. And after that, the system can turn to a photon or laser-photon rocket. 

In this case, the same system can be used as a communication tool and photon rocket. The same laser which makes the craft moving in space can also transmit data, and that laser can be used as the quantum channel, where the qubits are sent between Earth and that spacecraft. A free-electron laser is a multifunctional tool. 

()https://scitechdaily.com/the-paradox-of-a-free-electron-laser-without-the-laser-a-new-source-of-coherent-radiation/

()https://en.wikipedia.org/wiki/Photon_rocket