Conventional lasers amplify the radiation that is emitted when electrons move between certain energy levels in atoms or molecules. But the radiation in a free electron laser is generated by a beam of accelerated electrons. Magnets makes the electron beam follow a zigzag path, and the acceleration the electrons experience each time they change direction makes them emit radiation. Mirrors direct this radiation back into the electron beam and this leads to further emission. This self-amplifying effect can take place at any wavelength, and the free electron laser can be tuned by altering the speed of the electrons.

The DESY experiment successfully amplified light at ultraviolet wavelengths ranging from 80 to 180 nanometres, with its peak amplification at 98 nanometres. The international team of scientists at the Hamburg centre aim to achieve a similar gain with soft X-rays - that is, radiation with a wavelength below six nanometres - within a year. To do this, they will need to extend the current linear accelerator from 50 metres to 300 metres in length.

The free electron laser programme at DESY is part of the TESLA project proposal to build a next-generation linear collider for particle physics.