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Free Electron Lasers

Acronym: FEL

Definition: laser devices where light amplification occurs by interaction with fast electrons in an undulator

More general term: lasers

German: Freie-Elektronen-Laser

Category: laser devices and laser physicslaser devices and laser physics


Cite the article using its DOI: https://doi.org/10.61835/iqu

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A free electron laser is a relatively exotic type of laser where optical amplification is achieved in an undulator, fed with high energy (relativistic) electrons from an electron accelerator. Such devices have been demonstrated with emission wavelengths reaching from the terahertz region via the mid- and near-infrared, the visible and ultraviolet range to the X-ray region, even though no single device can span this whole wavelength range.

free electron lasers
Figure 1: Setup of an undulator, as used in a free electron laser. The periodically varying magnetic field forces the electron beam (blue) on a slightly oscillatory path, which leads to emission of radiation.

In the undulator, a periodic arrangement of magnets (permanent magnets or electromagnets) generates a periodically varying Lorentz force, which forces the electrons to radiate with a frequency which depends on the electron energy, the undulator period, and (weakly) on the magnetic field strength. Both spontaneous and stimulated emission occur, allowing for optical amplification in a certain wavelength range.

The greatest attractions of free electron lasers are:

  • their ability to be operated in very wide wavelength regions
  • the large wavelength tuning range possible with a single device
  • the spectacular performance in extreme wavelength regions, not reachable with any other light source

Compared with other synchrotron radiation sources (pure undulators and wigglers), FELs can generate an output with a much higher spectral brightness and coherence. This is very useful for a number of applications, including fields such as atomic and molecular physics, ultrafast X-ray science, advanced material studies, ultrafast chemical dynamics, biology and medicine.

The big drawback of FELs is that their setups are very large and expensive, so that they can be used only at relatively few large facilities in the world. A highly ambitious free electron laser project is pursued in Hamburg (European XFEL, originally within the TESLA project, now within a European project) [18]. That 3.4 km long XFEL generates hard X-ray output with unprecedented performance features: wavelengths down to 0.05 nm, pulse durations below 100 fs, and extremely high brilliance. The LCLS at SLAC has already achieved lasing wavelengths below 0.15 nm, corresponding to a photon energy of 10 keV. The substantially upgraded version SLAC-II (with an immensely improved radiance) has become operational in 2023.

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[1]L. R. Elias et al., “Observation of stimulated emission of radiation by relativistic electrons in a spatially periodical transverse magnetic field”, Phys. Rev. Lett. 36, 717 (1976); https://doi.org/10.1103/PhysRevLett.36.717
[2]D. A. G. Deacon et al., “First operation of a free-electron laser”, Phys. Rev. Lett. 38 (16), 892 (1977); https://doi.org/10.1103/PhysRevLett.38.892
[3]A. M. Kondratenk and E. L. Saldin, “Generating of coherent radiation by a relativistic electron beam in an ondulator”, Part. Accel. 10, 207 (1980)
[4]C. A. Brau, “Free-electron lasers”, Science 239 (4844), 1115 (1988); https://doi.org/10.1126/science.239.4844.1115
[5]K.-J. Kim and A. Sessler, “Free-electron lasers: present status and future prospects”, Science 250, 88 (1990); https://doi.org/10.1126/science.250.4977.88
[6]G. R. Neil and L. Merminga, “Technical approaches for high-average-power free-electron lasers”, Rev. Mod. Phys. 74, 685 (2002); https://doi.org/10.1103/RevModPhys.74.685
[7]W. Ackermann et al., “Operation of a free-electron laser from the extreme ultraviolet to the water window”, Nature Photon. 1, 336 (2007); https://doi.org/10.1038/nphoton.2007.76
[8]P. Emma et al., “First lasing and operation of an ångstrom-wavelength free-electron laser”, Nature Photon. 4, 641 (2010); https://doi.org/10.1038/nphoton.2010.176
[9]W. A. Barletta et al., “Free electron lasers: Present status and future challenges”, Nuclear Instruments and Methods in Physics Research A 618, 69 (2010); https://doi.org/10.1016/j.nima.2010.02.274
[10]J. N. Galayda et al., “X-ray free-electron lasers – present and future capabilities”, J. Opt. Soc. Am. B 27 (11), B106 (2010); https://doi.org/10.1364/JOSAB.27.00B106
[11]E. C. Snively et al., “Broadband THz amplification and superradiant spontaneous emission in a guided FEL”, Opt. Express 27 (15), 20221 (2019); https://doi.org/10.1364/OE.27.020221
[12]N. S. Mirian et al., “Generation and measurement of intense few-femtosecond superradiant extreme-ultraviolet free-electron laser pulses”, Nature Photonics 15, 523 (2021); https://doi.org/10.1038/s41566-021-00815-w
[13]E. Prat et al, “A compact and cost-effective hard X-ray free-electron laser driven by a high-brightness and low-energy electron beam”, Nature Photonics 14, 748 (2020); https://doi.org/10.1038/s41566-020-00712-8
[14]W. Decking et al., “A MHz-repetition-rate hard X-ray free-electron laser driven by a superconducting linear accelerator”, Nature Photonics 14, 391 (2020); https://doi.org/10.1038/s41566-020-0607-z
[15]A. Fisher et al., “Single-pass high-efficiency terahertz free-electron laser”, Nature Photonics 16, 441 (2022); https://doi.org/10.1038/s41566-022-00995-z
[16]The World Wide Web Library on Free Electron Lasers, http://sbfel3.ucsb.edu/www/vl_fel.html
[17]The LCLS (Linac Coherent Light Source) at SLAC (Stanford), https://lcls.slac.stanford.edu/
[18]The European X-ray laser project XFEL, https://www.xfel.eu/

(Suggest additional literature!)

Questions and Comments from Users


What is the energy level diagram of a free electron laser?

The author's answer:

I think that the concept of energy levels is not applicable to such a laser – at least not with substantial additional theory.


Could the concept of a free electron laser be reversed? For example, could you turn light incident on a gas or electron cloud into a stream of charged particles that could be turned into electrical potential via direct conversion?

The author's answer:

Yes, the energy transfer between electrons and the oscillating field can be in both directions, depending on the arrival time of the electrons in the oscillating field.

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