Lasing of an X-ray oscillator

Lasing of an X-ray oscillator 

X-ray Free-Electron Laser Oscillator (XFELO) is a concept that promises significantly higher stability and improved coherence properties compared with current free-electron lasers. The approach is based on recirculating X-ray pulses in a cavity to enable multipass amplification, analogous to optical lasers. However, achieving gain in an XFELO had so far remained elusive, largely because it requires low-loss cavity optics and exceptionally tight alignment tolerances.

I will report on the first lasing of an XFELO at hard X-ray wavelengths, recently demonstrated at the European XFEL facility [1]. Lasing was achieved at 6.952 keV in a 132.8 m round-trip diamond-based Bragg cavity synchronized to the 2.23 MHz electron-bunch spacing. The presentation will focus on the X-ray optical concepts, as well as the alignment and synchronization strategies that made this result possible. The impact of non-ideal optics and heat-load-driven effects under MHz operation, will also be discussed.

Finally, I will outline our strategy to further develop the method and the scientific opportunities enabled by XFELO technology.

References

Rauer, P., Bahns, I., Friedrich, B. et al. Lasing of a cavity-based X-ray source. Nature (2026).
https://doi.org/10.1038/s41586-025-10025-xExterner Link