New Progress Achieved in the Research of High-Repetition-Rate Fully Coherent Fre
埃飞电子 2025-07-23
New Progress Achieved in the Research of High-Repetition-Rate Fully Coherent Free-Electron Laser
Recently, the Free-Electron Laser Team of the Shanghai Advanced Research Institute, Chinese Academy of Sciences, has made progress in the research of a new mechanism for high-repetition-rate fully coherent free-electron lasers (FELs). They have experimentally verified for the first time the self-proposed new mechanism of "Direct Amplification Enabled Harmonic Generation" (DEHG-FEL) and successfully realized its lasing amplification and stable operation. This marks a crucial step on the path to achieving fully coherent extreme ultraviolet (EUV) and X-ray FEL light sources with a repetition frequency at the megahertz (MHz) level.
High-repetition-rate fully coherent EUV and soft X-ray FELs have important application prospects in cutting-edge fields such as advanced time-resolved spectroscopy, coherent diffraction imaging, and the study of ultrafast dynamics of nanostructures and nanodevices. However, the traditional external-seeded FEL scheme usually requires an ultraviolet seed laser with a peak power as high as the hundreds of megawatts level for driving, which makes its repetition frequency usually limited to the kilohertz level.
The Free-Electron Laser Team of the Shanghai Advanced Research Institute had previously verified the self-amplification mechanism of coherent energy modulation on the Shanghai Soft X-ray Free-Electron Laser Facility (SXFEL). In this work, the research team adopted a long modulation section and directly amplified the weak seed laser signal through the high-gain FEL process, thus obtaining a high-power and stable modulated laser. During this process, the electron beam obtained sufficient energy modulation, and after passing through the dispersion section, coherent micro-bunching at the optical wavelength scale was formed, thereby generating a fully coherent radiation output with harmonics up to the 12th order. The team further amplified the 7th harmonic to saturation, obtaining a saturated pulse energy of approximately 160 microjoules, with an energy stability of 5.5%. Through the harmonic cascading process, the team also successfully achieved the output of the 16th harmonic, and the output spectral bandwidth was close to the Fourier transform limit, indicating that the DEHG-FEL had maintained good longitudinal coherence.
The DEHG technology effectively amplifies extremely weak seed laser signals with its simple device configuration. In principle, it can reduce the seed laser power by two to three orders of magnitude and has the capabilities of stable, controllable, and high-order harmonic output, making it particularly suitable for high-repetition-rate external-seeded free-electron lasers. In addition, it has the potential to provide a solution for achieving high repetition rates for more complex two-stage seeding schemes (such as Echo-Enabled Harmonic Generation, EEHG). In the future, this scheme is expected to be combined with high-harmonic-generation technology (HHG) to provide new methods for ultrafast spectroscopy, imaging, and materials science research in the soft X-ray .
The relevant research results, titled "First Lasing and Stable Operation of a Direct-Amplification Enabled Harmonic Generation Free-Electron Laser", were published in "Physical Review Letters" and were selected as "Editor's Suggestion" articles.
