Progress has been made in the research of fully coherent free electron lasers

The free electron laser team of the Shanghai Advanced Research Institute of the Chinese Academy of Sciences has made progress in the research of fully coherent free electron laser, and based on the Shanghai soft X-ray free electron laser device, the new mechanism of echo harmonic cascaded free electron laser independently proposed by China has been verified, and the soft X-ray coherent radiation with excellent performance has been obtained. Recently, the relevant research results were published in Optica under the title of Coherent and ultra-short soft X-ray pulses from echo-enabled harmonic cascade free-electron lasers.

X-ray free electron laser is one of the most advanced light source large scientific devices in the world. At present, most of the X-ray free electron lasers in the world are based on self-amplifying spontaneous radiation mechanism (SASE), SASE has extremely high peak brightness and femtosecond ultra-short pulse width and other excellent performance, but SASE is vibrated by noise, the coherence and stability of its radiation pulses are not high, and it is not a “laser” in the X-ray band. One of the most important development directions in the field of international free electron lasers is to produce fully coherent X-ray radiation with conventional laser quality, and its important way is to use the operation mechanism of external seed free electron lasers. The radiation of the outer seed type free electron laser inherits the characteristics of the seed laser, with excellent characteristics such as full coherence, phase controllability and accurate synchronization with the external pump laser. However, limited by the wavelength and pulse width of the seed laser, the short wavelength coverage and pulse length adjustment range of the outer seed type free electron laser are limited. In order to further expand the short wavelength coverage of the seed type free electron laser, the international community has vigorously developed new free electron laser operation modes such as echo harmonic generation in recent years.

Outer seed type free electron laser is one of the main technical routes for the development of high-gain free electron laser in China, and all four high-gain free electron laser devices in China have adopted the external seed operation mode. Based on the Shanghai deep ultraviolet free electron laser device and the Shanghai soft X-ray free electron laser device, scientists have realized the world’s first echo type free electron laser optical amplification and the first extreme ultraviolet band echo type free electron laser saturation amplification amplification. In order to further promote the outer seed type free electron laser to short wavelength, the research team independently proposed a new mechanism of echo harmonic cascade of fully coherent free electron laser, which was adopted by the Shanghai soft X-ray free electron laser device as the basic scheme, and completed the whole process from principle verification to soft X-ray band light amplification. The results show that compared with the traditional seed-type operating mechanism, this new mechanism has excellent spectral characteristics, and the superior performance of this new mechanism in pulse length control and ultrafast pulse generation is further verified by using the ultrafast X-ray pulse diagnosis technology ( independently developed by researchers. The relevant research results provide a practical technical route for the generation of fully coherent free electron lasers in the sub-nanometer band, and will provide ideal research tools for X-ray nonlinear optics and ultrafast physical chemistry.

Echo harmonic cascaded free electron lasers have excellent spectral performance: the left figure is the conventional cascading mode, and the right figure is the echo harmonic cascade mode

The use of echo harmonic cascading enables X-ray pulse length adjustment and ultrafast pulse generation

At present, the Italian FERMI-FEL unit and the SwissFEL unit have proposed plans to use this new mechanism to further improve their radiation performance. The relevant research work is supported by the National Key R&D Program, the National Outstanding Youth Fund Project, the National Natural Science Foundation of China and the Shanghai Talent Program. (Source: Shanghai Advanced Research Institute, Chinese Academy of Sciences)

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