The Free electron Laser team of the Chinese Academy of Sciences has made progress in the research of fully coherent free electron lasers. Based on the Shanghai Soft X-ray Free Electron Laser Facility, the new mechanism of echo harmonic cascade free electron laser proposed by China has been successfully verified, and the soft X-ray coherent radiation with excellent performance has been obtained. Recently, The results were published in Optica under the title Coherent and ultra-short soft X-ray pulses from echo-enabled harmonic cascade free electron lasers.

The X-ray free electron laser is one of the most advanced light sources in the world. At present, most of the international X-ray free electron lasers are based on the self-amplifying spontaneous emission mechanism (SASE), SASE has a very high peak brightness and femto level ultra-short pulse width and other excellent performance, but SASE vibration by noise, the coherence and stability of its radiation pulse is not high, is not the X-ray band "laser". One of the most important development directions in the field of international free electron laser is to generate fully coherent X-ray radiation with conventional laser quality, and the important way is to use the external seed free electron laser operating mechanism. The radiation of the external seed free electron laser inherits the characteristics of the seed laser, and has excellent characteristics such as full coherence, phase control and precise synchronization with the external pump laser. However, due to the limitation of the wavelength and pulse width of the seed laser, the short wavelength coverage and pulse length adjustment range of the outer seed free electron laser are limited. In order to further expand the short wavelength coverage of the external seed free electron laser, new free electron laser operating modes such as echo harmonic generation are being vigorously developed in the world in recent years.

External seed free electron laser is one of the main technical routes to develop high gain free electron laser in China. At present, all four high gain free electron laser devices in China have adopted external seed operation mode. Based on the Shanghai Deep Ultraviolet Free electron Laser Facility and the Shanghai Soft X-ray Free Electron Laser Facility, scientists have successively achieved the first international echo type free electron laser light amplification and the first extreme ultraviolet echo type free electron laser saturation amplification. In order to further promote the external seed free electron laser to the short wavelength, the research team independently proposed a new mechanism of fully coherent free electron laser with echo harmonic cascade, 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 light amplification in the soft X-ray band. The research results show that compared with the traditional external seed type running mechanism, this mechanism has very excellent spectral characteristics, through the adoption of the researchers of independent development of ultrafast X-ray pulse diagnosis technology (https://doi.org/10.1016/j.fmre.2022.01.027), The superior performance of this new mechanism in pulse length control and ultrafast pulse generation is further verified. The relevant research results provide a feasible technical route for the generation of fully coherent free electron lasers in the subnanometer band, and will provide an ideal research tool for the fields of X-ray nonlinear optics and ultrafast physical chemistry.

The echo harmonic cascade free electron laser has excellent spectral performance: the left image is the conventional cascade mode, and the right image is the echo harmonic cascade mode

X-ray pulse length adjustment and ultrafast pulse generation can be realized by echo harmonic cascade