In order to meet users' experimental requirements, the two-color FEL has attracted wide attention from the high-gain FEL research community worldwide. In recent years, lots of methods have been proposed, and some experiments have been carried out for the generation of the two-color FEL. Yet they are mostly based on a self-amplified spontaneous emission (SASE) FEL which lacks longitudinal coherence.In this paper, we proposed a new method to generate fully coherent two-color soft XFELs (SXFEL) based on echo-enabled harmonic generation (EEHG). Key technologies were studied, and a three-dimensional FEL simulation was demonstrated.According to the requirements of the two-color FEL generation, it is necessary to transform the seed laser system into two-color lasers, in which the central wavelength and time delay of the two-color seed laser pulses can be continuously and independently adjusted. Hence we designed a two-color seed laser system. The basic technique is to split an 800 nm infrared laser into two identical beams and send them into two third harmonic generation (THG) systems. The central wavelength of the two output ultraviolet (UV) lasers can be tuned independently according to different angles of BBO crystal in the two THG systems, and the time delay can be adjusted through an optical wedge pair inserted into the THG system. By integrating the two UV lasers, we can obtain the two-color seed lasers required by the two-color FEL generation.Results and Discussions The central wavelength of the two-color seed lasers we measured experimentally was 264.85 nm and 266.28 nm, respectively. At the same time, we measured the pulse duration and the time delay of the two-color seed lasers. The width of a single UV laser pulse was about 170 fs, and the time interval between the two pulses was about 2 ps (Fig. 6). By adjusting the optical wedge pair, the time delay between the two UV beams can be further adjusted to 0-1 ps. The central wavelength difference can also be changed accordingly.In the numerical simulation, we adopted two-color seed laser pulses with their central wavelengths being 264.8 nm and 265.3 nm, respectively, and their time delay was about 500 fs. The FEL simulation results indicate that by using these two-color seed laser pulses, we could achieve two-color SXFELs with their wavelengths being 5.884 nm and 5.894 nm, respectively. In addition, their peak power was about 300 MW, and their time delay was consistent with that of the seed lasers (Fig. 8).It should be pointed out that in a practical FEL generation and amplification process, the energy chirp of the electron beam itself and the central wavelength difference of the two-color seed laser pulses cannot exceed the FEL gain bandwidth (for SXFEL, the value is about 2.0×10^-3). Otherwise, the two-color FEL pulses cannot be amplified simultaneously. In order to satisfy the EEHG optimization conditions, it is necessary to ensure that the pulse intensities of the two-color seed laser pulses are basically the same.

As the latest generation of X-ray light sources, X-ray free electron lasers (XFELs) have the advantages of extremely high peak brightness, full coherence, tunability, and ultrashort pulses. They have been applied to many state-of-the-art scientific research fields such as physics, chemistry, materials, biology, medicine, and so on.

Our scheme basically involves a conventional EEHG configuration, which consists of two energy modulation sections, namely, M1 and M2, two dispersion sections, namely, DS1 and DS2, and a long undulator section, namely, R. The electron beam obtained from the upstream of a linear accelerator (LINAC) will interact with seed1 in M1 to get an energy modulation with an amplitude of 7.5. Then the electron beam is sent to the strong dispersion section DS1 with R₅₆ at 7.84 mm to stretch the longitudinal phase space of the electron beam to form a periodic structure. Seed2 will imprint another energy modulation with an amplitude of 6 into the electron beam. The second dispersion section DS2 with R₅₆ at 0.18 mm will convert the energy modulation into harmonic density modulation, and the electron beam will then go through the radiator R to generate FEL radiation.

On the basis of the EEHG scheme and Shanghai SXFEL facility, a new method for generating fully coherent two-color SXFEL pulses was proposed in this paper. In the study, we designed and set up a two-color seed laser system and tested its performance. The results show that this key optical system can meet the requirements of two-color FEL generation. By using two-color seed lasers with their central wavelengths of 264.8 nm and 265.3 nm, respectively, as well as a time delay of about 500 fs, we performed a three-dimensional FEL simulation based on the practical parameters of the SXFEL facility. The simulation results indicate that we can eventually generate two-color SXFEL radiation pulses with their central wavelengths being 5.884 nm and 5.894 nm, respectively, as well as peak power being about 300 MW.