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Soft X-ray free-electron lasers (FELs) have gained significant attention as a research tool in X-ray ultrafast spectroscopy due to their ultra-high pulse brightness and ultra-short duration. Combined with an independent optical laser to perform pump-probe experiments with time resolution has wide-ranging application value and can have great impact on ultrafast dynamics research in fields such as energy catalysis, solid state physics, materials science, and biology. However, the inherent temporal and spatial jitter of soft X-ray FEL pulses significantly limits the time resolution in these experiments due to the low level of synchronization between the two independent light sources. Here, we present a spatiotemporal coupling device suitable for soft X-ray FELs. The device uses a self-designed four-blade slit device which is suitable for ultra-high vacuum environments to complete the spatial coupling between the two foci of both the soft X-ray FEL and optical laser, reducing the negative effects caused by spatial jitter of soft X-ray FEL beam spots. Based on this, a wavefront-splitting scheme is used to reflect and separate approximately 30% of the soft X-ray FEL beam for arrival time diagnosis. Based on the principle of transient decrease in the reflectivity of semiconductor material surfaces induced by X-rays, precise time measurement is achieved on a shot-by-shot basis through spectral encoding. After experiments, the data is rearranged according to the arrival time delay between the two pulses, effectively increasing the time resolution of the pump-probe experiment to the femtosecond scale.
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