Focused light field parameters are the core indices for the interaction experiments between ultra-intense ultrashort lasers and matter, and they are also a prerequisite for correcting wavefront distortion and optimizing the focusing performance via adaptive optics. Presently, several studies introduce the parameters of ultra-intense ultrashort laser devices. However, from an application perspective in physical experiments, there are very few reports on the sampling and measurement of the laser wavefront and focal point under vacuum conditions. In this study, a scheme for sampling and measuring the focused light field in a target chamber under vacuum conditions and exposure to a 10 PW laser device is presented. Through the fixing of some elements on the translation table, switching between parameter measurements and physical experiments is realized. Moreover, the measurement system has a high measurement accuracy and provides more accurate laser parameters for physical experiments.

The optical path of the sampling measurement system was designed and built. First, according to the wide spectrum characteristics of the laser pulse, an achromatic objective lens and a large-aperture achromatic lens were used to reduce the chromatic aberration that may be introduced by the system. Second, to ensure the optimality of adaptive optical wavefront correction, an image transfer system was designed to ensure the occurrence of an object-image conjugate relationship between the deformable mirror and wavefront detector. Subsequently, an ideal light source was used to calibrate the wavefront distortion introduced by the sampling measurement system. Finally, the focused light field in the target chamber was measured and optimized under air and vacuum conditions.

After completing the optical path, a semiconductor laser output from the optical fiber is used as the ideal light source to calibrate the sampling measurement system. The peak-valley (PV) value of the light source is 0.102 μm, and the RMS value is 0.014 μm, which is close to the measurement limit of the four-wave shear interferometer device. The size of point light source is 5.5 μm±0.5 μm, and the measured far-field focusing size is approximately 60 μm after 10 times magnification, which is close to the diffraction limit (Fig.4). Subsequently, wavefront measurements of the main laser are conducted under air and vacuum conditions before undergoing correction, and the difference in the results shows the necessity of vacuum sampling measurement (Fig.5). The wavefront correction of the 10 PW laser pulse is performed using a sampling optical path system. The deformable mirror (520 mm) reduces the peak-valley (PV) value to 0.5 μm and the root-mean-square (RMS) value to 0.07 μm. Under the same correction voltage, the laser focus point closest to the diffraction limit can be obtained under both air and vacuum conditions (Fig.6).

In this study, a sampling measurement system is designed and built to measure the laser-focused light field in vacuum. The design and calibration results show that the system introduces minimal chromatic aberration and wavefront distortion, and it can accurately measure the wavefront distortion and intensity distribution of the laser focal field. The results of the wavefront measurement in air and in vacuum using the 10 PW main laser show that the wavefront distortion measured by this system in vacuum is essentially consistent with that in air, and the slight difference in the Zernike coefficient indicates the necessity of the system. Using this system, the wavefront of the 10 PW laser pulse focus point is measured and corrected under air and vacuum conditions, and the focus point closest to the diffraction limit is obtained, which proves the effectiveness of the sampling measurement system. In summary, the proposed system can accurately measure the wavefront distortion and intensity distribution of a 10 PW laser focal field under physical experimental conditions and perform wavefront correction through an adaptive optics system to improve the laser focusing performance. It also provides accurate laser parameters and extreme physical conditions for investigating the interactions between strong light and matter.