High-power ultrafast femtosecond laser will form a unique plasma channel at the beam focus due to the nonlinear effect when propagating in a transparent optical medium, namely, filamentation. Filamentation can be used as the excitation source of the fluorescence spectrum for detecting material components, which is of great application value in monitoring atmospheric pollutants. However, when the optical system is used to remotely focus a femtosecond laser into a filament, the aberration in the system can lead to the irregular shape of the focused light spot, as well as a larger radius and poorer quality. As a result, filamentation intensity is weakened, and the detection sensitivity of the system is reduced, which is not conducive to remote detection of atmospheric pollutants with a low concentration. Therefore, determining how to effectively reduce the aberration of the femtosecond laser filamentation system and improve the quality of the focused light spot is essential for enhancing the filamentation intensity and the detection sensitivity of the system.

On the basis of the strong aberration compensation ability of the optical freeform surface, we propose a method introducing a transmission freeform surface phase plate into the femtosecond laser filamentation system to compensate for the system aberration. Firstly, by simulating and analyzing the aberration characteristics of the original system in the optical design software, we design the compensation freeform surface phase plate to reduce some main aberrations and overall wave-front aberrations. The results of the optimal design show that the system aberration is significantly reduced, and the light spot has a more regular shape and a smaller size. Then, we investigate the manufacturability of the designed freeform surface phase plate by tolerance analysis and manufacture the phase plate by single-point diamond turning successfully. Finally, we adjust the femtosecond laser filamentation system with a freeform surface phase plate and carry out the laser filamentation experiment. The experimental results demonstrate the effectiveness of the freeform surface phase plate, with the shape regularity of the focused light spot improved and its radius significantly reduced, and filamentation intensity of femtosecond laser filamentation system is greatly enhanced.

Firstly, a freeform surface phase plate is optimized in CODE V to compensate for the aberration of the femtosecond laser filamentation system when the laser is focused at 10 m. The shape of the focused light spot is more regular, and its radius is reduced after optimization (Fig. 7), which means the main aberration of the system, astigmatism in the X direction, and the overall wave-front aberration are greatly decreased (Fig. 8). Secondly, after the tolerance analysis of the designed freeform surface phase plate (Fig. 9), the phase plate is manufactured by single-point diamond turning. Then, the horizontal and vertical surface shapes of the phase plate are scanned by ZYGO's profilometer, and manufacturing errors in both directions can be achieved (Fig. 11). These errors can be converted into curvature radius changes to analyze the influence on the compensation results (Fig. 12). According to the analysis results, the performance of aberration compensation in experiments are predicted. The phase plate is installed in the original system to study the aberration compensation experimentally. The light spot images in the experiment show that the shape regularity of the compensated light spot is effectively improved, and its radius is significantly reduced (Fig. 15). However, due to the manufacturing error of the phase plate surface, the actual radius of the compensated light spot is larger than the simulation result, and the focusing position moves forward (Fig. 16), which confirms the previous prediction results. Finally, ultrasonic signals are used to characterize the filament intensity before and after compensation. The results show that the filamentation intensity after compensation is 56 times higher than that before compensation (Fig. 17).

In this paper, the transmissive freeform surface phase plate is used to compensate for the astigmatic aberration mainly existing in the femtosecond laser filamentation system. After the optimal design, the aberration in the system has been effectively compensated. The tolerance analysis of the design results, including surface shape and assembly and adjustment tolerance, is carried out, and the device is manufactured. The manufactured freeform surface is set into the system according to the design to verify its aberration compensation effect in the experiment. The light spot shape during focusing is more regular, and the RMS radius of the light spot at the focusing position is less than 0.5 mm when the femtosecond laser propagates linearly. Moreover, the ultrasonic signal intensity, which is utilized to characterize the filamentation intensity, is 56 times higher than the case when femtosecond laser propagates nonlinearly. Therefore, it is feasible to use optical freeform surfaces to compensate for the aberration in the femtosecond laser filamentation system, which is of great practical value for enhancing the filamentation intensity of femtosecond laser filamentation system at a long distance and improving its remote sensing detection sensitivity.