Results and Discussions In the ablation experiments with the burst-mode femtosecond laser, no burn marks occur on the edge of the ablation pit (Fig. 3). The ablation edge is consistent with the rectangular scanning pattern without distortion. There is no collapse caused by rising temperatures and water evaporation around the ablation pit, which shows that no obvious thermal effects appear in the process of ablation. From the perspective of the shape of the ablation pit, the taper of the ablation pit generated by the burst-mode femtosecond laser is small (Fig. 4), which is conducive to improving the thrombus removal rate during surgery. Moreover, the linear relationship between the depth of the ablation pit and the equivalent energy density is great in the range of low energy density, which is convenient to obtain the expected ablation depth by adjusting the laser parameters. Compared with that of the burst-mode femtosecond laser, the ablation depth of the traditional mode femtosecond laser is smaller under the same equivalent energy density. Additionally, there are obvious slopes and large taper in the ablation pit, which may lead to reduced thrombus clearance rate during surgery. The ablation thresholds calculated by the fitting curves of the traditional mode and the burst mode are about 1.093 J/cm² and 0.104 J/cm², respectively. The ablation threshold of the traditional mode is about 10.5 times that of the burst mode. The energy density required to obtain 250 μm ablation depth with the traditional mode is 31.87 J/cm², while that with the burst mode is only 19.19 J/cm². Therefore, the burst-mode femtosecond laser can effectively reduce the ablation threshold and improve the ablation efficiency.

Thromboembolism and thrombosis are the important causes of arterial occlusive diseases. Rapid thrombus removal is one of the key strategies in the treatment of arterial thrombotic diseases. Currently, the most thrombus removal is achieved through surgery. However, conventional thrombus removal procedures, such as open surgery and minimally invasive surgery, have low thrombus removal rate and the risk of vessel wall damage and rupture. As a new laser technology, the burst-mode femtosecond laser has the potential to solve these problems. It shows low thermal effect and high ablation efficiency in industrial precision machining, which is expected to provide a new solution for thrombus removal technology. However, there is a lack of experimental data to verify the actual ablation effects on thrombus with the burst-mode femtosecond laser. The ablation effect with the burst-mode femtosecond laser based on animal blood clot samples is studied in this paper. The experimental results show that the burst-mode femtosecond laser improves the ablation efficiency and reduces the ablation threshold, which has great clinical research and development potential.

In this study, the ablation experimental platform is established with femtosecond laser and high-speed galvanometer. The laser can output both traditional mode pulses and burst-mode pulses, and the repetition rate is adjustable. In addition, a pair of aperture stops and a pair of reflectors are set in the platform to adjust beam diameter and fold optical path, respectively. The fresh edible duck blood clots purchased in the market are used as ablation samples. During the ablation experiments, the glass tube containing the blood clot samples is inserted into the sample bracket fixed on the z-axis lifting platform. To facilitate detection and observation, a single-layer large-area scanning method is proposed to carry out the ablation experiments. According to repetition rate, pulse energy, scanning speed, and pulse output mode, 12 experimental groups are set up. The three-dimensional super depth of field microscope is adopted to observe and record the images and data of ablation pits. The ablation threshold is obtained by fitting multiple groups of experimental data. Through the comparison and analysis of the ablation results with burst-mode femtosecond laser and traditional mode femtosecond laser in the same equivalent energy density, the differences in the ablation effects under the above two modes are explored.

In this paper, the ablation effects of burst-mode femtosecond laser and traditional mode femtosecond laser are analyzed. Compared with the traditional mode femtosecond laser, under the same energy density, the burst-mode femtosecond laser has higher ablation efficiency and smaller ablation pit taper, which is beneficial to improving the thrombus removal rate. The ablation thresholds calculated by the fitting curves of the traditional mode and the burst mode are about 1.093 J/cm² and 0.104 J/cm², respectively. The ablation threshold of the traditional mode is about 10.5 times that of the burst mode. In Rayleigh length, the energy density of burst-mode femtosecond laser has a great linear relationship with the ablation depth, which is helpful to control the expected ablation depth by adjusting the laser parameters. This study shows that the burst-mode femtosecond laser has great clinical research and development potential to become a novel type of thrombus removal technology.