Research On The Dynamic Characteristics Of Femtosecond Laser Ablation Process Based On Optical Sensing

Due to the diversity and complexity of the interaction between femtosecond laser and material, femtosecond laser ablation has important application value in the related fields of micro-nano machining, laser propulsion and processing of energetic materials, and so on. Therefore, the dynamics process of femtosecond laser ablation has become an important research content for femtosecond laser application fields. The dynamics of plasma expansion and plasma shock wave is important subject, which remains to make it clear for laser shock forming, laser fusion, laser medicine and laser propulsion application mechanism.According to the complexity of femtosecond laser ablation crystal material produces micro plasma expansion and micro plasma shock wave progress, the dynamic characteristics of femtosecond laser ablation process have been researched from two aspects of theory and experiment using optical sensing measuring technology, and the influencing the dynamic characteristics parameter, including electronic density, impact pressure ect, which has been studied in the process of femtosecond laser ablation. The experimental system is established in this paper. The system characteristics is using the femtosecond laser pump probe technique; the high-speed photography experiment system has been established by using a ICCD shooting micro plasma dynamic process; a set of online monitoring system has been designed based on all fiber no-gel microstructure probe type optical fiber F-P acoustic emission; a new principle and new technology of monitoring micro plasma shock wave dynamics process has been obtained by using F-P acoustic emission sensor online monitoring system; A preliminary exploration of a new method of crystal material composition detection has been proposed.In this paper, the main contents are listed as follows:(1) The time-resolved pump probe system has been established to probe the dynamic process of femtosecond laser ablation, in order to research the complexity of the micro plasma produced by femtosecond laser ablation crystal material. The time-resolved shadow images have been analyzed for different micro plasma dynamic process produced under different femtosecond laser energy and different crystal materials. The variation rules of the propagation velocity of normal direction and tangent direction, propagation distance have been researched for different energy and different crystal materials micro plasma shock wave. The change regularity of impact wave velocity and impact pressure has been researched for different crystal materials. The basic influence factors of femtosecond laser micro plasma dynamic process have been established preliminary.(2) The micro plasma morphological evolution produced by femtosecond laser ablation of different crystal material has been studied through high speed photography method. The high-speed photography experiment device is simple and it is flexible operation, it can study the development process of laser micro plasma intuitively. The study found that femtosecond laser micro plasma presents the droplet shape in the space, and it is obvious asymmetry along the direction of laser beam. The size of laser micro plasma near the laser is larger than the other, and the horizontal direction distribution is symmetrical. The size of laser micro plasma in the horizontal direction and the vertical direction is increasing gradually with increase of laser energy and the increasing trend in slowly become smaller. What's more, the size of the plasma in the longitudinal change is obvious and the variation in the horizontal direction is the relatively smooth. These conclusions further verify the relevant conclusions of time-resolved shadowgraphs experiment. These conclusions have important guiding significance to the further study the interaction between high intensity femtosecond laser and materials. It is important to the applications of femtosecond laser.(3) The all fiber no-gel microstructure probe F-P acoustic emission sensor has been designed and manufactured based on the principle of F-P optical fiber acoustic sensing and laser plasma shock theory. It can be used to online monitor the corresponding frequency of high sensitivity. Meanwhile, the optical fiber sensing signal demodulation system is designed. Then, femtosecond laser micro plasma shock wave measurement system has been constructed based on the optical fiber sensing technology. The experimental results show that the sensing measurement system can been used to measure the weak acoustic emission high frequency signals of femtosecond laser plasma shock wave. Laser plasma shock wave acoustic signal can been measured directly in one or a plurality of femtosecond laser pulse. It can avoid measurement errors caused by laser shock wave repetitive or laser stability. It can improve the accuracy of the experiment.(4) For the first time, the online monitoring of the femtosecond laser micro plasma shock wave spatial and temporal progression has been proposed by using the optical fiber F-P acoustic emission probe. The dynamic of femtosecond laser micro plasma shock wave has been analyzed through online monitoring of femtosecond laser plasma shock acoustic signal. The sensing theory and method for laser plasma shock wave dynamics process monitoring has been established. The temporal variation rules and formation mechanism of micro plasma shock wave have been studied in experiments. The experimental results showed that: the intensity of femtosecond laser plasma shock wave acoustic emission signal increases with the laser energy increased; the frequency is in the range of 0~100KHz; the frequency range and peak position basically does not change with the change of laser energy. Peak frequency of different crystal materials have slight difference and peak frequencies of the same crystal materials remain unchanged.(5) A novel method for detection of crystal materials has been proposed.A novel femtosecond laser micro plasma shock wave acoustic emission signal analysis method has been proposed based on the time-frequency analysis. The characteristics of micro plasma shock wave acoustic emission signal have been studied using wavelet transform and energy statistics method. Through the characteristics analysis of the detected signal, it can preliminary realize the detection of crystal material composition.