Kinetics Of High-power Laser-induced Filamentary Damage In Transmission Optical Components

An important prerequisite for the realization of laser inertial confinement fusion is to have extremely high-powered large laser devices.The devices require a large number and variety of transmission optics,and the induced damage of these optics has been a challenge in increasing the output power of the laser system.Fused silica is the most widely used optical material in the system,and its laser-induced damage has been a focus of attention.Although many studies have been conducted on laser-induced damage of fused silica elements,few have focused on the energy deposition distribution,plasma shielding effect,stress wave transmission and multi-focusing of fundamental frequency nanosecond laser-induced fused silica filamentary damage.In this paper,we investigate the dynamics of 1064 nm nanosecond laser-induced damage(including core damage and filamentary damage)in fused silica experimentally using time-resolved pump-probe shadow imaging.The specific studies in this paper are as follows:1.We built a time-resolved pump-probe platform under laboratory conditions to observe the damage characteristics of fused silica with different laser energy and the influence of plasma shielding effect on damage by shadow imaging technology.The results show that the bulk damage in the material can be classified into core damage and filamentary damage according to their morphology.The filamentary damage formed earlier than the core damage,but had a smaller damage area.The core damage consisted of a wide range of crack propagation and a central melting zone,while the filamentary damage was composed of a series of discrete damage sites in series.The laser pulse contracted in the focal region led to an explosion in the fused silica,resulting in core damage.The temporal and spatial distribution of laser pulses and the self-focusing effect are the main reasons for the formation of filamentous damage.We found that the plasma shielding effect increased with the laser energy,and caused significant changes in the morphology of core damage and filamentary damage.At lower laser energy(<30 m J),the core damage was located at the front surface of the sample,and had a smaller area.At higher laser energy(>30 m J),the core damage moved to the back surface of the sample along the optical axis,and had a larger area.Interestingly,we observed that the diameter of the filamentary damage was not affected by the energy increase and the plasma shielding effect when the laser energy was lower than 70 m J.2.Two stress waves with different velocities appear during the formation of core damage,and the velocities of the two stress waves are 6.0 km/s and 4.0 km/s,which are consistent with the longitudinal and transverse speeds of sound in fused silica.To further investigate the propagation of stress waves,the velocity variations of stress waves at different time delays with different laser energy were obtained by varying the time delay of the pump-probe shadow imaging system.The experimental data fitting results show that the stress waves induced in fused silica have the same propagation law,but there are also some differences.The propagation of stress waves is divided into three main stages.Firstly,the stress waves are generated by an explosion in the focal region inside the fused silica and reach the peak velocity in a very short time,then they start to gradually decay and finally the velocity drops to the speed of sound in the fused silica when they start to propagate stably.However,higher laser energies induced stress waves with higher peak velocities and longer decay times to the speed of sound in fused silica.In addition,line stress waves occur after the first stress wave,while the second stress wave did not.The superposition of stress waves is generated by discrete damage sites that form filamentary damage.3.Multi-focusing phenomena were observed during 1064 nm nanosecond single-pulseinduced fused silica bulk damage,which is a particular nonlinear optical phenomenon with a strong dependence on laser energy.At laser energy of less than 20 m J,the bulk damage mainly appears as a single focus.Once the energy reaches or exceeds 20 m J,the laser pulse will create multi-focusing damage in fused silica,and the probability of multi-focusing damage will increase with the increase of laser energy.Also,as the laser energy increases,the fused silica changes from continuous breakdown to multiple discrete plasmas,and the distance between adjacent plasmas increases rapidly,which results in larger crack extensions and longer damage sizes due to multi-focusing.In addition,the temporal and spatial order of multi-focusing generation is obtained based on the velocity difference of multiple stress waves in multifocusing.The multi-focusing phenomenon of nanosecond single-pulse laser in fused silica is discussed and analyzed based on relevant theories.The main reason for this phenomenon is considered to be the combination of two factors:(1)the self-focusing effect of laser pulses,which causes the laser beam to converge and increase its intensity;(2)the reshaping behavior of laser pulses in the time domain,which causes the laser pulse to split into multiple sub-pulses with different peak intensities and arrival times.These two factors result in multiple focusing sites along the laser propagation direction.