Investigation Of Dynamics Of High Power Nanosecond Laser-induced Damage Of Fused Silica Optical Windows

As a widely used optical component,the laser-induced damage of fused silica has been a limitation of the output power of high power laser facilities for a long time.Previous studies mainly focus on the defect mechanism and concentrate on the ultraviolet wavelength laser pulse,while researches focus on the damage dynamics and the infrared wavelength laser pulse are rarely reported.In this thesis,dynamics of laser-induced damage of fused silica under irradiating of 1064 nm nanosecond laser pulse has been thoroughly studied with the help of multimodal analysis methods include time-resolved shadowgraphy,interferometry and fast photography.These experiments were all performed on a home-made pump-probe platform.Main results in this thesis are concluded as follows:1.Employing the time resolved shadowgraphy and interferometry apparatus,the rear surface damage process of fused silica has been insitu observed.The expansion of laser plasma has been numerically simulated based on the point explosion theory and2D hydrodynamic fluid model.Results show that the damage is first formed in the defect region and laser-induced plasma is formed in the air part surrounding the damage site.Numerical simulation results show that the internal pressure of plasma can reach as high as 600 MPa.The simulation also indicate the formation of internal shockwave which moving towards the sample surface in the time scale of hundreds of nanoseconds.At the same time,it can also be observed the opaque region expanding in a high speed in the bulk.This expansion process ceases quickly when the laser pulse is terminated.After the termination of laser pulse,the material response mainly includes the launch of stress waves and the birth and growth of fractures.The overall duration needed to the formation of fractures in bulk material is about several hundred nanoseconds.At about tens of nanosecond after the initiation of damage,the ejection of nanometric-sized neutrals in the air part can be observed.At about several hundred nanoseconds delay,the ejection of micrometric-sized particles can be obviously observed.With the increase of delay,the particle features gradually vary from spherical,small diameter?5²0 m?and high speed?>1km/s?to flake-like shape,large diameter?50²00 m?and low speed?¹0m/s?.The particle fly patterns include linear,helical and turning around like pattern.By comparing the particle ejection of fused silica,K9 glass,CaF₂ and NaCl,it can be concluded that the particle ejection is dependent on the material mechanical and thermal properties.Mechanisms include phase explosion,impact ejection and stress wave ejection are attributed to be responsible for the particle ejection process.2.Using the time-resolved interferometry and ICCD?Intensified CCD camera?fast photography,dynamics of nanosecond laser-idnuced damage at front and rear surfaces of fused silica,especially the effect of laser parameters on the damage process and expansion of laser plasma has been investigated.The results show that plasmas formed at the front and rear surface exhibit different self-emission properties.The front surface plasma includes a bright plasma core and relatively weak plasma periphery and its expansion follows the power law.The rear surface plasma shows an irregular shape and splits into the fast and slow components at²0 ns delay.The fast component's expansion follows the power law while the slow component's expansion follows the Drag force model.As the moving speed of the slow component is comparable to the ejected particles,we suggest that the slow component is the thermal emission of the superheated ejected neutrals.It is also shown that laser pulse energy and the focusing geometry affect the dynamics of surface damage.For the front surface damage,the higher the laser energy is,the plasma is more violently ionized and therefore there are more free electrons.The farther the laser focus located,the shockwave in air is more close to the cylindrical shape.The shockwave from the breakdown of air will impact the shockwave from the breakdown of material and form the stagnation layer.For the rear surface damage,the damage crater depth will increase suddenly and increase with laser fluence when the laser power density exceeds 117GW/cm².The damage mechanism transits from the surface damage to the filamentary damage.3.Based on the two-frame shadowgraphy and ICCD fast photography apparatus,the forming phases of bulk damage morphology has been identified and the effects of laser energy and focus length of lens have been studied.The results show that the formation of the final morphology of bulk damage can be identified into two phases.The line-like body is formed firstly and then the bloom-like head part forms.The birth and growth of the line-like body exhibit as the discrete breakdown dots.These damage dots then coalescing together through some thermal transport mechanisms and form the final visually continuous morphology.The morphology asymmetry along the laser direction is attributed to the shielding effect caused by the absorption of the plasma inverse bremsstrahlung mechanism and superheated bulk materials.The formation duration of the bulk morphology is tens of microseconds.The lateral length of the bulk damage site increase with the laser pulse and the focusing lens focus and varies pulse to pulse with all parameters fixed.Mechanisms in terms of moving breakdown,self-focusing and stimulated Brillouin scattering have been discussed and the self-focusing theory combing with absorptive defects are attributed to the main mechanism for the bulk filamentary damage.