The Mechanism Of Pulse Laser Interaction With Transparent Optical Materials And Application In Laser Machining

In this dissertation,ns laser,ms laser and the combined pulse laser that consists of ns laser and ms laser have been applied to interact with transparent optical materails.The underlying mechanisms as well as their applications in laser machining are studied.First,the influence of focal position on the interaction between ns laser and K9 glass is studied.The damage morphology can be divided into bulk damge inside the glass and surface damage at the glass surface.The corresponding damage thresholds are 12.1J/cm2 and 8.7J/cm2,respectively.A one-dimensional model with multi-inclusions is built to calculate the temperature rise under the irradiation of ns laser.The results show that the clusters formed by the aggregation of inclusions can active the thermal cooperation effect,which increases the maximum temperature rise.Besides,the calculated maximum temperature rise grows with the spot size,which is consistent with the experimentally obtained results that the damage threshold decreases with the increase of the spot size.Second,the photoresist is utilized as the absorber to carry out the experiment of multi-pulse laser-induced backside dry etching(MP-LIBDE).The influences of laser fluence,pulse number,film thickness,confining condition and surface modification on the etching features are studied.The results show that the etching process can be described as three steps:(1)pre-heating of the sample;(2)expansion of ablated material and delamination of film from the surface of fused silica;(3)cracking of film and ejection of ablated materials.The independency of etching rate on film thickness under water confinement can be explained by the constraint force from water.Near-surface modification due to the process of interaction of the decomposition products of ablated photoresist with the fused silica enables laser ething.Third,an inclusion matrix model irradiated by ms laser is built to calculate the temperature and thermal stress fields.The results show that inclusions act as heat sources and increase the temperature rise of regions around them.The uneven distribution of temperature inside the substrate makes the expansion of different parts differ from each other,which generates thermal stress.The x component of stress tensor is the primary factor that induces damage in glass.The damage at the rear surface of the substrate is generated by thermal stress.Both of numerical results and experimental results show that the damage region can extend inward from both front and rear surfaces with the increase of laser energy.Finally,a ms laser combined with a ns laser(called combined pulse laser,CPL)is applied to machining transparent materials.The effects of delay and laser fuence are studied.It is found that slightly modified site and heavily modified site(cone-shaped cavity)can be generated by CPL.When ns laser firstly irradiates K9 glass(?t<0),the groove generated by the pre-incident ns laser lowers the fluence needed for the post-incident ms laser to form SMS,while the fluence needed to form the cone-shaped cavity is improved due to the laser conditioning induced by the pre-incident ns laser.When ns laser irradiates during the ms laser-sample interaction(?t>0),the corresponding processing efficiency is higher than that of CPL with ?t<0.Moreover,an optimal delay for getting the highest processing efficiency of cone-shaped cavities is found.Three processes are associated with this phenomenon:(1)thermal cooperation effect of randomly distributed defects before ns laser irradiation;(2)overlapping effect that links individual modified sites together and grows to a larger modified site after the generation of visible modification by ns laser;(3)thermal radiation from the laser-supported combustion wave.The outcome of this dissertation will be helpful for application of ns laser,ms laser and the combined pulse laser in laser machining of transparent optical materials in future.