Research On The Damage Threshold Of Nanosecond Pulse Laser Based On Metal Reflective Films

With the continuous development and application of high power laser,the optical thin film elements in laser system have gradually become the key to limit its development,and the study of laser induced damage threshold（LIDT）of optical thin film is an important subject with a long way to go.Due to the high laser threshold of all-dielectric film system,most of the studies are focused on the damage mechanism of all-dielectric reflection film,while few studies on the damage process of metal reflection film.However,for some beam expanding systems or optical systems with relatively low energy density,the metal film layer has the characteristics of low preparation cost,low temperature rise in the preparation process and little influence on the surface shape of optical elements.However,the threshold value of metal film and its dielectric enhanced film system is seldom studied in practical research,which is lack of guidance and application to practical engineering.Therefore,the study starts from the nanosecond pulse laser damage threshold of metal reflecting film,and combines theoretical simulation and experimental exploration of temperature field and thermal stress field in the process of laser action on metal reflecting film,and analyzes the methods of metal protection,metal medium enhancement and metal multi-band reflecting film damage in comparison and comparison,the relationship between the reflectance of the working band and the damage threshold,provides a theoretical basis for the application of metal dielectric reflective film in engineering.The main research is as follows:（1）The laser threshold values of Al,Ag and Au reflected films with metal protection were analyzed and discussed.A nanosecond pulsed Gauss laser irradiation model of metal reflected films was established in Comsol to study the temperature field and thermal stress long field at532nm and 1064nm with pulse width of 5ns under 1-on-1 laser irradiation mode.The results show that the fracture strength of the protective medium film as the threshold condition is closer to the actual measured damage threshold than that of the melting point of the metal layer(Al_660.5℃,Ag_962℃and Au_1064℃).At 532nm band,the damage thresholds of Ag,Au and Al protective reflection films are 1.855J/cm²,0.690J/cm² and 0.289J/cm²,respectively.At 1064nm band,the damage thresholds of Ag,Au and Al protective reflection films are 2.962J/cm²,3.131J/cm² and 1.869J/cm².（2）In order to meet the optical system with higher reflectivity requirement in practical application,it is usually necessary to perform dielectric enhancement on metal Al and Ag films.According to the actual dielectric enhancement film system,the thermal stress change of metal Al and Ag films under the action of nanosecond pulsed Gaussian laser,the temperature field of metal reflection film and the power density of surface deposited beam were analyzed respectively.In combination with the threshold results obtained in the actual experiment,Combined with the threshold results obtained from the actual experiments,the damage thresholds of Al film at 532nm are 1.232J/cm² and 3.542J/cm² for monocycle and biocycle enhancement,indicating that the enhancement of the medium increases the reflectance at the same time,and the reason is that the stress release of the metal film is shared by the multilayer film.Therefore,the enhancement period is bound to reduce the probability of dielectric film fracture.The threshold of Ag film monocycle enhancement at 532nm is 4.112J/cm²,which is the threshold of Ag monocycle enhancement and Al biocycle enhancement at the reflectance of1064nm.The threshold values at 1064nm are 5.252J/cm² and 4.465J/cm²,respectively.The analysis shows that the absorption of Ag layer is smaller than that of Al layer,and the thermal stress of metal layer is correspondingly reduced,and the probability of fracture damage of the whole film is reduced.（3）Design and preparation of three bands Al and Ag dielectric enhanced reflection film system for YAG laser at 1064nm,532nm and 355nm,and study the laser damage threshold.Through theoretical simulation and experimental tests:The damage thresholds of 355nm nanosecond pulsed laser irradiation are lower than those of specific 532nm and 1064nm,and the test thresholds of 355nm working band are 1.699J/cm² and 2.735J/cm²,respectively.The reason is that at the band of 355nm,the absorption of Al and Ag films increases correspondingly,as well as the thermal stress of the films.The test thresholds of the Al enhanced films at 532nm and 1064nm are 2.692J/cm² and 4.363J/cm²,respectively,which are closer to the tensile strength used as the threshold in the simulation process.As the wavelength decreases,The laser damage threshold of both membrane systems showed a downward trend.By studying the laser damage threshold of several typical metal film systems,the theoretical simulation uses the mild tension of dielectric film layer as the damage condition to be closer to the actual test damage threshold,which provides a theoretical basis for the laser damage prediction of metal film systems in practical engineering applications.