Spectral Characterization Of The Damage Of Fused Silica Induced By UV Laser

High power laser irradiation damage of fused silica optical element,as the terminal,has become one of the bottlenecks restricting the output capacity of ICF device,and it is a hot issue in the research of laser damage at home and abroad.It is of profound significance to understand the damage mechanism of fused silica optical elements and study the factors affecting the damage threshold for guiding the repair of fused silica optical elements in engineering.Numerous surface modification or post-treatment processes can improve the laser damage resistance of fused silica by eliminating or alleviating damage precursors,such as chemical etching based on HF etching technique,reactive ion beam etching process and CO2 laser treatment.In this paper,we focus on the diagnosis of visible damage of fused silica irradiated by UV laser,and study the process of damage and the spectral characterization of defects that affect the damage threshold in detail.In this paper,optical microscopy,scanning electron microscopy,X-ray diffraction,photothermal spectroscopy and Raman spectroscopy were used to characterize the morphology and spectral characteristics of fused silica optical elements before and after irradiation damage.The process of laser-induced damage and the distribution of defects is studied.The results are as follows:(1)The photothermal absorption characteristics of fused silica irradiated by high frequency ultraviolet laser are studied by means of photothermal spectroscopy.The results show that the photothermal absorption signal takes place in a process from scratch,reaches the strongest and decreases.When the photothermal signal gradually decreases,the microscopic imaging shows that the sample has visible damage,which indicates that the component has signs before the visible damage occurs.When the photothermal signal first increases to a critical point,visual damage will occur.Therefore,we propose a method to predict the visual damage of components by combining the micro imaging and photothermal spectroscopy.If we find that there is pre damage to the components,it can save the consumption of components greatly,which is of great engineering significance.(2)According to the difference of the damage area measured by photothermal spectroscopy and optical microscopy,the laser damage area visible ratio(LDAVR)is defined,which is the ratio of the damage area obtained by optical microscopy and the damage area obtained by photothermal spectroscopy.The change of ratio with excitation parameters is studied.It is found that it has a saturation limit,and the ratio is not available when the damage reaches a certain degree after irradiation,we found that it did not damage laterally,but began to damage in depth,until it broke down the element.It was stable at a certain value of 0.8.So,we take this ratio as the criterion of component failure.When the ratio of optical elements is less than 0.5,they can still be used for a period of time,which provides a certain reference value for extending the life of elements in engineering.(3)The damage of fused silica irradiated by ultraviolet laser with different parameters was studied by Raman spectroscopy.The generation of interstitial oxygen molecule and interstitial nitrogen molecule was found,and a new Raman peak of 1602cm-1 was found.Then,the photothermal signal of the whole damage area was analyzed,and it was found that the strong photothermal absorption signal in the fracture area was caused by the generation of interstitial O2 and interstitial N2 molecules,resulting in a large amount of laser energy is absorbed.At last,it is found that there is low temperature cristobalite in the damage area by XRD,and the mechanical and thermal properties of cristobalite are unstable,both of which are the causes of the damage.In this paper,the different damage stages of fused silica irradiated by UV laser are characterized and analyzed.It is helpful to predict the damage degree of laser-induced fused silica,and provide some reference for repairing optical elements in time and improving the service life of optical elements,which has important scientific significance and practical application value.