UV Laser Damage Platform Construction And Damage Characteristics Of CaF₂ Crystal

With the rapid development of laser technology,the problem of optical elements damaged by high power laser in optical systems has attracted increased attention.Ultraviolet(UV)lasers have been widely used in industry,medicine,scientific research,military and other fields for their features such as high single photon energy,high resolution and less photothermal effect.At the same time,damage introduced by UV lasers has gained increased attention.As excellent UV window materials,fluoride crystals have high transmittance even in the deep ultraviolet range,and they can be used in fluorine environment with the resistance to fluorine,so they stand out of UV window materials.At present,research on damage introduced by UV laser are less than those on infrared and visible bands,and most of studies focused on fused silica Studying the damage of fluoride crystals introduced by UV laser not only helps us understand the fundamental process of interaction between UV laser and materials,but also provides important reference for laser processing and laser attack and defense.Excimer laser is typical UV laser source with high pulse energy,wide wavelength rage and high peak power.In this paper,193 nm ArF and 248 nm KrF Excimer were used to construct the UV laser damage plaand tform.CaF2 was selected as the research object to investigate the initial damage damage growth of CaF:2 crystal under 193 nm and 248 nm laser,respectively.A series of experiments were carried out following the aspect of damage morphology,laser-induced threshold,defect distribution characteristics and defect-induced damage mechanism.The main results are as follows:1)A method of transmitting signals under strong electromagnetic interference environment of the discharge excitation excimer laser was designed to improve the signal transmission.The system not only realized precisely online monitoring of the slow-changing gas pressure signal,but also measured the transient energy signal of pulse laser,so that the energy of excimer laser became more stable.2)A system was build with optical elements and hardware equipment based on the before-mentioned excimer laser.In analysis,size and energy density of the light spot projected onto the surface of the sample can be adjusted.A stable,adjustable and high quality laser source was provided for studying UV laser induced damage to CaF2.The position of the sample was controlled by a electric stage.A CCD camera was used to monitor the surface morphology of the sample when the laser irradiated on the sample.3)The laser-induced damage threshold(LIDT)was measured by 1-on-1 test method using an excimer laser source with pulse width of about 25 ns and wavelength of 248 nm and 193 nm,respectively.It is concluded that the fracture defect causes local modulation of the light field during laser irradiation,enhances the absorption of phonon energy,and reduces the laser induced damage threshold;structural defects make the calcium fluoride band gap hybrid,increase the absorption of crystal to the laser,so that reduce the laser-induced damage threshold;in analysis,during the irradiation of laser,the maximum laser fuence and the maximum electric field strength are both near the back surface,and the plasma generated on the front surface is located in the air,reducing the energy deposition on the front surface,the plasma generated on the back surface is located in the crystal of calcium fluoride,absorbing energy power and increasing the internal stress of the material,so that the back surface of the calcium fluoride crystal is damaged earlier than the front surface,and the damage is even worse;the 193 nm two-photon energy is higher than the band gap of CaF2,two-photon absorption phenomenon can be used to explain that LIDT of CaF2 irradiated by 193 nm laser is lower than that irradiated by 248 nm.4)The damage morphology shows the characteristics of the damage directly,and the damage law is summarized through comparative analysis.As the incident laser energy fluence increases,the damage gradually increases,which is manifested in the increase of damage points,plaques,and the increase of the damage area.The initial damage starts from the fracture,bending and shedding of the' cleavage plane of calcium fluoride crystals,and there is no obvious thermal melting phenomenon.As the number of laser pulses increases or the repetition frequency increases,the microscopic defects in the material absorb the laser energy and then spread.The localtemperature is risen,and the defect is broken and melted to form a damage pit.In severe cases,even a thermal explosion leaves a catastrophic damage.The chips were sputtered,the molten residue was scattered on the surface of the sample,and the stripped periodic strips were threaded under the peeled pieces.