Study On The Damage Mechanism Of Optical Elements Used In High Energy Laser System

With the development of high energy laser technology,high energy laser system is gradually moving from the laboratory to the battlefield.In the face of the harsh battlefield application environment,it is important and urgent to improve the anti-damage performance of optical components in high power laser system.The optical elements are irradiated by a few kW/cm~2 continuous high power laser for a long time.The partial surface of optical element absorbs excessive laser energy,leading to thermal damage,which hinders the high power laser practicability.At present,the study on the damage mechanism of optical elements at home and abroad mainly focuses on the photoionization effects induced by ultrashort pulse laser.The study on thermal effect of long pulse and continuous wave laser is relatively weak.Therefore,the purpose of this research is to study the thermal damage mechanism under high power continuous laser system,the main contents are as follows:The general thermal damage process of optical elements is studied.First,the absorption distribution of the optical element was measured by the weak absorption measuring instrument.Compared with the absorption peak caused by the defects of the substrate and the film,the scale and intensity of the absorption peak caused by contaminants were two orders of magnitude larger.This indicates that contaminant is the main cause of thermal damage.Then the thermal damage process of optical elements is studied experimentally.The thermal damage process is divided into four stages:local energy accumulation,film ablation,thermal stress accumulation and crack,element melting.Based on the theory of heat conduction and thermal stress,a general model of optical element thermal damage was established.The thermal damage is caused by contaminants adhering to the optical elements.The contaminants form locally strong absorption peaks,which absorb a large amount of laser energy and transfer it to the film through heat conduction.The temperature of film rise,then small area ablation began to appear.The ablated region reveals the silicon,leading to rapid accumulation of thermal stress.At the same time,the ablated region continues to heat the surrounding film,resulting in the surrounding film ablating.When the thermal stress of the silicon reaches the value of crack,the element begins to break and suffer catastrophic damage.According to the above physical model,the film ablation process,the thermal stress accumulation and the bursting process were studied.The theoretical results were in good agreement with the experimental results.The thermal damage behavior of the optical elements in the laser cavity was studied.Local damage of the resonating mirror will affect the laser oscillation,change the distribution of laser intensity in the cavity.Based on the Babinet principle and diffractive theory,the angle spectrum theory and fast Fourier transform method,the influence of absorption peak at the micro scale and the large scale are studied.It is found that the effect of small damage is very small.With the expansion of the damage area,the laser intensity distribution may be changed,therefore the laser power density irradiating on the damaged area decrease.According to the coupling relationship between the damage of optical element and the distribution of laser intensity,the thermal damage process of optical element in cavity is studied by the physical model.Several problems in the use of optical elements are explored.The cleaning effect of"First Contact"is better than that of manual cleaning,and the standard cleaning method is explored.The service life of clean optical element is evaluated according to the degradation law of the absorption.The scale law of optical elements thermal damage is discussed.If the radiation power linear density(W/cm)is the same,the damage behavior of the optical element is the same too.In addition,the emergency stop times for different scale systems have the multiple relation with the square of scale ratioThe thesis established a physical model of thermal damage for the optical elements used in high power laser system,and clarified the thermal damage mechanism in high power laser system.It is of great importance to improve the anti-damage ability of the optical elements and ensure the stable operation of high power laser system.