Theoretical Research On Pulsed Laser Detection Of Absorptive Defects On Optical Components

With the rapid development of modern laser technology,higher and higher requirements are put forward for the performance of optical components.Especially,large-sized laser components with diameters over 100 mm are widely used in high power and high energy laser systems,and the quality requirements of optical elements are even more stringent.But the laser damage threshold of optical component is an important factor of restricting the system operation.One of the key factors causing laser damage in these laser systems is the strong absorption defect on the surface/subsurface of the laser components,which usually with a micron size.How to accurately detect these defects in large aperture optical components has become an important part of the research on the performance of optical components themselves and the influence of the whole optical system.As a non-contact,high-sensitivity detection and absorption method,the photothermal technology provides a new idea for the detection of absorption defects.The thermal lens technology has also promoted the development of absorption defect detection.However,in the past,the detection of absorption defects of laser optical elements was mostly based on the thermal lens theory,combined with the single-point scanning method.This method is suitable for the small-sized laser components,but it is time-consuming and labor-saving for large-diameter laser components,so it is not suitable for the detection of large-diameter optical component defects.Therefore,this thesis aims at the absorption defects of large-aperture optical elements in high-energy systems,combined with thermal lens detection theory and CCD imaging technology,and finally proposes a thermal lens detection absorption type based on the combination of pulsed pump light and detection light of area array CCD defective method.Theoretical research work is mainly focused on the detection of absorption defects of optical components,and the specific content are as follows:(1)A thermal lens model of a single absorption defect excited by a pulsed laser is established.The Hankel integral transformation method is used to solve the heat conduction equation of single defect absorption.The temperature field distribution caused by defect absorption is derived,and the effects of the defect size and delay time on the temperature field is simulated.The results show that: the peak temperature rise in the center of temperature field decreases with the increase of delay time for defects of the same size;for defects of different sizes,under the same delay time,the peak temperature rise generated by absorption of large size defects is larger than that of small size defects,which also has an important impact on the photothermal signal generated.(2)Combining Fresnel diffraction theory,quantitatively characterizing the photothermal signal generated by the absorption of defect,and imaging the photothermal signal with CCD to achieve defect detection.The relationship between the photothermal signal and the detection distance,defect size and delay time are analyzed and the core parameters are optimized.The results show that there are an optimal detection distance and delay time to maximize the photothermal signal amplitude.Generally,short delays are used for small defects and long delays are used for large defects.The increase of the defect size makes the amplitude of the photothermal signal generated by the absorption gradually increase.(3)Based as the theoretical model of thermal lens with a single absorption defect,the temperature field and the distribution of photothermal signals of multiple independent defects are further derived.Quantitative analysis of the impact of various key parameters on defect detection when multiple defects are present.The results show that in order to achieve the simultaneous detection of multiple defects,the optimal detection distance should give priority to small defects.(4)The CCD is then used to characterize the photothermal signals of multiple defects,the defect samples are imaged,and considering the limitation of the CCD detection sensitivity,how to select the core detection parameter of delay time is further discussed.The results show that in the case of many different sizes of absorption defects on the surface / subsurface of the fused silica sample,the selection of the delay time should also consider the small radius defects first.When the delay time is short,the amplitude of the TL signal of the large defect is larger than that of the small defect.When the small defect is detected,the large defect can also be detected.At the same time,the delay time meets the requirements of CCD detection.