Laser-induced Damage On Fused Silica With Photo-acoustic Spectrum Analysis

The laser induced damage of optical elements has always been one of the important factors which limits the improvement of load capacity for high energy and high power laser system.Therefore,the study on laser induced damage mechanism and laws of optical components,the improvement of optical components processing technology and the extension of useful life for optical components is always the most important issue in high energy and high power laser technology development.However,the damage mechanism of optical components includes multi-photon ionization,avalanche ionization,impurity-induced damage and nonlinear effects,and the damage process has a coupling effect and the development of online,effective and convenient method for damage threshold measurement is required.Currently,four traditional test methods have been used on damage judgement and measurement,including the phase contrast microscopy,the photothermal deflection method,the CCD monitor and the photo-acoustic method.Compared to the traditional methods,the photo-acoustic method can realize online,convenient,efficient and low-cost test,but the detection accuracy is still a problem and needs to be improved..In this thesis a new judgment of photo-acoustic method is proposed,in which the judgment from the change of Photo-acoustic signal amplitude is improved to that of photo-acoustic spectrum analysis.The photo-acoustic spectrum analysis improves the accuracy of photo-acoustic method,provides a clue for early-damage judgement and can realize a convenient,efficient,low-cost,accurate and online test.This work may also provide a new direction for the research on damage mechanism of optical elements.In this thesis,the polyvinylidene fluoride(PVDF)piezoelectric film is used to collect the photo-acoustic signals generated during the damage process,which improves the photoacoustic detection sensitivity,the amplifying circuit and electromagnetism shield are designed to realize the fidelity amplification of photo-acoustic signal.The position of photo-acoustic probe attached on the sample and pasting technology is investigated.The photo-acoustic measurement is designed and the experimental setup is established.The best wavelet basis and reconstruction order are chosen to filter the electronic noise mixed in photo-acoustic signal,and the Fourier transform is used to extract the spectral characteristic peak of filtered photo-acoustic signal.The laser-induced damage of fused silica sample is judged with the 1-ON-1 procedure,based on photo-acoustic spectrum analysis.To evaluate the accuracy of photo-acoustic spectrum analysis,the confocal microscopy is used to observe the sample surface and the results of judgment of laser induced fused silica damage are consistent with that of the confocal microscopy.The damage threshold of fused silica sample made in the Lab is about 20.9 J/cm2 and the damage morphology observed with microscopy is gray haze,and the diameter and depth of the largest crater in gray haze is 6.7 ?m by 200 nm.The result shows that the photo-acoustic spectrum analysis improves the the measurement accuracy to the early-damage of fused silica and the reasearch in this thesis may provide a new solution for online and early-damage judgment for fused silica and other optical components..