Damage Identification And Property Study Of Optical Elements In Terahertz Band

Laser system to irradiate higher energy relies on the ability of optical components to resist laser damage,The laser damage resistance depends on the accurate measurement of laser damage threshold.The key in the test is to identify the damage of optical components.Terahertz time domain spectroscopy(THz-TDS)is a rapidly developing and new technology for nondestructive testing,the amplitude and phase of the sample in terahertz band can be extracted simultaneously,the refractive index and absorption coefficient of the sample can be achieved conveniently and quickly to analyze the sample physical and chemical properties.Based on this,terahertz time-domain spectroscopy technology was applied to the research of laser induced damage identification of optical components,and a new research idea of laser-induced damage identification was sought,the optical characteristics of optical components after damage was explored in terahertz band.The main research contents are as follows:(1)The transmission terahertz time-domain spectroscopy system was used to measure the undamaged region and damaged region of fused silica glass,K9 glass and Si sample,the terahertz time domain signal was obtained,based on Fourier transform,the amplitude,refractive index and absorption coefficient in terahertz frequency domain were calculated;By analysis and comparison,it showed that the identification of the laser-induced damage of three samples could be realized when the peak to peak value in terahertz time-domain and the amplitude spectrum in frequency-domain decreased,and the absorption coefficient increased.Using this recognition method,according to zero damage probability threshold calculation method in the international standard ISO11254,the laser damage threshold of Si sample was tested,and the result was 1.56J/cm2;In order to verify the correctness,microscope identification specified in international standard was utilized to measure the threshold of Si sample,and the result was 1.45 J/cm2,these two test results were similar,which showed that the damage identification of Si sample using terahertz time domain spectroscopy was feasible and effective.(2)Terahertz time domain spectroscopy was applied to the laser-induced damage identification test of TiO2,LaTiO3 and ITO thin film,which deposited on Si substrate,by the transmission terahertz time-domain spectroscopy system,the terahertz time domain signals of the damage region and undamaged region induced by laser were obtained,based on the calculation modal of optical parameters for thin film,the amplitude,refractive index and absorption coefficient in terahertz frequency domain were obtained;By comparing these parameter difference between the damage region and undamaged region,it found that the damage of TiO2 thin film and LaTiO3 thin film was identified when the peak to peak value in terahertz time-domain and the amplitude spectrum in frequency-domain decreased,and the absorption coefficient increased under the condition of no damage to the substrate,and the damage of ITO thin film was identified when the peak to peak value in terahertz time-domain and the amplitude spectrum in frequency-domain increased,and the absorption coefficient decreased under the condition of no damage to the substrate;According to zero damage probability threshold calculation method in the international standard ISO11254,this damage identification method was used to measure laser induced damage threshold of TiO2 thin film,LaTiO3 thin film and ITO thin film,the results were obtained as 0.64J/cm2,0.56J/cm2and 0.62J/cm2 respectively,by comparing the threshold of 0.55J/cm2,0.53J/cm2 and 0.57J/cm2 based on microscope identification specified in international standard,the results were similar,which showed that the damage identification of these three thin film samples using terahertz time domain spectroscopy was practical.(3)According to the way of increasing the laser energy density gradually,fused silica glass,K9 glass,Si sample and TiO2,LaTiO3 and ITO thin film deposited on Si substrate were damaged by laser irradiation,the damaged areas were measured by transmission terahertz time-domain spectroscopy system,using the terahertz time domain signal obtained and the amplitude in frequency domain achieved by Fourier transform,the damage change law was studied.It showed that with the increase of laser energy density,the peak to peak value in terahertz time-domain and the amplitude in frequency-domain gradually decreased,the damage degree of the damaged area was deepened;So the reduction extent of the peak to peak value in terahertz time-domain and the amplitude in frequency-domain could be utilized to determine the damage degree of the above samples.(4)According to refractive index and absorption coefficient of the damaged and undamaged region of fused silica glass,K9 glass,Si sample and TiO2,LaTiO3 and ITO thin film deposited on Si substrate,the reason for the change of refractive index and absorption coefficient after damage was preliminarily studied,it found that the change of microstructure of sample occurred,there were broken bond,ionic bond and band gap decrease.