Investigation On Laser Damage Characteristics And Acoustic Emissions Of Millisecond Laser-K9 Glass Interaction

Investigation on laser-induced damage of the optical materials has attracted much attention since the invention of laser,and plenty of the investigations are focused on the interaction between nanosecond laser and optical materials.In this dissertation,millisecond-laser induced damage characteristics are studied experimentally and numerically,and acoustic emission characteristics during the interaction are analyzed.The influences of focal lengths and focal positions on the damage probability and the damage morphology near the front and the rear surfaces of K9 glass are studied by millisecond laser damage test platform.And acoustic emissions generated during the interaction are detected.Results show that,when laser is focused at the front surface,damage is mainly caused by melting process.And conical pits extending from front surface to interior are observed.The size of these pits is in the order of hundreds of micrometers.Strong acoustic emission is accompanied with the formation of conical pit,and the spectrum of acoustic signal is mainly distributed in the frequency range of 10~5kHz.In this case,the duration of acoustic signal is positively correlated with pit size.But when laser is focused at the rear surface,stress damage is dominant,and the corresponding damage morphology is irregular with larger size in the order of centimeters.Weak acoustic emission during the interaction is detected with spectrum evenly distributed in the range of 10~20kHz.For longer focal lengths,damage probability decreases more slowly with the increasing distance between focal plane and sample surface,which is mainly due to the influence of focal length change on fluence spatial distribution.Thermodynamic model is established to simulate the irradiation of millisecond laser on K9 glass.Temperature field and stress field of K9 glass are calculated by the Finite Element Method.Results show that,stress damage is mainly caused by radial stress and hoop stress.For shorter focal length,spot size is smaller and the temperature rise of interaction center is higher,which causes molten damage to sample.In this case,the temperature gradient around interaction center is more obvious,and the size of pit is smaller,which is consistent with experimental results.This dissertation can provide some references for the study on damage mechanism and damage process of long pulse laser induced damage.