Research On On-line Monitoring Technology Of Laser Damage Threshold Based On Scattered Light

With the development of laser technology,high-power lasers such as fiber continuous lasers,pulse train lasers,and high-frequency quasi-continuous lasers are becoming more and more important in the processing industry,and their application fields are becoming wider and wider.The laser damage threshold is always one of the main research contents in the field of laser-substance interaction.Therefore,this paper has carried out research on the laser damage threshold online monitoring technology based on scattered light,as follows:In terms of theory,based on the theory of controlled heat transfer and geometric optics,two-dimensional and three-dimensional numerical calculation models are established using COMSOL software.Taking aluminum targets as examples,the relationship between the laser-induced target surface deformation and the laser damage threshold is calculated and analyzed.The central temperature rise curve was obtained,and the relationship between the two-dimensional temperature field distribution of the target surface and the change of damage morphology at different times of laser irradiation was analyzed.The small topographic changes on the target surface caused illumination light reflection and scattered light laser light with time.Evolution process.The relationship between the two-dimensional tracking result of the scattered light of the illumination light and the three-dimensional ablation hole morphology of the target when the laser is applied at 30?s,300?s,and 1ms is calculated.Etched holes scatter and the light distribution of scattered light changes significantly.It was concluded that when the depth of the ablation holes was 0.1 mm-1 mm,the light distribution gradually diverged and weakened to a minimum at 1 mm.In terms of system design,the laser damage threshold online monitoring system design and system static test analysis are required for the detection of reflected spots.Including the overall system design,hardware design(optical and electrical design)and software design.The optical design includes an expanded beam collimation and focusing system for the damage laser,and an illumination light collimation system.The circuit system design includes a photoelectric detector photoelectric conversion circuit,a weak signal preamplifier,a filter circuit,a difference operation circuit and a division operation circuit,etc.,and the signal is analog / digital converted.In order to meet the high-sensitivity response to the change of the scattered light field distribution of the illuminating light,a PIN four-quadrant photodiode is used as a photodetector innovatively.The core design idea of the system software part is based on the judging concept of the spot offset threshold and the quadrant amplitude threshold,including four parts: instrument control,calculation processing,system calibration,and laser emergency stop.After calibration of the monitoring system,static test analysis was performed on the results of the ablation morphology of the aluminum target,which verified the overall feasibility test of the system.In terms of experiments,a dynamic threshold monitoring experiment of laser ablation under the action of continuous laser was established.By changing the laser power applied,150 W,200W,250 W,and 300 W were used as typical test values.The relationship between the change of the molten pool on the target surface and the monitoring data under the action of continuous laser was analyzed.Experimental research is focused on two cases of target damage and target melting threshold.The device designed in this paper can make a reasonable choice for the power and time of laser in laser processing.