| Optically addressed spatial light modulators(OASLMs)are widely used in high power laser systems such as high energy lasers,laser processing,laser communications,and laser weapons.With the development of high power laser systems in the direction of high repetition frequency and the improvement of high throughput operation capabilities,the modulator is also facing the application requirements of improving laser damage resistance.In recent years,the research has mainly focused on the damage of the single layer structure in the modulator under high peak power laser irradiation.The systemic damage mechanism and theoretical explanation of the modulator under high power laser irradiation still need to be further improved.Therefore,this dissertation carried out systematic theoretical and experimental studies on the damage characteristics and performance degradation of the modulator,and proposed a control and design scheme for the high damage threshold OASLM,which provides technical support for improving the laser damage resistance of the modulator.At the same time,in order to further expand the application of the modulator,this thesis carried out the research on the surface exposure additive manufacturing technology based on the OASLM.Compared with the traditional point scanning method,the surface exposure scanning method has great advantages in terms of forming efficiency,forming accuracy and residual stress control level.However,the physical process of surface exposure additive manufacturing is still unclear,and it is necessary to deeply analyze the influence of process parameters on the forming effect.The major work and research results are as follows:1.In order to understand the systematic damage characteristics of OASLM,a thermodynamic damage model of high power laser incident modulator is established,and the damage effects of the combined effects of each layer structure on the device are studied.The simulation results show that under the irradiation of high peak power laser,under the influence of thermal stress,the hoop tensile stress of indium tin oxide(ITO)film almost exceeds its tensile fracture stress,resulting in fracture damage of ITO film.At the same time,the high temperature generated by ITO will cause the temperature of the polyimide(PI)alignment layer to exceed its melting temperature,resulting in the dissolution or shedding of the PI film layer,and the simulation results are consistent with the experimental test results.In addition,in order to improve the laser damage resistance of the device,the damage effect of the heat dissipation film layer on the device is analyzed by simulation.The results show that adding a heat dissipation film layer can greatly reduce the temperature of the ITO film and effectively alleviate the thermal damage.2.For the performance degradation of OASLM under high average power laser irradiation,theoretical and experimental studies on the performance degradation of the modulator are carried out,and the physical characterization of the performance degradation caused by the thermal effect of the modulator is improved.Theoretical research results show that under high average power laser irradiation,it is mainly due to the thermal deposition of the ITO film absorbing heat,and the heat cannot be dissipated in the liquid crystal layer in time,resulting in changes in the birefringence and threshold voltage of the liquid crystal,and resulting in modulator performance degradation.When the temperature rise is serious,the liquid crystal will even reach the clear point,making the modulator completely useless.The experimental results show that when the performance of the modulator is degraded,the performance degradation can be compensated by adjusting the driving parameters of the device,which is consistent with the theoretical research results,which improves the laser damage resistance of the modulator from 7.5 W/cm2 to 21 W/cm2.3.For the requirement that the OASLM can withstand high laser power irradiation,the simulation and experimental research on the heat dissipation structure of the modulator are carried out,and the heat dissipation effect of the device is improved by using sapphire as the liquid crystal cell substrate of the modulator.The simulation results show that the heat dissipation level of the modulator using the sapphire substrate has been greatly improved,but due to the serious thermal deposition of the bismuth silicate(BSO)crystal,there is a large temperature difference between the sapphire and the BSO crystal.This problem is improved after bonding the sapphire substrate.The experimental results show that the laser damage resistance of OASLM with unilateral sapphire structure is increased to 75 W/cm2.However,due to the difference in thermal expansion coefficients of the substrates on both sides of the liquid crystal cell,the deformation produced under higher power laser irradiation is inconsistent,which will cause the liquid crystal cell is unglued,and a vacuum area appears inside.OASLM with double-sided sapphire structure has improved the laser damage resistance to 100 W/cm2 and can work stably for a long time.4.To expand the application of OASLM,the melting forming model of 316L alloy steel surface exposure additive manufacturing is established,and the influence of laser parameters,powder layer thickness and pulse laser energy on the melting forming effect is studied.The model calculation results show that the selection of process parameters needs to consider many factors comprehensively,and the optimal combination of process parameters has been established from the aspects of printing efficiency and molding quality.Afterwards,the problems faced by OASLM in surface exposure additive manufacturing systems are studied.The simulation results show that the device has serious thermal deposition under repeated frequency laser irradiation,and the laser damage resistance of the modulator can be further improved through additional active heat dissipation measures.Highlights of this thesis are as follows:1.Based on Lambert Beer’s law,a thermodynamic damage model for OASLM is established,which successfully explains the systemic damage characteristics of the modulator,rather than the damage mode of a single structural layer.On this basis,the design scheme of the heat dissipation film is proposed,which effectively reduces the thermal effect of the ITO film under laser irradiation,and provides an improved design idea for the OASLM with high damage threshold.2.It is proposed to compensate for the performance degradation of OASLM through optimization of driving parameters.For the first time,systematic theoretical and experimental studies have been carried out on the temperature response characteristics,performance degradation,driving voltage,and current compensation of modulators,improving the characterization of performance degradation of modulators under high average power laser irradiation,providing important guidance for achieving OASLM with higher laser damage resistance.3.It is proposed to use sapphire as the substrate of the liquid crystal cell of the OASLM,which improves the laser damage resistance of the modulator by more than ten times,and has good optical performance and modulation ability,providing technical support for the optimal design of the modulator.4.The additive manufacturing model of surface exposure scanning mode is established,and the influence of process parameters on the forming results is analyzed.The understanding of the physical process of surface exposure forming is deepened,and the optimal parameters for surface exposure additive manufacturing are established.On this basis,the application requirements of OASLM in additive manufacturing are deeply understood,which lays a research foundation for the further improvement of the modulator’s laser damage resistance and the follow-up work of metal surface exposure additive manufacturing. |
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