FDTD Analysis On Influence Of Micro-Nano Surface On Near-Field Light Intensity Distribution In KDP Crystal

crystal is widely used as frequency converter and photoectric switch in optical project. Howerever, the defects on the surface or inside KDP crystal as a result from the process of growth and cutting will greatly affect the optical quality of KDP crystal such as laser damage threshold. For example, the different inclusions inside KDP crystal formed in the process of rapid growth and the micro-waviness on the surface of KDP crystal left by single point diamond turning. Therefore, making an in-depth study on the laser damage mechanism and degree affected by those defects not only can explain the causes of damage and provide the theoretical basis for increasing laser damage threshold, but also can guide technics of rapid growth improving and working parameters choosing. Thus, it is significant to reach the rigorous standard for devices of Inertial Confinement Fusion.In this paper, firstly presented the fundamental theory of Finite Difference Time Domain method that we used to analyze the subsequent problems; built the model of uniform plane electromagnetic wave irradiating on the surface of smooth and film-coated KDP crystal by 2-D FDTD method and campared the results with theoretical results; made preparatory work for subsequent analysis.Then, presented the formation and extraction of micro-waviness and built the physical model of defects such as micro-waviness, concave pit and inclusion. analyzed the law of how the defects impacting on near-filed light intensity distribution inside and on the surface of KDP crystal by FDTD. Results indicate: defects will distort the incident wave and lead to distribution of strength and weakness of light intensity inside KDP crystal; the degree of distortion has relation with feature parameters of micro-waviness, concave pits and inclusions.At last, carried out experiments of variable parameter processing using single point diamond turning (SPDT) and laser damage threshold detection of KDP crystal. Results indicate: major damage of KDP crystal is break and the degree of break on back surface is worse than front surface; roughness can not used to be the only criterion of assessment for effect of surface topography on laser damage threshold of KDP crystal; micro-waviness is a crucial influence factor of self-focusing damage.