High-power Laser Optical Elements "defects" Distribution Of The Beam Near-field Quality Statistical Laws

"Defects" on optics such as dust, impurities, scratches, pits and subsurface defects would affect the performance of high power solid-state laser system. It is the main limiting factor in increasing the load capacity of the laser. Finding proper parameters to describe the "defects" distribution, establishing a quantitative relationship between these parameters and near-field beam quality, and setting a method to obtain specifications of optics "defects" distribution are the most urgent things to do to increase the laser power. To achieve these goals, the major research work of this thesis can be listed as follows:1. Relationships between the "defects" distribution and the near-field beam quality were studied qualitatively in spatial domain: spatial statistical model of "defects"distribution was established, two important statistical parameters were abstracted, which were the total density of the "defects" and the power exponent of the "defects" distribution. Statistical property of the near-field intensity was studied theoretically, statistical parameters were obtained, which were the average intensity and the contrast of intensity. Relationships between the "defects" distribution and the near-field quality were researched numerically. The results showed that, the increase of the total density and decrease of the power exponent will increase the contrast of near-field intensity, which would cause the deterioration of the beam quality.2. Power Spectral Density (PSD) was utilized as the parameter to describe the "defects" statistical property in frequency domain. The relationship between the "defects" distribution PSD and one "defect" PSD was researched. Based on the spatial model of "defects" distribution, the relationships between "defects" PSD and spatial parameters were studied.3. Relationships between the "defects" distribution and the near-field beam quality were studied quantitatively in frequency domain: Based on the small modulation approximation, theoretical relation between the "defects" PSD and the near-field intensity PSD was established. Based on the theory of light propagation, the theoretical relation was validated in linear medium, nonlinear medium, and spatial filter. The results showed that, connection between the near-field intensity PSD and the "defects" distribution PSD was the propagation factor of system, this factor is related to system structure and operation parameters.4. Based on the theoretical relation between the "defects" distribution PSD and the near-field intensity PSD, methods to establish specifications of optics "defects" was obtained. Take TIL as an example, specifications of "defects" distribution PSD of key components of this system were deduced.