高功率激光系统中缺陷引起的近场调制分析

Analysis of Near-Field Modulations Caused by Defects in High Power Laser System

高功率激光装置中通过测量光束强度分布来评价近场光束质量。然而测量结果为一稳态分布,并不能真实反映经噪声扰动后光束的强弱调制演变特性,可能会忽略传输过程中存在的调制较为严重的区域。针对此问题着重研究了光束经噪声扰动后的近场传输演化特性,进而对近场分布测量的局限性进行深化理解和补充。同时,为简化分析单因素局部缺陷影响下不同传播距离处的近场分布变化规律,引入等效菲涅耳数来表征近场强度调制特性。结果表明,当实测近场分布中存在较弱调制信息时,测量位置之前可能存在更为严重的调制区域。以调制为π的相位型缺陷为例,其最大调制相对于入射光强度可增大9倍。

In high power laser device, near- field beam quality is evaluated through the intensity distribution. However, the measured distribution is a steady state, which can not reflect the evolution features of intensity modulation caused by noise disturbances. Some area encountering serious intensity modulation may be ignored in transmission process. Near-field transmission characteristics of the beam disturbed by noise disturbance are studied. Meanwhile, deeper understanding of the limitations of the measured near-field intensity distribution is obtained. To simplify the analysis of the near-field intensity distribution at different distance under the influence of single local defect, the characteristics of near-field intensity modulation is represented by equivalent Fresnel number. The results show that once a weak modulated information appears in the distribution of measured nearfield intensity, there may be a more serious modulation area before the measurement position. A highest modulation can be up to nine times for a phase type defects with π delay corresponding to incident intensity.