fresnel approximation meaning in English

菲涅耳近似
菲涅尔近似

On the other hand , the electric field and intensity distributions of the he11 mode in the hollow fiber are calculated by using the exact solutions of maxwell equations based on the vector model , and the diffracted near - and far - field distributions of the he11 - mode output beam under the fresnel approximation are studies . we derive an analytical expression on the far - field distribution of the he n - mode output beam in free space and discuss its applicable condition . we also analyze and compare the differences between the he11 - and lp01 - mode output beams , and find that the near - field distribution of the lpoi - mode output beam is a gaussian - like one , but the near - field distribution of the he11 - mode output beam is a doughnut - like one , whereas the far - field distribution of both the lp01 - and he11 - mode output beams are a doughnut one
此外，我们把中空光纤中lp _ ( 01 )模(标量模型)和he _ ( 11 )模(矢量模型)的场分布以及它们各自的输出光束的近场与远场分布进行了比较，发现两种模式的输出光束在近场分布有很大的区别， lp _ ( 01 )模输出光束在近场是一个高斯分布，而he _ ( 11 )模输出光束在近场是一个中空光束，在远场两种模式的输出光束分布是基本一致的，这正是由于在中空光纤中采用了弱波导近似以后所引起的误差。