掺杂半径的选择是部分掺杂光纤能够有效选模的关键。从模式的归一化模场分布出发,通过对模式场分布的比较,确定采用较小或较大的掺杂半径均不利于基模的选取。从饱和增益和功率分配两个角度详细分析了掺杂半径的变化对各模式的增益获取能力和功率提取能力的影响。纤芯掺杂半径的变化改变了模式场与增益粒子的相互作用机会,因而改变了各模式的增益获取能力及模式功率的输出大小。当掺杂分布与基模场分布达到最佳重叠并使得基模在强度上占据优势时,则基模获得的增益超过其他模式,输出功率达到最大。定量计算结果说明,阶跃光纤采用0.6左右的相对掺杂半径可以有利于提取基模。

The choice of doping radius is the key of ensuring the part-doped fibers to play an effective role in mode selections. This paper begins with the normalized mode distributions. By comparing the intensity distributions of different fiber modes,the use of smaller or larger doping radius is deemed not to be conducive to the fundamental mode selection. And then from the views of saturation gain and mode power allocation,the influences of doping changes on the mode ability to gain access and the ability to power extraction are analyzed in detail. The varying of doping radius changes the interaction opportunities between the mode field and the population inversion,and then changes the mode ability to gain access as well as its mode power output. When the gain doping optimally overlaps with the fundamental mode distribution,and the fundamental mode intensity gets the advantage over other modes,the access gain of the fundamental mode will exceed any other mode,and its power will reach the maximum value. Some quantitative results have indicated that selecting about 0.6 relative doping radius for step-index fiber can help us to efficiently extract the fundamental mode power.