在光纤研制过程中掺铒光纤(EDF)的纤芯折射率控制尤为关键。对采用改进型化学气相沉积(MCVD)技术沉积纤芯疏松层，并用溶液浸泡法，采用不同铝离子浓度的氯化铒溶液制作的四种掺铒光纤进行了折射率测试和电子探针微小分析(EPMA)。提出了掺铝将改变原疏松层中的二氧化硅和二氧化锗的比例，铝离子进入疏松层越多，最后得到的掺铒光纤纤芯的二氧化锗的摩尔分数就会越少，用氧化铝生成和二氧化锗挥发两个化学反应式进行了解释。掺铝和掺锗都会提高纤芯的折射率，但由于锗减少引起的折射率降低量大于铝提高引起的折射率提高量，导致掺铝后纤芯的整体折射率下降。

It is well known that the fiber core refractive index control in fiber fabrication is important, especially for erbium doped fiber (EDF). The refractive index will affect the cutoff wavelength of EDF, and then will bring much impact on the gain performance of EDF. In this paper, the refractive index was measured and the composition of several EDFs was analyzed by electronic probe microbeam analysis (EPMA). The EDFs were fabricated through modified chemical vapor deposition (MCVD) method combining with solution doping technology. It was found that the aluminum codoping would seriously affect the ratio of silicon to germanium in the core, the more aluminum incorporated into the porous layer, the less germanium would stay in the EDF. It was explained with the two chemistry equations of alumina creation and germanium oxide volatilization. Codoping aluminum and germanium into silicate both would increase the refractive index, but index decreasing brought by the amount of germanium decreasing was larger than the index increasing brought by the amount of aluminum increasing, which led to the refractive index decreasing along with the amount of aluminum increasing.