单调变化的渐变折射率波导的表面折射率大于波导基模的有效折射率，因而不能用传统的m-线方法测量。利用分析转移矩阵(ATM)方法研究了渐变折射率波导中的表面等离子波，提出了一种确定波导近表面折射率的实验技术，在波导折射率分布的拟合中，得到了比传统逆WKB方法更为合理的结果。

Surface refractive index of a monotonously graded-index waveguide, which is greater than the effective index of the fundamental mode and cannot be measured in conventional m-line spectroscopy, is experimentally determined by employing the surface plasma wave (SPW) resonance technique based on analytical transfer matrix (ATM) method. The index profiles predicted are more accurate than those obtained by the conventional inverse WKB (Wentzel, Kramers, Brillouin) method.