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有源光纤的两种非线性效应非线性极化（克尔效应）和增益饱和效应都将引起光信号频谱展宽。增益饱和导致包络失真，同时也影响到由非线性极化所造成的自相位调制。这种影响可用一个函数ｆ（ｚ）表示。本文从非线性薛定谔方程出发，导出了ｆ（ｚ）的级数表达式，并指出有源光纤因自相位调制的频谱展宽可按Ｂ＝２Ｆ（１＋μ）估计。由于有源光纤的ｆ（ｚ）远大于１，即使光纤较短，但其影响在高增益高输出功率的多级放大器级联的系统中将成为不可忽略的因素。
The two non-linear effects of active fiber nonlinear polarization (Kerr effect) and gain saturation effect will cause optical signal spectrum broadening. Gain saturation leads to envelope distortion, but also affects the self-phase modulation caused by nonlinear polarization. This effect can be expressed by a function f (z). Based on the nonlinear Schrödinger equation, this paper derives the series expression of f (z) and points out that the active fiber can be estimated as B = 2F (1 + μ) due to the spectral broadening of the self-phase modulation. As the f (z) of an active fiber is much larger than 1, even though the fiber is short, its effect will not be negligible in a cascaded multi-stage system with high gain and high output power.