In the abnormal dispersion region when the perturbation frequency is smaller than the cut-off frequency, where modulation instability is thought to occur, the gains of perturbations with different initial phases while with the same frequency will evolve to the same positive value through different ways. 在被认为有调制不稳定的扰动频率小于截止频率的负色散区,频率相同而初相位不同的扰动增益将经历不同形式的演化后趋于同一正值。