理论分析并仿真了单模光纤中前向和后向拉曼Stokes光功率随输入抽运光功率的变化规律，发现其具有明显的阈值特性。由此提出了一种新的单模光纤拉曼阈值的定义，将拉曼Stokes光功率随输入抽运光功率变化曲线的二阶微分的最大值对应的输入抽运光功率定义为单模光纤拉曼阈值，还给出了与之相应的单模光纤拉曼阈值的测量方法，通过对单模光纤拉曼阈值随单模光纤特征参数的变化规律进行仿真和拟合，分别获得了单模光纤中前向和后向拉曼阈值方程。搭建了实验平台，采用脉冲光抽运，对长度为24 km的单模光纤中前向和后向受激拉曼散射的阈值特性进行了实验验证，结果表明，实验结果与理论分析和仿真结果相符合，验证了所提出的阈值定义和测量方法的有效性，实验中还发现，在不同的光纤特征参数下，单模光纤后向拉曼散射阈值比前向时平均提高了24.9%，与理论仿真结果的25.3%相符合，表明单模光纤中前向受激拉曼散射更容易产生。

The power evolutions of forward and backward Raman Stokes wave versus input pump power in a single mode fiber (SMF) are theoretically analyzed and simulated, which demonstrates an obvious threshold characteristic. A novel definition for Raman threshold in a SMF is proposed. The input pump power corresponding to the maximum of the second-order derivation of the power evolution curve of Raman Stokes wave versus input pump power is defined as the Raman threshold. A practical measurement approach for Raman threshold in a SMF is also demonstrated based on the proposed Raman threshold definition. By simulating and fitting the dependence of Raman threshold on the fiber characteristic parameter, the theoretical threshold equations for the forward and backward Raman scattering in a SMF are obtained, respectively. An experimental setup is established to verify the proposed definition and measurement method. The threshold characteristics for the forward and backward Raman scattering in a SMF with a length of 24 km are studied by using a pulsed pump. The experimental results agree well with the theoretical predictions and verify the effectiveness of the proposed definition and measurement method. The experimental results also demonstrate that the threshold of backward Raman scattering is approximately 24.9% in average higher than that of forward case at different fiber characteristic parameters in a SMF, which is coincident with the theoretical value of 25.3% obtained by simulation. The forward stimulated Raman scattering is relatively easier to occur in a SMF.