In this study, we proposed an experimental scheme for measuring the nonlinear phase shift generated by cross-phase modulation using a Fizeau interferometer. Subsequently, a formula was comprehensively obtained for evaluating the phase shift under different probe light powers, control light powers, and bias currents. The results denote that the nonlinear phase shift exhibits a monotonically increasing relation with the control light power and the bias current, and in contrast it exhibits a monotonically decreasing relation with the probe light power. Thereby, the operating points of cross-phase modulation for the semiconductor optical amplifier were optimized. Further, we proposed a method for determining the optimal phase modulation points that affect the nonlinear phase shift, and the optimal phase modulation parameters for a phase shift of π/2 were obtained. The probe light power is 0.29 mW, the control light power is 0.5 mW, and the bias current is 276 mA.