在高功率激光驱动器中，调频到调幅（FM-to-AM）效应是制约聚变点火的一个关键物理量，因而需要定性分析和定量评估。基于晶体的可接受带宽，从光谱畸变和瞬时频率两个角度研究了频率转换过程中FM-to-AM效应的物理机制。利用瞬时频率定性分析了相位调制和强度调制之间的相关性，得到了晶体最佳匹配角度情况下三倍频强度调制频率的变化规律。根据推导出的表征调制度的解析模型，定量分析了三倍频输出强度调制特性，量化了输入带宽和功率密度对调制度的影响，通过数值模拟验证了该模型的可靠性。所得结果为抑制三倍频过程中FM-to-AM引起的强度调制提供了理论指导。

In high power laser driver, frequency modulation to amplitude modulation (FM-to-AM) effect is a key parameter that could restrict fusion ignition, so qualitative investigation and evaluation are necessary. Based on acceptable bandwidth of the crystal, the physical mechanism of FM-to-AM effect in the process of frequency conversion is investigated from aspects of spectrum aberration and instantaneous frequency, respectively. Using instantaneous frequency, the correlation between phase modulation and intensity modulation is qualitatively analyzed, and the variation regularity of modulation frequency regarding the third harmonic generation (THG) intensity at the optimal phase matching angle is presented. With analytical derivation of modulation degree model, the modulation characteristics of output THG laser are quantitatively analyzed and the influences of input bandwidth and power density on THG intensity modulation are quantitatively studied as well. By numerical simulation, the reliability of the model is verified. All obtained results can give theoretical guide to suppress intensity modulation induced by FM-to-AM effects in THG process.