基于体光栅的耦合波理论,推导了飞秒脉冲通过无吸收、透射型体光栅时衍射光强的频谱表达式和瞬时表达式。在此基础上,讨论了二者随折射率调制度的变化。发现在一定的折射率调制度范围内,衍射脉冲的瞬时变化为:开始是单脉冲,然后中心向下凹陷分裂为二个脉冲,而后是三个脉冲。如果此时折射率调制度进一步增加,则脉冲数目又回复到一个脉冲。在一定的折射率调制度范围内,上述过程是重复发生的。同时发现,在折射率调制度处于1.0×10-3-1.0×10-2区间时,双脉冲间隔随折射率调制度增大而增大。对衍射光强谱随折射率调制度的变化进行了数值模拟,并对衍射脉冲的瞬时变化进行了解释。

Based on the volume grating coupled wave theory,temporal and spectrum expressions of a non-absorption transmission volume grating read out by a femtosecond pulsed laser beam are deduced. Then their changes with the refractive index modulation are discussed. It is showed by numerical simulation that,when the refractive index is increasing in a certain range,at first the temporal diffracted pulse will be one,then the center of the pulse will decrease and split into two pulses,then into three pulses. If increasing the refractive index modulation further more,the number of pulses will return to two,then to one. The above processes will be repeated if the refractive index modulation increased to another certain level. Moreover,if the refractive index modulation changes from 1.0×10-3 to 1.0×10-2,the interval between two diffraction pulses will increase with the increasing of the index. And the simulation of the changes of diffraction intensity spectrum with the refractive index are also mode.