Femtosecond pulse shaping by modulating the refractive index modulation of volume holographic grating

Based on the modified Kogelnik's coupled-wave theory, time- and frequency-domain diffractions of a femtosecond pulse from transmitted volume holographic gratings (VHGs) are theoretically studied. Results show that when the refractive index modulation of the VHG changes in a certain range, the number of temporal diffracted pulse will evolve from one to two, then to three, and this pulse number evolution is periodic. This particular phenomenon can be explained by diffraction intensity spectrum and the overmodulation effect of refractive index modulation of transmitted VHG. Moreover, we find centers of all temporal diffracted pulses translate along the negative time axis, and the translation is irrelevant to the refractive index modulations. We will use time delay of volume grating to give a reasonable explanation.