Pulse splitting by modulating the thickness of buffer layer of two-layer volume holographic grating

Based on Kogelnik's coupled-wave theory and matrix optics, generation of femtosecond double pulses by modulating thickness of the buffer layer of two-layer volume holographic grating (TL-VHG) is discussed. Expressions of diffraction field when a femtosecond pulse incidents on the TL-VHG are deduced. Simulation results show when thickness of the buffer layer increases from 6mm to 11 mm or even larger, one incident pulse splits into double femtosecond pulses with the same duration and peak intensity, and pulse interval is linearly proportional to the thickness. The reason of these phenomena is due to the interference of diffraction waves reconstructed from two gratings and phase shift resulting from the buffer layer thickness. Time-delay of diffracted double pulses is explained by group time delay of periodic media. It is shown that the slope of the pulse interval with respect to the thickness of buffer layer is 2 times of that of pulse time-delay. Furthermore, we demonstrate it is possible to control the output double pulses' duration and pulse interval by varying the grating thickness.