Research On The Modulated Femtosecond Double Pulses Based On Dynamic Diffraction Of Volume Holographic Grating

Due to its ultrashort pulse duration and ultrahigh peak power,femtosecond pulses has been a powerful tool to reveal the interaction between light and materials,and have been widely used in the fields of physics,chemistry,biology,etc.However,in some practical applications,we need femtosecond pulses with specific waveforms,such as femtosecond dual pulses,so it is necessary to shape the femtosecond pulse output from the femtosecond laser.The shaped femtosecond dual pulses have very important uses in many fields,such as pump-probe,femtosecond micro-machining,quantum coherent controlling,chemical reaction controlling,ultrashort pulse detecting.In this thesis,based on Kogelnik coupled wave theory,by studying the diffraction of femtosecond pulse and chirped femtosecond from the sinusoidal volume holographic grating(VHG),the femtosecond double pulse with adjustable pulse interval and pulse waveform are obtained.At the same time,the physical mechanisms of modulation of the femtosecond dual pulses are studied.The main research contents are as follows:1.The dynamic diffraction characteristics of femtosecond pulsed diffracted by volume holographic grating are studied.Based on the Kogelnik's coupled wave theory,the coupled wave equations of the femtosecond pulse diffracted by the volume holographic grating are derived,and the diffracted and transmitted fields and intensities are acquired.The dynamic diffraction characteristics of the femtosecond pulse passing through the volume holographic grating—energy oscillation and pulse splitting—are found and analyzed by interference of fields.2.Based on the diffraction of a femtosecond pulse by a volume holographic grating,femtosecond double pulses with adjustable pulse interval are acquired.Based on Kogelnik's coupled-wave theory,it is found that when a femtosecond pulse incidents on the transmitted VHG,four fields,two transmitted and two diffracted waves emerge inside the VHG,then two transverse standing waves along grating vector direction will be generated inside the VHG.Due to each standing wave propagates along specific grating layers,thus two standing waves have two different velocities along propagation depth.If thickness of the VHG is large enough,two transverse standing waves will separate from each other,and on the output plane of the VHG,femtosecond dual pulses are generated on both diffraction and transmission.Results show that the pulse interval of the dual pulses is determined by the refractive index modulation and thickness of the VHG,and independent of the grating period.While the waveform of the dual pulses depends on the incident femtosecond pulse and is independent of the VHG parameters.3.Based on the diffraction of chirped femtosecond pulse by a volume holographic grating,femtosecond double pulses with adjustable pulse waveform are acquired.The coupled wave equations of a femtosecond linear chirped pulse diffracted by a VHG are derived,and find that the femtosecond dual pulses with pulse duration and peak intensity are modulated by chirped parameter of incident femtosecond pulse.Results show that when the chirped parameter is larger than 0,dual pulses are broadened,while when the chirped parameter is smaller than 0,dual pulses are condensed.However,the broadened and condensed values of each pulse are different,so femtosecond dual pulses with different pulse duration are acquired.The influence of material dispersion on the diffraction is also studied,it is found that the dispersion will increase the broadening and condense effects.