Study On Band-stop Filtering Technology Based On Volume Bragg Gratings

In the high-power laser system,intensity and phase fluctuations in laser beam induced by dust,defects on optical elements etc.will grow rapidly during the propagation in laser mediums,and further lead to the beam filamentation.This physical phenomenon is known as the small-scale self-focusing(SSSF),which will limit the useful power of laser systems either by creating damage or by limiting the focusability of laser beam and is a principal restriction in the realization of high-energy/power performance.Therefore,it is of great practical significance to study how to effectively control the SSSF effect.Traditional method is to use the spatial low-pass filter,including the focused 4f spatial filter and the non-focusing angular filter based on volume Bragg gratings(VBGs),to remove the medium and high spatial frequencies in the laser beam.However,the traditional filters are gradually limited with their inherent drawbacks as the high-power laser system develops.For high-power lasers,previous research has shown that the specific range of medium-high frequencies(MHFs)which has the greatest nonlinear gain will cause the start of SSSF effect in laser beam.Based on this,a band-stop filtering technique based on VBGs was applied in this thesis.It can be designed to only filter the MHFs which possess the fastest growth in the laser beam.This provides a new idea for controlling the SSSF effect in high-power laser systems and overcoming the defects of traditional low-pass filter as well.In this thesis,the suppression of SSSF effect with band-stop filter(BSF)based on VBGs in the high-power laser system is studied theoretically.The formation condition and exact definition of hump volume Bragg grating(HVBG)are presented with the coupled-wave theory,and the diffraction characteristics of HVBG are analyzed in detail.The mathematical model for computing the propagation and amplification of laser beam in nonlinear mediums is built.The filtering processes of BSF based on HVBGs and combined single-peak VBGs in a two-stage amplifier laser system are simulated,respectively.In addition,the theoretical model of FM to AM conversion of VBG is set up,and the FM to AM conversion effects caused by HVBG and single-peak VBG are analyzed and compared.Results show that the HVBG or combined single-peak VBG both can suppress the SSSF effects in the laser beam by controlling the growth of specific MHFs that may has large nonlinear gain.For the HVBG and VBG with the same filtering characteristic frequency,the FM to AM conversion caused by HVBG is weaker than that of the single-peak VBG when broadband beam passes through.