Spectral Beam Quality Study Based On Volume Bragg Grating

Volume Bragg grating(VBG)is a kind of refractive index phase grating,and the refractive index changes periodically in the base material.According to the different structure,it can be devided into transmitting VBG(T-VBG)and reflecting VBG(R-VBG).The physical nature of both two are the same:The process of Bragg diffraction can be seen as elastic scattering of light waves by acoustic waves.The Bragg diffraction not only has selectivity to the incident angle,but has selectivity to the wavelength,this is the unique diffraction characteristic of VBG.Spectral beam combining based on VBG use the unique diffraction characteristics to combine laser beams with different wavelengths in the spectral domain to achieve power scaling.In accordance with the beam diameter superposition before and after combination,spectral beam combining belongs to shared-aperture beam combination,which mitigate these physical restrictions in single laser output,i.e.,thermal effect in gain medium,nonlinear effect damage and so on,is an effective approach to improve laser power while maintain the beam quality.The beam quality is an important basis for the evalution of laser propagation and focusing,which is closely related to the interaction of high energy lasers with matter.Thus,the output beam quality after combination is essential in spectral beam combining system.The dissertation focuses on the beam quality analysis of high power spectral beam combining system,which are thoroughly studied from two aspects:VBG and sub-beams.The influence factors include properties of diffraction,spatial dispersion,thermal mechanical of reflecting and transmitting VBG(R-VBG and T-VBG),as well as beam size,spectral width and divergence of sub-beams.The main contents and results of this dissertation are as follows:(1)Based on the diffraction efficiency of infinite planar beams,we derive the average efficiency formula of the actual finite beam with spectral width,divergence and finite beam size.The influence of spectral width,divergence and beam size on the average efficiency is analyzed.The results show the spectral width is large,or beam divergence and beam size are large,the average efficiency is small.(2)It is theoretically predicted and experimentally proved the spectral directivity of diffracted wave when incident beam is deviated from Bragg wavelength.On the basis of angular directivity of diffracted wave in Bragg angle-mismatched readout,we study the spectral directivity of diffracted wave,and propose the angular spatial dispersion and spectral spatial dispersion to characterize the spatial dispersion properties of a VBG,its significance lies in the fact the angular and spectral spatial dispersion will introduce additional divergence to the sub-beams.The results show that the directivity of diffracted wave in Bragg-mismatched readout agrees well with the Ewald sphere vector model(ESVM).Compared with T-VBG and R-VBG,the difference in angular spatial dispersion is not significant,but the spectral spatial dispersion of T-VBG is much bigger than R-VBG.For R-VBG,the spectral spatial dispersion is small when its grating vector angle is near orthogonal inclination,and is zero for normal R-VBG.(3)Considering the coupling of the incident beam conditions and dispersion of a VBG,a single VBG beam quality analysis model is established.We study the influences of beam spectral width and divergence on single diffracted beam quality,as well as the influences of beam wavelength instability and optical axis jitter on combined beam quality,respectively,the results show the interaction of beam spectral width,divergence and spatial dispersion of a VBG will introduce additional divergence to single diffracted beam,while beam wavelength instability and optical axis jitter mainly affects the far field overlap ratio between sub-beams.In the single VBG beam quality analysis model,the diffracted beam quality is closely related to grating groove density,beam size,beam spectral width and initial beam quality,when grating groove density and beam size is small(i.e.the grating period is small),or the spectral width and initial beam quality become worse,the deterioration of beam quality after diffraction is significant.(4)Using finite element method,a heat conduction analysis model which describes interaction of high power laser with VBG is proposed,and theoretically one can obtain the temperature distribution at any position of a VBG.We analyze theoretically and experimentally the impact of thermal effect of a VBG on diffraction efficiency and beam quality of the diffracted beam.The results show that the transmitted beam contributes much more significantly to the heating of the VBGs;Laser-induced heating of VBGs under high-power radiation will cause change of VBG parameter,mainly reflected in the groove density(spatial period or spatial frequency)changes duo to thermal expansion of VBG,this lead to red shift of resonance wavelength for a VBG with drift of 7?pm/K;Another detrimental effect is that the "heated" VBG can cause surface deformation and produce a"positive lens",this lead to different affect on the wave-front of transmitted and diffracted beam,causing an inaccurate measured beam quality once they are combined.The simulation of cooling for VBGs shows good results,therefore the cooling measures should be taken into account in the combining system.(5)We investigate the feasibility analysis of a 100 kilowatts VBG-based spectral beam combining system.Considering the current manufacturing technology of VBG and development of high quality beam,towards 100 kilowatts application target,we set the boundary conditions for VBGs and sub-beams under certain assumptions.The results show that there are still some difficulties in the fabrication of high efficiency multiplexed VBG.By appropriately optimize the spectral channel spacing of a combining system,to date we experimentally obtain highest combining output,to the best of our knowledge,while the spectral density does not significantly decrease.Through the above several aspects of the work,we have a more profound understanding of the influence factors of beam quality in high power spectral beam combining system.We theoretically and experimentally give a comprehensive analysis of the effects of spectral width and beam divergence,dispersion of VBG and thermal effect on laser beam quality.This provides guidance for the application of VBG in higher power combining system.