【作者】 沈本剑；

【导师】 谭吉春；

【作者基本信息】 国防科学技术大学 ， 物理学， 2012， 博士

【摘要】 高功率水平、较好光束质量的激光在激光泵浦、激光切割与加工、惯性约束聚变ICF等领域具有重要应用。谱合成是获取高功率激光光束的有效途径之一,空间滤波是控制激光光束质量重要措施之一。本文针对谱合成技术和空间低通滤波技术展开深入研究,为高功率激光的获取及光束质量控制提供参考。由于热效应的影响,单个激光器的输出功率有限。谱合成技术利用衍射元件将多束波长不同的激光光束合成为一束光束,合成光束在近场和远场均能保持高度的重叠性,且不需控制各光束间的相对相位,具有操作简单的优势,是实现高功率激光输出的有效方式之一。论文阐述基于透射体布拉格光栅和反射体布拉格光栅的谱合成技术,并对两种谱合成方式进行对比分析。结果表明：单色光入射时,体布拉格光栅的衍射旁瓣是限制谱合成光谱间距和谱合成效率的主要因素。准单色光入射时,入射光束的光谱宽度与体布拉格光栅波长半宽度的相对大小是限制谱合成效率的主要因素,体布拉格光栅衍射旁瓣是限制谱合成光谱间距的主要因素。在准单色光入射的情况下,当入射光束的光谱宽度为体布拉格光栅波长半宽度的40%,且透射体布拉格光栅对入射光束中心波长的衍射效率为100%时,透射体布拉格光栅对入射光束的衍射效率为90%；当入射光束的光谱宽度为体布拉格光栅波长半宽度的50%,且反射体布拉格光栅对入射光束中心波长的衍射效率为99.99%时,反射体布拉格光栅对入射光束的衍射效率高达99.9%。准单色光入射时,透射体布拉格具有色散作用不利于进行谱合成。为解决谱合成中装置体积庞大、光谱间距较大等问题,分别提出解决方案。针对多光束谱合成中采用常规反射体布拉格光栅造成的谱合成装置体积过大的问题,分析采用多重反射体布拉格光栅实现多光束谱合成技术的可行性。为实现pm量级光谱间距的谱合成技术,提出相移反射体布拉格光栅谱合成模型。为抑制反射体布拉格光栅的衍射旁瓣,提出基于切趾反射体布拉格光栅的谱合成技术。并对体布拉格光栅的热稳定性进行分析。计算结果表明：通过合理的参数设计,采用多重反射体布拉格光栅可有效减小谱合成装置的体积。利用相移反射体布拉格光栅可实现谱合成间距为pm量级的多光束谱合成,但对各光束入射角度的控制精度要求较高。切趾技术可有效抑制反射体布拉格光栅的衍射旁瓣,引入Sin切趾后,反射体布拉格光栅一级衍射旁瓣衍射效率最大值由45%降低为0.8%。当反射体布拉格光栅的波长半宽度为0.5nm,入射光束的光谱宽度为0.2nm时,为获得95%的谱合成效率,引入Sin切趾后的谱合成光谱间距由无切趾时的0.94nm减小为0.61nm,有利于在有限谱合成光谱带宽内增加谱合成光束数目,实现高功率谱合成输出。利用反射体布拉格光栅的角度偏振衍射特性和较高的激光损伤阈值,采用反射体布拉格光栅进行双光束合成以实现高转换效率的径向偏振光输出。体布拉格光栅温度的变化会影响体布拉格光栅的性能,在谱合成中应根据需要对体布拉格光栅的温度进行控制。空间低通滤波器具有滤除光束中非线性增长较快的空间频率成份、改善光束质量等作用,对激光光束进行空间低通滤波是控制激光光束质量的重要措施之一。在ICF等高功率激光系统中,特别是对于长脉冲高功率激光系统,针孔滤波器的等离子体堵孔效应是高功率长脉冲激光应用首先要解决的问题之一。为实现对高功率激光光束质量的控制,论文对高功率激光空间低通滤波技术展开研究,建立柱面透镜空间低通滤波理论,并实验验证柱面透镜空间低通滤波技术的可行性。分析计算结果表明,柱面透镜焦面处光强最大值随柱面透镜组间距的增加而减小。柱面透镜仅对畸变光束的一个方向聚焦,可有效减小焦面处的光功率密度,避免等离子体堵孔效应。全息柱面透镜具有对大截面光束进行空间低通滤波的潜力。对于调制频率高于柱面透镜狭缝等效截止频率的畸变光束,采用柱面透镜空间低通滤波器与采用针孔直径和狭缝宽度相同的针孔滤波器的滤波效果相同,均可有效抑制引起光束畸变的高频角谱分量；对于调制频率高于柱面透镜狭缝等效截止频率而低于针孔截止频率的畸变光束,针孔滤波器的滤波效果优于狭缝宽度与针孔直径相同的柱面透镜空间低通滤波器。理论分析和实验结果表明,采用焦线相互垂直的两组柱面透镜可实现对畸变光束的二维空间低通滤波,获取较好光束质量的光束。为实现对高功率激光光束质量的控制,避免柱面透镜焦面处的等离子体堵孔效应,提出基于反射体布拉格光栅和柱面透镜的空间低通滤波结构。通过结合非聚焦空间低通滤波技术与柱面透镜空间低通滤波技术的优势,利用反射体布拉格光栅对畸变光束进行预处理,降低引起入射光束畸变的高频角谱分量的光强,从而进一步减小引起光束畸变的高频角谱分量在柱面透镜焦面处的光功率密度,避免等离子体堵孔效应,为高功率激光空间低通滤波技术提供参考。更多还原

【Abstract】 High-power laser with great beam quality has been significantly used in laserpumping, laser cutting and drilling, Inertial Confinement Fusion (ICF) system and so on.Spectral beam combining technology is an effective approach for achieving high powerlaser output, and the beam quality of the high power laser can be controlled or refinedby low-pass spatial filtering technologies. In this dissertation, the spectral beamcombining technology and low-pass spatial filtering technology for achieving highpower laser with great beam quality are studied, which provides a reference andrecommendations for achieving high power laser with great beam quality.The ultimate output power of single laser is limited due to thermal effects. It is aneffective approach for achieving high power laser output by spectral beam combiningtechnology, which incoherently combines several lasers with different wavelength into asingle near-diffraction-limited beam with the same aperture by using dispersive opticalelements and does not require phase control of each single laser. The spectral beamcombining approaches based on the transmitting volume Bragg grating (TVBG) andreflecting volume Bragg grating (RVBG) are studied and compared theoretically. Theresults show that in the spectral beam combining system with plane wave incidence, thesidelobes of the TVBG and the RVBG are the main factor which limits the spectralseparation between each laser channel and the spectral combining efficiency. In thespectral beam combining system with polychromatic beam incidence, the competitionbetween the spectral width of incident beam and the spectral selectivity of the TVBGand RVBG limits the spectral combining efficiency, and the sidelobes of the TVBG andRVBG are still the main factor which affects the spectral separation. For the TVBG,when the spectral width of the incident polychromatic beam is equal to40%times ofspectral selectivity of the TVBG, the diffraction efficiency is90%when the diffractionefficiency of the center Bragg wavelength is100%. For the RVBG, when the spectralwidth of incident polychromatic beam is equal to50%times of spectral selectivity ofthe RVBG, the diffraction efficiency is99.9%when the diffraction efficiency of thecenter Bragg wavelength is99.99%. Comparing with RVBG, there is additionalfar-field divergence for TVBG with polychromatic beam incident due to the dispersivecharacteristic of the grating, which limits the applications of TVBG in spectral beamcombining system.Several recommendations are proposed for solving the limitations of the spectralbeam combining technology based on the RVBG. In order to reduce the scale of thespectral beam combining system when the number of combined laser beam is huge, thefeasibility of spectral beam combining technology based on the superimposed reflectingvolume Bragg grating is demonstrated theoretically. For achieving the spectral separation near the pm levels, the spectral beam combining model based on thephase-shifted reflecting volume Bragg grating is established. For suppressing thesidelobes of the RVBG in spectral beam combining system, the spectral beamcombining model based on the apodisation reflecting volume Bragg grating is proposed.The thermal effects of the volume Bragg grating are considered. The results show thatthe scale of the spectral beam combining system can be effectively reduced byreasonable design the superimposed reflecting volume Bragg grating. By using thephase-shifted reflecting volume Bragg grating, the spectral separation near pm levelscan be achieved with the cost of incident angular controlling. The sidelobes of thereflecting volume Bragg grating can be effectively suppressed by using apodisationtechnologies. The peak diffraction efficiency of the first side lobe is suppressed from45%to0.8%after using the Sin apodisation approach when the diffraction efficiency ofthe no-apodisation RVBG is99.9%. The spectral separation between the two lasersources is also reduced effectively. For the RVBG with spectral selectivity is0.5nm,and the spectral width of the incident polychromatic beam is0.2nm, in order to achievethe spectral combining efficiency at95%, the spectral separation of the RVBG is0.94nm. However, the spectral separation is reduced to0.61nm after using the apodisationtechnology in RVBG. Using the apodisation technology, it is an effective approach toachieving high power laser output by increasing the number of combined beams in atotal spectral bandwidth. Using the angular diffraction characteristic and high laserinduced damage threshold performance of the reflecting volume Bragg grating, radiallypolarization beam is achieved theoretically with high combining efficiency bycombining two perpendicular linearly polarized beams with RVBG and phasecontrolling technology. The characteristic of the volume Bragg grating can be affectedby its temperature, and the temperature of the volume Bragg grating should becontrolled in spectral beam combining system.Low-pass spatial filter technology is an effective approach to refine or control thebeam quality of the laser beam. In high power laser system such as ICF, the pinholespatial filter limits the ultimate output power of the laser system with long pulse widthbecause of plasma closure of the pinholes. In order to control or refine the beam qualityof the high power laser system, the theory of low-pass spatial filtering based on thecylinder lens is established, and compared with the theory of pinhole spatial filter. Theresults show that the peak intensity of the incident beam in the focal plane of thecylinder lens decreases with increasing of the cylinder group distance. The peak powerin the focal plane of the cylinder lens is effectively reduced comparing with that of thelens with the same focal length in pinhole spatial filter. The peak power in the focalplane of the cylinder is smaller than that of the lens with same focal length in pinholespatial filter, and the pinhole closure can be effectively avoided in the same high powerlaser system. The filtering of high power laser with large beam aperture can be achieved by using holographic cylinder lens because it is convenient to achieve large scaleholographic cylinder lens. The results of filtering the deformed beam are completelysame by using the cylinder lens or lens with the same width of gaps and pinhole whenthe modulation frequency of deformed beam is larger than the cutoff frequency of thegap in low-spatial filter based on the cylinder lens, and the performance of pinholespatial filter in better than cylinder lens spatial filter when the modulation of thedeformed beam is larger than the cutoff frequency of the pinhole but smaller than that ofthe gaps. Numerical and experiment results show that the low-pass spatial filter can beachieved and the beam quality can be controlled or refined by using two groups ofcylinder lens with perpendicular focal line.In order to control or refine the beam quality of the high power laser which wouldcause plasma pinhole closure of the cylinder lens low-pass spatial filter system, a newfilter model by combining the RVBG and cylinder lens is proposed. In this new filtersystem, by using the RVBG to pre-filter the deformed beam, the intensity of the highangular frequency in the gap which located at the focal plane of the cylinder lens isreduced to avoid plasma pinhole closure. This new configuration provides a referenceand recommendations for low-pass spatial filtering technology in high power lasersystem.更多还原