高功率法拉第隔离器多模退偏特性

理论分析了隔离器在高功率多模激光入射时的自退偏特性.结果表明：线性双折射致退偏与圆双折射致退偏均与入射光功率的平方成正比,线性双折射致退偏与光斑大小无关,而圆双折射致退偏随光斑半径的增大而减小.当光斑半径与旋转晶体半径相比很小时（R0/r0≥80）圆双折射引起的退偏大于线性双折射引起的退偏,当光斑半径接近旋转晶体半径（R0/r0≤3）时,与线性双折射引起的退偏相比,圆双折射引起的退偏可以忽略.对比单模激光入射,在同种条件下,多模入射使得线性双折射致退偏减小了0.4倍, 最小圆双折射致退偏减小为0.05倍.     

高功率隔离器单模与多模工作退偏特性

以传统结构隔离器为模型,应用琼斯理论,比较分析了单模与多模激光入射时高功率隔离器热致退偏特性。研究结果表明:高功率隔离器热致退偏可分为线性双折射致退偏与圆双折射致退偏;两种退偏均与入射激光功率的平方成正比;在光功率一定的条件下,线性双折射致退偏不随光斑半径的变化而变化,圆双折射致退偏随光斑半径的增大而减小;在其它条件相同的条件下,多模光场能够有效地降低隔离器热致退偏,多模时线性双折射致退偏仅为单模时退偏的60%,圆双折射致退偏也比单模时要小,减小幅度随光斑半径的增加而增加。     

重频应用下等离子体电光开关热退偏损耗分析

 基于有限元数值方法,给出电光晶体KDP在高平均功率激光负载下温度场分布和应力场分布。在此基础上得到了折射率随温度变化、电光系数随温度变化、及应力双折射引入的退偏损耗。数值模拟显示：电光系数随温度变化和应力双折射是引起开关退偏损耗的主要因素。当入射激光平均功率为40 W、辐照时间为420 s时,KDP晶体最高温度为38.43 ℃,电光系数随温度变化及应力双折射引入的最大退偏损耗分别为2.38%和4.04%。实验测量了应力双折射导致的退偏损耗,实验结果和理论结果符合较好。     

LD端面泵浦Cr~4∶YAG被动调Q激光器输出特性研究

对激光二极管端面泵浦Cr4+∶YAG被动调Q Nd∶YAG激光器输出特性进行了实验研究.实验研究发现,激光器输出功率及脉冲重复频率随谐振腔长度增大而增大.为解释这一实验现象,测量了泵浦光斑在激光晶体内尺寸,同时计算了激光晶体及Cr4+∶YAG晶体内的基模激光光斑半径随谐振腔长度变化.分析结果表明:激光晶体内泵浦光斑尺寸远小于激光晶体内基模光斑半径,腔模间交叠效率较低;当腔长增加时,激光晶体内的基模激光光斑减小,腔模间交叠效率增加,从而导致输出功率及脉冲重复频率随腔长增加而增加;另外,Cr4+∶YAG晶体内光斑半径也随谐振腔长度减小,引起Cr4+∶YAG晶体漂白时间缩短,导致脉冲重复频率随腔长增加而增加.     

在YAG:Nd晶体中位错对激光特性的影响

本文用双折射貌相法和化学腐蚀法测定了六根晶体棒中的位错分布和位错密度,用光学方法检验了晶体的光学质量,晶体的激光性能是在一台Q开关YAG激光装置上测定的。实验表明当位错密度增加到一定程度时,位错形成的应力场会引起明显的双折射,以致探测光通过晶体时波面会发生畸变。在干涉仪上可观察到干涉条纹的增加。同样,在位错应力场区消光比亦下降。因此,作为结构缺陷的位错也是晶体光学不均匀的来源之一。晶体激光性能的实验结果指出:位错对激光输出特性有很大的影响,从激光近场光斑图看出,在位错应力场区,因为高的激光阈值而不能产生激光振荡,并且由于应力双折射效应产生光的退偏性。由于存在高位错密度,晶体的激光发散度增加,倍频效率下降。当晶体中存在缀饰位错(decorated dislocation)时,激光输出特性的变化尤为明显.     

磁光晶体Bi-YIG的磁致退偏效应

建立了磁光晶体磁致偏振特性测试系统。对Bi YIG晶体样品在大范围磁场下的退偏现象进行了测试。结果表明,达到磁饱和或接近磁饱和时,Bi YIG晶体的偏振性能最优;达到磁饱和后,随着磁场的增强,出现了磁致退偏效应。根据实验结果分析了磁致退偏效应的产生机理,并说明了磁致圆二向色性及磁致线双折射是产生退偏的原因。     

高功率光隔离器永磁系统的理论研究与优化

基于Halbach阵列永磁体的特点以及光隔离器对磁场的需求,设计了两种适用于高功率隔离器的永磁系统,分析了其磁场分布、磁场非均匀性对隔离度的影响,研究了磁体装配误差对磁场的影响。研究结果表明:通过引入斜向磁化永磁体,选择合适的磁化角度和磁体长度,可以提高永磁体的磁场强度,大幅度减小旋转器所需磁光晶体长度;受磁场非均匀性影响的隔离度与磁光晶体的孔径、长度以及入射激光的光斑半径有关,当晶体长度一定时,减小晶体半径和入射光的半径可以显著提高隔离度。在入射光半径为1.5mm、磁光晶体半径5mm时,对应的隔离度分别为105.8dB和45.4dB。     

LD端面泵浦Cr4:YAG被动调Q激光器输出特性研究

对激光二极管端面泵浦Cr⁴⁺:YAG被动调QNd:YAG激光器输出特性进行了实验研究.实验研究发现,激光器输出功率及脉冲重复频率随谐振腔长度增大而增大.为解释这一实验现象,测量了泵浦光斑在激光晶体内尺寸,同时计算了激光晶体及Cr⁴⁺:YAG晶体内的基模激光光斑半径随谐振腔长度变化.分析结果表明:激光晶体内泵浦光斑尺寸远小于激光晶体内基模光斑半径,腔模间交叠效率较低;当腔长增加时,激光晶体内的基模激光光斑减小,腔模间交叠效率增加,从而导致输出功率及脉冲重复频率随腔长增加而增加;另外,Cr⁴⁺:YAG晶体内光斑半径也随谐振腔长度减小,引起Cr⁴⁺:YAG晶体漂白时间缩短,导致脉冲重复频率随腔长增加而增加.     

具有热补偿的内腔KTP光参量振荡器

在电光调Q Nd:YAG激光抽运内腔KTP光参量振荡器中,由于在抽运光的谐振腔内插入了起偏器,当工作在高重复频率时,Nd:YAG棒热致双折射效应致使起偏器产生退偏损耗,导致抽运光能量下降和光束质量变差,最终光参量振荡器的激光输出能量也将下降.为了补偿热致双折射引起的起偏器退偏损耗,提出了基于λ/4波片的具有热补偿的内腔光参量振荡器结构,实验结果表明在高重复频率工作时,具有热补偿的内腔光参量振荡器较无补偿内腔光参量振荡器的激光输出能量提高了20%左右.     

高功率二极管泵浦模块热效应补偿研究

分析了作为放大器应用的高功率二极管泵浦模块NdYAG圆棒的热效应,包括热透镜效应和热致双折射所引起的退偏效应及其补偿方法.在光路中加入合适的负透镜可以对特定工作条件下激光棒的热透镜进行较好的补偿;使用两个增益和荧光分布等参数完全一致的泵浦模块作为放大级,在其间加入90°石英旋转片和4F像传递系统可以较好的补偿热致双折射所引起的退偏效应.并且对以上方法进行了实验验证,取得了良好的补偿效果,提高了泵浦模块的提取效率和激光输出功率.     

Generation of millimeter wave Gaussian beams and their propagation in a dielectric waveguide

Linearly polarized fundamental mode Gaussian beams were generated and coupled to a hollow circular oversized dielectric waveguide by placing the waist of the beam at the guide entrance. The transmission properties of the waveguide were characterized as a function of frequency for a variety of coupling conditions. These conditions included changes in the input beam waist radius, angle of incidence, and displacement perpendicular and parallel to the guide axis. It has been found that: 1.) power transmission is maximized when the waist of the input beam is centered at the guide input, injected normally, and has a radius of 0.43 times the waveguide radius, 2.) power transmission decreases rapidly with increasing angle of incidence and the rate of that loss increases with frequency, 3.) the waveguide preserves the linear polarization of the input beam, 4.) power transmission in the fundamental waveguide mode is not greatly affected by moderate displacements in the input beam position, and 5.) upon exit from the waveguide the launched EH₁₁ mode propagates as a fundamental mode Gaussian beam in the quasi-far field. The results compare favorably to the transmission theory of Belland and Crenn and approximately to the near and far field mode pattern theory of Degnan.     

Effective dichroism in forward scattering by inhomogeneous birefringent medium

The Mueller matrix model for inhomogeneous medium characterized by simultaneous linear and circular birefringence for single scattering in the forward direction is derived. We find that the presence of inhomogeneity results in the occurrence of new polarization effects. In particular, forward scattering by inhomogeneous birefringent medium exhibits linear and circular dichroism. We show that in the case of weak depolarization (when Cloude's entropy is less than 0.5), the initial parameters for linear and circular birefringence can be obtained from deterministic Mueller matrix associated with the largest eigenvalue of Cloude's coherency matrix. Sample calculations are given for quartz.     

Single rod efficient Nd:YAG and Nd:YALO lasers with average output powers of 46 and 47 W in diffraction limited beams with M<1.2 and 100 W with M<3.7

Using a new resonator concept guaranteeing fundamental mode operation, flashlamp pumped Nd lasers with average output powers of 46 and 47 W for the two materials were realized with beam qualities better than 1.2*DL. Due to the absence of thermally induced birefringence the efficiency of the Nd:YALO laser was up to 1.85% and thus 1.5 times higher than that of the realized Nd:YAG laser. The Nd:YALO laser output is linear polarized. Average output power of 100 W could be extracted from a Nd:YAG single rod oscillator with a beam quality of better than 3.7×DL.     

圆波导梳状慢波结构的耦合阻抗

 对目前慢波结构的模拟计算中存在的问题进行了深入分析，并提出了新的计算耦合阻抗的方法。研究了圆波导梳状慢波结构基波的耦合阻抗,其值可以在较宽频带内大于1 W。该结构内孔径为波长的1/4~1/3，是同一频段螺旋线慢波结构的5倍左右，能容纳比螺旋线大25倍的电流，能够承受比螺旋线大25倍的平均功率。互作用计算表明采用这种慢波结构的行波管在Ka波段可获得的最高功率达170 kW。     

高斯光束经环形光栅的衍射特性分析

基于傅里叶-贝塞尔变换计算高斯光束垂直入射环形光栅时的衍射远场分布,分析了其衍射远场光强分布的一般规律,并与平面波入射时的情况进行了比较.计算结果表明：当光栅半径为1.5倍高斯光束束腰半径时, 随着光栅环数的增加,中央亮斑半值全宽先减小后增大、中央亮斑所包含的功率占总功率的比值减小、中央主极大光强值减小,三者的变化趋势与平面波入射时的趋势一致;中央亮斑半径、次极大光强值变化趋势与平面波入射时的变化趋势不同.当环数小于5时,高斯光束经过环形光栅的衍射场光强变化无规律;当环数大于10后两种情况下衍射场光强变化都不明显;当环数趋于无穷时中央亮斑半径、中央亮斑半值全宽、次极大光强值趋于圆孔衍射(环数等于1)时的值,中央亮斑所包含的功率占总功率的比值约等于圆孔衍射时的1/2,中央主极大光强值约等于圆孔衍射时的1/4.     

X波段内同轴膜片加载相对论返波振荡器的特性研究

 把同轴膜片作为相对论返波振荡器的慢波结构，从Maxwell方程组和Floquet定理出发，导出了电磁波在同轴膜片加载相对论返波振荡器中的色散关系。通过编程计算得出TM_0n模式的色散曲线，数值分析了周期长度和导体半径对器件工作频率的影响，运用PIC程序模拟了返波振荡器中注波互作用过程。讨论了纵向聚焦磁场、电子注到慢波结构的距离、槽深和槽宽对输出功率和效率的影响。研究表明：增加器件的周期长度，可以减小器件的截止频率和同一模式下的工作频率； 调节内导体半径的大小，几乎不影响器件的工作频率。周期长度取11 mm，槽深取2 mm，槽宽取3.6 mm，漂移长度取7.5 mm的结构，在520 kV电压、8 kA电流和0.7 T的轴向聚焦磁场条件下，器件能输出2.8 GW的峰值功率,功率效率约33.2%。该慢波结构加工方便，具有广阔的应用前景。     

Induced focusing and conversion of a Gaussian beam into an elliptic Gaussian beam

We have presented an investigation of the induced focusing in Kerr media of two laser beams, the pump beam and the probe beam, which could be either Gaussian or elliptic Gaussian or a combination of the two. We have used variational formalism to derive relevant beam-width equations. Among several important findings, the finding that a very week probe beam can be guided and focused when power of both beams are well below their individual threshold for self-focusing, is a noteworthy one. It has been found that induced focusing is not possible for laser beams of any wavelength and beam radius. In case both beams are elliptic Gaussian, we have shown that when power of both beams is above a certain threshold value then the effective radius of both beams collapses and collapse distance depends on power. Moreover, it has been found that induced focusing can be employed to convert a circular Gaussian beam into an elliptic Gaussian beam.     

Thermal effects in high-power Nd:YAG disk-type solid state laser

An investigation of thermal effects in a high-power Nd:YAG disk-type solid state laser pumped with different pump beam transverse profiles is carried out by numerical simulation based on the finite element method (FEM). Impact of the heat sink on the thermal effects is included in the simulation. The distribution of first principle stress, thermally induced birefringence, including the distribution and variation of the birefringence loss, are studied. The characteristics of the phase variation are analyzed with consideration of the temperature gradient, deformation, strain and thermal stress. Thermal lensing is explored as a function of pump power and of the radius pumped with different pump beam transverse profiles. The non-parabolic part of optical phase distortion is simulated. Furthermore, the characteristics of the bi-focus of the disk laser are also studied. Experiments on the maximum tensile stress distribution and depolarization loss are carried out. The presented calculations are in qualitative agreement with the experimental observations.