不同波形脉冲激光的时域误差对相干合成的影响

相干合成技术是实现高功率、高亮度光纤激光系统的重要技术途径.然而, 脉冲激光阵列中常常存在时域误差,这将影响脉冲激光的相干合成效果. 建立了脉冲激光存在时域误差时的相干合成理论模型,并在不同波形(方波、三角波、正弦波) 的脉冲激光存在时域误差时,对相干合成光束在远场的脉冲波形、峰值功率、 光强分布和桶中功率(PIB)等特性进行了数值计算和对比分析.计算结果表明: 方波脉冲激光相干合成光束的脉冲波形受时域误差影响严重,光强分布和PIB随着时域误差 的增大发生线性变化;三角波脉冲激光相干合成光束的脉冲波形和峰值功率受时域误差 影响严重,光强分布和PIB在时域误差较大时随着时域误差的增大发生较为剧烈的变化; 正弦波脉冲激光相干合成光束具有较好的输出特性,在两路正弦波脉冲激光相干合成中, 将两脉冲之间的时延控制在脉冲持续时间的10%以内,就能取得良好的合成效果.     

高功率光纤激光的时间部分相干性对相干合成的影响

 将多个光纤激光器的输出光束进行相干合成是获得高功率、高光束质量激光的有效途径。利用准部分相干光(PPCB)模型,计算了高功率光纤激光器阵列发出的部分相干光在远场的能量分布,分析了激光的时间部分相干性对相干合成的影响。计算结果表明,随着单根激光器输出光束线宽的增大,远场光斑的图样基本保持不变,但峰值功率和Strehl比随之减小。对于采用的算例,要保证远场光斑的Strehl比大于0.8,单根光纤激光器的线宽不能超过5 nm。     

基于光斑二阶矩的阵列光束倾斜相差自适应控制方法

针对阵列光束相干合成中存在的倾斜相差大的问题,提出了基于合成光束远场光斑二阶矩的阵列光束倾斜相差自适应控制方法。以合成光束远场光斑二阶矩作为评价函数,理论上模拟了采用随机并行梯度下降算法实现7路光束的倾斜闭环控制过程。实验上搭建了7路光纤激光相干合成系统,利用自适应光纤准直器对倾斜相差进行校正。以合成光束远场光斑的二阶矩作为评价函数,采用随机并行梯度下降算法,实现了7路光束的倾斜的闭环控制,合成光束模拟远场光斑的桶中功率由0.05 V提升至1.95 V。实验中将倾斜扰动的增益系数变为与二阶矩相关的函数,实现了自适应变增益系数的倾斜闭环,在一定程度上提升了倾斜控制的带宽。从理论上和实验上验证了基于光斑二阶矩的倾斜相差自适应控制方法在光束合成及合成孔径探测领域应用的可行性。     

基于自适应桶中功率评价函数的光纤放大器相干合成实验研究

光纤激光光束合成技术被公认为是实现高亮度、高光束质量激光束的优秀方法, 而激光阵列间的倾斜像差是影响合成效果的重要因素. 针对当前光束合成中存在的倾斜校正量受限问题, 提出了基于自适应桶中功率(PIB)评价函数的倾斜控制策略. 搭建了两路2 W光纤放大器的相干合成实验平台, 利用自制的压电环光纤相位调制器和自适应光纤准直器分别实现锁相和倾斜像差校正, 验证了基于自适应PIB的倾斜控制策略的可行性, 并实现了相干合成. 关键词： 光纤激光阵列 倾斜控制 自适应桶中功率 相干合成     

Long-exposure far-field properties of the array of mutually-injected fiber lasers

In this paper, the long-exposure far-field properties of the array of mutually-injected fiber lasers are studied to understand the effect of the multiple phase-locked states on coherent beam combining of the array. It is found that the long-exposure far-field will degenerate into the superposition of the intensities of the elementary lasers when all the phase-locked states of the array are exported with the same probability. It means that the interference pattern will be greatly weakened in the long-exposure far-field because of the export of multiple phase-locked states of the array. This result implies that the multiple phase-locked states of the array go against coherent beam combining of the array, and only some of these phase-locked states should be exported. Then, the effects of the high-order phase-locked states on the far-field are discussed. The result suggests that only the in-phase state should be exported (i.e., all the other phase-locked states should be suppressed) in order to realize high-efficiency (> 80%) coherent beam combining when the number of the elementary lasers of the array is smaller than 19.     

Simulataneous coherent and spectral addition of fiber lasers

A new approach for the simultaneous coherent and spectral addition of a two-dimensional array of fiber lasers is presented. A combining efficiency of over 80% and a combined beam profile with M(2)=1.15 were experimentally obtained with an array of four fiber lasers. This approach can lead to significant upscaling in fiber laser additions.     

两路120 W固体激光高光束质量相干合成

通过主动相位控制实现了两路固体激光器的高光束质量相干合成输出,总输出功率达到240W。建立了两路120W板条激光放大器,通过光束整形获得了高光束质量方形光斑,并实现了高占空比光束拼接,填充因子高于92.4%。研制了光轴一致性探测与控制系统,采用基于压电陶瓷的快反镜实现了光轴的高精度闭环控制,两束激光光轴一致性优于2μrad(RMS)。设计了基于现场可编程门阵列(FPGA)的相位探测与控制系统,用随机并行梯度下降算法(SPGD)锁定了两束激光的活塞相位。相位闭环后,远场光斑峰值提高到开环状态的1.7倍,为理想值的84%。合成光束主瓣包含67%的激光总能量,光束质量(BQ)为1.1。     

Numerical analysis of coherent combination for fiber lasers and application to beam steering

A coherent combination model of fiber lasers has been established. The relative beam quality factor has been introduced and studied as a proper beam quality factor for the combined coherent beam. The disturbance of the phase shift, polarization degradation, distribution characteristic, the wave-front aberration and the tilt error has been numerically studied to investigate their effects on the beam quality. This model has been also used to analyze the control of beam steering. The results have shown that the impact of the phase shift on the combination is more serious than that caused by polarization degradation, and the coherence degradation caused by the phase grating is more severe than that caused by the phase shifter.     

多单元光纤激光阵列的倾斜控制实验研究

本文结合光纤激光阵列技术的发展趋势,介绍一种基于大模场面积光纤的自适应光纤光源准直器的研制进展,搭建七单元光纤激光阵列,进行平行发射体系下阵列间的倾斜控制实验,并初步探索了基于目标在回路的共形发射体系下阵列间的倾斜控制.最后,在三级全光纤放大链路的主振荡-功率放大系统中,完成了两路10W量级光纤激光阵列的倾斜控制及锁相.为在实际大气环境下应用光纤激光阵列技术提供了参考.     

系统误差对相干偏振合成效率的影响

基于偏振合成技术,以提高系统的合成效率为目的,针对合束过程中存在的几种主要系统误差进行了系统分析和定量计算。结果表明,相对于重叠误差、角度倾斜和偏振合束器的吸收损耗而言,半波片的调整误差是导致合成效率下降的主要因素,尤其是在系统向多路合成拓展时,其影响更加显著。若偏振合束器自身损耗为0.015,光束倾斜角为2°,重叠误差为5%,则16路光束的偏振合成效率仍高于92.5%;当半波片方位角出现0.045rad的偏差时,即使不考虑其他误差,系统合成效率低于74%。     

Diffractive-optics-based beam combination of a phase-locked fiber laser array

A diffractive optical element (DOE) is used as a beam combiner for an actively phase-locked array of fiber lasers. Use of a DOE eliminates the far-field sidelobes and the accompanying loss of beam quality typically observed in tiled coherent laser arrays. Using this technique, we demonstrated coherent combination of five fiber lasers with 91% efficiency and M2=1.04. Combination efficiency and phase locking is robust even with large amplitude and phase fluctuations on the input laser array elements. Calculations and power handling measurements suggest that this approach can scale to both high channel counts and high powers.     

类视网膜阵列结构的光纤相控阵方法研究

出射光束阵列布局对光纤相控阵的高功率激光相干合成性能有着重要影响。针对现有光纤相控方法的阵元间距难以突破半个波长,导致能量合成效率低的问题,提出了类视网膜多环形光纤相控方法,设计了具有圆形对称分布的结构,在保持中心阵元以及最外环阵元位置不变的情况下,通过粒子群算法优化环间其余阵元间距以获得最佳的远场相干合成性能。仿真结果表明,与传统光纤相控方法相比,提出的阵列布局可将能量集中度从0.562提升至0.921,峰值旁瓣水平从0.212压缩至0.043。     

Mutual injection locking and coherent combining of three individual fiber lasers

Mutual injection locking and coherent combining of three individual erbium-doped fiber lasers are reported. Theoretical model of the three fiber lasers under mutual injection is also analyzed. Under free running state, the far field beam profile is a simple intensity superposition as three incoherent beams. Under mutual injection locking, interference lobes with high contrast ratio are obtained in the far field of three fiber lasers, which shows that the three fiber lasers have good interference characteristic under mutual injection. The experimental results are in good agreement with the theoretical results. According to this method, we can also realize coherent beam combining in more fiber lasers.     

基于迈克尔逊型耦合腔光纤激光器相干合成的理论与实验研究

为使光纤激光器在自由空间中进行有效的相干合成,提出了耦合臂上带有会聚透镜的迈克尔逊型耦合腔,并将外腔配置为f－f结构以增强内部反馈.利用高斯光束的ABCD定律分析了外腔的传输特性,详细研究了外腔偏离f－f结构对相互注入耦合效率的影响,指出输出光纤端面与透镜前焦平面的偏差是影响反馈耦合效率的关键因素.基于此特别设计的迈克尔逊型耦合腔,实验上实现了两路光纤激光的有效相干合成.当透镜被移走时,阵列输出功率下降为原始值的78.5％,且相干态变得不稳定.     

134-W phase locking of two-dimensional four-fiber lasers with improved self-imaging resonator

We experimentally demonstrate the phase locking of a two-dimensional （2D） array of four fiber lasers using an improved self-imaging resonator with a spatial filter. The high visibility interference round stripes of the coherent beam profile are observed. The coherent output power of the fiber array exceeds 134 W. Tile entire system operates quite stably, and no thermal effects observe in the spatial filter, indicating that the coherent output power can be increased using this method.     

高功率光纤激光部分相干合成的可行性及效果分析

提出部分相干合成的概念,论证了谱线宽度和偏振特性并不是影响高功率光纤激光部分相干合成的瓶颈.与完全相干合成的理想情形相比,随着光束线宽的增大,部分相干合成光束的远场光斑图样基本保持不变,但峰值强度和斯特尔(Strehl)比随之减小,光斑能量越来越分散.利用数值计算的方法对部分相干合成与非相干合成光束的远场效果进行了比较分析.计算结果表明,尽管线宽的存在降低了部分相干合成的效果,但与非相干合成相比,部分相干合成的光束仍具有较大优势.     

基于多层电介质光栅光谱合成的光束质量

基于电介质光栅的光谱合成是实现高功率高光束质量激光的重要途径. 在电介质光栅的光谱合成系统中, 光栅色散效应是影响合成激光光束质量的重要因素. 本文推导了单光栅和双光栅光谱合成系统中由于光栅色散引起M²因子的变化公式; 详细讨论了这两种合成系统中单路激光线宽、单路激光光斑半径、相邻两路激光波长差、相邻两路激光间距以及光栅周期对光束质量的影响. 研究表明对于单光栅合成系统, 在合成过程中若保持光束质量M²因子的大小不变, 则单路激光带宽随光斑半径的增加而减小; 在双光栅光谱合成系统中, 在保持光束质量的前提下, 单路激光带宽可随光斑半径的增大而相应增加. 数值计算表明, 若要满足合成光束的光束质量M² ≤1.2的要求, 在单光栅系统中激光线宽需窄于亚纳米量级, 在双光栅系统中激光带宽可为亚纳米. 本文为高功率、高光束质量的光纤激光光谱合成系统的搭建提供了理论指导.     

Experimental research on application of Hartmann micro-lens array in coherent beam combination of two-dimensional laser array

We demonstrate experimentally the application of a phase error detection method in the coherent beam combination (CBC) of a laser array. The method is based on the Hartmann micro-lens array. Both the piston and tilt errors can be detected and corrected simultaneously by combining this method with adaptive optics-correcting technology. The far-field intensity pattern of the combined beam has high energy concentration and good beam quality. The power encircled in the main lobe of the far-field pattern is 41.3%, and the contrast of the pattern reaches 81.8%. Experimental results show the great potential of the Hartmann phasing method for use in the CBC of a large number of laser beams.     

基于随机并行梯度下降算法的光束相干合成技术

介绍了随机并行梯度下降算法的基本原理,对算法流程进行了仿真验证,并对其中随机扰动幅度和增益系数两个关键参数进行了仿真分析。分析结果表明,这两个参数存在一个最适区间,只有在此区间内取值时算法才能有效收敛。以仿真分析为依据开展了光纤激光的相干合成实验,结果表明光束相干合成效果显著,有效地验证了仿真分析的结果。     

基于随机并行梯度下降算法的多波长激光相干合成

由于受到受激布里渊散射(SBS)等非线性效应的限制,单频激光放大器的功率在百瓦量级,以非单频激光多波长激光作为种子源能够有效地抑制SBS并提高放大器功率.与单频激光相干合成相比,非单频率、多波长激光的相干合成有望将输出激光功率成量级地提高.基于随机并行梯度下降(stochastic parallel gradient descent,SPGD)算法,实现了四波长激光的相干合成.在系统闭环时,四路多波长激光合成后的主瓣能量能够提高3倍,达到理想值的75%.验证了多波长、非单频激光相干合成的可行性,为高功率相干合成的发展提供了新的途径.