Coherent beam combination of 137 W fiber amplifier array using single frequency dithering technique

We demonstrate coherent beam combination of 137 W fiber amplifier array using single frequency dithering technique. Four polarization-maintained fiber amplifiers are tiled into 2×2 array and the output power of each amplifier is about 35 W. Single frequency dithering algorithm is run on a signal processor based on field programmable gate array (FPGA) for phase control on the fiber amplifiers. When the phase control system goes into closed-loop, the fringe contrast of far-field intensity pattern is improved by more than 87% from 0 in open-loop, and the residual phase error is less than λ/20.     

Coherent beam combination of an optical array using adaptive fiber optics collimators

A novel adaptive-fiber-optics-collimator (AFOC) compensating both piston-type and tip/tilt-type phase errors of output beam is introduced, and has been employed in experiments of coherent beam combination (CBC) of a delta distributed fiber array. Feedback control is realized using stochastic parallel gradient descent (SPGD) algorithm. Excellent CBC effect has been achieved when piston and tip/tilt errors among beamlets corrected. The necessity of wavefront tip/tilt control in CBC is verified. Experimental results exhibit great potential applications of this kind of AFOC in fiber amplifier arrays.     

Bandwidth analysis and improvement of the beam phasing of fiber amplifiers using the stochastic parallel gradient descent algorithm

We present the numerical analysis and improvement of the bandwidth of the beam phasing of fiber amplifiers using the stochastic parallel gradient descent (SPGD) algorithm. Time-varying phase distortions are incorporated into the dynamic simulation. It is shown by numerical calculation that the bandwidth of the standard SPGD algorithm is dependent on the phase distortions and on the number of lasers. The time-averaged cost function will decrease with an increase in the amplitude and frequency of phase distortions, and in the number of lasers. A cross-grouped SPGD algorithm is proposed, which offers attractive performance due to its robustness to various time-varying phase distortions and faster convergence rate for beam phasing of larger number of lasers. Dynamic simulation shows that the control bandwidth can be improved using the cross-grouped SPGD algorithm.     

Coherent beam combining master oscillator-power of hybrid phase control in amplifier configuration

A novel scalable architecture for coherent beam combining with hybrid phase control involving passive phasing and active phasing in master oscillator-power amplifier configuration is presented. Wide-linewidth mutually injected passive phasing fibre laser arrays serve as master oscillators for the power amplifiers, and the active phasing using stochastic parallel gradient descent algorithm is induced. Wide-linewidth seed laser can suppress the stimulated Brillouin scattering effectively and improve the output power of the fibre laser amplifier, while hybrid phase control provides a robust way for in-phase mode coherent beam combining simultaneously. Experiment is performed by active phasing fibre laser amplifiers with passive phasing fibre ring laser array seed lasers. Power encircled in the main-lobe increases1.57 times and long-exposure fringe contrast is obtained to be 78% when the system evolves from passive phasing to hybrid phasing.     

Coherent beam combining of hybrid phase control in master oscillator-power amplifier configuration

A novel scalable architecture for coherent beam combining with hybrid phase control involving passive phasing and active phasing in master oscillator-power amplifier configuration is presented. Wide-linewidth mutually injected passive phasing fibre laser arrays serve as master oscillators for the power amplifiers, and the active phasing using stochastic parallel gradient descent algorithm is induced. Wide-linewidth seed laser can suppress the stimulated Brillouin scattering effectively and improve the output power of the fibre laser amplifier, while hybrid phase control provides a robust way for in-phase mode coherent beam combining simultaneously. Experiment is performed by active phasing fibre laser amplifiers with passive phasing fibre ring laser array seed lasers. Power encircled in the main-lobe increases1.57 times and long-exposure fringe contrast is obtained to be 78% when the system evolves from passive phasing to hybrid phasing.     

模拟退火算法光纤放大器相干合成

提出了利用模拟退火算法实现相干合成的思路。对利用模拟退火算法实现多路光纤放大器相干合成进行了数值模拟,验证了方法的有效性,并分析了算法收敛速度与合成光束数目的关系。进行了两路W量级光纤放大器相干合成的实验,结果表明,模拟退火算法能够有效控制各路光纤激光的相位,系统闭环将目标圆孔内的能量提高了1.8倍,并使得目标圆孔内能量大于理想值80 %的概率从19.4 %提升到了51.3 %,取得了较为明显的相干合成效果。     

模拟退火算法光纤放大器相干合成

提出了利用模拟退火算法实现相干合成的思路。对利用模拟退火算法实现多路光纤放大器相干合成进行了数值模拟,验证了方法的有效性,并分析了算法收敛速度与合成光束数目的关系。进行了两路W量级光纤放大器相干合成的实验,结果表明,模拟退火算法能够有效控制各路光纤激光的相位,系统闭环将目标圆孔内的能量提高了1.8倍,并使得目标圆孔内能量大于理想值80%的概率从19.4%提升到了51.3%,取得了较为明显的相干合成效果。     

The 260-W coherent beam combining of two compact fibre amplifier chains

Coherent beam combining of two fibre amplifier chains with a total power of 260 W in a compact system using the stochastic parallel gradient descent (SPGD) algorithm is demonstrated. A 150 MHz linewidth fibre laser is built and introduced for high-power amplification to mitigate stimulated Brillouin scattering (SBS). Compact high-power amplifier chains are built with low power all-fibre system and high-power bulk free-optics fibre amplifiers. When the total power is about 260 W, active phase-locking of two high-power amplifiers is demonstrated using the SPGD algorithm. In closed-loop, the power in the main lobe increases 1.68 times, the visibility is increased from 0 to 0.62, and the phase residual error is less than λ/10.     

Coherent combining of a seven-element hexagonal fiber array

The coherent combination of the outputs from a seven-element hexagonal fiber array with a hexagonal prism is described. The fill factor of the coherent array achieves 0.6. A total of 0.40 of the energy is contained in the central lobe and the Strehl ratio is 0.91. A theoretical model of the hexagonal fiber array is established based on Fraunhofer diffraction theory. The far field distribution of the hexagonal fiber array is presented. The effects of fill factor and phase errors on far field distribution are analyzed and simulated.     

基于偏振鉴相的相干合成技术

推导了偏振相干合成后任意方向检偏的光强受束间相位差调制的解析表达式,并通过实验验证了表达式的正确性。开展了基于偏振鉴相的相干合成实验研究,用梯度下降算法锁定了两束激光的相位,获得了稳定的输出功率,验证了基于偏振鉴相的相干合成技术可行性;分析了基于偏振鉴相的相干合成技术的优点和对激光器的要求;提出了一种基于偏振鉴相的可定标放大全口径相干合成方法,采用多探测器和多级并行相位调制的方法,解决了随机并行梯度下降（SPGD）算法闭环带宽随合成光束数增加而快速退化的问题。     

光纤激光相干合成高速高精度相位控制器

基于随机并行梯度下降算法（SPGD）和现场可编程逻辑阵列（FPGA）设计制作了相干合成（CBC）相位控制器。理论分析表明,该控制器单次迭代速率大于1.125 MHz,对于2路和16路相干合成,其平均控制带宽的理论值分别大于70 kHz和9 kHz,与现有的SPGD算法相位控制器相比有了量级上的提高。利用该控制器进行了验证性实验,表明该控制器能够实现高速高精度相位控制。当利用相位控制器对两路激光的相位进行锁定时,目标圆孔内能量提高了1.51倍,远场光斑对比度提高了5.29倍。     

激光场相干叠加光束合成技术

光束拼接激光相干合成中,拼接占空比不高导致合成光束能量集中度不够理想,为了解决这个问题,提出了激光场相干叠加光束合成技术路线,设计了基于半反半透合束镜的多级激光场相干叠加光束合成方案。研究了子光束之间的光强差和波面误差对相干合成的影响,结果表明,激光场相干叠加光束合成中,对参与合成子光束的光强差、相位差的容差均不难满足：仅考虑单一因素时,子束光强相差三倍时合成效率仍可达90%,相位误差小于/5时,合成效率可达到90.5%。搭建了基于半透半反合束镜的两束激光场相干叠加光束合成实验装置,实验验证了激光场相干叠加光束合成技术的可行性,在闭环情况下得到了稳定的合成光束输出,合成效率可达95%以上。     

Coherent and incoherent combining of fiber array with hexagonal ring distribution

Beam combining of fiber array with hexagonal ring distribution is studied in detail. The theoretical analysis and numerical calculation results are given to illustrate the propagation properties of the resulting beam through free space. A comparison between the coherent and the incoherent case shows that high peak irradiance and good beam quality for coherent combining can be obtained in the far field. The effect of phase errors and the beam quality M² factor are also studied. Results indicate that the element numbers should be increased to achieve high power and the space between adjacent elements should be reduced to maintain good beam quality, which is basically the same for both coherent and incoherent beam combining.     

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。     

LaB6在低压强氮气和氦气中的放电特性

研究了LaB6在1~10 Pa氮气和氦气中的直流和脉冲放电特性以及放电过程对电极的影响。结果表明，电极直径为5 mm的LaB6氦气放电管在脉冲工作状态下可以长期稳定放电。在脉冲电压为2.2 kV、脉冲宽度10 ms、频率13.3 Hz下，脉冲峰值放电电流超过120 A。氦气放电管在放电过程中，阴极表面有离子的清洗和活化作用，可以使电极的表面逸出功降低，提高放电管的发射能力和稳定性。LaB6作为气体放电电极具有寿命长、延迟时间短、放电电流大等优点，可用于重复强流脉冲气体放电的高压高速开关器件。     

基于偏振自适应和主动相位控制的相干合成的实验研究

进行了非保偏放大器与保偏放大器的相干合成的实验研究。通过随机并行梯度下降（SPGD）算法对非保偏放大器进行偏振自适应控制,使非保偏放大器输出激光的消光比达到11.5 dB,与保偏放大器在同一偏振方向上的光功率占总功率的93.4%。利用单抖动法进行非保偏放大器与保偏放大器的主动相位控制,实现相干合成。实验结果表明:SPGD算法能够有效实现偏振自适应控制,偏振自适应控制前后相干合成远场的条纹对比度从80.1%提高到87.2%,相干合成的效果提升明显;通过增加参与合成的放大器路数,并在各路激光中引入多级功率放大器,能够得到更高的合成功率输出。     

基于偏振自适应和主动相位控制的相干合成的实验研究

进行了非保偏放大器与保偏放大器的相干合成的实验研究。通过随机并行梯度下降（SPGD）算法对非保偏放大器进行偏振自适应控制,使非保偏放大器输出激光的消光比达到11.5 dB,与保偏放大器在同一偏振方向上的光功率占总功率的93.4%。利用单抖动法进行非保偏放大器与保偏放大器的主动相位控制,实现相干合成。实验结果表明：SPGD算法能够有效实现偏振自适应控制,偏振自适应控制前后相干合成远场的条纹对比度从80.1%提高到87.2%,相干合成的效果提升明显;通过增加参与合成的放大器路数,并在各路激光中引入多级功率放大器,能够得到更高的合成功率输出。     

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240 W。建立了两路120 W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2 rad（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。     

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。     

两个光纤激光器的相干合成及其闭环控制

 采用了并联主振荡-功率放大的相干合成方案,实现了光纤激光器的相干叠加。主振荡的输出光通过保偏光纤分束器分为两路,其中一路加入保偏光纤相位调制器以实现闭环控制,两个保偏光纤放大器分别对两路输出光进行放大,放大后的输出光再由一个2×2的保偏光纤耦合器会聚相干。合成输出光的一部分通过光电转换后进入数据采集卡,采集卡将采集到的数据送入计算机进行实时处理并将得到的反馈信号作用于相位调制器,相位调制器实时控制两路信号的相位差,从而实现整个实验系统的相干合成输出。系统实现闭环控制后,两路光纤放大器的相位差为2π的整数倍,输出光强始终保持最强。该相干合成方案简单易行,性能稳定。