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。     

Experimental demonstration of coherent beam combining over a 7 km propagation path

We demonstrate coherent combining (phase locking) of seven laser beams emerging from an adaptive fiber-collimator array over a 7 km atmospheric propagation path using a target-in-the-loop (TIL) setting. Adaptive control of the piston and the tip and tilt wavefront phase at each fiber-collimator subaperture resulted in automatic focusing of the combined beam onto an unresolved retroreflector target (corner cube) with precompensation of quasi-static and atmospheric turbulence-induced phase aberrations. Both phase locking (piston) and tip-tilt control were performed by maximizing the target-return optical power using iterative stochastic parallel gradient descent (SPGD) techniques. The performance of TIL coherent beam combining and atmospheric mitigation was significantly increased by using an SPGD control variation that accounts for the round-trip propagation delay (delayed SPGD).     

Multimode fiber laser beam cleanup based on stochastic parallel gradient descent algorithm

We present experimental research on multimode fiber laser beam cleanup based on a stochastic parallel gradient descent (SPGD) algorithm. The multimode laser is obtained by injecting a 1064 nm central wavelength single mode fiber laser into a multimode fiber and the system is setup by using phase only liquid crystal spatial light modulators (LC-SLM). The quality evaluation function is increased by a factor of 10.5 and 65% of the laser energy is encircled in the central lobe when the system evolves from open-loop into close-loop state. Experimental results indicate the feasibility of the multimode fiber laser beam cleanup by adaptive optics (AO).     

Coherent beam combining of 137 W 2 × 2 fiber amplifier array

We demonstrate coherent beam combining of two-dimensional fiber amplifier arrays with a total of 137 W output power using stochastic parallel gradient descent (SPGD) algorithm. Compact all-fiber polarization-maintained single frequency fiber amplifier chains are developed and four fiber amplifiers are arranged to 2 × 2 laser array with a fill factor of 70% in the near-field. Active phase control is implemented by a digital signal processor (DSP) based SPGD controller. The fringe visibility of the coherent combined beam profile is as high as 81% when the system is closed-loop controlled despite perturbations of the environment.     

Coherent beam combining of two fiber amplifiers using stochastic parallel gradient descent algorithm

We demonstrate the coherent beam combining of fiber amplifiers using a stochastic parallel gradient descent (SPGD) algorithm, producing a total output power of 23 W. Active phase control on each amplifier is performed by running the SPGD algorithm on a DSP chip with updating rate of almost 100 kHz. Experimental results show that power encircled in the target pinhole in closed-loop mode is 1.85 times more than that in open-loop mode, which is 92% of the ideal case. The fringe contrast of the far-field fringe pattern is as high as 92% when the system is in closed-loop mode. The whole system in closed-loop mode performs well in a long-time observation.     

基于随机并行梯度下降算法光纤激光相干合成的高精度相位控制系统

随机并行梯度下降(SPGD)算法是实现大功率主振荡功率放大器相干合成的有效手段之一.分析了SPGD算法用于相干合成的基本原理,设计了基于SPGD算法的相位控制系统,给出了基于该SPGD控制器的相干合成实验结果.理论分析表明,SPGD算法控制器的迭代速率为200kHz,对于两路相干合成的平均控制带宽大于12.5kHz,最优控制精度可达1/179个波长.实验表明,SPGD控制器能够实现高速的相位控制.在相位噪声幅度大于1/10个波长、频率为3kHz的情况下,有效地实现了两路激光的相位锁定,相位残差控制在1/25个波长内.     

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.     

随机并行梯度下降算法拟合大气湍流廓线统计模式的研究

掌握湍流基本规律及其廓线结构分布,是大气光学理论与应用研究中的关键问题之一。利用随机并行梯度下降算法对大气湍流廓线统计模式拟合进行了研究。在已获合肥地区整层大气折射率结构常数C2N平均廓线的前提下,以广义Hufnagel-Valley湍流模型为基础,拟合获得了该地区不同时间与不同季节的湍流模式廓线。研究发现,该方法获得的该地区湍流模式廓线在整层大气高度上均能很好地同实测平均廓线相符合,且两者所表征的整层湍流特征参量也能保持非常好的一致性。对寻求基于Hufnagel-Valley模型的通用湍流廓线模式拟合方法进行了有益的探索。     

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。     

Numerical and experimental study on coherent beam combining of fibre amplifiers using simulated annealing algorithm

We present the numerical and experimental study on the coherent beam combining of fibre amplifiers by means of simulated annealing (SA) algorithm. The feasibility is validated by the Monte Carlo simulation of correcting static phase distortion using SA algorithm. The performance of SA algorithm under time-varying phase noise is numerically studied by dynamic simulation. It is revealed that the influence of phase noise on the performance of SA algorithm gets stronger with an increase in amplitude or frequency of phase noise; and the laser array that contains more lasers will be more affected from phase noise. The performance of SA algorithm for coherent beam combining is also compared with a widely used stochastic optimization algorithm, i.e., the stochastic parallel gradient descent (SPGD) algorithm. In a proof-of-concept experiment we demonstrate the coherent beam combining of two 1083~nm fibre amplifiers with a total output power of 12~W and 93% combining efficiency. The contrast of the far-field coherently combined beam profiles is calculated to be as high as 95%.     

Active coherent beam combining of diode lasers

We have demonstrated active coherent beam combination (CBC) of up to 218 semiconductor amplifiers with 38.5?W cw output using up to eleven one-dimensional 21-element individually addressable diode amplifier arrays operating at 960?nm. The amplifier array elements are slab-coupled-optical-waveguide semiconductor amplifiers (SCOWAs) set up in a master-oscillator-power-amplifier configuration. Diffractive optical elements divide the master-oscillator beam to seed multiple arrays of SCOWAs. A SCOWA was phase actuated by adjusting the drive current to each element and controlled using a stochastic-parallel-gradient-descent (SPGD) algorithm for the active CBC. The SPGD is a hill-climbing algorithm that maximizes on-axis intensity in the far field, providing phase locking without needing a reference beam.     

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。     

自适应光学中SPGD算法关键参数实时调节方法

介绍了随机并行梯度下降（SPGD）算法及其在相干合成中的应用,针对实验中算法关键参数难以调节的难点,提出采用软硬件结合的新方式,实现对实验数据的在线采集和分析以及对SPGD算法关键参数的自动实时调节。开展了4路光纤激光相干合成实验,对不同调节方法进行对比。实验中采用新方式有效调节了SPGD算法中增益系数和随机扰动幅度的取值,合成效果显著。     

Experimental investigation of segmented adaptive optics for spatial laser array in an atmospheric turbulence condition

We describe a concept of using segmented adaptive optics (AO) to correct the higher-order wavefront aberrations (HOWAs) in an atmospheric turbulence condition. The effects of piston and tip-tilt corrections are analyzed by numerical simulation. An experiment test-bed is built for verification. Seven spatial beamlets are combined into a hexagonal array by use of a pyramid, and an active segmented mirror is used for phase modulation. The AO control is implemented by stochastic parallel gradient descent (SPGD) algorithm. Experimentally, after segmented AO is enabled, the HOWA in warmed air environment has been effectively corrected and the peak intensity of far-field has been enhanced by 5 times.     

Adaptive correction of vortex laser beam in a closed-loop system with phase only liquid crystal spatial light modulator

We propose and demonstrate the wave front correction of a vortex laser beam by using dual phase only liquid crystal spatial light modulators (LC-SLMs) and a stochastic parallel gradient descent (SPGD) algorithm. One phase only LC-SLM is used to generate vortex laser beam by loading spiral phase screen onto the wave front of input quasi-Gaussian beam. The other phase only LC-SLM under SPGD controller based on the subzone control method adaptively compensates the wave front of vortex laser beam. Numerical simulation and experimental results show that after correction, vortex doughnut like beam is focused into a beam with airy disk pattern distribution in the far field. The adaptive corrections of vortex laser beam with different optical topological charges are studied. The results show that the optical topological charge has little influence on adaptive correction. The powers in the main lobe of far field intensity distributions of vortex laser beams with different optical topological charges are all greatly improved by adaptive correction. The technique proposed in this paper can be used in optical communication, relay mirror and atmospheric turbulence correction.     

Stochastic parallel gradient descent laser beam control algorithm for atmospheric compensation in free space optical communication

Atmospheric turbulence seriously degrades the performance of free space optical (FSO) communication systems, especially the coupling efficiency. In this paper, we propose a stochastic parallel gradient descent (SPGD) algorithm to compensate the atmospheric turbulence in FSO communication system. Theoretical analysis and work flow of the algorithm are given. Both simulation and experimental results indicate that the coupling efficiency can be increased from 12% to more than 80% after 270 iterations or more than threefold, which implies a significant improvement of the performance.     

Coherent Beam Combining of Three Watt-Level Fiber Amplifiers Using a DSP-Based Stochastic Parallel Gradient Descent Algorithm

We present an experimental study on coherent beam combining of three watt-level fiber amplifiers using a stochas- tic parallel gradient descent （SPGD） algorithm. Phase controlling is performed by running the SPGD algorithm on a digital-signal-processor （DSP） chip with a voltage updating rate of 16500 times per second. Energy encircled in the target pinhole is 2.62 times more than that in an open loop. The combining efficiency is as high as 87%.     

上下行传输孔径接收光强起伏特性分析

为了研究地星上行和星地下行激光链路孔径内接收光强信号的典型特征,基于大气湍流理论和位相屏方法,计算了短波和中波红外激光在特定大气相干长度条件下,孔径内接收激光功率与总功率的比值及起伏情况。根据统计结果讨论了上下行大气通道传播特性的差异,结果表明相同外界条件下,直径50 cm孔径内接收的下行激光信号强度大于上行激光信号强度1个数量级以上,其信号的起伏程度也低于上行激光;中等湍流下,下行激光孔径接收光强的概率分布函数服从对数正态分布,最大概率接收功率比与无湍流条件下的功率比值一致,分别为0.42%(1.315 m)和 0.26%(3.8 m)。     

自适应光学随机并行梯度下降算法波前整形规律仿真

本文首先介绍了基于Zernike模式的SPGD算法对大气湍流畸变波前的整形原理,通过推导得到了关于性能指标的简明表达式,使SPGD算法收敛速率得到明显提升。然后建立了自适应光学随机并行梯度下降算法波前整形系统模型,主要对SPGD算法收敛速率、整形能力和整形效果随波前畸变量和变形镜模型的变化规律作了较为详细的仿真研究,整体定性结果表明：三者的变化规律有一定的相似性,同时利用最小二乘法得到了关于整形能力和整形效果变化规律的定量表达式,若从自适应光学波前整形系统的实时性和简单性考虑,在保证一定整形效果的情况下,选择37单元变形镜对畸变波前的3~27（25）阶Zernike像差进行整形即可。     

Coherent beam combining of fiber amplifier based on adaptive polarization and active phase control

Coherent beam combining (CBC) of a polarization-maintaining fiber amplifier and a non-polarization-maintaining fiber amplifier is presented. The experiment is based on adaptive polarization control and active phase control. The stochastic parallel gradient decent (SPGD) algorithm is used for the adaptive polarization control in the non-polarization-maintaining fiber amplifier and the active phase control is realized by single frequency dithering (SFD) algorithm. When the adaptive polarization control system and the phase control system go into closed loop, the fringe contrast of far-field intensity pattern is improved to more than 87.7%. A scalable architecture for CBC of two styles fiber amplifiers is also proposed.