High mode purity of high-power fiber lasers is strongly required in 3-dimensional printing, high brightness beam combing, and high accuracy material processing. Mode decomposition (MD) is an effective technique for diagnosing the mode composition of a high-power fiber laser. In particular, dynamic mode evolution could be commonly generated during power scaling process due to thermal and nonlinear effects. Consequently, the fast and accurate mode decomposition (MD) method is strongly required. The non-iterative fast mode decomposition based on matrix operation method is theoretically a promising technique to achieve ultra-fast MD with high accuracy. However, this technique of realizing MD is mainly limited by the noise of the light-spot image in a practical system. In this report, the effect of the image noise on the noniterative fast mode decomposition technique is carefully investigated. Simulation results show that the decomposition accuracy decreases as the intensity of noise increases. Nevertheless, the effect of image noise on the non-iterative fast mode decomposition method depends not only on the noise intensity but also on the coefficient matrix condition number of the matrix equations. Furthermore, the higher the condition number of coefficient matrix of linear equations is, the more influence of image noise on the non-iterative fast MD accuracy. The results presented could give instructive reference for further optimizing the non-iterative fast mode decomposition technique used in practical high-power fiber lasers.
In this paper, an innovation coherent beam combining (CBC) architecture to generate the structured light beams array was proposed and experimented. The simulation and experimental results reveal that the optical vortex beams array (OVBA) with multi-modes can be generated effectively in the far field. The OVBA is composed of multiple sub-OVB in the intensity distribution. Furthermore, the number of OVBs can be modulated by changing the fill factor of the laser array in the near field. In particular, the performance of a OVBA copier was observed, which may deepen the understanding of creating the structured light fields by CBC technique. The experiment results were in excellent agreement with the simulation results. This work could provide valuable and practical reference on generation and manipulation of high power structured light beams
In this report, we will introduce our recent advances in developing the deep-learning-based coherent fiber laser array systems for power scaling and spatial light structuring. Our motivation is to construct a deep-learning network for estimating the thermal and environmental induced phase errors, and further compensate the phase errors by the phase control servo with the assistance of the network outputs. Technical progresses in terms of the network optimization, two-stage control scheme, and optical field information acquisition will be covered. Moreover, the prospects and challenges towards the future implementation of intelligent control for CBC systems will be discussed.
In this letter, a two-stage phase control technique is proposed to increase the control bandwidth of the target-in-the-loop (TIL) system. In this technique, the first stage phase control is enabled by LiNbO3 phase modulator to compensate the phase noises in the fiber amplifiers, and the second stage phase control is enabled by the liquid crystal (LC) to compensate the phase noises induced by the atmospheric turbulence. We built a TIL coherent beam combining system with 3-channel coherent fiber lasers over a 40 m atmospheric propagation path. In our experiment, the stochastic parallel gradient descent (SPGD) algorithm was employed for phase control. When the phase control system was in the close loop, the performance of laser beam projection was significantly improved, and the phase locking bandwidth for transmitter side phase distortions reached 1 kHz. This method can be used for applications such as energy transmission and free-space optical communication.
A novel method for active coherent beam combining by Particle Swarm Optimization (PSO) algorithm is demonstrated in this paper. The principle of this method is introduced, and its advantages are presented in detail. In the simulation, 37 fiber lasers are coherently combined by employing stochastic parallel gradient descent algorithm with 91 steps, and then, the combined beams are combined by PSO algorithm with only 30 steps. And the result shows that the more laser elements, the more remarkable PSO algorithm is in the CBC system. Because of the high control ability of PSO algorithm in coherent beam combining combined with traditional algorithm, it is scalable to phase-locking system in a large number of fiber lasers.
The pulse shape from the pulsed amplifier always distorts because of gain saturation effect, an effective method is active control the output pulse shape by reshaping the pulse shape of the laser seed. We demonstrated a new method for active temporal pulse shape control of fiber amplifier with adaptive proportional control. We numerically researched three proportional control methods, including static proportional control, adaptive proportional control and piecewise adaptive proportional control. The results show that proportional control can generate arbitrary temporal pulse shape with high accuracy and less iterations even if parameters of the fiber amplifier are unknown.
Structured optical field (SOF), which includes vortex beams, non-diffraction beams, cylindrical vector beams and so on, has been under intensive investigation theoretically and experimentally in recent years. Generally, current research focus on the extraordinary properties (non-diffraction propagation, helical wavefront, rotation of electrical field, et al), which can be widely applied in micro-particle manipulation, super-resolution imaging, free-space communication and so on. There are mainly two technical routes, that is, inner-cavity and outer-cavity (spatial light modulators, diffractive phase holograms, q-plates). To date, most of the SOFs generated from both technical routes involves with single monolithic beam. As a novel technical route, SOF based on array beams has the advantage in more flexible freedom degree and power scaling potential. In this paper, research achievements in SOF generation based on array beams are arranged and discussed in detail. Moreover, experiment of generating exotic beam by array beams is introduced, which illustrates that SOF generated from array beams is theoretically valid and experimentally feasible. SOF generated from array beams is also beneficial for capacity increasing and data receiving for free-space optical communication systems at long distance.
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