Research On Efficient Coherent Combining Technology Of Fiber Laser Phased Array

Constructing a lightweight and modular fiber laser phased array(FLPA)to coherently combine multiple medium-power lasers is an important technique that could realize high-energy laser output.At present,coherent beam combining(CBC)technology is limited to industrial applications because of its low efficiency.Therefore,the efficient CBC technology of FLPA is systematically studied in this paper.This paper focuses on the construction principle of the FLPA system,the coherent combining of array beams in the atmosphere,and the efficient phase-locked control strategy,to maximize the efficiency of CBC under different application scenarios and application requirements.The main research contents of this paper are as follows:In the first section,based on the scalar diffraction theory,the calculation model of the CBC system is deduced;The definitions and characteristics of various common beam quality evaluation indexes are introduced;Then,the influence of FLPA’s parameters on the combined beam’s quality is simulated.Based on the simulations,the criteria for designing high efficient FLPA system are clarified,and the potential of amplitude modulation to improve the combined beam’s quality is realized;Finally,based on the theory of atmospheric turbulence,the concept of turbulent atmospheric phase screen is introduced,which provides a numerical method for the atmospheric transmission and coherent combining of array beams.In the second section,a dense 19-element FLPA system is designed,which allows a fill factor of 0.903.The design principle of each part of the system is given,and the performance of the key devices is analyzed.Based on the 19-element FLPA system,the efficient coherent combining of array beams is realized in an indoor environment.In the third section,an efficient coherent combining method based on beamlets amplitude modulation is proposed.Starting from the physical connotation of efficient CBC,the multidimensional optical parameter regulation mechanism is introduced.After the phase-locked and tip/tilt control of the FLPA beam,the amplitude coefficient is modulated.In this way,The PIB of the combined beam could further be increased by14.3%,which proves the great potential of amplitude modulation to improve the PIB of the combined beam.In the fourth part,the atmospheric propagation and coherent combining of array beams are studied theoretically and experimentally.Firstly,the aberration decomposition method of atmospheric turbulence is introduced,and the advantages of atmospheric turbulence low-order aberration compensation are verified;Secondly,in the 2.1km near-horizontal atmospheric transmission environment,the stable aiming and efficient coherent combining of 19-element beamlets to the target under different turbulence intensity is realized.When the system enters the closed-loop state,the PIB of the combined beam could be improved by up to about 13.4 times higher than that in the open-loop state;Finally,by splicing two FLPA modules,the atmospheric transmission and coherent combination of 38-element array beams are realized and the corresponding PIB could be improved by up to about 29 times after aberrations compensation,which verifies the array element expansion ability of the FLPA system.In the fifth section,an efficient CBC method based on deep learning is studied.The working principle and ability of convolutional neural network(CNN)in the phaselocking task are introduced;The limitation of Non-uniqueness on the generalization performance of CNN is analyzed,and a spiral phase modulation method is proposed to solve this limitation;Based on the simulation data,a CNN for efficient CBC is trained.The simulation results show that in the 7,19,37,and 61 elements FLPA system,the phase-locked speed of CNN is always much faster than that of the traditional SPGD algorithm,which can better meet the high-speed and high-precision phase-locked requirements of CBC tasks.This paper focuses on the high-efficiency CBC method of the FLPA system,including theoretical analysis,numerical simulation,and experimental research.Starting from the connotation of optical field regulation and high-speed phase-locked strategy of the FLPA,a variety of innovative technical methods such as efficient CBC based on amplitude modulation method and high-speed phase-locked strategy based on deep learning method are proposed,which lays a solid foundation for subsequent research.