Technique Of Aberrations Control For Adaptive Fiber Laser Array Propagating Through Atmosphere

Fiber laser array technique,as a method of coherent combining,is a promising way to generate laser output with high lightness and high quality and will act as a key role in future high power laser systems.This technique brings in abilities of adaptive optics into fiber laser technique.With the development of fiber-integrated aberration correctors,the adaptive ability of the fiber laser array could get further promotion.Present researches on fiber laser array mainly focus on realization of coherent beam combining(CBC)indoor.Further development of the fiber laser array will face challenges when propagating through real atmosphere with long range.Aberrations in such transmission system through atmosphere include aberrations within the fiber laser array,like phase noises and tip/tilt errors,and the turbulence-induced dynamic aberrations located on the path from the fiber laser array to the target.Correcting these aberrations is critical for obtaining combined laser beams on the target surface with best beam quality and highest power intensity.Target-in-the-loop techniques with blind optimization control are the main existing methods to correct the turbulence induced aberrations in fiber laser array and have disadvantages of poor efficiency and low convergence rate,especially when facing applications with long range and large array scale.This paper integrates wavefront recover techniques and aberrations correction methods based on pre-compensation,which work fine in traditional adaptive optics system,with fiber laser array to explore new correction methods.This paper is focused on issues of correcting the turbulence induced aberrations in atmosphere.Researches in this paper mainly contain five parts as below.First,mathematical model of the fiber laser array propagating in atmosphere is built up and factors influencing the CBC of the fiber laser array are discussed.Far-field metric functions of the CBC and incoherent combining of the fiber laser array with different array scale without aberrations compensation under turbulence with different intensity are investigated.Functions of the aberrations correction in sub-apertures are discussed based on numerical simulation.Metric functions of the CBC with low order aberrations correction in sub-apertures,including pistons,tip/tilts and defocuses,are analyzed.Necessity and cost-benefit of correcting the higher order aberrations in sub-apertures are discussed.Results prove the necessity of correcting pistons and tip/tilts in sub-apertures for fiber laser array propagating through atmospheric turbulence.For strong turbulence,correcting higher order aberrations in sub-apertures is also necessary.Secondly,correction of tip/tilts in sub-aperture of the fiber laser array for both fiber laser beam coupling and emission is researched with experiments and numeric simulation.Promotion of the coupling efficiency of the fiber laser array with tip/tilts correction in sub-apertures is achieved with numeric simulation,where adaptive fiber optics collimators(AFOCs)act as the tip/tilts devices and stochastic parallel gradient descent(SPGD)are adopted as the control method with coupling efficiency as the metric.Reception and transmission of fiber laser beams using two-element AFOC array under simulated turbulences are demonstrated.Coupling efficiency of the fiber laser beam,with wavefront disturbed by the simulated turbulence,from space into single mode fiber(SMF)is promoted with control of SPGD,meanwhile the far-field concentration ratio of the outgoing laser beams of the SMFs are also promoted.Results show that promotion of the overall coupling efficiency of the fiber laser array and pre-compensation of the tip/tilts of the outgoing laser beam could be achieved simultaneously based on the active coupling control in sub-apertures.Thirdly,CBC of the fiber laser array with tip/tilts control based on adaptive space-to-fiber laser beam coupling is demonstrated experimentally with a 7-element AFOC array.Mathematic model the of tip/tilt control based on adaptive space-to-fiber laser beam coupling is built up and analyzed.The far-field power-in-the-bucket(PIB)metric is used for phase-locking.Parallel tip/tilt control is implemented via maximizing the received power in each sub-aperture with fixed two-channel SPGD control.The average of normalized coupling efficiencies of seven AFOCs increases from 0.76 without tip/tilt control to 0.94 under control.In CBC,the PIB metric increases by 4.6 times and the phase residual error is less than ?/15.Results show that tip/tilts correction processes based on the active coupling control in each sub-aperture are mutually independent and also parallel to the phase-locking.This feature give abilities of real-time correction of the turbulence-induced aberrations to the fiber laser array,regardless of the array scale.Fourthly,new aberrations correction methods of the fiber laser array is proposed.This method is based on pre-compensation of the measured aberrations in the transmission path and can be applied for long-range applications where the convergence rate and control banwidth of the existing blind optimization methods degrade sharply.Aberrations could be measured by a wavefront sensor we propose here,which is based on the AFOC array and integrates with the fiber laser array.Recovery of the turbulence induced aberrations with this new wavefront is researched with numeric simulation.The best Zernike mode number of the modal recovery method under array with different parameters and turbulence with different intensity is discussed.Based on the recovered aberrations,piston aberrations are calculated and pre-compensated in the fiber laser array,together with tip/tilt aberrations.Performance of the pre-compensation correction are analyzed,results show that such method could promote the CBC efficiency of the fiber laser array under atmosphere turbulence.Finally,experimental recovery of the aberrations of glass plates with the 7-element AFOC array is demonstrated.Sub-aperture averaged local phase gradients of the aberrations are inferred from the deviation of the voltages applied on the AFOCs between the two processes of active coupling control before and after the glass plates are introduced.Aberrations then could be recovered with modal estimations.Root-mean-square values of the aberrations of the two glass plates,excluding the whole tip/tilt aberrations,are 0.433 ?m and 0.647 ?m,the recovery errors are less than 0.0749 ?m and 0.116 ?m correspondingly.CBC of the fiber laser array with pre-compensation of the measured glass plates aberrations are achieved experimentally.Results show that such technique could correct the piston and tip/tilt aberrations simultaneously,and get far-field metric almost the same as ideal correction.