Propagation And Control Of High-Power Fiber Laser Arrays

Coherent beam combination(CBC)of fiber laser array is an effective way to achieve high power and high beam quality laser beam.At present,important breakthroughs have been made in key technologies in the field of fiber laser coherent synthesis.Under laboratory conditions,high average power of coherent beam combination has been improved to 10 k W,and more than 100 fiber lasers of coherent beam combination has been achieved.On one hand,when the coherent array beam is applied in free optical communication,laser phased array radar and other fields,it will be affected by a series of atmospheric effects,resulting in the degradation of the beam quality in the target.The research on the scaling rules of array beam propagating in the atmosphere is able to provide scientific optimization design for relevant engineering systems.On the other hand,limited by the withstand power of key devices,the power improvement of the structured light field encounters a bottleneck.The rapid development of regulation technology of array beam provides a potential technical approach for the generation of high-power structured light field.Based on the above analysis,theoretical and experimental studies on atmospheric propagation characteristics and light manipulations of coherent array beams are carried out systematically.The main research contents are as follows:The propagation characteristics of airborne partially coherent array beams propagating in the atmospheric turbulence and engine plume are studied.Two step calculation method for calculating of laser beam propagation in engine plume and atmospheric turbulence is proposed,and the analytical expression of cross-spectral density function of airborne array laser beam propagation in atmosphere is derived.The results show that profile of spectral density and the spectral degree of coherence of partially coherent array gradually change to an elliptic shape after propagating through a jet engine plume region.The beam broadening of coherent array beams is severe even propagating through a small distance in the engine plume.It has been demonstrated that when the propagation distance in the jet engine turbulence is between 1 and 10 m,the influence of jet on the beam broadening reaches the degree of moderate turbulence or even strong atmospheric turbulence.It is also found that low spatial coherence can suppress the influence of engine plume.The propagation characteristics of high-power coherent array beams are studied by considering the influence of atmospheric turbulence and thermal blooming.A fourdimensional simulation model of higher-order mode(HOM)fiber laser array and partially coherent array beam propagation in atmospheric turbulence under the thermal blooming effect is established for the first time.The influence of thermal blooming effect on the propagation characteristics of partially coherent array beam and high order mode array beam is studied by numerical simulation.The results show that the far-field spot of HOM fiber laser array is similar to the intensity distributions of initial arrangement,and the focal field tailoring can be realized by changing the initial arrangement of HOM array beam in the initial plane.However,the initial arrangement will influence the energy focusability of HOM array beam in the far field.In addition,considering the combined effect of atmospheric turbulence and thermal blooming,the thermal distortion parameters of partially coherent array beam are derived.It is found that the turbulence effect can suppress the distortion caused by the thermal blooming effect at the cost of beam broadening of laser beam.The tight focusing characteristics of cylindrical vector beams(CVBs)generated by coherent beam arrays are studied,and an experimental system of combined CVBs is established.Firstly,a theoretical model of combined CVBs is established,and the effects of initial polarization direction,truncation parameter and exit sub-aperture radius on the intensity distributions of tightly focused light are discussed.Secondly,the experiments of generating CVBs based on CBC technology are carried out.It is found that flexible tightly focused field tailoring can be realized by adjusting the initial polarization rotation of each sub-aperture.The feasibility of coherent array to generate CVBs is verified compared with the theoretical model.The inverse design method of generating structured light based on coherent array beams is proposed,and the generation of arbitrary array beams and quasi-non-diffraction beams at any position is realized,and experimental research on the generation of special structured light fields is carried out.Firstly,according to the theory of light field propagation and transformation,the designed phase for generating array beams and quasinon-diffraction beams is generated.Based on the designed phase,the feasibility of generating structured array beams is analyzed numerically.Secondly,the verification experiment of quasi-non-diffraction beams generated by the coherent array beams is carried out,and the Gaussian array beam and the vortex array beam are generated in the far field by phase modulation.The generated arbitrary shape-invariant structured beams are verified and are in excellent agreement with the results of inverse design method.