Study On The Target-in-the-Loop Coherent Beam Combination Technology Of Fiber Lasers

Brightness enhancement capability of single fiber laser/amplifier is limited by the thermal effect of the optical fiber waveguide,mode instability,photon darkening,and many nonlinear effects.Coherent Beam Combination(CBC)is a well-used technology to obtain higher brightness laser output.With respect to the high-power tiled coherent combination system,the long-distance application and improvement of the target power density are currently international difficulties in the field of CBC.In this paper,the target-in-the-loop(TIL)CBC technology is selected as the research object,and systematically theoretical and experimental studies are carried out.1.By comprehensively considering the characteristics of the array beams,the transmission system parameters,atmospheric turbulence,the target characteristic parameters and control algorithum,a TIL-CBC full numerical model is first established.Using this numerical model,we simulated the closed-loop control effect under different turbulence strength with different target parameters(static finite aperture target,static expansion target,and dynamic expansion target).Rultes show that,to the cooperative target with limited aperture,the Power-in-the-bucket(PIB)is a well used evaluation function for TIL adaptive phase control;to the non-cooperative target with expanded aperture,atmospheric turbulence has an important effect on the control effect.Based on the extended Huygens-Fresnel principle,the influence of target on the temporal coherence of high-power narrow-linewidth fiber laser array under TIL synthesis is theoretically and numerically analyzed.Studies have shown that as long as the initial optical path difference is zero,the target characteristic has little influence on the temporal coherence of the narrow linewidth laser source,and the ideal coherent situation can still be achieved on the target surface.2.We successfully developed a conformal projection system that can withstand high power output,and conducted a series of indoor CBC experiments based on the conformal projection system to verify its performance and reliability.Firstly,AFOCs that can withstand kW-level high power output while maintaining near-diffraction limit beam quality were successfully developed.By improving the structure of AFOC,while maintaining the control range above ± 30?m,its first resonance frequency enhanced from 200 Hz to about 3kHz.A large-scale fiber laser beam quality measurement method based on PD and CCD was invented.The method was proved to be convenient and effective.This method provides an effective solution for the performance characterization of conformal projection system.Secondly we successfully designed a 3-element-triangle arrangement conformal transmission system and a seven-element-hexagonal arrangement high power fiber laser conformal transmission system with maximum array fill factor of 96.7% and maximum truncation factor of 1.12.Based on the two conformal transmission systems,we analyzed the effects of beam tilt,beam defocus,and beam truncation on the coherent combining efficiency,and verified the effectiveness of the self-produced conformal projection system.3.A series of TIL-CBC experimental studies are carried out based on the self-developed seven-element hexagonal fiber laser conformal projection system.First,a TIL-CBC experiment based on six-element fiber laser array through 150 m propagation distance is carried out.When the active phase control is on,the energy proportion in the central lobe on the target surface increases from 22.3% to 33.4%.The experiment verifies the feasibility of PIB metric function for fiber laser array TIL-CBC system.Secondly,we successfully achieve a 6-element fiber amplifier array TIL-CBC under a km-level propagation distance for the first time interiorly.When the tip-tilt control and piston phase-locking control are both on,the energy proportion in the central lobe on the target surface increases 3.1 times to 21.1%,and the normalized peak intensity increases 4 times.Tirdly,based on the conformal projection system,the application of TIL-CBC technology in new fields such as atmospheric parameter characterization and hollow beam generation are initially explored.Finally,a spectral combining scheme based on TIL technology is proposed,and theoretical analysis and preliminary experimental verification are carried out.4.Based on the CBC technology,the fiber laser beam array beam control is studied theoretically and experimentally.Firstly,we theoretically analyze the feasibility of generating quasi-vortex beams by phase modulation of the combined optical field using CBC technology.Then the orthogonality of generated modes is analyzed from the perspective.Secondly,the vortex beams with momentums of ±1 are first achieved experimentally based on 6-channel fiber laser array.