Study On Coherent Polarization Beam Combining System Of High Power Fiber Lasers

Brightness scaling ability of a monolithic fiber amplifier/laser is limited by many factors,such as thermal effects,nonlinear effects,mode instability,fiber damage,high brightness pump sources and so forth.Coaxial coherent beam combination(CBC)can further scale the laser brightness.Nowadays,two issues are strongly required to break through in coaxial CBC systems,one issue is to achieve higher brightness element employed in coaxial CBC systems,and the other issue is to optimize the combining efficiency of the coaxial CBC systems.In this paper,we select one of the typically representative coaxial CBC systems(coherent polarization beam combination(CPBC))to perform systematically theoretical and experimental study.1.By comprehensively considering the two majority limitations(stimulated Brillouin scattering(SBS)and mode instability(MI)effects)in single frequency/narrow linewidth fiber amplifiers,the achievable approaches to obtain all-fiberized,high brightness,narrow linewidth,near linear-polarization fiber lasers are carefully explored.The brightness scaling potentials and limitations with different gain mediums and different narrow linewidth seeds are compared.The feasibility of the MI suppression techniques of coiling the gain medium,reducing the core size of the active fiber,changing the pumping wavelength are investigated.Based on the systematical study,the approach to achieve 2 k W level fiber laser with high brightness,narrow linewidth and near linear-polarization is proposed.2.Comprehensive studies are performed based on high power coherent polarization beam combination(CPBC)system by considering multiple detrimental factors in a practical system and some experimental validations are implemented.Firstly,a generally theoretical model is established based on one combination unit of CPBC system,and the recurrence relation to analyze the combining efficiency of CPBC system with large number elements is presented.With some approximations,the approximate expression of the combining efficiency of CPBC system with large number elements is presented.Then,the influence of phase and optical path controllers,the characteristics of each combination element,the characteristics of combination components(fiber endcap,collimator,half wavelength plate,polarization beam combiner)on the combining efficiency of CPBC system is investigated in detail,and the metrics of CPBC system with large number elements on the detrimental factors above-mentioned are examined.Based on the practical conditions,the significant issues that should be investigated and validated in high power CPBC system with large number elements are figured out.The engineering methods are proposed and validated to evaluate the influence of significant issues on the combining efficiency of CPBC system.3.The significant issues in high power CPBC system with large number combination elements are carefully studied and validated.The phase noise suppression property of the phase controller is experimentally investigated and the phase noise distribution characteristic of high brightness,narrow linewidth,and polarization maintained amplifiers is examined.The feasibility of homemade high brightness,narrow linewidth,and polarization maintained amplifiers used in high power CPBC system is validated.Besides,some further optimizing methods to decrease the residual phase error of phase controller are proposed.The optical path control system is designed based on high power,multi-channel CPBC system,and its feasibility is preliminary validated with complex spectral structures.Based on the validated results,the feasibility of the optical path control system used in high power,multi-channel CPBC system is illustrated and some further optimizing methods are proposed.The dynamical tilt aberration of the homemade high brightness,narrow linewidth and polarization-maintained amplifiers is tested,and its feasibility used in high power,multi-channel CPBC system is proved.The thermal lens effects of the combination components used in high power CPBC system are analyzed based on high brightness,narrow linewidth and polarization maintained amplifiers.By analysis,we show that the thermal effect induced by commercial collimator will seriously limit the brightness scaling ability and extendibility of the CPBC system.In order to radically overcome the influence of thermal effects induced by combination components,high power collimator with defocus compensation property is associated developed after carefully investigating and selecting the lens windows and defocus compensation manner.Further,the significant parameters of the developed collimator,such as defocus compensation range,compensation precision and so forth,are investigated.Based on the analyzed results,the feasibility of the developed collimator used in high power,multi-channel CPBC system is validated.4.High power CPBC platform is established by using fiber amplifiers with different types of gain fiber and different power-level.The brightness scaling ability of the CPBC technique in single frequency laser regime,picosecond pulsed laser regime,narrow linewidth laser regime,and multi-wavelength laser regime is validated,and the combination technique is driven to new development level by carefully optimizing the system.The output characteristics of the CPBC system are examined by using the theoretical models and engineering methods.Based on the high power experimental results,the directions of further optimizing the CPBC system are pointed out,and the referenced optimizing targets of the related control systems are provided.