Study On Coherent Polarization Beam Combining Technology Of Fiber Lasers

Acquiring high brightness laser source is the all-along research hot-pot in laserregion. Fiber lasers and amplifiers have attracted intensive attention due to theiressential advantages of compact structure, excellent beam quality, and convenient heatmanagement. However, several obstacles, including thermal effects, nonlinear effects,and the ability of high brightness pump diodes will ultimately restrict the enhancementof the output power of a single fiber laser. In order to further enhance the brightness ofthe output beam, many coherent beam combination and aperture filling configurationsare presented. As a new and scalable configuration, coherent polarization beamcombing (CPBC) technique is analyzed theoretically and validated experimentally in anall-around way. The extendibility and power enhancement of the CPBC technique isanalyzed, and the applicability of the CPBC system is also extended. The main work ofthe dissertation is listed as follows:1. Based on the combining efficiency of the CPBC system, the theoretical model ofCPBC system is established.Firstly, the available extended structures of CPBC system are presented and theexpression of the combining efficiency of the CPBC unit is established, which containsthe overlap error, tilt error, amplitude error, and the residual phase error of the phasingcircuit. Secondly, three fundamental structures are extracted from all the availableconfigurations, and based on the analysis of CPBC unit, their theoretical models andexpressions of the combining efficiency are deduced, furthermore, the extendibility ofthe CPBC technique is analyzed, and the optimal combining configuration of the CPBCsystem is demonstrated. Finally, based on the optimal structure, the evaluatedexpression of the combining efficiency is established, which contains the loss PBC,overlap error, tilt error, dynamic degeneration of polarization and dynamical phase errorof the phasing circuit. In order to validate the feasibility of the approximate expression,for the Gaussian beam profile, the evaluated expression is elaborated.2. The feasibility of the CPBC configuration is validated experimentally.Firstly, the feasibility of the single frequency dithering technique in CPBC systemis analyzed, and then the experimental platform of the CPBC system is establishedaccording to the optimal structure. The Watt-level and10Watt-level CPBC system isvalidated experimentally. The CPBC system is extended to eight channels for the firsttime and the combining efficiency of the whole system is92%. Experimental resultsshow that, with the increase of channels and enhancement the power of each channel,CPBC system can remain high combining efficiency (>90%) and excellent beam quality.At present, the output power of the CPBC system is60W, which is the maximum outputpower in open published literature. The output power of the system is restricted by polarization-maintained fiber amplifiers and pump sources; the whole system has greatpotential in scaling to higher power level.3. Based on the experimental platform established, some extendable study isaccomplished.In order to overcome the enhancement limitation of the single-frequency amplifier,CPBC of multi-tone fiber amplifiers is validated experimentally; The CPBC system isapplied to pulsed-region and4-channel70ns-pulsed lasers are coherently polarizationcombined successfully; Based on the ring cavity passive phasing technique, thefeasibility of CPBC by passive phasing technique is validated experimentally. Thesignificant crystal apparatus in CPBC system are tested experimentally. Theexperimental results reveal that by selecting the material of the crystal, the temperaturechange of the significant apparatus in CPBC system can be neglected in high powerdensity (>1700W/cm2), and the beam quality of the injected beam is not degenerated.