| The continuous development of high-power fiber lasers not only brings new challenges to laser transmission systems,but also puts forward new requirements for laser beam energy distribution.Negative curvature hollow core fibers(NC-HCFs)guide light through air cores,which have advantages such as low nonlinear effects,high optical damage threshold,and mode adjustability.They have great application potential in the field of high-power single-mode laser transmission.The all-fiber signal combiner can improve the output power of the laser by combining multiple laser beams into a single fiber,as well as regulating the laser mode components to output high-power multi-modes laser with high beam quality or special energy distribution,which has important research significance for further expanding the application fields of high-power fiber lasers.Subsequently,this thesis focuses on the transmission and combining of high-power lasers,and conducts relevant research on NC-HCFs and all-fiber signal combiners.The main work includes:(1)Single-mode low-loss transmission and fabrication feasibility are important characteristics of high beam quality laser transmission NC-HCFs.By regulating the coupling of core high-order modes and cladding leakage modes,an NC-HCF structure was designed to achieve single mode or single polarization low loss transmission at 1μm band.For single mode fibers,the high-order modes extinction ratio was up to 4.65×10~4,and the large diameter tolerance rate of the cladding tubes improved the feasibility of the fiber fbrication.For polarization-maintaining negative curvature hollow core fiber(PM-NC-HCF)structures,the loss of fundamental mode was as low as 0.005 d B/m,which was the first PM-NC-HCF with a loss of less than 0.01 d B/m at 1μm band.(2)Improving output power while maintaining high beam quality through all-fiber signal combiners is of great research significance.To further improve the output beam quality of the 3×1 signal combiner,lateral mode coupling in a tapered fiber bundle(TFB)and mode excitation coupling at the splice of the TFB and the output fiber were analysed.For the first time,an output fiber with a core diameter of 34μm was used in a 3×1 signal combiner,which achieved a total laser output of 8.89 k W with a transimission efficiency of97.2%.The output beam had a Gaussian distribution and a M~2 factor of 2.32,which was close to the theoretical limit value of 2.(3)The adjustabale ring mode(ARM)laser consisting of a central beam and a ring beam has a broad prospect in the field of laser processing.An all-fiber ARM(6+1)×1 signal combiner was proposed.The influence of(6+1)×1 TFB structure on the independent propagation characteristics of the central beam and the ring beam,as well as the output beam quality were analysed.According to theoretical analysis,an ARM(6+1)×1 signal combiner was fabricated to achieve an ARM laser with a Gaussian-liked central output beam in a 50μm fiber core.To further improve the central output power of the ARM laser and solve the problem of a too high central energy density under ultra-high power.An ARM(6+3)×1signal combiner was proposed and fabricated,which achieved an ARM laser with an 8.8k W uniformly distributed central output beam in a 100μm fiber core.Furthermore,a cascated combiner composed of a 3×1 signal combiner and a(6+1)×1 signal combiner with series connection by a connective fiber was proposed and fabricated.It achieved an ARM laser with an 8.8 k W flat top distributed central output beam in a 50μm fiber core,providing broader application scenarios for high-power lasers. |
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