Study On High-power Fiber End-cap And All-fiber Signal Combiner

Fiber lasers have been applied broadly in industrial manufacture and national security due to their essential characteristics such as high conversion efficiency,excellent beam quality,convenient heat management and compact configuration.Due to the rapid increasing output power in recently years,fiber laser has been one of the most important development orientations of high power lasers.However,the power scaling of fiber laser is limited by the end facet damage,the nonlinear effects,and the thermal effects.High power fiber end-cap can effectively solve the problem of end facet damage by increasing the damage threshold of output fiber end facet for a fiber laser.All-fiber signal combiner is one of the most important solutions to breakthrough the power scaling limitation of a single fiber laser by combing several high power fibers into a single fiber to realize a higher output power.This thesis is mainly focusing on the design and manufacture of high power fiber end-cap and all-fiber signal combiner.The main work included is as follows:1.Parameters for signal laser transimmting in a fiber end-cap like spot diameter,feedback light loss,and beam waist deviation are theoretically calculated and experimentally comfirmed.High power few-mode and multi-mode fiber end-caps are experimentally fabricated whose capabilities are tested with 3.01 kW and 6.26 kW fiber lasers,respectively.A monolithic fiber end-cap collimator is studied by theory and experiment,which realize high power direct free-space fiber-fiber couping with measured coupling loss less than 0.3 dB.2.Based on the three-layer medium waveguide models,mode properties of a tapered single-core fiber and a tapered multi-core fiber are theoretically analized.An approximation fomula is derived for fundamental core mode cutoff condition of a tapered multi-core fiber.Based on single-core fiber tapering technology,a single mode field adaptor between single-mode fiber and large mode area fiber is fabricated with measured transmission loss less than 0.4 dB.Multi-core photonic crystal fiber adaptors are experimentally fabricated based on controlled air hole collapse technology and selected air hole collapse technology,respectively.Low-loss splicing is realized between seven-core photonic crystal fiber and double cladding fiber and the lowest loss is 0.22 dB,which breaks the limitation of hundrad-watt-level supercontinuun generated in photonic crystal fibers.3.A theoretical model of core-coupling signal combiner(COSC)is built and relevant experimental researches are carried out.The theorecital model of COSC is built based on guided mode decomposition method and excited mode properties in the output fiber are analyzed for different deviation cases of the input fiber.Incoherent laser beam combining by a COSC is simulated based on beam propagation method(BPM).A 7×1 COSC is fabricated for high power single-mode fiber laser beam combing and the combing output power is 6.08 kW with beam quality M~2-10.2.Incoherent combining of supercontinuum laser by a COSC is also simulated and experimentally comfirmed by combining seven fiber laser with 802 W and spectrum covering 1060-1700 nm.4.A theoretical model of cladding-coupling signal combiner(CLSC)is built and relevant experimental researches are carried out.The theoretical model of a CLSC is built based on mode evolution properties of a tapered multi-core fiber.The beam quality of incoherent beam combining by a CLSC is analyzed based on analytical method.Theoretical results show that the limitation of beam quality is M~2-1.75 for a 3×1 CLSC and M~2-2.70 for a 7×1 CLSC.Factors that may affect beam quality of output laser are also simulated.A 7×1 CLSC is fabricated for high power single-mode fiber laser beam combing and the combing output power is 6.26 kW with high beam quality M~2-4.3,which are the highest output power and best beam quality for all-fiber signal combiner to the best of out knowlesge.