Analysis And Applications Of High-power Fiber Lasers

High-power lasers have long been an important research topic in laser optics. As a special kind of solid-state lasers,the fiber lasers find prospective applications in medical,optical communication,lidar,optical sensing,material processing,and scientific researching areas because of its compactness,high efficiency,and high beam quality.Due to the various advantages of fibers,the high-power fiber lasers are attracting more and more researching interests nowadays.The main obstacles in developing a high-power fiber laser are the heating effect and fiber nonlinearity.To avoid such problems,we can either improve the cavity structure of a single laser using fibers with large core areas,thus less fiber nonlinearity,or simply combine several lasers together to reach a higher power.The discussions in this thesis include an analysis of the characteristics of a novel ytterbium(Yb) doped large-core segmented-cladding fiber(SCF),both as a fiber amplifier and as a fiber laser.It also includes an analysis of laser combining schemes,which is mainly about the characteristics of the coherent beam combining(CBC),as well as the feasibility of devising an all-fiber CBC scheme with better performance.The major work and results of this thesis are as follows:1.Establish a numerical model for simulating an Yb-doped large-core SCF amplifier by combining the rate equations of a two-level model and the mode field distribution calculated by finite-element method.In particular,we have investigated the influence of the structural parameters on the signal gain and noise level.The results show the necessity of choosing appropriate structural parameters of the SCF to make a balance between the gain,the noise figure,and the single-mode performance,due to the single-mode operation principle of a SCF.We also give the optimal parameters of the amplifier.2.Simulate a steady-state Yb-doped large-core SCF laser by adding feedback conditions to the established numerical model of the fiber amplifiers.We present an analysis of the SCF lasers including the threshold,the efficiency,the signal to noise ratio,as well as the optimal cavity parameters.The results show that,being different from a SCF amplifier,a SCF laser has much higher efficiency and better single-mode performance within a broad range of cavity parameters.By appropriately choosing the cavity parameters,the Yb-doped SCF laser can operate with near-quantum-limit efficiency.3.Calculate the characteristics of the far-field spot of general hexagonally arrayed coherent beam combining devices based on the Fraunhofer scalar diffraction theory.For ideal coherent beam combining devices,the calculation gives some general considerations on the devices to obtain higher power convergence,in which a major conclusion is to arrange the unit lasers as tightly as possible.For combining devices with some deviations,we plot the variation of the power percentage of the main peak of the beam with the change of each parameter.The result leads to the conclusion that one must pay special attention to the phase difference between unit lasers to obtain higher main peak efficiency.We also propose configurations by which arrayed CBC devices can deliver a flat-top beam,as well as the feasibility of changing the angular direction of the main peak like phased array radars.4.Make some improvement on the co-cavity CBC scheme based on the injection locking theory.We have proposed an improved 2-way co-cavity CBC experiment in which the injection from a fiber DFB laser helps stabilize the spectrum of the CBC laser.We have also devised a 2-way ring-cavity amplifying experiment to verify the coherence improvement by substituting the amplifying fibers of original MOPA structures into ring lasers.The experimental results have confirmed the effects of injection locking,with which we can develop all-fiber CBC devices with the advantages of narrower linewidth,better stability,and the ability of self-organizing.The innovative work and results in this thesis are as follows:1.Analyzed the characteristics of the novel Yb-doped large-core SCF lasers and highlighted the difference between the SCF laser and the SCF amplifier,as well as the influence of SCF structural parameters on the laser performance.The optimal cavity parameters are also calculated.2.Analyzed the influence of the arrangement of unit lasers on the combined beam for arrayed CBC devices.The results show that the power convergence can exceed that of the unit laser with some geometry.For applications as a lidar,the calculation leads to the conclusion that the phase difference between the unit lasers has maximum influence on the main peak efficiency as well as on the Streh1 ratio.We also proposed the concept of devising CBC devices with the ability of angular scanning or flat-top beam output.3.Made some improvement on the co-cavity CBC scheme based on the theory of injection locking.The spectrum of a co-cavity CBC device is greatly stabilized with the help of the DFB injection.We also attempted substituting the fiber amplifier in a MOPA laser into ring-cavity lasers,by which the coherence between different arms greatly increased.The idea helps in fabricating all-fiber CBC devices with better performance.