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Mode Decomposition And Mode Control Of High-power Fiber Lasers

Posted on:2017-11-03Degree:DoctorType:Dissertation
Country:ChinaCandidate:L J HuangFull Text:PDF
GTID:1318330536967149Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
The power scaling of the nearly diffraction-limited laser amplification in optical fiber have been developed in numerous applications ranging from industrial processing to national defense.However,the further power growth has been restricted by nonlinear effects and other factors.To increase the nonlinear threshold effectively and immediately,people have demonstrated large mode area(LMA)fiber with enlarged core size.However,when the LMA fiber is operated under high power level,the mode competition and coupling become even more complex and serious due to a large number of eigen modes supported in it.Those would lead to a poor output beam quality and even the appearance of the mode instability(MI),which is further obstacle of the beam quality.To improve the power scaling ability of fiber lasers with nearly diffraction-limited output,the physical processes of the mode competition and MI in high power fiber amplifiers(HPFA)are investigated by the mode decomposition scheme in the thesis.Besides,a novel design of the LMA fiber is proposed based on the selected loss mechanism.Furthermore,the excellent performance of the designed LMA fiber is verified experimentally by the demonstration of the HPFA with nearly diffraction-limited output.The work included in the thesis is organized as follows:An accurate and rapid mode decomposition(MD)on the output beam of a fiber is achieved in experiment through an algorithm named stochatsic parallel gradient descent(SPGD).By optimizing the initial value of the variables of SPGD algorithm,the online real-time MD on a changing beam output from fiber with M2 factor is realized firstly by applying the numerical analysis,which provides an easy and reliable way on the real-time mode diagnosis of HPFA.With the MD based on SPGD algorithm,the mode competition process in the fiber laser is studied in details with different launching conditions and launched pump power.The MD results prove the factor that the selected excitation of the fundamental mode(FM)benefits a good beam quality,while the weight of the higher order modes(HOM)reduces as pump power increasing.By using a high speed camera,the mode diagnosis of the output beam from a HPFA with MI based on a step index fiber(SIF)is demonstrated for the first time.The research shows,after the onset of MI,the dramatically dynamical mode coupling among the FM and HOMs starts with the obvious periodicity and tends to be random as the power increasing.Finally,the adaptive control of the fiber output beam is demonstrated,where the feedback is the real-time MD result.One could expect either a pure eigen mode or beam quality optimization by the adaptive adjustment,which is regarded as a novel routine on mode control for HPFA.On the numerical simulation,the characteristics of the HPFA under different values of HOM loss as a function of various parameters(including pump directions,pump wavelength and inner-cladding diameter)are studied.The calculation results show the output power of HOM decreases with the growth of HOM loss.Besides,along with the HOM loss increasing from 0 d B/m,the efficiency of the HPFA starts to drop sharply,then climbs slowly and converges finally.This can be explained by the interaction between the mode loss and local gain with inhomogeneous distribution.Also,the analysis confirms that the backward pumping and tandem pumping could expect efficient single mode(ESM)output with higher efficiency.Through the theoretical study,the influence of the interaction between the mode loss and location gain on HPFA output features has been understood deeply,while the principle of the LMA fiber design based on the selected loss mechanism has been explicated.All the physical insights help to provide the guidelines on HOM suppression in HPFA.A novel LMA design based on the selected loss mechanism is performed,whose mode properties,including effective mode area of FM,the losses of FM and HOM,and the overlap factor,are investigated theoretically by the finite element method.The results show strong suppression on HOM and nonlinear effects.In experiment,the novel designed LMA fiber with core diameter of 25 ?m is fabricated.The test results indicate a remarkable ability on HOM suppression.Finally,a HPFA demonstrated by the designed LMA fiber generates output power of 800 W with beam quality factor M2 of near 1.1.The power scaling of nearly diffraction-limited laser output in LMA fiber has been redistributed.
Keywords/Search Tags:Fiber laser, mode decomposition, mode control, beam quality, stochatsic parallel gradient descent algorithm, large-mode-area fiber
PDF Full Text Request
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