Mode Control Of High Power Fiber Laser

Transverse mode instability(TMI)has become the premier bottle neck for the power scaling of the near diffraction limited fiber laser and amplifier.The main objective reported in this thesis is to suppress the TMI via analyzing the mechanism of the TMI in the coileld active fiber amplifier and achieve mode control and near diffraction limited output in high power fiber amplifier.Based on this objective,the following four aspects of research work are carried out:(1)A theoretical model of the thermally induced refractive index changes in the coiled active fiber is established and is untilzed to calculate the thermally induced mode loss evolution and its impact on the output characteristics of the coiled fiber amplifier.(2)A theoretical model of TMI in the coiled active fiber amplifier is estabilished and the mode coupling in the coiled active fiber amplifier is studied.(3)A theoretical model of stimulated Raman scattering(SRS)induced TMI in the coiled active fiber amplifier is estabilished to analyze the interaction of the SRS effect and TMI effect in high power coiled fiber amplifier.(4)A theoretical model of TMI in the multiwavelength-pumped fiber amplifier is established and a scheme of 915 nm and 976 nm wavelength multiplexing LD counter-pumped fiber amplifiers is proposed to suppress the TMI and realize high power near diffraction limit optical fiber laser output.The main work of this thesis is as follows.(1)A theoretical modelling is carried out on the thermally induced mode bending loss evolution in large mode area gain fiber and its effect on the output characteristics of the fiber amplifier.First we establish the thermally induced refractive index changes based on the heat conduction equation and conformal mapping.Then the mode loss evolution in the conventional step index 20/400 fiber is calculated via the finite element program.The results shows that the heat load will induce quasi-exponential decrease of the core mode and proper bending radius should be selected according to the local heat load so as to achieve the effectively single mode operation.Besides,a novel model of fiber amplifier considering thermally induced mode loss evolution is established.The results show that the counter-pumping fiber amplifier shows much better HOM suppression than the co-pumping fiber amplifier.For the case of spirally coiled copumping fiber amplifier,High transfer efficiency and beam quality could be obtained via launching the signal power from the outside ring.(2)We investigate the TMI effect in the coiled fiber amplifier.Firstly,based on the local mode theory and taking into consideration of the thermally induced refractive index change,we established the theoretical model of the TMI in the coiled fiber amplifier.Then the TMI in the 20/400 fiber amplifier is investigated.The result shows that the heat load will not only decrease the HOM loss but also increase the mode coupling between the FM and HOM.The thermally induced core laser leakage will be triggered when the output power is above the TMI threshold,in which,the HOM is excited in the front section of the gain fiber and leaks into the inner cladding at the rear section of the gain fiber.In the counter-pumping fiber amplifier,the thermally induced core laser leakage is mitigated since counter-pumping scheme has better HOM suppression and higher gain situation,which leads to higher threshold of the TMI effect.Finally,the simulation result are verified through experiment.The results prove the existence of thermally induced core laser leakage in the co-pumping amplifier and the maximum power is increased to 3.1 kW via 915 nm counter-pumping scheme,which is 18 % higher than that of the 915 nm co-pumping scheme.(3)The SRS induced TMI effect in the coiled fiber amplifier is investigated numerically.Firstly,based on the analysis of the TMI in the coiled fiber amplifier and consider the heat load induced by the SRS effect,we establish the theoretical mode of the SRS induced TMI in the coiled fiber amplifier.Based on this model,the SRS induced TMI effect in the co-pumping fiber amplifier is investigated.The results show that the amplification of the Raman power,which generate additional heat load,will not only lead to the HOM loss decrease at the rear end section of active fiber,but also enhance the mode coupling between the FM and HOM.This results in the output of HOM power from the YDF and thus tightly coiling the fiber could not suppress the SRS induced TMI.Besides,we analyze the impact of the parameter such as YDF length,seed power and the pumping direction on the SRS induced TMI.The results show that shortening the YDF length will suppress the SRS effect,and as a result suppress the SRS induced TMI.Varing the seed power will leads to different TMI mechanism that increasing the seed power will increase the average signal power in the YDF and leads to the SRS induced TMI while decrease the seed power will suppress the SRS induced TMI but lead to the TMI induced by Yb heating.Counter-pumping fiber amplifer could mitigate both the Yb-induced TMI and SRS induced TMI since it has higher gain saturation and higher SRS threshold.(5)The TMI effect in the multi-wavelength pumped fiber amplifier is investigated numerically and experimentally.Firstly,based on the small signal gain of the multwavelength pumped fiber amplifier and the analysis of the SRS induced TMI,we establish the general model of the TMIeffect in the multi-wavlength fiber amplifier.Then we simulate the TMI effect in the multi-wavelength co-pumped fiber amplifier and verify the simulation with experiment.Both the simulation and the experiment results show show that adopting multi-wavelength pumping could increase the TMI threshold,compared with 976 nm co-pumping fiber amplifier.Besides,we simulate the TMI effect in the 915 nm counter-pumped fiber amplifier,976 nm counter-pumped fiber amplifier and multi-wavelength(915 nm + 976 nm)counter-pumped fiber amplifier.The results show that the output power of the 915 nm counter-pumped fiber amplifier is limited by the SRS induced TMI,while the 976 nm counter-pumped fiber amplifier is limited by the Yb induced TMI.However,for the 915 nm and 976 nm multi-wavelength counter pumping fiber amplifier,both SRS TMI and Yb induced TMI are suppressed which indicates that the 915 nm and 976 nm multi-wavelength counter pumping could be utilized to achieve higher power near diffraction limited fiber amplifier.The best 976 nm power ratio of the total pump power is 60 %.