The Study On Large-Mode-Area Single-Mode Ytterbium Doped Fiber

High-power fiber lasers have been developed rapidly in recent years due to their good performance and various applications in high-end manufacturing industries,which greatly invigorated a huge fiber laser market.Ytterbium-doped fiber is the key part of a high-power fiber laser,but the advanced manufacturing technology is in the hands of Western countries,resulting in slowly development of the domestic active fibers market because of foreign market monopoly.In particular,high-energy Yb-doped fiber used in the military field has adopted embargo measures abroad.In order to break the Yb-doped fiber monopoly from foreign technology,the work is to study the key technologies of large-mode-area Yb-doped fibers industrialization based on the present fiber preparation technology,then prepared and obtain large mode area,high doping,low background loss as well as low photodarkening effect of Yb-doped fiber.In this paper,the MCVD particle deposition mechanism and the rare earth solution doping mechanism are revealed based on the theory of deposition and doping from the MCVD processing researches.The steps of the processing and the influence of it on the optical fiber preform are introduced in detail.Based on precise controlled discharge volume,ion doping concentration control and three key preparation factors for deposition efficiency,as well as the main points of the fabricated processing are presented to achieve the high uniformity,high doping concentration,and high deposition efficiency.Then,optical fiber preform is drawing by the design of numerical aperture and mode area of core,in addition,the fiber absorption,loss and photodarkening performance were measured.For the 10/130,20/130,30/250 and 20/400 Yb-doped fibers with great commercial potential,100W Q-switched lasers,100W picosecond pulsed MOPA lasers and 500W oscillating fiber lasers were built to characterize the fiber performance,respectively.For high-energy Yb-doped fiber lasers with thousand or ten thousand watts,the requirements for rare-earth doped fibers are more stringent,such as require good refractive index profile,lower fiber loss and lower photodarkening effect for the performance of the lasers.The numerical simulation of the fluid by Ansys CFX shows that a relatively uniform refractive index profile was done when the gas flow rate in the reaction tube is around 0.12m/s.Through studing the F-doped mechanism in quartz glass,high Yb³⁺doped concentration with 1.23 wt%and the low numerical aperture with 0.062 were obtained.The in-situ solution doping process of MCVD was reasearched to achieve an Yb-doped fiber preform of a six layer doped core with the diameter of 2.8mm and good uniformity.After material purification,process treatment and lateral uniformity of refractive index profile,the background loss of Yb-doped fiber was successfully reduced to 7.3dB/km,and the difference of core lateral uniformity refractive index was only 1.5×10^-4.Through the research and improvement of the processing technology,the photodarkening effect can be complete suppression.Finally,the all-fiber oscillating structure was built,a 2kW single-mode laser was output with a slope efficiency of 82.5%,a corresponding beam quality of 1.29,which verified the excellent performance of the Yb-doped fiber.The 30/600?m fiber has been tested by the××research institution to obtain 10.155kW laser output,which further proves that the optimized Yb-doped fiber has the application potential of high-energy laser weapon,which is of great significance for breaking the monopoly of foreign fiber technology.In order to achieve a better beam quality in high-energy fiber lasers,four types of fibers were fabricated to explore the solution of large mode area single mode operation based on the MCVD process.Firstly,the four-core fibe is prepared by grinding method,an 11.47W laser is output,and the corresponding laser beam quality is 1.5.Then,the 50/400 Yb-doped fiber with 60%doping ratio has obtained the 87.1%laser output efficiency,and the beam quality is 2 when the output power is 310W,which demonstrating the potential of partially doped fiber in high power fiber lasers.Moreover,the NA 0.052 core diameter of 20.5/400fiber has output the 1.8kW laser output with 79.6%slope efficiency by constructing the MOPA platform.The output beam quality was close to the diffraction limit before the output power is 1.3kW.The center-sunken cladding-trenched fiber with the NA of 0.043 and core diameter of 40?m is prepared with the maximum mode area of 1460?m².The laser slope efficiency is 79.2%with the beam quality of 1.1 while the output power is 456W in the MOPA platform,which verifies that the fiber has the good single mode characteristic.