Design、Fabrication And Performance Of Ytterbium Doped Fiber For High Power Fiber Lasers

High power fiber lasers and amplifiers have been developed rapidly in recent years due to their advantages of high efficiency,small size,good beam quality,easy thermal management and high stability.It is widely used in industrial processing,military,medical,scientific research and many other fields.However,the photodarkening（PD）and nonlinear effect hinder the further steady improvement of the output power of the fiber laser.In this paper,we mainly study the PD and nonlinear suppression of Yb-doped fiber used in high power fiber laser,and finally achieve the high-power and stable output of fiber laser.The basic structure,composition,numerical aperture,mode field diameter and normalized frequency of Yb-doped fiber are discussed and simulated based on the design of Yb-doped fiber.The preform preparation technology,drawing technology and testing technology of Yb-doped fiber for high power fiber laser are described.The matters needing attention and the chemical reaction in the preparation process of Yb-doped fiber are further analyzed and the Yb-doped fiber is tested and analyzed by using related testing technology.In terms of the characterization of the PD test,the development of the PD test at home and abroad is discussed,and a PD test system is built.Based on the steady-state rate equation of Yb-doped fiber laser and theoretical model of the oscillator,the pump mode,pump power and length of the test fiber are optimized.The GR&R method is used to evaluate the test system,5.9%of the evaluation results show that the system is in good condition and meets the requirements of repeatability and stability test.In view of the common problem of PD in Yb-doped fiber for high power fiber lasers,it is verified that Ce doping can improve the PD inhibition performance of Yb-doped aluminosilicate fiber and the doping concentration of Ce is optimized.It is found that a small amount of P（P₂O₅～0.27 mol%）doping can significantly improve the PD inhibition performance of Yb/Ce co-doped aluminosilicate fiber,but it does not reduce the cladding pump absorption of the fiber.It is found that co-doping a certain concentration of Ce₂O₃（～0.05 mol%）,P₂O₅（～0.83 mol%）and Al₂O₃（～1.61 mol%）in the fiber core,the Yb/Ce/P co-doped 20/400μm aluminosilicate fiber shows excellent PD suppression performance,and PD loss is about 0 d B/m at 633 nm under the existing test conditions.No additional core attenuation is obviously introduced into fiber through doped each component,it is proposed that the PD suppression mechanism of prepared Yb/Ce/P co-doped aluminosilicate fiber is the coordination of each component.Through low refractive index fluorine doped quartz units embedded into the inner cladding of the preform that reducing the effective pump area of the circular inner cladding of the fiber,which has been proved to be an effective way for enhanced cladding pump absorption of Yb-doped double cladding fiber even with a circular inner cladding.A modified model is proposed to evaluate and calculate the cladding pump absorption of the inner cladding modified circular fiber.The magnitude of the cladding pump absorption depends on the number and diameter of the fluorine-doped quartz units embedded in the inner cladding.Compared with conventional octagonal Yb-doped fiber,the circular Yb-doped fiber can achieve better cladding alignment and fusion with the passive circular fiber,the nonlinear SRS threshold increases by 31.3%in the all-fiber laser oscillator with the enhance of cladding pump absorption.Aiming at the problem that the output laser of conventional Yb-doped aluminosilicate fiber in the short wavelength region of spectrum synthesis is limited by the ASE and nonlinear effect,an Yb-doped 25/400μm phosphosilicate binary fiber is prepared.Compared with conventional Yb-doped aluminosilicate fiber,the 1046 nm laser performance test of MOPA structure proves that Yb-doped phosphosilicate binary fiber has better ASE and nonlinear effect suppression results in short wavelength.Compared with aluminosilicate materials,the composition of phosphosilicate increases the asymmetry of coordination,and leads to the blue shift of emission peak,which is the main reason why Yb-doped phosphosilicate fiber is more suitable for short wavelength laser output.It is found that the pedestal structure design of fiber core can realize the low numerical aperture control of fiber core and achieve excellent beam quality output in high power laser.