The Study Of High Power Long Tapered Fiber Amplifiers

Fiber laser has the advantages of compactness,high slope efficiency,good beam quality,excellent heat management and stable performance.It has a wide range of applications in industrial processing,material handling,biomedicine and intelligent manufacturing.With the development of advanced applications,high power fiber lasers with high beam quality has been proposed.However,the output power of traditional single-mode fiber lasers has been greatly challenged by nonlinear effects and modal instability.Compared with uniform fiber,long tapered fiber has larger mode field area,higher pump absorption,can maintain good beam quality while suppressing nonlinear effects.In this dissertation,the beam quality characters,nonlinear effects and modal instability of long tapered ytterbium-doped fiber amplifier will be studied systematically and experimentally.Firstly,the development history of high power fiber laser is introduced.The main limiting factors of power scaling of diffraction limited fiber lasers are figured out.It is suggested that the long tapered fiber is a promising solution for suppressing nonlinear and obtaining higher output power.The development history of tapered fiber is also reviewed,and the current research status of long tapered fiber as gain medium is summarized.The significance of long tapered fiber in high power fiber laser power scaling and beam quality maintenance is explained.Secondly,based on the ray theory,the qualitative analysis of the mode variation in tapered fiber is made.The eigen equation of the meridional model in ray theory is derived by using the self-reproducing condition of rays and Fresnel’s formula.The transmission angles of the meridional rays are given by the roots of eigen equation.Based on this,we have made the theoretical study on the propagation angle in long tapered fiber and derived an important and intuitive conclusion that the product of the transmission angle of rays and the fiber core diameter is constant for a certain long tapered fiber.Then,based on the wave equation,we derive the theoretical model to describe mode interplay behaviors in long tapered fiber and give the general expression of the coupling coefficient including the change of the core diameter and other factors.By using this model,the simulation of the mode evolution in long tapered fiber is studied in detail.It is worth mentioning that our model can easily extended to include more influencing factors in order to consider the effect of more other perturbation factors on the mode evolution in long tapered fibers.The rate equation of ytterbium ions is derived based on the four-level approximation.Our result indicates that the two-level approximation can be used in the case where the order of the pulse width of external signal field is much greater than the relaxation time between1/2 and 3/4 energy level.Thirdly,the models for study of stimulated Raman scattering（SRS）and stimulated Brillouin scattering（SBS）in long tapered fiber amplifier is carried out.Based on the nonlinear Schr?dinger equation,the SRS model of long tapered fiber is established.The amplitude model is simplified as a power model by reasonable approximation,and the SRS in long tapered fiber is simulated.The simulation results show that the SRS intensity of 20μm to 45μm long tapered fiber is 1 magnitude weaker than which in 20μm uniform fiber when they have almost the same beam quality.Based on the relationship between SBS gain coefficient and fiber dimensions,the theoretical model of SBS in long tapered fiber is established.The results show that the long tapered fiber can broaden the SBS gain spectrum and suppress SBS.Then,a semi-analytical model of transverse temperature distribution in gain fiber is established.A semi-analytical model of transverse temperature distribution in fiber is established based on Green’s function method.The transient and steady-state distributions of fiber temperature in time domain are given.Based on this model,the static model instability caused by photodarkening in gain fiber is studied theoretically.At the same time,the temperature distribution with different amplitude and phase components of LP₀₁1 and LP₁₁1 mode is calculated,and the source of the frequency shift hypothesis in the following study of modal instability is explained.A threshold model of semi-analytical thermally-induced modal instability in long tapered fiber is given and the modal instability threshold in long tapered fiber is calculated.Using this threshold model,we study the modal instability threshold of long tapered fiber with different shapes,and compare them with the threshold of ordinary uniform fiber.The results show that when the effective core diameter of the long tapered fiber is equal to the diameter of the uniform fiber,the modal instability threshold of the tapered fiber amplifier is higher than that of the uniform fiber amplifier in the forward and bidirectional pumping conditions.Unlike the uniform fiber amplifier,the long tapered fiber amplifier has lower modal instability threshold in the case of backward pumping.Finally,the long tapered fiber amplifier is studied experimentally,which proves its advantages in SBS and TMI prevention.A single-frequency amplifier experiment was carried out using a long tapered fiber with an equivalent core diameter of 33.65μm.The SBS threshold was 2.76 times of which in commercial 20/400μm uniform fiber while maintaining good beam quality（M²¹.25）.In the experiment of modal instability,the threshold of the amplifier is 313W,while which of the commercial 30/250μm uniform fiber amplifier was below 300W;using a piece of 20/400 to 30/600 tapered fiber,we have achieved a maximum output power of 1.47kW,compared with ordinary 25/400 fiber,the use of long tapered fiber increased TMI threshold.The results show that the long tapered fiber can increase the nonlinear effect threshold by increasing the effective core diameter,and ensure the beam quality and higher modal instability threshold by the continuous gradient core structure.If a reasonable structure and doping design are carried out,the long tapered fiber is expected to achieve the higher power laser output while ensuring the near-diffraction-limit beam quality.The research has provided technical support for the research of high power fiber lasers with excellent beam quality.