Study On Period Doubling And Chaos In Snatiotemporal Mode-locked Lasers

Mode-locked fiber laser has the advantages of good output beam quality,low cost,flexible structure design,high-efficient heat dissipation and high tolerance to the working environment.What’s more,mode-locked laser is also an ideal platform to study the dynamic of nonlinear system.Researchers have conducted a lot of research on the traditional modelocked single-mode fiber laser.However,due to the limited optical power in single-mode fiber,multimode fiber was introduced to fiber lasers.With the help of multimode fiber laser cavity,spatiotemporal mode locking involving multiple transverse have been realized in recent years,and some high-dimensional nonlinear spatiotemporal dynamic processes have been found.Different from the traditional longitudinal-modes locked laser,spatiotemporal mode-locked laser locks many transverse and longitudinal modes simultaneously.Currently,the investigations on the highdimensional nonlinear dynamic process in spatiotemporal mode-locked laser are not sufficient,and there is a lack of in-depth research on how the laser cavity parameters affect these spatiotemporal dynamic characteristics.In this thesis,a typical nonlinear phenomenon,period bifurcation to chaos,in multimode fiber lasers is studied with the help of theoretical analysis,numerical simulations and experiment.This study will be helpful to understand the complex spatiotemporal dynamics in spatiotemporal modelocked multimode lasers and to discover new dynamic processes in highdimensional nonlinear systems.The main work contents are as follows:1.The combination of linear and nonlinear optical effects on pulse transmission in multimode optical fibers is analyzed theoretically and numerically.In particular,the influence of fiber parameters on the spatial beam self-cleaning in graded-index multimode fiber is studied.It is found that the refractive index profile parameter has the greatest influence on it.When the fiber refractive index profile parameter is optimized to make the inter modal dispersion close to zero,the self-cleaning effect can be promoted.2.Based on the nonlinear polarization rotation technique and the principle of spatiotemporal mode-locking,a ytterbium-doped multimode fiber laser is built,where spatiotemporal mode-locking with at 1030nm band is realized.By adjusting the cavity parameters such as pump power,waveplate state and spatial coupling,the experimental phenomenon of period doubling to chaos is found.It is investigated by lots of measures,including spatial sampling.The chaotic state is validated by 0-1 test.Other nonlinear phenomena are also found,such period-3 and multiple periods pulsation,multiple pulses mode locking and period-2 with multiple pulses.3.A lumped numerical simulation model is constructed according to the experimental cavity structure.The numerical simulation results qualitatively agree with the experimental results,including stable mode locking and period doubling to chaos by increasing pump power.Moreover,by varying the length of gain fiber,the simulation results of period bifurcation in different states are realized.Using the 0-1 test,the chaotic state of the simulation results is verified,which shows that different spatial components have different spatiotemporal dynamics.