Research Of Square-Pulse Based On Figure Of Eight Cavity Passively Mode-Locked Fiber Laser

Square-wave pulses generated in passively mode-locked fiber lasers can be applied to optical fiber sensing,signal processing and other related fields,and gradually have become a research hotspot in the field of fiber lasers in recent years.In this paper,square-wave pulses generated by figure-of-eight cavity passively mode-locked fiber lasers are studied in both theoretical and experimental aspects.Firstly,the conditions and influencing factors of square-wave pulse propagation in optical fibers are studied theoretically.The partial differential equation of optical transmission in optical fibers is solved numerically,and the square-wave pulse output is obtained.Then,based on the passive mode-locking technologies of nonlinear optical loop mirror and nonlinear amplifying loop mirror,Yb-doped and Er-doped passive mode-locked fiber lasers are constructed respectively.The output of square-wave pulse is obtained by experiment,and its output characteristics and steady-state characteristics are studied.The experimental results are partly compared with the theoretical results.The main work of this paper is as follows:1.The complex cubic-quintic Ginzburg-Landau equation describing the propagation of optical pulse in optical fibers is numerically solved by spectral method.By selecting appropriate parameters in the equation,the square-wave pulse output is obtained in the positive and negative dispersion regions respectively.The influence of dispersion value on square-wave pulse in the positive dispersion region and the influence of nonlinear effect in the negative dispersion region on square-wave pulse are calculated,respectively.The coupled nonlinear Schrodinger equation describing the propagation of optical pulse in optical fibers is solved numerically by using the split-step Fourier method.The numerical solution of square-wave pulse is obtained.The influence of polarization state on the distribution of two perpendicular polarization components of square-wave pulse is further analyzed by changing the parameter values.2.The Yb-doped passively mode-locked fiber laser is constructed based on the mode-locking technology of nonlinear optical loop mirror.Stable mode-locked square-wave pulse at 1 μm band is obtained.The output characteristics of the square-wave pulse and the existence of various steady-states are further studied.Another Yb-doped passively mode-locked fiber laser is built based on the mode-locking technology of nonlinear amplifying loop mirror.The stable mode-locked square-wave pulse is also obtained,and the square-wave pulses obtained by the two mode-locking technologies are simply compared and analyzed.Experiments show that the polarization state of square wave pulse in the cavity has a great influence on its output characteristics.3.The erbium-doped passively mode-locked fiber laser is constructed based on the nonlinear optical loop mirror technology,which works in the negative dispersion region and the positive dispersion region respectively.By increasing pump power and adjusting polarization state in the cavity,the output of tunable square-wave pulse is obtained,and the output characteristics of the square-wave pulse are studied.The influence of nonlinearity on the output characteristics of square-wave pulse is studied,and the experimental results are compared with the numerical results in Chapter 2.At the same time,the influence of the polarization state of square-wave pulse on the distribution of two polarization components perpendicular to each other is further studied.