Generation And Transmission Amplification Of Mode-locked Pulse In Fiber Laser

The passive mode-locked fiber laser is widely used in laser processing,optical sensing,precision industrial processing and other fields because of its compact structure,excellent beam quality,excellent stability and high cost efficiency.Among them,nonlinear optical loop mirror(NOLM)has been widely concerned for its excellent nonlinear characteristics and high response speed.With the study of the mode-locked behavior of fiber laser,some strange pulse phenomena appear in the dissipative soliton pulse,such as dissipative soliton,noise soliton,rectangular and Gaussian composite soliton,optical rogue wave and so on.The generation mechanism and characteristics of optical rogue wave is one of the advanced issue.On the basis of NOLM mode-locked ytterbium-doped fiber laser,the formation and evolution of optical rogue wave pulse are studied by theoretical analysis and experimental research.In addition,in this paper,the main oscillation of the dissipative soliton pulse is amplified directly to explore the amplification characteristics of picosecond dissipative soliton pulse.The main research contents are as follows:In theory,based on the generalized nonlinear Schrodinger equation,the analytical solution of the second-order optical rogue wave is given through Darboux change,and then the physical model of optical rogue wave is constructed.The generation process of optical rogue wave in Yb-doped fiber is simulated by split-step Fourier methods.At the same time,small-signal gain and group velocity dispersion coefficient are analyzed by using the control variable method.It is found that with the increase of small-signal gain,the effect of optical rogue wave is better,while the effect of group velocity dispersion coefficient on optical singular wave is negative,that is to say,with the increase of group velocity dispersion coefficient,the effect of optical rogue wave is worse.After that,we build the transmission model of the optical pulse in the ring cavity,and simulate the influence of different fiber length on the time and frequency domain when connecting the passive fiber,and the influence of gain coefficient,fiber length and incident pulse energy on the pulse characteristics when connecting the gain fiber,and analyze the nonlinear effect on them.In the experiment,we first set up a NOLM mode-locked ytterbium doped fiber laser,which is connected with high nonlinear fiber,and realized the output of rectangular dissipative soliton pulse.When the pump power increases,the pulse width increases linearly and the peak power decreases.After that,the output of optical rogue wave pulse is realized by changing the polarization state.The pulse is unstable and the peak power is very high,and the distribution of the peak power conforms to the L-type statistics.Seven Stokes Raman peaks are observed in the spectrum of optical rogue wave,and the spectral width becomes wider with the increase of pump power.In order to study the characteristics of picosecond pulse amplification,we first tested the self-made seed source,and found that with the increase of the input current,the pulse width remained basically unchanged,and the peak power of the pulse became larger and larger.After that,the seed source is connected to 1000 m passive optical fiber.The results show that the pulse width is broadened and the peak power is significantly reduced.The phenomenon that the top of the spectrum becomes flat is found by testing the spectrum.In this paper,the generation and characteristics of dissipative pulse in mode-locked fiber laser are studied by theory and experiment,including two pulse forms: rectangular pulse and optical rogue wave pulse.At the same time,the amplification of picosecond dissipative soliton pulse is also studied.It is found that in the process of the generation of dissipative pulse,the nonlinear effect seriously affects the output characteristics of the pulse,especially the small-signal gain and group velocity dispersion coefficient play an important role in the evolution of optical singular wave pulse;the self phase modulation of picosecond pulse in the amplification process will lead to the pulse spectrum superposition,which needs to be considered in the follow research.The research content of this paper is of great significance to understand the physical mechanism and pulse characteristics of optical rogue wave,as well as the amplification characteristics of picosecond dissipative soliton pulse.The above research results have certain reference value for optical communication,optical information processing,medicine,precision machining and other fields.