Research On Modulation Of Soliton Pulses In Passively Mode-locked Fiber Lasers

With the development of laser,the ultrafast optics has developed very rapidly.Although solid-state lasers can output pulses with ultra-high peak power and ultra-short pulse width,solid-state lasers have huge volume,complex structure and high price.In contrast,fiber lasers are cheap and highly integrated,so they have broader application prospects.However,due to the peak power clamping effect and other reasons,the output pulse peak power of the fiber laser is low and the pulse width is wide.Increasing the peak pulse power is one of the key problems in fiber laser research.Increasing the pump power can significantly increase the pulse peak power,but the pulse peak power greater than the threshold will cause the pulse to split into multiple pulses and soliton molecules.In addition,due to fiber birefringence effect and other reasons,the pulse sequence generated by fiber laser is uneven and presents periodic distribution.Therefore,exploring the modulation of fiber laser intracavity parameters on soliton molecules and periodic pulses is helpful to solve the problem of unstable multi pulse output of fiber laser,thus laying the foundation for designing stable fiber laser.The improved fiber laser based on nonlinear polarization rotation technology can effectively study the output pulse signal.In this thesis,the modulation of soliton molecules,dark soliton molecules,periodic and bifurcated pulses and dissipative solitons by different parameters in the cavity of a nonlinear-polarization-rotation mode-locked fiber laser is studied.By setting different fiber resonator parameters,the inherent formation mechanism of these phenomena is explored,so as to better design stable and high-performance fiber lasers.Based on the nonlinear polarization rotation technique,the characteristics of dissipative solitons are studied by using an improved fiber laser.The research contents and innovation achievements of this thesis are summarized as follows:1)Nonlinear polarization rotation mode locking is equivalent to artificial saturable absorber.The process of soliton molecules in passively mode-locked fiber resonator is studied by using this technology,and the formation mechanism of soliton molecules in laser is explored.Transmittance is very important for nonlinear polarization rotation mode locking,and must be considered in fiber laser design.The theoretical analysis shows that when the intersection angle of the polarization direction of the two polarizers is π/2,the linear transmittance can be 0.The coupled nonlinear Schrodinger equations are used to calculate the evolution of soliton molecules.Soliton molecules of different orders can be obtained by changing the small signal gain coefficient or phase delay.It is proved that both small signal gain coefficient and linear phase delay can affect the generation of higher-order soliton molecules.Different intracavity parameters of fiber lasers can modulate the pulse intensity,pulse spacing and order of soliton molecules and dark soliton molecules.By setting appropriate cavity parameters,stable output soliton molecules and dark soliton molecules can be obtained.2)Aiming at the phenomenon of non-uniform periodic pulse sequence output by fiber laser,the evolution law of periodic pulse is explored.The modulation effects of different intracavity parameters of fiber laser on periodic pulses are studied.When the linear phase delay is near the turning point of nonlinear polarization,the positive feedback region can become the negative feedback region.At this time,the soliton in the fiber resonator interacts with the linear wave,resulting in the periodic change of the pulse amplitude.Decreasing the pump power or increasing the polarization conversion power can eliminate the periodic change of the pulse.The large number of cycles(Maximum 212 cavity periods)pulse caused by the periodic interaction between soliton wave and dispersive wave is explored.When the small signal gain coefficient is relatively small,the periodic phenomenon of first splitting and then polymerizing can be obtained by selecting the appropriate linear phase delay.The bifurcation phenomenon is due to the periodic mutual attraction and mutual repulsion of pulses in the local region.Although the soliton pulses in other regions are also affected by the soliton pulses in the bifurcation region,they generally maintain the original evolution trajectory and transmission without bifurcation.3)The modulation effects of different intracavity parameters on the dissipative solitons in conventional fiber lasers are investigated.In view of the shortcomings of the traditional nonlinear polarization rotation fiber laser structure,a new structure is used to study the properties of dissipative solitons.The signal-to-noise ratio of the fiber laser with this structure is 64 d B at the fundamental frequency of 26.8 MHz,which shows that the output signal is very stable.Increasing the pump current,the3 d B bandwidth of the rectangular structure spectrum gradually widens.The spectrum and pulse shape of the output signal of the fiber laser have changed significantly after the dispersion Fourier transform.Different shapes of signals can be obtained by changing the pump current and polarization state.When the pump current is set to 297 m A and 257 m A respectively,the output signal is a right triangle and an ordinary triangle spectrum,and the pulse shape and spectrum shape are symmetrically distributed.