Research On Fundamental Frequency-raman Composite Soliton Pulsed Fiber Laser

With the further study of fiber lasers,high-power?high-energy ultrashort pulse has become one of the forefront of current research.However,with the increase of the peak power and energy of the pulse,the significant non-linear effects such as stimulated Raman scattering in optical fibers become the main factors limiting the stability of high-power and high energy pulses.However,the Raman scattering mechanism still exists and the pulse energy can not be scaled.If not suppressing stimulated Raman scattering,but increasing the energy transfer from fundamental frequency soliton pulse to Raman pulse by improving Raman gain,the fundamental frequency-Raman composite soliton pulse would be realized which will be a simple and effective solution to realize high-energy ultrashort pulse.Based on the nonlinear optical loop mirror(NOLM)mode-locked Ytterbium doped fiber laser,the formation and evolution of fundamental frequency soliton pulses in the fib er cavitly was studied under the influence of strongly stimulated Raman scattering by the theoretical,numerical and experimental methods.By optimizing the fiber cavity parameters,the fundamental frequency-Raman composite soliton pulse output is realized by optimizing the fiber laser cavity structure.The main research contents are as follows:In theory,based on the generalized nonlinear Schrodinger equation,the formation and evolution of fundamental soliton pulses under strong stimulated Raman scattering was simulated with the introduction of gain saturation model and saturable absorber mode-locking model are introduced to simulate the evolution of fundamental soliton pulses under strong stimulated Raman scattering.On the basis of the stable mode-locked fundamental frequency dissipative soliton pulse,the intracavity Raman gain is gradually increased,and the stability of the soliton pulse is gradually decreased.At the same time,Raman pulses appear at the front of fundamental-frequency soliton pulses.By changing the cavity length,Raman gain and saturated absorption characteristics,we find that fundamental frequency pulses and Raman pulses interact with each other,and there is a critical condition for both of them existing at the same time.In the experimental aspect,we first established a fundamental frequency NOLM mode-locked Ytterbium doped fiber laser to realize the fundamental frequency pulse output.The influence of the length of the NOLM loop on the related characteristics of the fundamental frequency pulse is studied,and the dissipative soliton resonant mode-locked pulse output is realized.The pulse width increases linearly with increasing pump power,and the peak power remains substantially unchanged.When the length of the NOLM loop is short and the pump power is increased,the fundamental-Raman dissipative soliton resonant mode-locked pulse output is preliminarily realized.In order to further increase the nonlinearity in the cavity,we introduce a 95 m high nonlinear fiber into the cavity to further increase the intracavity Raman gain and study the mode-locked laser output characteristics under the different length of NOLM loops.It is found that when the length of the NOLM loop is 240 m,a stable soliton bunch pulse output can be realiz ed.The width of the soliton bunch increases linearly with the increase of the pump power,and the peak power remains unchanged,which is also in the dissipative soliton resonance state.In the spectral test results of laser pulses,the first order Stokes Raman light is produced,and the pump power is increased,and the second order Stokes Raman light appears successively.When the length of NOLM ring is shortened,the peak power in the cavity increases,and the stimulated Raman scattering is stronger.At th is time,higher order Stokes Raman light appears in the spectrum,but the pulse stability gradually decreases.The repetition period of the pulse and the cavity length are no longer matched,and the average output power of the pulse is significantly enhanc ed compared to the fundamental frequency.In this paper,the evolution of fundamental frequency soliton pulses under the influence of strongly stimulated Raman scattering is studied by combining theory with experiments.It is found that the effect of stimulated Raman scattering will reduce the stability of the fundamental frequency pulse and limit the increase of pulse energy.However,when the Raman gain is increased and the fundamental frequency soliton pulse energy is transferred to the Raman pulse energ y,the fundamental frequency-Raman composite soliton pulse can be formed.The peak power of the pulse is higher,the pulse is narrower and the spectrum is wider.The content of this thesis has important research significance for understanding the nonlinea r dynamic process and pulse evolution behavior in the cavity of the mode-locked pulsed laser.This kind of laser pulse with wide spectrum and high peak power has wide application prospects in laser remote sensing detection,industrial processing and wide spectrum detection technology.