Research Of Pulse States Based On Mode-locked Fiber Laser And Amplifier

Passively mode-locked Tm-doped fiber laser,as an essential way to generate 2?m laser pulses,not only has the advantages of compact structure,economic,stable performance,and so on,but also possesses the merits of flexible structure and controllable parameters,consequently providing ideal platform for the exploring of complex nonlinear dynamics of mode-locking.The mode-locked pulse also exhibits rich nonlinear dynamics in fiber amplifier.However,because 2?m lasers were developed late compared with 1?m-1.5?m band,the research reports on laser pulse nonlinear dynamics in this spectral band are still less,and the pulse states of mode-locking are relatively lacking.Meanwhile,due to the different optical properties?such as group-velocity dispersion,nonlinear coefficient,gain bandwidth of rare earth ion doped fiber,and so on?of fiber in this band compared with 1?m-1.5?m band,there are some intrinsic differences for their nonlinear dynamics.Therefore,this thesis studies the 2?m nonlinear dynamics of mode-locking by employing the passively mode-locked Tm-doped fiber laser as the leading research object and employing Tm-doped fiber amplifier as secondary research object.The main contents of this thesis are listed as following:1)The pulse properties and dynamics characteristics of conventional soliton,stretched pulse,and dissipative soliton are experimentally and numerically studied based on hybrid mode-locked Tm-doped fiber laser.For the conventional soliton widely exists in 2?m mode-locked fiber lasers,by inserting a home-made all-fiber Lyot filter into the mode-locked oscillator and optimizing its filter period,the Kelly sidebands are effectively suppressed,and the wavelength of soliton can be precisely tuned from1952.6 nm to 1971.6 nm through adjusting the temperature of the Lyot filter.Besides,the necessity of Kelly sidebands suppression is revealed by investigating the impact of Kelly sidebands on conventional soliton amplification,including direct pulse amplification and chirped pulse amplification.When the conventional soliton is directly amplified,Kelly sidebands will be quickly amplified due to its high gain coefficient,which greatly depletes the pump energy and seriously prevents the effective amplification of the main pulse part.When chirped pulse amplification technique is adopted,depending on the dispersion sign of the pre-stretch fiber,the temporal distributions of Kelly sidebands become further far away or close to the main pulse part.In the former case,the gain coefficients of Kelly sidebands are still high,and thus the amplification of main pulse part is limited.In the latter case,because the temporal distributions of Kelly sidebands are greatly overlapped with the main part of the pre-stretched pulse,the pulse profile presents multiple interference peaks,and these peaks generate nonlinear phase shift during the amplification process.These two phenomena lead to a wide pedestal when the amplified pulse is compressed by dispersion component,and consequently limit the pulse peak power.2)The multi-pulse states of mode-locking are investigated.The generation mechanisms of multi-pulse mode-locking are analysed,and the dynamics of multi-pulse mode-locking is studied from three aspects.In the first part,based on hybrid mode-locking,we report the generation of soliton pair which is composed of two identical conventional solitons.In the second part,based on the NPR mode-locking characterized by high damage threshold,the soliton splitting threshold is reduced by compensating the net cavity anomalous dispersion and enhancing the cavity nonlinear coefficient.Consequently,stable harmonic soliton with 4.6 GHz(⁸23^th)repetition rate is obtained.In the last part,the pulse properties and evolution characteristics of noise-like pulse are studied.3)Coexistence of different types of mode-locked pulses is studied based on dual-wavelength mode-locking.Two sorts of coexistence patterns are discovered.The first one is the coexistence of noise-like pulse and harmonic soliton which is achieved by optimizing the cavity length,PC,and pump power of NPR mode-locking.The coexistence patterns can stably exist and self-start with increasing of the pump power by appropriately setting the PCs.Because the soliton splitting threshold is reduced by dispersion compensation,the harmonic mode-locking obtained at 1955.3 nm has a varied repetition rate from 324 MHz to 1.138 GHz.The NLP generated at 1983.2 nm operates at either fundamental repetition rate of 4.765 MHz or second harmonic state.Experimental investigations show that the coexistence patterns are caused by the wavelength-dependent phase delay of the mode-locked fiber cavity.Moreover,dual-wavelength noise-like pulses are also obtained by adjusting the PCs.The second one is the coexistence of dissipative soliton and stretched pulse based on a hybrid mode-locked Tm-doped fiber laser with near-zero group-velocity dispersion and strong third order dispersion.The coexistence patterns are obtained by carefully adjusting the PCs and pump power.Its dynamics is experimentally and numerically explored.By optimizing the filter period of comb filter induced by fiber birefringence,dissipative soliton with pulse durations of 4.18 ps and stretched pulse with pulse duration 468 fs are respectively obtained.Wavelength-dependent property of the net cavity group velocity dispersion leaded by the strong third order dispersion plays a key role for the coexistence patterns.Additionally,because the stretched pulse has a low splitting threshold compared with the dissipative soliton,coexistence of dissipative soliton and bound states of stretched pulses is also observed.4)The impacts of third order dispersion on mode-locking are investigated.Firstly,we investigate the impact of third order dispersion on soliton pulse in a hybrid mode-locked fiber laser with near-zero group-velocity dispersion.This section includes three parts.The first part compares the characteristics of soliton pulse under different net cavity group-velocity dispersions,and analyses the physical meaning of spectrum asymmetry caused by third order dispersion.The second part reveals and analyses a pulsating phenomenon,which occurs when the mode-locking switches from femtosecond pulse to picosecond pulse during increasing of the pump power.The last part demonstrates the bound states of two pulses.Unlike the usual bound states,the two pulses in here have different pulse durations,peak powers and center wavelengths,and the bound states switch into the single pulse regime with increasing of the pump strength.Its dynamics characteristics and formation mechanism are analysed by simulation.Secondly,the impact of third order dispersion on dissipative soliton resonance is investigated in the frame of cubic-quintic Ginzburg-Landau equation.The simulation finds that the dissipative soliton resonance under third order dispersion effect still stably exists with nearly the same pulse amplitude,but with different pulse durations.Depending on chromatic dispersion,the pulse duration can be notably enlarged or reduced by third order dispersion.Especially in large anomalous dispersion region,the pulse duration can be dramatically increased even with weak third order dispersion.The dependence of pulse duration on third order dispersion is prone to be enhanced with increasing of nonlinear gain.Another notable role of third order dispersion on dissipative soliton resonance is that it alters the symmetry of pulse profile,where one pulse edge is steepened,while the other pulse edge is stretched.