Research On Output And Stability Characteristics Of Passive Mode-Locked Fiber Laser Based On Two-Dimensional Nanomaterials

Ultrafast fiber laser has the advantages of simple and compact structure,excellent heat dissipation,and high beam quality.It has been widely researched and applied in many fields,such as optical communications,biomedicine,material preparation,elaboration,and terahertz wave generation.The methods of generating ultrashort pulses in passively mode-locked fiber lasers are mainly divided into nonlinear polarization rotation,nonlinear optical loop mirror,Mamyshev regenerator,semiconductor saturable absorber mirror and material-based saturable absorber.Especially,the passive mode-locking technology of materials is the simplest and most effective way to obtain ultrashort pulses.The saturable absorption characteristics of the material directly determine the performance of the laser pulse,which requires the material to have the traits of broadband spectral absorption,ultrafast response speed,easy integration,and low cost.With the development of materials,new two-dimensional nanomaterials including graphene,topological insulators,transition metal sulfides,MXene,black phosphorus,carbon nitride,etc.have become ideal materials for saturable absorbers.At the same time,these materials have flexible and tunable photoelectric response characteristics.In order to further improve the tunable characteristics of material-based mode-locked fiber lasers,the saturable absorber parameters of the materials can be actively adjusted by means of external electric fields,and then the pulse output characteristics can be controlled.In this paper,the output and stability characteristics of passive mode-locked fiber lasers with two-dimensional nanomaterial saturable absorbers are theoretically and experimentally explored:carbon nitride and carbon nanotubes are used to discuss the influence of saturable absorber parameters on pulse output characteristics;using Ti Te₂ obtained ultrashort and high energy pulses;using the metal phase 1T'-Mo Te₂ to discuss the influence of the external light field method on the pulse stability.This low cost,high efficiency external active control material-based mode-locked fiber laser experimental program may promote the further mature development of material-based passive mode-locking technology.The main research contents of this paper are as follows:1.the influence of material-based saturable absorber parameters on the output characteristics of mode-locked pulses is theoretically discussed.The parameters discussed of saturable absorber mainly include:modulation depth,saturation power,anti-saturation coefficient;laser parameters mainly include gain and dispersion.The simulation results show that the modulation depth has the effect of narrowing the pulse and enhancing the pulse stability;the anti-saturation absorption has the effect of switching the pulse type and clamping the peak power.2.based on carbon nitride(C₃N₄)and single-walled carbon nanotube(SWCNT)materials,the influence of saturable absorber on the type of pulse output is discussed.A thin-film saturable absorber was prepared using carbon nitride material and polyvinyl alcohol(PVA).In the 1.5?m,the mode-locked and Q-switched pulse were obtained by changing the pump power.A thin-film saturable absorber was prepared using single-walled carbon nanotubes.When a monolithic saturable absorber was built into the laser,a soliton pulse output was obtained,the pulse width was 439 fs,and pulse was split into bound solitons by increasing pump power.Noise-like pulse output is obtained when two-layer saturable absorber is inserted,and the envelope width of noise-like pulse can be increased from 15 ps to 98 ps with pump power.The measured saturable absorption characteristic results show that the single layer mainly exhibits the saturable absorption characteristic,and the two layers exhibit the anti-saturable absorption characteristic.The theoretical simulation proves that increasing pump power and introducing anti-saturation absorption effect will affect the pulse output type.3.the saturable absorber mode-locked fiber laser based on transition metal sulfide materials obtains narrow pulse and high energy pulse output respectively.Using metallic phase transition metal sulfide material Ti Te₂,using PVA and Ti Te₂ combined thin film saturable absorber to obtain 315 fs ultrashort pulse output in fiber laser applications;by changing the cavity length,using evanescent field type saturable absorber,edge polishing fiber combined with Ti Te₂,obtains a high energy pulse output of 20.8 n J.At the same time,the evanescent field type saturable absorber,combined with tapered fiber and quantum dot WS₂,obtains a stable pulse output in a 1.5?m fiber laser.The pulse has polarization-dependent characteristics.The pulse can be split two parts in the time domain by using polarization beam splitting.4.the saturable absorption characteristics of materials are adjusted by the external light field method,and the influence of this method on the mode-locking stability characteristics of the material-based saturable absorber fiber laser is discussed.(1)The photon energy in the 1.5?m band is similar to the band gap energy of semiconductor Mo Te₂.At the same time,Mo Te₂ has a semiconductor phase 2H and a semi-metallic phase 1T'.A stable semiconductor phase and a semi-metal phase thin-film saturable absorber are prepared respectively,test and analyze its saturable absorption characteristics.Under the same conditions,semi-metals have lower saturation flux and greater modulation depth.(2)The evanescent field type saturable absorber is prepared by combining the edge-polished fiber and the semi-metal phase 1T'-Mo Te₂.By calculating the relationship between the refractive index of the material in the edge-polished fiber waveguide structure and the refractive index of the covering layer,and simulating the optical field distribution,the results show that the use of a 200?m PVA layer can effectively enhance the light-material interaction in the device,thereby improving the saturable absorption characteristics.(3)The saturable absorber parameters of the material are adjusted by the external light field method,and the stability of the mode-locked state is discussed.The pulse stability is monitored by time dispersion stretching technology.It is found that the jitter of the pulse intensity mainly comes from the phase of the sub-pulse cannot be locked.Experiments have found that the application of an external laser on 1T'-Mo Te₂ can lock the phase of the sub-pulse and improve the stability of the mode-locked output pulse.At the same time,the power of external laser and photon energy effect on the saturable absorption characteristics of1T'-Mo Te₂ and pulse stability characteristics is discussed.