Research On High Repetition Rate Mode-locked Fiber Lasers Based On Novel 2D Material Saturable Absorber

High-repetition rate（HRR）mode-locked fiber lasers have attracted extensive attention from researchers due to their high repetition rate,narrow pulse width,high stability,and compact structure.HRR mode-locked fiber lasers have great potential and good application prospects in laser communication,time-frequency metrology,optical frequency comb,nonlinear microscope,biological sampling,laser ranging,high-speed electro-optic sampling,laser processing and many other high-grade,precision and advanced fields.It is worth noting that the HRR mode-locked fiber laser constructed by coupling two-dimensional materials as saturable absorbers plays one of the most important roles.Since 2008,various new two-dimensional materials SA,led by graphene,have exploded.Other common 2D materials include carbon nanotubes（CNTs）,black phosphorus（BPs）,transition-metal dichalcogenides（TMDs）,topological insulators,MXene,antimonene,and bismuthene,etc.Compared with traditional saturable absorbers such as dyes or semiconductor saturable absorbers,2D materials not only have good saturable absorption characteristics,but also have the advantages of low preparation difficulty,cost saving and easy compatibility.Besides,these materials have excellent nonlinear optical properties with large nonlinear polarizability and large nonlinear refractive index.In addition,due to the unique layered physical structure of 2D materials,they also have irreplaceable advantages such as wide operating band,controllable modulation depth,and fast recovery time.Based on the reasons above,this paper uses a new two-dimensional material,bismuth oxide tellurium（Bi₂O₂Te）,as a saturable absorber to build a mode-locked fiber laser,which realizes various types of mode-locking,and produces high repetition rate pulse output based on harmonic mode-locking（HML）.On the other hand,a high-repetition fiber laser based on dissipative four-wave mixing（DFWM）was built by using a Mach-Zehnder interferometer（MZI）as a comb filter.At the same time,the laser output results are discussed,and the main research contents are as follows:1.Design and build an erbium-doped fiber laser based on a two-dimensional material Bi₂O₂Te saturable absorber,and observe the pulse output of fundamental frequency solitons,bound state solitons,Q-switched mode-locked and harmonic mode-locked.Among them,the pulse repetition frequency of the fundamental frequency is 7.48MHz,which corresponds to a pulse interval of 133.71ns in the time domain（corresponding strictly to the cavity length）.The pulse center wavelength is 1559.44 nm,the 3-d B bandwidth is 2.09nm,and the signal-to-noise ratio（SNR）is 67.25d B.Secondly,the second-order,third-order and fourth-order bound state solitons were observed respectively,and their spectral center wavelength and repetition frequency were basically consistent with the fundamental frequency solitons.Spectrum peaks showing a modulation period can be observed on the spectrum,and the intervals between adjacent peaks are 1.31nm,1.3nm,and 1.29nm;multiple peaks appear on the autocorrelation trace,and the peak-peak intervals were 6.2ps,6.2ps,and 6.3ps,which matches the spectral modulation period after Fourier transform.Finally,a high repetition frequency harmonic mode-locked pulse output is generated.The pulse center wavelength is 1558.9nm,and the 3-d B bandwidth is 1.31 nm.During the process of increasing the pump power from 79m W to 466m W,the 28th order,50th order,79th order,126th order,168th order,205th order and 238th order harmonic mode-locked output were observed sequentially,and the pulse intervals were 4.8ns,2.7ns,1.7ns,1.1ns,0.80ns,0.65ns,and 0.56ns,corresponding to repetition rates of 209.37MHz,373.88MHz,590.72MHz,942.17MHz,1.25622GHz,1.53288GHz,and 1.77965GHz.2.A high-repetition fiber laser based on the DFWM principle was built,and MZI was embedded in the resonator to generate it.The extra-cavity extinction ratio of the introduced MZI comb filter is 12.2d B,and the extinction ratio after being introduced into the resonant cavity is>40d B.Observing the high repetition frequency output based on DFWM,by tuning the optical delay difference between the two arms of the MZI,the stable output optical signals of the free spectral range（FSR）are 0.21nm,0.43nm,0.96nm and 2.55nm respectively,corresponding to the high repetition frequency pulse output with pulse intervals of 38.9ps,19.0ps,8.5ps and 3.2ps respectively,the calculated repetition frequencies are 25.7GHz,52.6GHz,118GHz and 313GHz in turn.