The Research On Graphene-based Material Mode-locked Fiber Laser And Nonlinear Pulse Dynamics

Mode-locked fiber lasers have broad applications such as in industry,medicine,defense and so on.It also can be a powerful tool to promote the development of fundamentals.Meanwhile,it is the frontiers of nonlinear optics research as well.The research of mode-locked fiber lasers has great value to fundamentals and social economy.Mode-locked fiber lasers can be considered as an ideal playground for exploring nonlinear dynamics.In the past decades of years,Varieties of laser configurations possesses varieties of pulse shapes and dynamics,especially within fiber lasers which can generate various soliton pulses.We can observe tremendous nonlinear phenomenon with it.After successful exfoliation of single layer graphene in 2004,the two-dimensional materials represented by graphene have exhibited unique and extraordinary physical and chemical properties and are active in scientific frontiers,such as in the fields of electronic devices,flexible electronics,sensors,biomedical applications,composites,energy storage and photonics and so on.In the optical and photonics,graphene also shows remarkable optical properties,including broadband absorption due to linear dispersion of Dirac electrons,uniformly linear absorption of 2.3% through single atomic carbon layer,saturable absorption as consequence of Pauli blocking,ultrafast relaxation time in picosecond level,gated-variable optical conductivity and so on.Here we base on the platform of fiber lasers,and studied deeply in nonlinear pulse dynamics of mode-locked fiber lasers.We fabricated saturable absorber based graphene oxide which is the chemical derivative of graphene,and applied it in the application of mode-locked fiber laser as a passive amplitude optical modulator.Moreover,we have demonstrated graphene based electro-optical modulator,realizing active modulation of light amplitude by graphene.The main research work in this thesis including following parts:1.Nonlinear pulse dynamics in passive mod-locked fiber lasersTheoretically,we utilized the nonlinear Schrodinger equation to study the evolution and formation of the conventional solitons,dispersion-management solitons,self-similar solitons and dissipative solitons.We analyzed the roles of dispersion and nonlinear effects in generation of soliton pulses.Meanwhile,we introduced all kinds of saturable absorbers,including real ones and artificial ones,and their operation mechanism.We emphasized the important role of saturable absorbers,which acted as nonlinear loss devices in all-normal-dispersion mode-locked fiber lasers.Different saturable absorber transmittance curves will influence the final type of dissipative solitons,including conventional dissipative solitons,dissipative soliton resonance and noise-like pulses.By means of numerical simulation,in the view of transmittance curve of saturable absorber,we firstly studied and demonstrated the evolution processes from conventional dissipative soliton to dissipative soliton resonance and noise-like pulses based on the nonlinear Schrodinger equation and transmittance of saturable absorbers.We highlight the crucial role of reverse saturable absorption in the process and provide important theoretical support in designing mode-locked fiber lasers with ultrahigh pulse energy.2.All-normal-dispersion ytterbium-doped mode-locked fiber lasers based on graphene oxideWe utilized graphene oxide as basic material to prepare graphene oxide based saturable absorber through vertical evaporation method and drop-casting method,and introduced the prepared saturable absorber into the ytterbium-doped fiber laser.In the condition of ~22 m cavity length,we demonstrated conventional dissipative soliton generation through adjusting of polarization controller.Furthermore,we obtained noise-like pulse output under higher pump powers.Based on the experiment phenomenon and combined the characteristics of dissipative solitons in all-normal-dispersion regime,we proposed and analyzed the all-normal-dispersion mode-locked fiber laser based on graphene related materials,which usually have low modulation depth,relies heavily on potential nonlinear polarization rotation.The laser usually delivers pulses with long duration,introducing of narrow spectral filter will improve the pulse width.Moreover,in the condition of long cavity length of ~216 m,we firstly observed dissipative soliton resonance in graphene oxide based real saturable absorber ytterbium-doped mode-locked fiber laser delivering pulses with ultrahigh energy of ~160 nJ.We in-depth analyzed that both the potential nonlinear polarization rotation and interference effect in etalon have provided the necessary reserve saturable absorption effect.3.Graphene based electro-optical modulatorWe studied the physical mechanism of variation of optical conductivity as applied voltage.We demonstrated the graphene based electro-optical modulator consisting of two single layer graphene fabricated by chemical vapor deposition method and aluminum oxide film grew through electron beam evaporation on a silica substrate.We measured the transmittance of the modulator under different voltages by a continuous wave fiber laser,and realized modulation depth of ~7.5%.Furthermore,a supercontinuum source was applied,we observed that the modulator had a broadband operation range from 1100-1700 nm.