Filtering And Pulse Dynamics In Fiber Lasers

Pulsed fiber lasers with high stability and tunability are broadly applicated in optical commu-nications,micro-fabrication,optical information processing,medicine and sensing.As one of the important branches of nonlinear optics,soliton pulse dynamics in fiber lasers enriches the nonlinear science.Filters are modulation components of fiber lasers,which are irreplaceable in the generation of lasing,the formation of solitons,pulse modulation and reduction of noise.Simultaneously,the filters are necessary in tunable and narrow bandwidth fiber lasers.On the one hand,the effect of filtering on the pulse formation and dynamical propagation is great.On the other hand,the traditional filters such as fiber grating and bandpass filters,cannot meet the further demand of spectral narrowing.Therefore,it is urgent to study the new filters and the effects on pulse generation and transmission.In this thesis,the study is based on the dynamic fiber grating and bandwidth tunable filters.The scalar and vector pulse tracing models are employed to study the generation,evolution and transformation of Q-switched and mode-locked pulses.Further study explores the pulse dynamics and filtering in dynamically coupled resonators by utilizing effective gauge theory,topological photonics and non-Hermitian theory.(1)Q-switched fiber laser with narrow bandwidth based on dynamically induced sat-urable absorption filterThe linear cavity with heavily-doped erbium-doped fiber(EDF)is built,which generates un-stable Q-switching pulses under the effect of spectral filtering provided by fiber Bragg grat-ing(FBG).The 3 dB bandwidth of the optical spectrum one order of magnitude smaller than the filtering bandwidth of the FBG,manifesting the saturable absorption and narrow spec-tral filtering of the EDF.Later,the dynamically induced saturable absorption filter is made by adopting circulator-EDF-FBG structure.The bandwidth change is studied both in theory and experiment.After adopting three lengths(7 cm,10 cm and 20 cm)of EDF,the laser stably generates Q-switching pulses with narrow bandwidths which is the narrowest in the Q-switched fiber lasers with fiber-based saturable absorbers.(2)Hybrid mode-locked Er-doped fiber laser for narrow bandwidth pulsesIn this part,we use hybrid mode-locking technique for obtaining stable mode-locked fiber laser based on dynamically induced saturable absorption grating.Firstly,the three regimes of fiber laser mode-locked by NPR comprising of stable-,period-N-,and multiple-soliton regimes are systematically studied and the corresponding phenomena and mechanisms are also analyzed by numerical simulation.Moreover,narrow bandwidth fiber laser mode-locked by the combination of NPR and dynamically induced saturable absorption grating are con-structed,generating nearly transform-limited mode-locking pulses.The evolution of pulse energy is numerically simulated through pulse tracing model.The work provides a easy and feasible way to realize mode-locking pulses with narrow bandwidth.(3)Effect of spectral filtering on the characteristics of output pulsesThe former work manifests the narrow spectral filtering effect in pulsed fiber lasers.In this work,the effect of spectral filtering on pulse state transition,pulse characteristics(including peak power,single pulse energy,pulse duration and spectral bandwidth)and single pulse shape are systematically studied through scalar pulse tracing model.The simulation results are clearly coincident with those in published experimental work.In addition,the primary research on the influence of spectral bandwidth on polarization of vector solitons in a mode-locked fiber laser is carried through vector pulse tracing model.The simulation work can provide the reference on the experimental research of tunable fiber laser.(4)Pulse modulation and transmission in modulated fiber loopsPulsed fiber laser is commonly designed as single circular cavity.In this work,the two fiber loops with length difference are connected by a coupler.In every fiber loop,there is a phase modulator to control the phase of pulses.The system is expanded along position and time axes,forming discrete synthetic photonic lattice.Through modulating the signal of pulse generator,the gauge field is introduced in the synthetic photonic lattice,realizing the pulse transmissions which are highly similar with those in one-dimensional discrete waveguides.Further research obtains the topological edge state in finite period of gauge field.The work transforms the temporal pulse into spatial domain,which reduces the experimental difficulty of photonic lattice and promotes the development of temporal pulse modulation and shaping.(5)Spectral filtering of non-Hermitian coupled fiber loopsWhen the gain and loss are introduced in the coupled fiber loops,the component belongs to a non-Hermitian system.We theoretically calculated the bandwidth and shapes of comb trans-mittance spectrum with the change of gain and loss.An ytterbium-doped fiber laser based on double fiber loops is constructed,generating single frequency lasing with the help of FBG and double fiber loops.The results initially verify the filtering of double fiber loops.