Research On Mode-locking Methods And Pulse Characteristics Of Ultrafast Fiber Lasers

Ultrafast fiber lasers attract extensive attention due to their characteristics of fast heat dissipation,easy integration,high peak power,ultrafast pulse duration,and broadband output spectrum,which are widely used in the fields of communication,medical treatment,industry and military.The realization of ultrafast pulses depends on Q-switching or mode-locking technology in lasers.The pulses achieved by Q-switching technology are limited by the photon lifetime in the laser cavity,which can only reach the order of microsecond to nanosecond,while mode-locking technology can achieve ultrafast pulses with picosecond to femtosecond pulse widths.The widely used active mode-locking technology relies on electro-optic,acousto-optic or mechanical modulators,which have complex structure and high cost.As the key devices of passive mode-locking technology,the saturable absorbers have been widely concerned and developed in the past decades.Various saturable absorbers based on the semiconductor saturable absorption mirror,nonlinear polarization rotation,nonlinear optical loop mirror have been studied and applied to achieve ultrafast pulses output in fiber lasers.However,it is difficult to be widely used because of the complex preparation process,limited working range and low stability.With the increasing demands for different types of ultrafast lasers in different application fields,researches concentrate on finding saturable absorbers with simple preparation,flexible adjustability and high stability,so as to realize ultrafast fiber lasers with lower cost,diverse functions,compact structure and stable performance.Therefore,this dissertation focuses on the research of new saturable absorbers.We prepare the saturable absorbers based on Fe₃O₄nanoparticles and multimode fiber interference structures,and apply them in different fiber lasers to realize ultrafast pulses,which are measured and analyzed in detail.And we also study and discuss the formation principle and evolution process of the polarization domain pulses and the self-mode-locked pulses in a long-cavity fiber laser.The main contents of the dissertation include:（1）The saturable absorption properties and the application of Fe₃O₄ nanoparticles are investigated experimentally.A"sandwich"saturable absorber integrated with Fe₃O₄nanomaterials and two optical fiber connectors is prepared.Based on the saturable absorber,an erbium-doped fiber laser is built,which can achieve Q-switched pulses output.By reducing the nanoparticles concentration and the thickness of the material film,the optical absorption modulation depth of the material decreases.Based on the modified Fe₃O₄ based saturable absorber,stable mode-locked solitons and Q-switched mode-locked pulses are realized in an erbium-doped fiber laser.In order to explore the saturable absorption property in other waveband and the magnetic field response characteristics of Fe₃O₄ nanomaterials,magnetic fluid material,which contains Fe₃O₄ nanoparticles,is deposited on a taper fiber and encapsulated using a capillary glass tube to obtain a new saturable absorber based on evanescent field effect.A mode-locked ytterbium-doped fiber laser based on the magnetic fluid saturable absorber is achieved at 1030 nm.The mode-locked pulses in different wavebands obtained in our experiments all have picosecond pulse widths,which are significantly improved compared with the nanosecond or microsecond Q-switched pulses in previous works using the same materials.The transmission properties of the new saturable absorber under different magnetic field intensities are measured in detail.Meanwhile,a mode-locked fiber laser based on magnetic fluid and nonlinear polarization rotation is built and its response characteristics to magnetic field are studied.（2）Multimode fiber saturable absorbers based on the nonlinear multimode interference effect is investigated.A graded-index multimode fiber is spliced with two pieces of single-mode fibers to prepare a new saturable absorber based on the fiber interference structure.A flexible saturable absorber is obtained by putting the multimode fiber in the groove of an in-line manual fiber polarization controller,which can introduce additional nonlinear phase shift and relieve the strict requirement on the multimode fiber length when acting as a saturable absorber in previous works.An ytterbium-doped fiber laser based on the multimode fiber saturable absorber is built.By adjusting the polarization controller,dissipative solitons,noise-like pulses and Q-switched mode-locked pulses are obtained in the fiber laser.Based on the dispersive Fourier transform technique,we study the real-time spectral evolution of the stable Q-switched mode-locked pulses,which exhibits periodic breathing property.The real-time buildup process of the Q-switched mode-locked state is also captured,which shows that the continuous-wave in the laser gradually evolves into the stable Q-switched mode-locked pulses through unstable self-pulsation,relaxation oscillation and rogue Q-switching stage.Similarly,a new saturable absorber based on few-mode fiber is prepared using the same method.The stable mode-locked pulse is also achieved in an ytterbium-doped fiber laser based on this few-mode fiber saturable absorber.（3）Polarization domains and self-pulse phenomenon are observed in a 130 m long erbium-doped fiber laser without any mode-locking and nonlinear modulation devices.Polarization domains in different states and harmonic polarization domains are obtained by adjusting the intra-cavity polarization controller and the pump power.By fine-tuning the intracavity polarization controller,we study the detail evolution of the polarization domains with the cavity birefringence decreasing.It is found that when the birefringence is gradually close to zero,the polarization domain splits and eventually evolves into a stable giant self-mode-locked pulse.Based on the experimental results,we analyze that the giant self-mode-locked pulse originates from the multiple sub-domain pulses,which results from the strong coherent cross-coupling between two orthogonal polarization modes and other nonlinear effects in the long fiber cavity.