| Since the first implementation of mode-locked pulse using Nd: glass laser by De Maria and Stetser et al.in 1966,mode-locked laser pulses are favored by researchers in materials,biomedicine,photonics,etc,due to their excellent and intriguing properties of ultrashort duration,high peak power,wide spectral bandwidth and high beam quality.Mode-locked fiber lasers are fundamental building blocks of numerous photonic systems,successful applied in industry,medical treatment and scientific research,which provide new opportunities and challenges for high integration of photonic systems.As a critical component to obtain mode-locking,the development and demands of saturable absorbers have encountered new problems and bottlenecks.In recent years,nonlinear switching technique,expecially,the nonlinear multimode interference(NL-MMI)effect in mutilmode fiber brings gospel for saturable absorber to achieve high damage threshold and high integration.In this dissertation,using mdoe-locked fiber laser as carrier,saturable absorption characterisitcs of NL-MMI effect have been systematically investigated,and then the techniques of stable mode-locking based on NL-MMI effect are demonstrated theoretically,numerically and experimentally,and high energy mode-locked pulses are generated in experiment.Finally,fancy nonlinear phenomena,such as “rogue wave”,multi-peak pulses are observed,and the regulatory mechanism of polarization mode dispersion for pulse energy and envelop characteristic are numerical simulated.The main contents of this dissertation are as follows:1.The Q-switching mode-locked fiber laser based on NL-MMI effect have been investigated,and the technical method for generating stable mode-locked pulses have been obtained.The Q-switched mode-locking based on NL-MMI effect has been obtained experimentally for the first time,and the regulation of variation of modulation depth with 30% in Q-switched envelop has been achieved.The dynamics evolution of rectangle envelops of Q-switched pulses has been observed.The technical method for restaining Q-switching instibilities in mode-locking operation has been analized theoretically,and the functions of controlling mode-locking operation stability by modulation depth of saturable absorber and pump power have been discussed.2.The technical method for generating stable mode-locked pulses have been verified and stable soliton located at 1034 nm and 1560 nm has been obtained experimentally.The chirped dissipative soliton at 1 ?m used high-doped gain fiber and conventional soliton located at 1566 nm by controlling parameters of saturable absorber have been obtained.Dissipative soliton at 1570 nm with a time-bandwidth product of 24.6 has been achived and the factor resulting in instability of chirped dissipative soliton has been done numerically.3.High energy mode-locked pulses are obtained,and the physical mechanism of transmison between conventional soliton and stretched pulse in NL-MMI effect based mode-locked fiber laser is revealed.The obtained maximum pulse energy of conventional soliton is 0.38 n J,which breaks the theoretical limit of conventional soliton energy,and realizes the stretched pulse with pulse energy as high as 4 n J.The mechanism of controlling total cavity dispersion by principle modes in multimode fiber is proposed,and the function of controlling mode-locking operation by polarization state is analyzed numerically.The dynamics of soliton have been characterized,using the ultrafast measurement method of time-stretch Fourier transform technique,and the influence of multimode fiber on redistribution of pulse energy and the restrain effect of walk-off have been observed.4.The fancy nonlinear phenomena in the transition region of conventional soliton to stretched pulse are discovered.The phenomenon of optical “rogue wave” is observed experimentally,and the ratio ?=2.5 between maximum pulse intensity and significant wave hight is obtained,which reveals the physical mechanism of high energy conventional soliton transiting to higher energy stretched pulse.The phenomenon of continuous evolutions from dual-peak,triple-peak to single-peak pulse is observed.The laser physics of polarization mode dispersion controlling “rogue wave” and multi-peak pulse generations is proposed,and a corresponding theoretical model is established.The numerical simulation results show that “rogue wave” occurs with the ratio factor ? as high as 4.5 when the polarization mode dispersion is increased to 0.074 ps.When the polarization mode dispersion is set as 0.046 ps,the dynamic evolutions of dual-peak and multi-peak pulses occur. |
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