Research On All Fiber Mode Locked Laser Based On Multimode Interference

In recent years,the development of mode-locked fiber laser is very fast,especially in the breakthrough of mode-locked technology,which is developing towards the direction of ultra-short duration,high beam quality and peak power.The mode locking method based on nonlinear multimode interference effect(NL-MMI)has become a research hotspot in recent years.Compared with two-dimensional materials,the NL-MMI effect has the advantages of high damage threshold and all-fiber structure.Compared with the nonlinear polarization rotation(NPR)and the nonlinear amplifying loop mirror(NALM),the structure is simple,independent of polarization state,and has the characteristics of tunable nonlinear saturable absorption.In this thesis,the research of single mode-multimode-single mode fiber(SMS)mode-locking based on NL-MMI effect is carried out as follows:1.First of all,we introduced several current mainstream mode locking methods,compared the differences between NL-MMI mode locking and two-dimensional material mode locking,NPR and NALM in detail,and introduced the formation mechanism of mode-locking in detail.Then the dispersion and nonlinearity in cavity are discussed respectively,and the influence on the mode-locked pulse is studied.The principle of mode locking based on NL-MMI effect is explored,and the necessary conditions of mode locking based on multimode interference effect are derived.2.Then,the high order harmonic mode-locking based on NL-MMI effect in the 1.5?m negative dispersion region is realized,a fiber ring cavity structure is constructed,the polarization controller(PC)is used to bring in additional nonlinear phase shift,which reduces the requirement for the precise length of the multimode fiber and realizes the traditional soliton mode-locking.On this basis,according to the theory of soliton area,the soliton splitting is realized by adding a single-mode fiber into the cavity.By using the saturable absorption characteristic of SMS,the phenomenon of pulse self-walking and pulse instability is solved.Finally,the harmonic of order 880 is realized.Under the condition of the fundamental frequency of 1.28 MHz,the maximum repetition frequency can reach 1.127 GHz,and the pulse width is 960 fs.3.After that,the advantages of NL-MMI effect in high-energy soliton transmission are explored.The anti-saturation absorption characteristic of SMS structure is used to generate square wave pulse.Combined with the high damage threshold of SMS structure,the dissipative soliton resonance(DSR)pulse is successfully output with increasing pump power.The energy of DSR pulse can be increased from 6.26 n J to 8 n J,and the output power is 10.51 m W at 1129 m W pump power.4.Finally,we explore the role of polarization state in NL-MMI mode-locking.Whether in the harmonic mode-locking experiment or the dissipative soliton resonance experiment,we find that the nonlinear transmittance can be adjusted by changing the polarization state,and then changing the modulation depth and unsaturated loss of SMS structure.The characteristics of all fiber and adjustable nonlinear transmittance also provide more choices for the integration and diversity of fiber lasers in the future.