Study On Passive Mode-locked Fiber Laser Based On Nonlinear Multimode Interference

Passive mode-locked fiber lasers have the advantages of small size,strong anti-interference,and excellent beam quality.They turn into a popular research due to their significant applications in many areas,such as high-performance fiber sensing,high-precision spectral measurement analysis,high-precision precision machining,laser medical,high-capacity optical fiber communication,laser medical devices and other aspects.There is a very important nonlinear optic device in passive mode-locked fiber lasers: a saturable absorber(SA)that converts continuous wave into pulse.To date,various types of saturable absorbers(SA)have been reported.This paper mainly studies the passive mode-locked fiber laser based on two-dimensional material SA and optimized nonlinear polarization rotation(NPR)effect,which realizes different pulse outputs.A multimode fiber hybrid structure SA based on nonlinear multimode interference(NL-MMI)structure is proposed.The traditional femtosecond soliton and various multi-soliton outputs are obtained,which enriches the passive mode-locking mode.This paper mainly studies the performance of fiber lasers under the interaction of evanescent field and laser in different two-dimensional materials and analyzes their advantages and disadvantages.Molybdenum disulfide(Mo S2)and selenium bismuth oxide(Bi2O2Se)are used to prepare SA mode-locking devices,and test its pulse output performance after accessing the fiber laser.Due to the process of material transfer to the evanescent field of the tapered fiber,the prepared two-dimensional material SA does not perform well in mode-locking performance.It can only increase the loss of the fiber laser,and the pulse output phenomenon is less.With disulfide Molybdenum,selenium and bismuth oxide we realize the output of dark pulses and 16 th harmonic bright and dark pulse pairs,respectively.The experimental investigation of NPR passive mode-locked fiber laser with adjustable attenuator(VOA)to obtain a variety of soliton phenomena and the stability of the output spectrum and ultra-wide spectral output of the laser at different pump powers were studied.This paper proposes a multi-mode optical fiber hybrid structure SA based on NL-MMI,which has the advantages of simple structure and low cost,and eliminates the problem of directional selection during access.By continuously optimizing the length and parameters between multi-mode fibers to achieve the best performance of the SA mode-locked device,and experimentally researching the output performance of the SA passive mode-locked fiber laser based on the multi-mode fiber structure.In different mode-locked states,different mode-locked pulse outputs were observed—traditional femtosecond solitons,non-traditional soliton rains,higher harmonics(HML),soliton clusters with different pulse interval distributions,different orders,and pulse spacing Various soliton output phenomena such as inconsistently bound solitons,and noise-like impulses(NLP)with poor coherence are analyzed and discussed in theory.By means of optimizing the parameters in the cavity,the third harmonic dual-wavelength dark-bright pulse pair output was obtained experimentally and it was verified that the bright-dark pulses are orthogonal to each other in terms of polarization.The research work in this paper will provide a certain reference scheme for the realization of mode-locked fiber lasers,and it will provide rich guidance for the application of multimode fiber hybrid structure mode-locking technology based on nonlinear multimode interference.