Investigation On Soliton Phenomena In Novel Two-dimensional Materials Mode-locked Fiber Lasers

Passive mode-locked fiber laser has multiple characteristics such as passive modulation,ultrashort pulse width and fiber structure.It has many advantages such as no need for additional opto-electronic modulation devices,narrow pulse width,high peak power,high repetition rate,good stability,compact structure,easy alignment,flexible design,convenient integration,etc.It will gradually play an increasingly important role in a series of important disciplines such as precision material processing,precision measurement,optical fiber communication,national defense and military,nonlinear optics,medical.Especially in the field of medical,the improvement in various parameters of ultrafast fiber lasers compared to traditional lasers has greatly promoted the progress and development of medicine.Ultrafast fiber lasers have wide spectral characteristics and short coherence length which have great significance in improving the spatial resolution and contrast of images in the field of medical imaging.In medical treatment,the extremely short pulse width of ultrafast fiber lasers can effectively improve surgical accuracy and reduce intra-operation bleeding.Therefore,it exhibits benefits in expending various surgical methods and reducing the side effects caused by traditional surgical methods.Recently,two-dimensional（2D）materials have been used to prepare a variety of ultra-fast optical devices and apply in passive mode-locked fiber lasers due to their special layered structure advantages and excellent optical properties.It is of great importance in exploring the dynamic basis and characteristic advantages of the formation of various types of solitons in mode-locked fiber lasers,due to its applications of mode-locked fiber lasers with different types of solitons such as light sources,especially in the medical field which has more refined requirements such as pulse band,pulse power,pulse width and pulse frequency.In this thesis,we explored the nonlinear saturable absorption characteristics of three novel two-dimensional materials with air stability,prepares ultrafast optical modulation devices based on different methods,and applies them in erbium-doped fiber lasers to explore the generation of multiple soliton type mode-locked pulses.Conduct basic soliton dynamics research on different types of solitons,explore the characteristics and advantages of different solitons,such as the low chirp characteristics of traditional solitons that can obtain shorter pulse widths,the chirp broadening of dissipative solitons that can obtain wider spectral widths and larger single pulse energy,noise like mode-locked pulses that have large single pulse energy and high incoherence,and the high repetition frequency characteristics of harmonic mode-locked pulses.Ultrafast fiber lasers are classified according to different types of solitons in order to further meet the precise needs of medicine and provide technical support for light sources with different medical needs.In the study of Pd Se₂,a dissipative soliton mode-locked laser based on Pd Se₂ saturable absorber was successfully constructed,and it was used as a laser seed source to build an Optical Coherence Tomography（OCT）system aiming to improve the performance of the OCT system.The specific research results are as follows:1.Based on the liquid phase stripping method,the Tellurene nanosheets is prepared and coated on the tapered region of the tapered fiber to prepare a tapered saturable absorber（SA）.Based on the evanescent field effect,the interaction between the Tellurene nanosheets and the light in the tapered fiber is realized to realize the saturable absorption modulation.The ultrafast optical modulation device based on tapered fiber has strong heat dissipation ability and large nonlinear effect.When it is applied to erbium-doped fiber lasers,the large energy dissipative soliton mode-locked pulse is realized firstly.In addition,with the increase of pump power,high power noise-like mode-locked pulse output is obtained.In order to further explore the nonlinear absorption characteristics of Tellurene,in the experiment,Tellure-Polyvinyl alcohol（PVA）thin film was prepared based on the liquid phase stripping method and spin coating method,and it was placed on both ends of the fiber jumper to make a sandwich type saturable absorber,which was applied to the study of passive mode-locked erbium-doped fiber lasers.The stable mode-locked pulse laser output is realized in the fiber laser based on Tellure-PVA SA.In the experiment,the fundamental frequency,8th harmonic and 19th harmonic mode-locked phenomena are observed through the nonlinear characteristic regulation.The above experimental results show that Tellurene nanosheets have good nonlinear absorption characteristics.Tellurene taper type SA based on evanescent field effect has great potential in obtaining high energy pulse,which has important reference significance for exploring the use of Xenes as ultra-fast optical devices and building high power,high energy mode-locked fiber lasers.2.Based on the liquid phase stripping method and spin coating method,the ferromagnetic semiconductor CST was prepared acting as CST-PVA sandwich SA.The CST-PVA SA is used in the erbium-doped fiber laser.By adjusting and optimizing the length of the single-mode fiber and the erbium-doped fiber in the erbium-doped fiber laser,the polarization state of the resonator and the pump power.A variety of different types of mode-locked pulses are obtained.These include low-threshold conventional soliton mode-locking,dual-wavelength dissipative soliton mode-locking,and harmonic mode-locking adjustable from fundamental frequency to 49th order.The mode-locking threshold of low threshold traditional soliton mode-locking is 10.1 m W,which is the lowest value based on two-dimensional material mode-locking at present.Experimental results show that excellent CST nanocrystals can be prepared by liquid phase stripping method,and CST has good air stability,which has good application value in exploring soliton dynamics and harmonic mode-locking in fiber lasers.3.Firstly,the energy band characteristics of Pd Se₂ are theoretically analyzed based on the density functional theory;Then,Pd Se₂ nanoflakes are prepared based on the liquid phase stripping method and deposited on the purchased tapered fiber to prepare Pd Se₂ tapered fiber SA,and a ring cavity erbium-doped fiber laser is built based on Pd Se₂ tapered fiber SA.By adjusting the length of the gain fiber and single-mode fiber in the laser cavity,two different lasers with positive and negative net dispersion in the cavity are obtained,respectively,and the traditional soliton and high energy dissipation soliton mode-locked pulse are obtained.Because the diameter of the tapered fiber directly affects the nonlinear effect in the resonant cavity,Pd Se₂ tapered fiber SA was successfully prepared by self-made tapered fiber in the experiment.Benefiting from the high nonlinear characteristics of the self-made tapered fiber,single-pulse,double-pulse and three-pulse mode-locked output,as well as rich optical soliton phenomena such as 3rd,4th,6th,10th and 11th harmonic mode-locked pulses were realized.The experimental results prove the excellent performance of Pd Se₂ crystal in ultrafast optical devices,which can be widely used in the field of ultrafast photonics,and has important reference significance for the design of ultrafast photonics devices based on 2D materials.Finally,the OCT system was successfully constructed based on the dissipative soliton mode-locked laser as the seed source,a laser output with a pulse width of 390fs was achieved through multi-level amplifier and pulse width compression.