Multi-switchable Generation Between Dissipative Solitons And Noise-like Pulses In A Fiber Laser Based On Nonlinear Polarization Rotation

Noise-like pulses?NLPs?have wide applications in the fields of machining,optical sensing,supercontinuum generation,and optical coherence tomography due to its characteristics of high energy,wide spectrum bandwidth and low-coherence.In addition,as one of the special mode locked pulses,NLP has also received much attention in the basic physics research.Therefore,NLP has aroused intense research interests both in practical applications and in the study of physical phenomena.Passively mode-locked technique is the main technical mean to generate ultrashort pulses.So far,nonlinear polarization rotation?NPR?technology and saturable absorbers?SAs?with nonlinear optical characteristics are the two mainstream methods to realize passive mode locking.Among them,NPR has the advantages of simplicity,wavelength insensitivity,high thermal damage threshold,fast response time,and no degradation under long-term light.Therefore,it has been widely used in various types of pulse fiber lasers.The 45°tilted fiber grating?45°TFG?has the advantages of high polarization dependent loss,low transmission loss,and compact structure.It can be used as an excellent fiber polarizer to form the NPR mechanism to explore the soliton dynamics and new physical phenomena in fiber laser.In this dissertation,we pay major attentions on net normal dispersion erbium doped fiber laser?EDFL?based on NPR,which mainly includes the generation of dissipative soliton?DS?and NLP,and some switching phenomena between these two types of pulses in the same physical laser cavity.The main works and research results are as follows:1.We have studied the DS EDFL based on the 45°TFG.When the pump power is300 m W,this fiber laser can generate DS by properly adjusting the polarization controllers?PCs?.When the pump power is increased to 405 m W,the dual-pulse bound state of DS can be observed.The pulse width of the individual DS is about 3 ps if a Gaussian pulse shape is assumed.The pulse separation is approximately 5.8 times of the pulse width.In addition,both 2^ndand 4^th harmonic mode locking of the DS are also observed.The repetition frequencies are 84.74 MHz and 169.84 MHz,respectively.2.In addition to the multi-pulse phenomena of DS,this fiber laser can also achieve switchable generation from DS to NLP by only adjusting the PCs or increasing the pump power.The NPR mode-locking mechanism consisting of a 45°TFG and two PCs plays an important role in switching between these two pulse types.After the DS mode locking is obtained,the switching from DS to NLP can be achieved by simply adjusting the PCs or increasing the pump power.It should be noted that the NLP could be maintained when the orientations of the PCs are fixed and the pump power increases.To the best of our knowledge,the demonstrated laser features the highest fundamental repetition rate NLP generation by far.In addition,it is the first time achieved the NLP from an EDFL based on 45°TFG.3.By optimizing the cavity length and finely adjusting the orientations of the PCs,we have observed the multi-switchable generation between DS and NLP.When the orientations of PCs are fixed,by monotonically increasing the pump power,the output pulse state of this fiber laser can switch multiple times between DS and NLP.Both the multi-shuttle behavior between DS and NLP and the NLP to DS transition by increasing pump power have been observed experimentally for the first time.This phenomenon in our fiber laser manifests a novel pulse evolution process while all previous reports NLP is the final status whenever increase the pump power in the NLP fiber laser.This multi-shuttle behavior between DS and NLP can be attributed to the transmission of NPR mechanism together with the gain filtering effect.