The Research Of Mode-locked Fiber Lasers Based On Nonlinear Polarization Rotation And Saturable Absorber Technologies

In recent years,passively mode-locked fiber lasers have attracted extensive attention and research at home and abroad due to their competitive properties including compact structure,low cost,ultrashort pulse width,large pulse energy,high peak power and so on.The large-energy mode-locked fiber lasers exhibit various special potential applications in the fields of laser micro-nano machining,optical frequency comb,laser sensors,laser etching,etc.For obtaining high-energy mode-locked pulses,mode-locked fiber lasers with different soliton types including dissipative soliton,breathing soliton,self-similar pulse,noise-like pulse and so on have been investigated successfully.The large energy mode-locked fiber laser has become a hot spot in the field of laser technology research.Furthermore,the two dimensional material has received great investigation in the fields of electronics,optoelectronics,biomedicine,nanochemistry and so on.It has been proved that two-dimensional material is the effective modulator for obtaining ultrafast fiber laser operating between visible and mid-infrared ranges,which promotes the rapid development of solitons.The two dimensional material has been utilized as saturable absorber widely to demonstrate Q-switched or mode-locked lasers.Meanwhile,graphene,topological insulators,transition metal dichalcogenides,black phosphorus,MXene and so on are comprehensively employed for demonstrating passively mode-locked fiber lasers.The exploration of new two dimensional materials with excellent saturation absorption characteristics has great significance for the investigations of mode-locked fiber lasers.Unlike traditional mode-locked and Q-switched fiber lasers,the Q-switched mode-locked pulse has been investigated and developed experimentally in recent years.Q-switched mode-locked lasers are based on the combination of both Q-switching and mode-locking techniques in order to generate bursts of ultrashort pulses.Compared with traditional mode-locking,this technology can generate higher peak power pulse.Purely active scheme based on a single saturable absorber has also been proved for the successful generation of Q-switched mode-locked pulses.Although investigation groups around the world are very interested in the technical and commercial applications of fiber lasers,there are still relatively few investigations on Q-switched mode-locked fiber lasers,and there is still a lot of room for improvement.This paper focuses on passively mode-locked erbium-doped fiber lasers,nonlinear polarization rotation and saturable absorber technologies are combined to investigate lasers including large energy mode-locked,dark-bright-bright soliton pairs,multi-wavelength mode-locked and Q-switched mode-locked pulse lasers.The main research work is as follows:Firstly,based on a long laser cavity and high power pump sources,we successfully demonstrated an erbium-doped mode-locked fiber laser with enhanced pulse energy by using the nonlinear polarization rotation technology.Meanwhile,the maximum pulse energy was as high as 881.34 nJ.To our knowledge,881.34 nJ is the maximum pulse energy obtained within erbium-doped mode-locked fiber lasers so far.In addition,experimental phenomena including broadening of the pulse width,splitting of the pulse shape and increase of the pulse repetition rate for decreasing the influence of the high peak power and large energy were also investigated.Secondly,the CGT polyvinyl alcohol film was successfully prepared,and the morphology and nonlinear saturation absorption characteristics of the Cr₂Ge₂Te₆ was investigated.By using it as a saturable absorber in erbium-doped fiber laser,dark-bright-bright soliton pairs were generated for the first time.The threshold power of mode-locked laser was 110 mW,and the pulse repetition rate was 5.64 MHz.The experimental results fully prove that CGT nanosheets have excellent nonlinear optical properties,which provides a preliminary experimental basis for the investigation of new bright-dark soliton pairs.In addition,the ZrSe₂ polyvinyl alcohol film was successfully prepared,and the morphology and nonlinear saturation absorption characteristics of the ZrSe₂ was investigated.By using it as a saturable absorber in erbium-doped fiber laser,multi-wavelength mode-locked laser was generated for the first time.The threshold power of mode-locked operation was 224 mW,and the pulse repetition rate was 3.38 MHz.The experimental results fully prove that as a transition metal dichalcogenide,ZrSe₂ nanosheets have adjustable absorption peak and excellent saturation absorption performance,which can be used as an efficient saturable absorber for investigating multi-wavelength mode-locked fiber lasers.Finally,by using dispersion compensating fiber as a saturable absorber in erbium-doped fiber laser,Q-switched mode-locked pulse was generated for the first time.The cavity was gradually optimized by adjusting the length of dispersion compensating fiber.When the length of dispersion compensating fiber was 5 m,the Q-switched mode-locked pulse at the Q-switching envelope frequency of 72.84 kHz was obtained under the pump power of 637 mW.The maximum Q-switching envelope energy was 204 nJ,and the width of a single pulse at the peak position of the temporal envelope was 455.7 fs.To our knowledge,455.7 fs is the shortest width of a single pulse at the peak position of the temporal envelope so far.These results indicate that dispersion compensating fiber has excellent performance in obtaining Q-switched mode-locked pulse.