Numerical Study Of A Hybrid Mode-locked Erbium-doped Fluoride Fiber Laser

Mid-infrared 3 ?m ultrashort pulse laser is located at atmospheric transparent window,moreover,it is at the peak of the absorption spectrum of water molecules,which has important application value in the military,civilian and scientific field.The high erbium-doped concentration fluoride fiber laser has become one of the most effective methods to generate the 3 ?m ultrashort mid-infrared laser because it has the characteristic of high efficiency and wide transmission spectrum range.At present,the main methods to obtain the erbium-doped fluoride ultrashort pulsed fiber laser in 3?m is by mode-locked technique such as nonlinear polarization rotation(NPR)and saturable absorber(SA).Among them,the mode-locked fiber laser based on NPR with a simple and compact structure,the pulse duration can be reached to dozens of femtosecond,but it is easy to be disturbed by the environment;the mode-locked fiber laser based on SA is easier to be self-started and the mode-locked is stable,however,the width of the pulse is hard to achieved below than 100 fs.So,how to obtained erbium-doped fluoride ultrashort pulse laser in 3 ?m with better performance by using the advantages of the mode-locked technology based on NPR and SA sufficient is an important research direction.In this thesis,the theoretical model of the fiber laser based on hybrid mode-locked has been established and the output characteristics of the hybrid mode-locked erbium-doped fluoride fiber laser have been researched numerically.The main work is as follows:(1)Based on Maxwell's equations,the evolution process of the optical pulse transmission in the cavity has been derived,and based on passive fiber and gain fiber,the corresponding evolution equations of optical pulse transmission are obtained successively,meanwhile,the coupled Ginzburg-Landau equation has been solved numerically,then the mode-locked principles of NPR,semiconductor saturable absorber mirror(SESAM)and hybrid mode-locked have been elaborated in turn,and the related theoretical model are established.(2)Based on the established theoretical model,the output characteristic of the erbium-doped fluoride fiber laser based on NPR,SESAM,and hybrid mode-locked have been researched numerically.Among them,the erbium-doped fluoride fiber laser based on NPR mode-locked can be obtained the uniformed optical soliton mode-locked pulse with the pulse width is 510.7 fs and the pulse energy is 0.71 n J;based on the SESAM mode-locked technology,a uniformed optical soliton mode-locked pulse with the pulse width is 547.2 fs and the pulse energy is 0.79 n J can be obtained;based on the hybrid mode-locked technology of NPR and SESAM,a uniformed optical soliton mode-locked pulse with the pulse width is 155 fs and the pulse energy is 2.29 n J can be obtained,and the corresponding peak power is 14.78 k W.From the result of the simulation,we can see that compared to single mechanism mode-locked,the output pulse performance of hybrid mode-locked will be much better.(3)The effect of important parameters on the output pulse performance of the hybrid mode-locked erbium-doped fluoride fiber laser has been researched in detail,including the length of the gain fiber,the saturated energy of the gain fiber,the linear phase bias,and the small-signal gain.The numerical simulation shows that in the certain range,the longer gain fiber,the higher saturated energy and small-signal gain,the pulse width of the realized hybrid mode-locked pulse will be narrower and the 3d B spectral width will be wider,correspondingly,the peak power and the pulse energy will be higher,therefore the stable ultrashort mode-locked pulse with high pulse energy can be easily achieved.