Self-frequency Shift Study Of Mid-infrared Soliton Based On Fiber Cascade Structure

Mid-infrared laser has important application prospect and research significance in military,biomedical,atmospheric monitoring,optical imaging,and many other fields,and has become a hot research topic in recent years.Compared with the traditional laser,the wavelength tunable short pulse laser based on soliton self-frequency shift effect has the characteristics of narrow pulse width,high peak power,compact structure,high stability,and can overcome the spectral limitations of rare earth ion mode-locked fiber laser.Therefore,it is an excellent wavelength tunable laser source.At present,there are still some problems such as narrow tuning range,low Raman efficiency and structural responsibility.In view of the above problems,this thesis focuses on the self-frequency shift phenomenon of soliton in mid-infrared Raman laser with fiber cascade structure.The research contents are as follows:Firstly,the formation of optical solitons in optical fibers and the related mechanisms of self-frequency shift of solitons in optical fibers are described,including dispersion,self-phase modulation,soliton splitting,stimulated Raman scattering,etc.Then,the generalized nonlinear Schrodinger equation of optical pulse transmission in optical fiber is derived,and the numerical model of soliton self-frequency shift phenomenon is established by solving the equation.Secondly,the self-frequency shift process of infrared Raman solitons in fluorotellurite fiber is studied theoretically,and the effects of pumping wavelength and core diameter on the self-frequency shift effect of solitons are studied by numerical simulation.By analyzing the results of numerical simulation,the different effects of soliton order at low power and high power are illustrated.In the condition of1.9 ?m and 2.8 ?m high power seed pulses,a wide band tunable Raman soliton laser covering 1.9?4.1 ?m and 2.8?5.2 ?m has been realized by using 4 ?m fluorotellurite fiber as nonlinear medium.The cascade structure of fluorotellurite fiber and chide fiber was designed to further enhance the SSFS effect in the mid-infrared band and achieve wavelength conversion of 1.9?7.5 ?m and 2.8?8.1 ?m.Finally,the cascaded structure of thulium-doped fiber amplifier and highly nonlinear germanium doped fiber is set up.By optimizing the length of thulium-doped fiber,the Raman conversion efficiency in thulium-doped fiber reaches 91%,and the wide tunable Raman soliton between 1.96 ?m and 2.88 ?m is realized.When the central wavelength of the Raman soliton is 2.82 ?m,It can also maintain more than 40% Raman conversion efficiency.The work in this thesis can provide effective analysis for optimizing the Raman soliton laser and provide a meaningful idea for expanding the wavelength range of the self-frequency shift of the soliton and improving the Raman conversion efficiency.