Cascaded Raman Self-Frequency Shift Study Of Mid-Infrared Soliton Based On 2μm Femtosecond Laser-Pumped Nonlinear Fiber

Mid-infrared femtosecond lasers have revolutionized many fields,including biomedical,materials processing,gas molecule detection,and military.A wavelength-tunable laser offers a high degree of flexibility to meet wavelength requirements in multiple fields.Among several ways to achieve wavelength tuning,Soliton self-frequency shift（SSFS）uses Intrapulse Raman Scattering（IRS）in optical fibers to achieve a wide range of continuously tunable wavelengths,inheriting a series of advantages of fiber laser,such as high beam quality,high stability,easy integration.However,the current wavelength-tunable laser based on the SSFS effect is difficult to obtain pure high-power Raman solitons,which makes it difficult to be applied in practical production.With this goal,this dissertation will study the mid-infrared Raman soliton generation based on a 2μm femtosecond pulsed laser source with a cascaded structure of SSFS system.The main research contents are as follows:（1）The evolution of pulses when group velocity dispersion and self-phase modulation act alone and together is studied based on Generalized nonlinear Schr?dinger equation（GNLSE）,and the mechanism of optical soliton generation and related theories are analyzed.Then,the time-domain GNLSE is solved analytically using the distributed Fourier method,and the mid-infrared SSFS process of indium fluoride fiber is numerically simulated based on GNLSE.The effects of pump peak power and fiber length on SSFS are studied for pump wavelengths of 2μm and 2.8μm,respectively.The continuously tunable Raman solitons from 2 to 2.9μm and 2.8 to 4.2μm are obtained when the pump peak power is 750 k W and the fiber length is 10 m.（2）Firstly,a 2μm stretched soliton and dissipative soliton mode-locked laser source with hybrid mode-locked structure are built,with the former having a 3 d B bandwidth of17.5 nm,an output power of 2.4 m W,a repetition frequency of 10 MHz,and the latter having a 3 d B bandwidth of 35.4 nm,an output power of 5.7 m W,and a repetition frequency of 10.4 MHz.Then,using a 2 m Tm-Doped Fiber Amplifer（TDFA）to amplify the stretched soliton signal light,a wavelength tuning from 1967.5 to 2238.1 nm is achieved,with a maximum conversion efficiency of 90.9%and a maximum output power of 270.6 m W for the furthest Raman soliton,corresponding to a limit conversion pulse width of 259.1 fs Finally,when the dissipative soliton signal light is amplified by using a4 m TDFA,a supercontinuum with a wavelength range of 1938～2339 nm and a 3 d B bandwidth of 401 nm is achieved.（3）The amplified pulses of the stretched solitons are injected into 1.5 m 64 mol.%germanium-doped fiber,and the wavelength tuning from 1966 to 2561.5 nm is achieved,with the furthest Raman soliton conversion efficiency of 36.2%and the single pulse energy of 8.1 n J.When the length of the germanium-doped fiber is increased to 3 m,the central wavelength of the furthest Raman soliton is frequency shifted to 2651.2 nm.When the germanium-doped concentration is increased to 94 mol.%,the center wavelength of the furthest Raman soliton is 2823 nm.Finally,the wavelength tuning from 1966 to2545.9 nm is achieved by using 2 m In F₃ fiber as the Raman frequency shift medium.The furthest Raman soliton has a pulse energy of 9.1 n J,a limiting transition pulse width of72.8 fs,and a peak power of 125 kW.