Research On 2-4 ?m Tunable Pulsed Fiber Laser Based On Cascaded Soliton Self-Frequency Shift

The 2?4?m mid-infrared pulsed laser has important application prospects in gas detection,material processing and military fields.Compared with single-wavelength pulsed lasers,wavelength tunable pulsed lasers have the advantages of high integration and flexibility.The soliton self-frequency shift(SSFS)effect is an effective means to realize the flexible tuning of the pulse wavelength.The high-frequency component of the pulse is used as the pump light through the Raman scattering effect in the fiber,and the energy is continuously transferred to the low-frequency component of the pulse.The center wavelength of the Raman soliton continuously moves to the long wave direction,which can realize the mid-infrared wavelength tunable Raman laser.At present,the wavelength tunability in the mid-infrared range has been realized through various nonlinear fibers,and the maximum tuning range is 2?4.3?m.However,the generated laser still has some problems,such as low output power and low Raman conversion efficiency and other issues.In response to this problem,this article focuses on the construction of ultrashort pulse mode-locked fiber lasers based on cascaded fiber SSFS effect,and conducts research on tunable lasers with wavelengths of 2 to 4?m.Based on this,the main research contents of this paper are as follows:1.Two forms of the generalized nonlinear Schr(?)dinger equation(GNLSE)of the pulse transmission equation in the mid-infrared band are established and the corresponding solution methods are given.At the same time,the gain term is introduced into the time domain GNLSE to realize the GNLSE with gain.The research is mainly based on the time-domain GNLSE and solved by the split-step Fourier method.The effects of pulse width,pulse peak power,fiber length and gain on the SSFS effect are compared,and the negative correlation between the pulse width and the SSFS effect is obtained.The conclusion that pulse peak power,fiber length and gain are positively related to the SSFS effect.2.Based on the theoretical results,we built a 2?2.9?m wavelength tunable fiber laser based on cascaded fibers,and studied the possibility of realizing 2.8?4?m wavelength tunable fiber laser through theoretical simulation.First of all,through the combination of hybrid mode locking and dispersion management technology,a weak Kelly sideband soliton with a center wavelength of 1950.7 nm and a pulse width of692.9 fs is generated;the thulium-doped fiber amplifier(TDFA)is used as a primary amplification and synchronous wavelength expansion to achieve the Raman soliton with the center wavelength of 1985.2?2294.7 nm,and when the center wavelength is 2163.5nm,has the shortest pulse width of 434.8 fs.The expanded pulse is injected into a 94mol.%doped germanium core fiber,and the wavelength of 2054.9?2862.7 nm Raman soliton is realized through the high nonlinearity of the fiber.When the center wavelength is 2862.7 nm,the narrowest pulse width is 273.61 fs.The energy is 3.94 n J,and the Raman soliton conversion efficiency is 30.5%;then the obtained?2800 nm pulse is amplified by Er³⁺ZBLAN fiber amplifier to increase the pulse energy to 9.24 n J;theoretically we predict based on fluorotellurate(TBY)The further wavelength expansion characteristics of the fiber realize the Raman soliton laser with a wavelength of 2800-4047 nm,and the shortest pulse width is predicted to be 61 fs.3.The Ho³⁺-Pr³⁺co-doped ZBLAN mode-locked fiber laser was built,and based on gold nanowires(GNWs)as a saturable absorber,a fundamental-order mode-locked pulse and a second-order harmonic mode-locked pulse with a center wavelength of2864.9 nm were generated.The pulse width and energy are 12 ps and 19 ps,3.7 n J and2.1 n J,respectively.This is the first time that a mode-locked laser based on a saturable absorption material in this band has been observed to produce harmonic mode-locking.