Study On Ultrashort Pulse Laser Source Based On Soliton Self-frequency Shift And Their Properties

Soliton Self-Frequency Shift(SSFS),which derives from intrapulse stimulated Raman self-scattering resulting in continuous energy transmission from the higher frequency components as a pump to the lower ones.Increase of the wavelength of the soliton takes place in a way that frequency-shift per unit length is proportional to pulse width,dispersion of fiber and spectrum of Raman gain.This phenomenon provides us with a mechanism of tuning wavelength of output pulse by varying the input power.This thesis is aimed to explore different methods for ultrashort pulse-width measurement first and develop the tunable ultrashort laser source at 2?m based on SSFS and especially,raise a means to enhance the maximum frequency-shift of soliton at certain power.In the beginning,discussion is made about the applications of SSFS in tunable lase,optical analog-to-digital converter and optical buffering,and the developments of tunable laser base on SSFS to date and ways to enhance the SSFS are summarized.Then the main idea and the innovation are stated.Methods for ultrashort pulse-width measurement is given,and numerical investigations based on MATLAB are performed on interferometric autocorrelation(IAC)to show the IAC signals under different initial pulse chirps.A real autocorrelation apparatus is designed and made to test pulse-width of a certain ultrashort fiber laser and a right result is obtained compared to the reference value.Then theories of soliton and SSFS are elaborated and demonstrated,and the split-step Fourier method used to realize the numerical simulations of nonlinear Schrodinger equation(NLSE)is performed based on MATLAB.The impact on soliton transmission in fiber due to soliton order and higher order dispersion are analyzed,as well as SSFS due to dispersion coefficient,soliton order and pulse-width.Finally,we investigate the characteristics of wavelength tunability in large mode photonic crystal fiber(LMA25)pumped by ultrashort pulse at 1.55?m.131fs pulse duration of soliton is measured by IAC at 1.83 ?m.By varying the power of input pulse,a tunable range of 1.6-2.1?m is achieved at the output.Then a thulium doped fiber(TDF)is cascaded to LMA25 and integrated to form a stable tunable ultrashort laser ranged from 2.014?m to 2.172?m.This novel cascade structure brings enhancement on the maximum frequency-shift about 100-160nm of soliton compared to that in the LMA25.Further study shows that Tm3+ absorbs the residual 1.55?m pump and generates stimulated radiation at 1.9?m which enhances Raman effect to bring extra frequency-shift to the soliton.