Study Of All-normal Dispersion Fiber Lasers And Automatic Mode-locking Techniques

Compared with Q-switched pulses and electric-driven laser pulses,mode-locked pulses have narrower duration and higher peak power.Therefore,they have been widely used in supercontinuum generation,nonlinear microscopic imaging,optical frequency comb,THz wave generation,precision machining and so on.Mode-locking fiber lasers have drawn much research interest for their high conversion efficiency,low threshold,compact structure and excellent thermal management contrast to their solid-state counterparts.One of the most important research fields of the mode-locked fiber lasers is achieving high pulse energy.Limited by the soliton area theorem,the pulse energy of the conventional soliton fiber lasers only reaches 0.1 nJ.The stretched-pulse fiber lasers designed by dispersion map can promote the pulse energy to nJ level.Operating in self-similar regime or all-normal-dispersion(ANDi)regime,the lasers can obtain higher pulse energy.ANDi fiber lasers have attracted extensive attention since they can provide high pulse energy with large nonlinearity tolerance,free of dispersion compensation and easy alignment.Nonlinear polarization rotation(NPR)has been successfully applied in all kinds of pulsed fiber lasers as an equivalent saturable absorber.NPR has many advantages over other types of saturable absorbers,such as wavelength insensitivity,fast response time,high damage threshold and no degradation under long time illumination.In this thesis,all the work is about the operating characteristics of ANDi fiber lasers based on NPR.The main contents of the thesis are as follows:1.The pulse propagation equations in fibers are overviewed,including nonlinear Schrodinger equation in standard fibers,Ginzburg-Landau equation in gain fiber and coupled mode equations in birefringent fibers.The numerical solution steps of the pulse propagation equations are listed in details.A linear polarization representation method is proposed to solve the coupled mode equations.This method is verified by numerical simulations.Compared with circular polarization representation method,this method can allow larger step to accelerate the calculation process and save the calculation time.2.Based on the coupled Ginzburg-Landau equations and Jones matrix,a spectral transmission formula is derived theoretically,which indicates that the whole laser cavity can work as an equivalent filter because of the fiber birefringence.The peak transmission wavelength,the modulation depth and the average transmission can be adjusted by rotating the waveplates.The bandwidth has an inverse proportional relationship with the fiber birefringence.With the equivalent filter,the fiber lasers can operate in multiwavelength mechanism and tune the central wavelength.The coupled Ginzburg-Landau equations are numerically simulated by the linear polarization representation method under different fiber birefringence.The simulation results indicate that the larger the fiber birefringence is.the smaller the stability region will be.Therefore,the fiber birefringence should be considered in laser design.3.A cross-splicing method is proposed to compensate the fiber birefringence.Based on this method,an NPR-mode-locked polarization-maintaining fiber ring laser is demonstrated.The characteristics of the laser are simulated numerically.Simulation results indicate that with this method,the laser can deliver stable dissipative solitons:otherwise no stable pulses can be achieved.In experiments,when the beat number NB is compensated to 35,stable dissipative solitons are obtained with pulse energy of 2.1 nJ.If adopting co-splicing,only continuous wave and weak modulation phenomenon generate in the laser cavity.4.A magneto-optical polarization controller at 1 ?m is designed to set up an ANDi fiber laser.In order to provide large-enough adjustable magnetic field for magneto-optical crystal,a magnetic yoke is made with silicon steel and copper coil.The relationship between the magnetic field strength and the electric current is measured by Hall sensor.The Q-switched mode-locking regime and continuous-wave mode-locking regime of the ANDi fiber laser are studied.Stable dissipative solitons are obtained with pulse energy of 3.34 nJ at average power of 213 mW.The long-term stability of the laser is continuously tested,which indicates that the mode-locking states is maintained over 5 hours with no obvious change in spectrum and the power fluctuation of only 0.6%.5.The automatic mode-locking techniques of passively mode-locked fiber lasers are studied.A noval automatic mode-locking method is proposed to realize the self-starting of NPR-mode-locked ANDi fiber lasers.The principle,state criterion and implementation of the proposed method are analyzed systematically.The characteristics of the first order reflection light of the birefringent filter are analyzed theoretically and experimentally.With this method,the lasers can restore to the mode-locking state from any initial states within minutes.At mode-locking state,the output pulses are dissipative solitons.6.The noise-like pulses from NPR-mode-locked ANDi fiber lasers are investigated.In experiments,the noise-like pulses are obtained when no birefringent crystal is inserted in the cavity.The average power of 4.3 W is obtained at pump power of 11.3 W.corresponding to optical-optical conversion efficiency of 38%.The energy of noise-like pulses envelop is 104 nJ with 3dB bandwidth of 47.8 nm.The dependence of spectral width and output power on pump power is studied.The characteristics of noise-like pulses are simulated by calculating the coupled Ginzburg-Landau equations.The simulation results are consistent with the experimental results.7.The proposed automatic mode-locking method is applied in noise-like pulse fiber lasers.A Lyot birefringent filter with bandwidth of 25 nm is inserted into the laser cavity.The light filtered out from the Lyot filter works as feedback signals.With this method,the noise-like pulse fiber laser can be started automatically.