| In recent years,with the expansion of the laser wavelength to the long wave,the mid-infrared fiber laser has gradually played an important role in the fields of biomedicine,gas detection and military jamming,and is known as one of the three main development directions of the laser in the future.So far,the longest wavelength that can be achieved in the silica-based fiber is 2μm.The method for achieving long wave output by nonlinear effect does not depend on rare earth doped ions,which brings the wavelength flexible and tunable.Because there is no radiative transition,it has the advantages of high quantum efficiency and low heat effect.However,at present,the developed high-power fiber laser output wavelength is 1μm,and it is difficult to transfer the power to the long wave effectively through nonlinear effect.In view of the above problems,this thesis focuses on the generation and efficient amplification of 2μm high power nanosecond fiber lasers,and applies them to stimulated Raman scattering and optical parametric oscillation.The main research contents are as follows:1.The simulation model of thulium doped fiber was constructed,and the influence of fiber length and cross relaxation in the process of efficient amplification was studied through the simulation model.Then,by optimizing the algorithm structure and considering the time domain factor,the simulation program of the nanosecond pulse amplification in the fiber was realized.Through this program,we studied the change of pulse shape in the amplification.2.A passively Q-switched laser based on Cr2+:Zn Se was built in the experiment.The pulse width and the repetition rate were 640 ns and 61 k Hz respectively by using fiber loop mirror and shortening cavity length.The power was amplified to 41 W subsequently by cascade MOPA amplification system,and tried to use it for plastic cutting.3.The gain-modulated nanosecond pulse laser source was attained experimentally.The pulse distortion and the modulation instability were observed in its subsequent amplification.The cause of the 10 ns spike in the front of the pulse was revealed by the simulation and experiment.Then,combining with pulse shaping technology,the distortion was effectively controlled and the ideal nanosecond square pulse was realized.4.The gain-modulated pulse laser obtained above was used in the stimulated Raman scattering experiment.The performances of different PRF,pulse width and pulse shapes in Raman conversion were compared.Finally,the second order Raman laser was obtained at 2.42μm,which is the farthest Raman laser wavelength that realized in silica-based fiber by now.When the passive Q-switched laser was used for parameter conversion,it was found that the long pulse laser would cause damage to the Zn Ge P2 due to the serious heat accumulation.The work in this thesis can provide theoretical and experimental reference for the generation and efficient amplification of 2μm nanosecond pulse,and provides a meaningful empirical method for its nonlinear application. |
