Study On All Polarization-maintaining Femtosecond Fiber Amplification System With High-Power Ultrashort Laser Pulses

The fiber optic femtosecond laser system,which takes LMA-YDCF as its gain medium,has the advantages of a high average power,an all-fiber production,a compact structure,a low cost and a long-term stability.Based on this fact,the development ofall-fiber femtosecond laser systemwith LMA-YDC F has a high value in military,industrial and medical fields.This paper aims at the study on the full polarization-maintaining Yb³⁺doped fiber femtosecond laser amplification system with a high power and high quality,and meanwhile focuses on other issues like the the spectrum modulation during the amplification,as an achievement of which,it obtains an optimal scheme of improving the average power and quality of output pulse.In the study,it is found out that the spectrum modulation and nonlinear chirp accumulation are the key factors that cause the quality degradation of the output pulse.Therefore,by selecting the appropriate polarization maintaining fiber combiner fabrication process and optimizing the amplifier stage structure,the problem of pulse spectrum modulation is solved,which also reduced the accumulation of nonlinear chirp and improved the quality of output pulse.The main work of this paper is summarized as follow.First of all,it summarized the development history of ultrashort pulse lasers and amplifiers,introduced the common techniques of femtosecond fiber lasers and amplifiers,and analyzed the general situation and development trend of domestic laser market.Secondly,from the aspect of the rate equation and the nonlinear Schrodinger equation of Yb³⁺doped fiber,it introduced the algorithm for solving the above equations,namely the step-Fourier algorithm and the fourth order Runge-Kutta algorithm.In the meantime,it discussed emphatically about the effect of spectrum modulation on amplified output respectively under the condition of Gaussian and super-Gaussian type in the input pulse spectrum.Thirdly,it briefly introduced the structure of all-fiber amplifier and its core devices,analyzed the application and classification of fiber combiners,presented the two kinds of manufacturing techniques of fiber-optic combiner which refers to the end-face pump technique and side-pump technique,as well as the advantages,disadvantages and the applicable environment of the two production processes.Except for this,it analyzed emphatically on the defects of?2+1?×1 polarization maintaining beam combiner and qualitatively explained the cause of spectral modulation and time-domain lobe side lobes introduced by the beam combiner,and finally proposed two optimization and improvement schemes for structural defects.Lastly,taking the advantage of the chirped-pulse amplification technique,this papar developed a high power full polarization-maintaining femtosecond laser amplification system with dissipative soliton mode-locked femtosecond laser as the seed source.In the experiment study with a gain medium of Nufern PM 20/130,it analyzed the influence of the extended fiber length and spectral modulation on the output pulse,found out the problem of spectral modulation introduced by polarization maintaining combiner,and improved the pulse quality through the process optimization and enhancement.Afterwards,it solved the problem of spectral periodic modulation and time-domain side lobes by replacing the gain fiber?Nufern PM25/250?and polarization maintaining fiber combiner,and optimizing the amplifier structure,as an achievement of which,Finally,a high-quality ultrashort laser source with an average power of 31 W under pump power 46 W with 70%slope efficiency and a pulse duration of 249 fs at the repetition rate of 111 MHz after compression,corresponding to single pulse energy up to 180 nJ and pulse polarization ratio of 18dB,was obtained.