Theoretical Research On Ultrashort Pulse Amplifier Based On Normal-Dispersion Active Optical Fiber

Ultrashort pulse lasers have played an important role in military,communications,medical treatment and fine processing fields because of its remarkable advantages such as high repetition rate,wide spectrum and high peak power.Consequently,ultrashort-pulse fiber lasers with high pulse energies have attracted much attention because of their broad application prospects.Ultrashort pulse fiber lasers with high peak powers are desired in practical researchs and productions.For the high-peak-power ultrashort pulse lasers,the saturable absorbers in fiber lasers require very high damage threshod.The problem can be solved by using fiber power amplifiers.In this manuscript,the numerical model for ultrashort-pulse fiber amplifier is established.The program codes for simulation,taking into account the rate equations and pulse propagation equation,are obtained by using software MATLAB.The evolution of pulse in the normal-dispersion fiber amplifier is numerically analyzed detailedly.In this case,the model takes into account the effect of the variation of gain spectrum on the pulse amplification during.The main conclusions of this work are as follows:(1)In the process of pulse amplification in the normal-dispersion fiber,the limited gain bandwidth can limit the spectral broadening of parabolic pulse,and degrade the linear chirp,which leads to an asymmetric pulse;The shift of center frequency of the seed pulse has a considerable effect on the output pulse.The main reason is that the tuning of the relative position of the gain spectrum in the Er-doped fiber and pulse spectrum can lead to different levels of amplification for different frequency components.(2)For the seed pulses with the same pulse energy,the pulse with shorter width in temporal domain will lead to an output pulse with narrower spectral width.The main reason is that in the fiber amplifier with the same active-fiber length,the initial pulse with higher peak power has a stronger GVD-induced broadening,which can make the spectral broadening and amplification weaker;The initial chirp of seed pulse can shorten the temporal range of the linear chirp.Compared the seed pulse with the negative chirp,the initial pulse with the same and positive chirp will have a greater negative effect on the formation of parabolic pulse.(3)The third-order dispersion in the optical fiber amplifier can lead to pulse distortion.If the third-order dispersion is negative,the leading edge of the pulse exhibits oscillations,and the center of the pulse drifts toward the leading edge.If the third-order dispersion coefficient is positive,the effect on the pulse is opposite.Moreover,the stronger the effect of group velocity dispersion is,the weaker the effect of third-order dispersion is.The more the nonlinear coefficient is,the stronger the effect of third-order dispersion is.