Study On Pulse Width Of High Power Acousto-optic Q-switched Yb-doped Fiber Laser

The high-power nanosecond pulsed fiber laser has the characteristics of high peak power,high single pulse energy and narrow pulse width,and has important applications in industrial processing,laser communication and other fields.To obtain high peak power pulses,compressing the pulse width is an effective way.In this paper,the output pulse width of acousto-optic Q-switched pulsed fiber laser with MOPA structure is studied theoretically and experimentally.The main content of this article is as follows:Firstly,the output pulse width expression is deduced from the laser rate equation.There is no analytical solution from the expression analysis.Therefore,the numerical value is set to obtain a series of numerical solutions.To the conclusion,the pulse width is proportional to the photon lifetime in the cavity and ?nin/?nt is almost inversely proportional.Furthermore,the effects of parameters such as pump power,AOM performance,cavity length,fiber grating reflectivity,and modulation frequency on the Q-switched pulse width are analyzed theoretically and the influence of the fiber amplifier on the pulse width is analyzed.Then,an acousto-optic Q-switched fiber laser was built for theoretical analysis and verified by experiments.The experimental results show that when the other parameters are not changed,the pulse width of the resonant cavity output decreases with increasing pump power,and finally tends to remain unchanged;when the output coupling of the acousto-optic modulator is placed in the first-order diffraction position,the pulse The width is narrower than the zero-order diffraction position;the shorter the resonant cavity length,the narrower the output pulse width,but when shortening the length of the gain fiber,it has an optimal length,making the output pulse width narrowest;the output fiber grating reflectivity The higher the pulse width of the output,the narrower;when the parameters of the resonator remain the same,the higher the acousto-optic Q-switch modulation frequency,the narrower the pulse width of the laser output.Experiments and theoretical analysis are basically the same.According to the experimental results,the parameters of the resonator were optimized,and an acousto-optic Q-switched all-fiber laser was built.When the modulation frequency was 20 kHz,the narrowest measured output pulse width was 54 ns,and the 3 dB bandwidth at the center wavelength of 1064 was less than 0.4 nm.Finally,in order to further test the amplifying characteristics of the output pulse of the optimized acousto-optic Q-switched fiber laser and the influence of the amplifier on the pulse width,a two-stage power amplifier was built using the MOPA structure.Among them,the power pre-amplifier pre-amplifies the signal optical power,and the signal optical power is amplified to 4 W.Three gain fibers with different lengths are intercepted.It is found that the pulse broadening degree is larger as the gain fiber is lengthened.The amplifier slope efficiency increases as the pump power increases,but the final amplification efficiency tends to be flat.After the signal light is amplified by the pre-amplifier,the output pulse is broadened,but the pulse sequence is stable and the pulse shape is unchanged,and the spectrum is basically unchanged except for the power.In the main power amplifier,the signal light power is further amplified,and the output pulse width is evenly broadened as the output power increases.When the modulation frequency is 170 kHz,the signal light average power is amplified to 177.5 W,and the pulse width is broadened from 160ns to 300 ns.However,there is no change in pulse shape and sequence,and the 3 dB bandwidth of the output spectrum at the center wavelength of 1064 is less than 1 nm.