The Research On Pulse Characteristics Of Acousto-optic Q-switched Fiber Laser

High power pulsed fiber laser has been widely used in many fields such as optical fiber sensing,material processing,military and national defense because of its advantages of good beam quality,high conversion efficiency and high reliability.To obtain peak power,compression of pulse width and increase of pulse energy are the most effective ways.At the same time,due to the existence of multi-peak pulse phenomenon,the processing effect and efficiency of laser in the field of precision machining are seriously affected,so it is of great significance to study the smoothness of output laser pulse.In this paper,the output pulse characteristics of acousto-optic q-switched fiber laser are studied theoretically and experimentally.The main research is as follows:1.The q-switched technology of nanosecond pulse fiber laser and its related theories are studied.The effects of pump power,core diameter,gain fiber length and output transmittance on output pulse width are analyzed by using the rate equation theory of fiber laser.The effect of fiber amplifier on pulse characteristics is also studied.2.The seed source system of asymmetric acousto-optic q-switched fiber laser is built and studied and analyzed according to the above theory.The experimental results show that the pulse width of the fiber laser decreases with the increase of the pump power,and finally tends to a constant value.In a certain range,the output pulse width decreases as the length of the resonator decreases,and reaches a minimum value.However,as the length of the resonator continues to decrease,the pulse width starts to increase again.Therefore,there is an optimal length,making the output pulse width the narrowest.Increasing the reflectivity of fiber Bragg grating will also cause the output pulse width to narrow.When the parameters in the resonator remain constant,the larger the repetition frequency of the acousto-optic modulator,the narrower the pulse width of the seed source output.3.Further study is made on the pulse characteristics of the amplification stage of the acousto-optic q-switched fiber laser and the changes of the pulse characteristics during the amplification process,and the fiber laser with the first-stage power amplification structure is built.The experimental results show that the output pulse width increases with the increase of the amplifier pump power.When the length of the gain fiber is cutdown step by step,the pulse broadening amplitude is smaller.The reasons of pulse width broadening are analyzed from many aspects.At the same time,a MOPA structured pulsed fiber laser was built,which enriched the study on the output pulse characteristics of acousto-optic q-switched amplification stage.4.Based on the experimental system of asymmetric resonator seed source,the multi-peak phenomenon of the output pulse of the system is studied,and the reason of this phenomenon is analyzed experimentally,and the solution is put forward.The experimental results show that the multi-peak number of pulses can be reduced and the pulse output waveform can be optimized by increasing the pump power,reducing the repetition frequency and reducing the duty cycle ratio of AOM driving signal.The smoothness of the output pulse of the asymmetric resonator seed source system is improved.