Theoretical Analysis On Rational Actively Mode-locked Thulium-doped Pulse Laser

2 ?m band pulsed laser shows a promising prospect in the application of medical,defense,machinery processing and optical communications and other fields.The most frequently used mode-locking method of actively mode-locked fiber laser is to make the parameters of laser cavity periodically changed by introducing external signal.Due to the limitation of the modulator frequency,the repetition frequency of the output laser pulse is limited and high repetition rate ultrashort pulse cannot be realized.Rational harmonic mode-locking technique is an improved scheme based on modulation mode-locking technique.In this paper,the rational number actively mode-locking method is studied,and the theoretical model of the rational number actively mode-locking thulium doped pulse laser is established,and the influence of parameters on the output pulse characteristics of the mode-locked laser is analyzed through simulation in the hope of narrowing the output laser pulse width as well as improving the pulse repetition frequency,then the method of solving the limitation of modulation frequency is discovered and the 2 ?m ultrashort laser pulse based on rational number actively mode-locked is obtained.The main points of this paper are as follows:1,Establish a numerical simulation model of rational actively mode-locked thulium-doped pulse fiber laser.The model is composed of three parts:model of thulium-doped fiber amplifiers,pulse propagation model in common single mode fiber and model of modulator.And the functions of the three parts in the formation of 2 ?m rational harmonic pulse laser is analyzed in detail.2,The waveform stability of 2 ?m pulse laser is analyzed both in time domain and frequency domain under different frequencies.Simulation results show that high repetition rate mode-locked pulses output can be achieved with small modulation frequency.The modulation frequency of laser output 18 times in time domain is fm =3.6fc,which is 3.6 times of fundamental frequency.And the stability of laser in frequency domain is analyzed,results show that the stability of rational actively mode-locked method is worse than actively mode-locked method,and the stability decreases with the increase of order.3,Variation characteristics of ultrashort pulse output power,width and energy parameters of rational actively mode-locked thulium doped fiber laser with active fiber length,doping concentration,pump power and nonlinearity of single mode fiber and other parameters are discussed.Simulation results show that the output power increases linearly with the increase of doping concentration as well as pulse width narrowing within a certain degree of thulium doping.Increasing the length of thulium doped fiber within certain range can increase the output pulse power and reduce the pulse width.The nonlinear effect of single mode fiber mainly affects output pulse width,and the narrowest pulse width can be obtained by optimizing the length of fiber.Increasing the pump power and the modulation depth of modulator is conducive to the improvement of laser pulse power and narrowing of pulse width.