Study On Nonlinear Polarization Rotary Mode-lock Fiber Laser And Its Pulse Width Characteristics

In recent years,passive mode-locked fiber lasers have been widely used in optical communications,military defense,precision manufacturing,biological microscopy and many other fields due to their narrow pulse width,high peak power,simple structure and low cost.The methods for obtaining narrow pulses include saturated absorber mode locking,nonlinear polarization rotary mode locking,figure 8 ring mirror mode locking,etc.,the nonlinear polarization rotary mode-locked fiber lasers have an indispensable position in many mode-locked fiber lasers because of ultra-fast response speed and high output stability.In order to realize the output of narrow pulse width and high repetition laser pulse,this paper mainly conducts experimental research on nonlinear polarization rotary mode-locked fiber laser and its pulse characteristics.Several near-infrared nonlinear polarization rotation(NPR)mode-locked fiber lasers were constructed,and the effects of intracavity dispersion and device polarization degree on pulse width and other characteristics were analyzed,and a variety of pulse types were obtained.The main research contents are as follows:(1)A theoretical model of passive mode-locked fiber laser was established.The process of passive mode locking was introduced,and the working principle of nonlinear polarization rotary mode locking was analyzed in detail.The transmission equation of pulsed laser in optical fiber was introduced,and the evolution of pulse under the influence of dispersion and nonlinear characteristics was elaborated,and the characteristics of soliton,broadened pulse,self-similar pulse,dissipative soliton and noise-like soliton based on this equation were analyzed.(2)Experimental study of nonlinear polarization rotary mode-locked ytterbium fiber laser under positive dispersion.A laser pulse with a spectral center of about 1035 nm,a pulse width of 350 ps,a repetition rate of 23 MHz and a spectral width of 7.2 nm was preliminarily obtained.By using a space-discrete optical isolator with higher polarization accuracy,the dissipative soliton pulse output was obtained,and a laser pulse with a spectral center of about 1030 nm,a pulse width of 126 ps,a repetition rate of 27 MHz,and a spectral width of 11.4 nm was realized.Finally,negative dispersion was introduced by adding a grating pair to the laser system,further compressing the pulse width to 202 fs.(3)Experimental study of nonlinear polarization rotary mode-locked erbium-doped fiber laser under negative dispersion.A laser pulse with a spectral center of about 1560 nm,a pulse width of 1.01 ps,a repetition rate of 32 MHz and a spectral width of 6.1 nm was preliminarily obtained.By exchanging the position of the space-discrete opto-isolator and the half-waveplate,the polarization degree of the laser system was increased to narrow the pulse width,and a laser pulse with a spectral center of about 1537 nm,a pulse width of 610.6 fs and a spectral width of 23 nm was obtained.Further,by twisting the angle of incidence of the opto-isolator to increase the polarization degree of the laser system,the output pulse width was further narrowed,resulting in a soliton pulse with a pulse width of 168 fs and a spectral center around 1543 nm,showing two significant peaks,at 1556 nm and 1531 nm,with a repetition rate of 32 MHz.Under this system,noise-like pulses,pulse splits,multi-pulse splits,etc.were observed.