Research Of Square-Pulse Generation Based On Mode-Locked Yb-Doped Fiber Laser

Square wave pulse has important applications in the signal processing system,optical fiber sensing,high power laser punching etc,and square wave pulse generated in a passively mode-locked fiber laser has many advantages.In this paper,we first study the mode-locked Yb-doped fiber lasers operating in the dissipative soliton resonance region,and study the formation process of the flat-top square wave pulse by numerical study.Then,the Yb-doped fiber lasers outputting stable flat-top square wave pulses based on two different mode-locked methods are studied by experiment.In this dissertation,the main contents and achieved results are presented as follows:1.The theoretical analysis of the mode-locked Yb-doped fiber lasers operating in the dissipative soliton resonance region is carried out.The evolution of the flat-top square wave pulse is studied by numerical simulation.It is found that the width of the square wave is linearly increased as the pump power increases,while the peak power remains constant.2.The mode-locked Yb-doped fiber laser based on nonlinear polarization rotation is constructed and the output of flat-top square wave pulse is successfully realized.When the cavity length is 1501.8 m and the pump power is 543.2 mW,the maximum width and single pulse energy of the obtained square wave pulse are 812.4 ns and 42.34 nJ,respectively.The relationship between the width of the square wave pulse and the cavity length is studied.When the cavity length is shortened to 1001.8 m and 501.8 m,the square wave pulses with maximum widths of 431 ns and 164 ns are obtained,respectively.For the cavity length of 1001.8 m,the square wave pulse with width of 1.25 ?s in a specific dissipative soliton resonance region is obtained by adjusting the polarization controllers carefully.3.The mode-locked Yb-doped fiber laser based on nonlinear optical loop mirror is constructed and the cavity length is increased by a 500 m long single-mode fiber.The flat-top square wave output with pulse width of 218.4 ns is obtained.The pulse width and single pulse energy change linearly with the increase of the pump power while the peak power keeps constant,indicating that the laser is operating in the dissipative soliton resonance region.