The Study Of Mode-locked Yb-doped Fiber Laser Based On Equivalent Saturable Absorber

The laser in the 1?m waveband has great potential for scientific research and applications such as laser distance measuring,optical coherent communication,atmospheric radar detection,image processing,etc.The main research on ultrafast optics is located on the field of passive mode-locked technology.The devices and technologies of passively mode-locked including nonlinear amplifying loop mirror(NALM),nonlinear optical loop mirror(NOLM)and nonlinear polarization rotation(NPR)technology.Therefore,it is urgent to explore the combinate effects of different optical devices on the generation of mode-locked pulses and other optical characteristics(spectra,repetition frequency,pulse duration,etc.),which have a great significance for the development and application of mode-locked fiber lasers.The mode-locked fiber laser that realizing switchable and tunable single/dual wavelength output attracts the interests of researcher who worked in many fields,and has become research hotspot.In this Thesis,we theoretically and experimentally explored the influence of Lyot filter and NALM on the realization of mode-locked wavelength switchable and tunable characteristics.The main research work of the paper are as follows:1.Single wavelength switchable mode-locked ytterbium-doped fiber laser based on fiber Lyot filter is studied.The fiber Lyot filter is fabricated by 18-cm long polarization maintain fiber,a polarizer and polarization controller(PC).According to the equation of transmittance,transmittance spectra of Lyot filter under different polarization states are simulated,which support the single and dual-wavelength switchable mode-locked in the theoretical analysis.The fiber Lyot filter and polarization-independent isolator are fused into the cavity,and the mode-locked fiber laser based on NPR technology is built.By adjusting the polarization controller under low pump power,we obtained dissipative soliton pulses with a center wavelength of 1036.6 nm,gives a bandwidth of 7.6 nm,repetition rate of 26.39 MHz,optical signal-to-noise ratio of 68 d B,and pulse duration of 6.8 ps.By increasing the pump power and adjusting the polarization controller,the center wavelength of the mode-locked spectra is switched,and continuous wave components caused by high pump power are observed in the mode-locked spectra.2.Wavelength tunable dissipative soliton mode-locked fiber laser based on NALM is studied.The NALM and the unidirectional loop are connected through a 3 d B coupler to form figure-of-eight cavity.We can obtain stable dissipative soliton within the pump power of 150-650 m W.When the pump power is 240 m W,we obtained dissipative soliton pulses with a center wavelength of 1064.1 nm,gives a bandwidth of 7.7 nm,repetition rate of 18.8MHz,optical signal-to-noise ratio of 71.2 d B,and pulse duration of 867 fs.By adjusting the PC,the dissipative soliton mode-locked state is achieved within the wavelength tuning range of 1032.8-1065.1 nm,and the mode locking is very stable.Twisting PC to make modelocked to reach a stable state under the pump power of 150 m W.Only increase the pump power,the central wavelength of the mode-locked is blue-shifted within 1037.4-1041.9 nm,so we obtained wavelength tuning range of 4.5 nm.3.Single and dual-wavelength switchable mode-locked fiber laser based on NALM is studied.In order to figure out the influence of the filtering effect on the mode-locked fiber laser,a piece of polarization maintain fiber is insert into the NALM on the basic of the previous experimental study.Therefore,a pulse switchable mode-locked fiber laser is constructed.When the pump power is 210 m W,we obtained single and double pulse switchable mode locked fiber laser by adjusting the polarization controller.Continue to increase the pump power,the pulse is split.We obtained single,double,and triple pulse switchable output by adjusting the PC in the mode-locked state.