Synchronous And Asynchronous Dual-wavelength Mode-locked Ytterbium Doped Fiber Laser

Fiber lasers have attracted much attention in the laser community due to their unique advantages,such as stability,compact structure,high beam quality,low coupling loss and so on.With the maturity of various fiber devices,fiber lasers gradually become commercialized and are widely used in various fields.Therefore,the research on fiber lasers is always a hot topic in laser fields.Multi-wavelength lasers are widely used in fiber optical communications,optical fiber distributed sensing,laser radar and so on.In this thesis,we have demonstrated asynchronous and synchronous dual-wavelength mode-locked ytterbium-doped fiber lasers based on nonlinear polarization rotation(NPR)effect.The center wavelengths of the asynchronous dual-wavelength mode-locked laser are at 1040 nm and 1060 nm under the cavity length of ~123 m.The corresponding pulse widths are 2.12 ns and 2.52 ns while the pulse repetition rates are 1.6872 MHz and 1.6875 MHz,respectively.The center wavelengths of synchronous dual-wavelength mode-locked laser are at 1023 nm and 1072 nm with a pulse width of 1.99 ns.The repetition rates is locked at 1.6872 MHz.By adjusting the polarization controllers,the shorter wavelength laser can be tuned from 1025 nm to 1036 nm and the longer one can be tuned from 1074 nm to 1085 nm,while the interval between the two wavelengths remains unchanged.The switching between synchronous and asynchronous dual-wavelength mode-locked states,as well as single-wavelength and dual-wavelength mode-locked states can be realized without using any other complex devices.These works are important for furtherly studying the multi-wavelength mode-locked ytterbium doped fiber lasers.In this thesis,the impact of cavity length on dual-wavelength mode-locking is also studied.It's found that when the cavity length is short,synchronous dual-wavelength mode-locking can only be achieved at higher pump power,and the asynchronous dual-wavelength mode-locking is unstable.When the cavity length is long,the laser is prone to pulse splitting,which affects the mode-locking stability.Therefore,it is important to find the suitable cavity length and pump power to optimize the experiment results through experimental analysis.Synchronous multi-wavelength mode-locked fiber lasers are widely used in ultrafast laser spectroscopy,terahertz generation and detection and so on.Passive injection is one of the most effective ways for synchronizing mode-locked pulses.In this thesis,synchronization of two fiber lasers is realized by injecting a ytterbium-doped fiber laser into an erbium-doped fiber laser.It is found when the mode-locked Yb-doped fiber lasers worked at 1040 nm or 1060 nm,the cavity mismatch lengths of the two lasers are 4.5 mm and 3 mm,respectively.When the Yb-doped fiber laser worked at asynchronous dual-wavelength mode-locking state,the cavity mismatch lengths of the two lasers are 8 mm and 5 mm respectively.It can be seen that the cavity mismatch length of asynchronous dual-wavelength injection is larger than that of single-wavelength injection,and the synchronization is more stable.In this thesis,a mode-locked erbium-doped fiber laser based on single-walled carbon nanotubes and fiber focusers is also demonstrated.The nonlinear parameters of the carbon nanotube saturable absorber can be effectively controlled.Combing cross phase modulation between different-wavelength lasers with broadband nonlinear absorption of carbon nanotube saturable absorbers,we expect to achieve synchronization of two mode-locked fiber lasers at 1 ?m and 1.5 ?m.