Research On 2?m Fiber Laser Based On The Combination Of Lyot Filter And NALM

In recent ten years,2 ?m fiber laser has become the focus of scientific research because of its unique advantages in the fields of medical treatment,material processing,laser communication and atmospheric monitoring.Multi-wavelength operation and mode-locked operation are two widely used operating states in 2 ?m fiber laser.In order to realize the multi-wavelength state and the mode-locked state in a fiber laser,it is necessary to flexibly control the longitudinal mode spacing in the laser resonator.So there has been great technical difficulties.In this thesis,Lyot filter and nonlinear amplifier loop mirror(NALM)technology are combined to design and realize a 2 ?m fiber laser which can produce multi-wavelength laser and mode-locked pulse laser.By appropriately changing the length of the polarization-maintaining fiber,the fiber laser can achieve single-wavelength and dual-wavelength switchable mode-locked pulsed laser output.The laser has an important application prospect in scientific research,remote sensing,space optical communication and other fields.In this thesis,the basic principles of Lyot filter and NALM are analyzed.Combined with the filtering characteristics of Lyot filter and the intensity dependent loss characteristics of NALM,a design scheme of 2 ?m fiber laser based on the hybrid structure of Lyot filter and NALM is proposed.Based on the proposed scheme,a 2 ?m fiber laser experimental device is set up in this thesis,which uses a double clad thulium doped fiber as the gain medium,a 793 nm semiconductor laser as the pump source,and two extrusion polarization controllers(PC)and a section of polarization-maintaining fiber to form a Lyot filter.By adjusting the state PC and the pump power,the multi-wavelength and mode-locked states in the laser are studied in detail.The experimental results show that when the fiber laser works in the multi-wavelength state,NALM plays the role of amplitude equalizer(AE),which can effectively suppress the mode competition effect in the laser cavity,and the output multi-wavelength flatness is good.The laser can form stable laser with 15 wavelengths(within full width at half maximum),and the wavelength interval is 0.93 nm.When the laser operates continuously for an hour in the multiwavelength state,the maximum and minimum drift of the wavelength are 0.45 nm and 0.10 nm,respectively.And the wavelength can be freely tuned in the range of 2012.00?2039.00 nm.When the fiber laser works in the mode-locked state,the function of NALM changes into a saturable absorber(SA).The central wavelength and width of spectrum of the mode-locked pulse laser are 2025.15 nm and 19.57 nm,respectively.The pulse width,repetition frequency and pulse energy are 1.76 ns,4.66 MHz and 9.33 n J,respectively.The signal-to-noise ratio is 40 d B.Under the condition of continuous operation for an hour in the mode-locked state,the shift of the central wavelength and the change of the spectral width of the laser spectrum are 0.40 nm and 0.71 nm,respectively.The change of signal-to-noise ratio is only 0.04 d B.The repetition frequency remains unchanged.This shows that the laser can run stably for a long time under mode-locked state.In addition,by adjusting the length of polarization maintaining fiber in Lyot filter,switchable single-wavelength and dual-wavelength mode locking operation are realized.