Research On 2μm Band Multi-wavelength-tunable Thulium-doped Fiber Laser Based On Novel Filter Device

Multi-wavelength Thulium-doped fiber laser(MWTDFL)operates at the 2 μm band,locating at eye safety band and in the atmospheric absorption window with the transmittance over 80%.Those make this band widely used in free space communication,environmental monitoring,optical fiber sensing and other fields.In addition,the MWTDFL based on the novel all fiber filter has strong anti-electromagnetic interference ability,high stability,simple structure,easy implementation,and it is an ideal light source for laser space communication.Under the support of National Natural Science Foundation of China,based on the principle,design,and manufacture of all fiber interferometer,the 2 μm band wavelength-tunable MWTDFL has been studied theoretically and experimentally in detail.The main innovative achievements are listed as follows:1.A MWTDFL based on cascaded single-mode-four-mode-single-mode fiber interferometer(SFS-IF)is proposed and verified.The transmission characteristics of cascaded SFS-IF based on different lengths of four-mode fibers are simulated and analyzed.Since the four-wave mixing effect,the strong mode competition in the cavity is suppressed.By adjusting the polarization state in the cavity,from dualwavelength to eight-wavelength operations can be obtained tunably,and the number and spacing of wavelengths can be switched flexibly.31 lasing lines in the range of1973.89～1995.99 nm can be obtained by optimizing the designed filter.By monitoring the laser within 50 min,the wavelength drift is less than 0.02 nm,and the peak power fluctuation is less than 0.706 d B.2.A MWTDFL based on four-mode fiber Sagnac ring filter is proposed and verified.The transmission characteristics of Sagnac ring filter are simulated and analyzed by transfer matrix analysis method,and confirmed by experimental results.By adjusting the polarization state in the cavity,eight single-wavelength operations can be obtained,with the optical signal-to-noise ratio(OSNR)of 55 d B and the tunable range of 28.51 nm.By changing the polarization state in the cavity,five switchable dual-wavelength operations can be obtained,with the maximum and minimum wavelength intervals of21.78 and 1.50 nm,respectively.3.A wavelength-interval-switchable MWTDFL based on nonlinear dual-pass MachZehnder interferometer(NDP-MZI)filter is proposed and verified.By analyzing NDP-MZI filter using transfer matrix analysis method,the filter can switch different wavelength intervals,and has the function of power equalization to suppress wavelength competition.By adjusting the polarization state in the cavity,20 lasing lines with opposite phases and the same wavelength interval(0.46 nm)can be switched flexibly.The wavelength drift is less than 0.04 nm,and the peak power fluctuation is less than 0.612 d B within 60 min.By changing the polarization state in the cavity,60 lasing lines with an adjacent wavelength interval of 0.23 nm can also be obtained.4.A wavelength-switchable single-longitudinal-mode(SLM)Thulium-doped fiber laser based on three-channel superimposed fiber Bragg grating(FBG)is proposed and verified,and enabled by a compound-subring-cavity and an unpumped TDF-based saturable absorber.By adjusting the polarization state in the cavity,three singlewavelength in SLM operation can be obtained and switched flexibly.By monitoring the output spectra within 50 min,wavelength drift is less than 0.01 nm,and the peak power fluctuation is less than 0.641 d B.5.A MWTDFL with a high OSNR and stability is proposed and experimentally demonstrated at 2.05 μm.A superimposed polarization-maintaining FBG is utilized as a multi-channel narrow bandwidth filter to select the lasing wavelengths in the laser cavity in the 2 μm band.Polarization hole burning effect can effectively suppress the wavelength competition caused by uniform gain broadening in the thulium-doped fiber.By adjusting the polarization state in the cavity,the tunable output of single-,double-,three-and four-wavelength operations can be obtained.The OSNRs of four switchable single-wavelength operations are as high as 72 d B at pump power of 2.89 W.Wavelength drifts are less than 0.01 nm and peak power fluctuations are less than0.162 d B within 50 min.In addition,four combinations of dual-wavelength operations are obtained,and the polarization states between each dual-wavelength lasing were orthogonal.The OSNRs are higher than 68 d B,and the polarization extinction ratios are higher than 33 d B.When the gain and loss are balanced in the laser cavity,the proposed laser also has the capability of four three-wavelength operations and one four-wavelength operation.