Research On Multi-wavelength Thulium-holmium Co-doped Fiber Laser

Multi-wavelength fiber lasers have been researched extensively in recent years for itsattractive advantages of multiple wavelengths, narrow line-width, high stability and tunablewavelength spaces, of which the multi-wavelength fiber lasers around2μm can achievepotential applications in atmospheric transmission, gas sensing, ladar and newwavelength-division-multiplexed (WDM) fiber communication systems. Hence, the researchof switchable narrow-linewidth multi-wavelength fiber ring lasers around2μm hassignificance to the above fields. Under the support of National Natural Science Foundationof China, this thesis is devoted to study the related technology of multi-wavelength fiberlaser around2μm, including theoretical analysis and experiment research. The mainachievements of this thesis are listed as follows:(1) A simplified mode beating filter model based on multimode fiber (MMF) ispresented, and its filtering transmitted spectrum is simulated by the Rsoft, which shows thatthis model can be used as a multi-wavelength filter. A switchable multi-wavelength thuliumholmium co-doped fiber (THCDF) ring laser based on the spatial mode beating filter isproposed and experimentally demonstrated. We use the dual-stage amplification systembased on the Er3+-Yb3+co-doped fiber as the1570nm pump source, THCDF with a lengthof2m is used as the gain medium, thus single-,dual-and triple-wavelength appeared underthe room temperature by adjusting the angle of polarization controller(PC). Differentwavelength spaces are obtained by inserting different length of MMF. Moreover, thewavelength stability performance is tested, which shows that the single-wavelength powerfluctuation is less than0.4dB, while dual-wavelength power fluctuation less than0.6dB.(2) A switchable multi-wavelength thulium holmium co-doped fiber ring laser based onthe polarization maintaining fiber Sagnac filter is proposed and experimentally demonstrated.Comb filter effect is produced by the Sagnac loop which is constituted by a3dB coupler andpolarization maintaining fiber (PMF). A new mechanism to enhance the polarization holeburning effect is analyzed which is exploited by a segment of ordinary THCDF coiled roundthe plates of the three-loop polarization controller. The THCDF coils introduce stress in theTHCDF and therefore a change in the refractive index. About7wavelengths simultaneouslasing with a spacing of0.65nm is achieved. (3) A novel comb filter based on a dual-pass Mach-Zehnder interferometer (MZI)incorporating a Sagnac loop mirror consists of one2×23-dB coupler and a segment ofmultimode fiber is proposed and theoretical analyzed. The transmission spectra of theproposed filter reveal multi-peaks features: a envelope peak with long period containsmulti-peaks with short period, thus shows that filter effect is overlapped by the reflectionspectra of the Sagnac interferometer and the dual-pass MZI. A switchable multi-wavelengththulium holmium co-doped fiber ring laser based on the above filter is proposed andexperimentally demonstrated. We have experimentally demonstrated high-performancedual-wavelengths simultaneous oscillations with tunable wavelength spaces, higherside-mode suppression ratio, and lager output power.(4) A theoretical introduction of saturable absorber around2μm based on standing-wavesaturation effects as a transient fiber Bragg grating (FBG) is presented. The factors affectingthe bandwidth and the reflectivity of the transient FBG are analyzed. The linewidth andreflectivity as the function of doped fiber length and doping concentration werecorrespondingly simulated by Matlab software. This work can be used as a reference fortransient Bragg fiber gratings formed by unpumped doped fiber near2μm...