The Investigation Of Single Longitudinal Mode Tm³⁺-doped Fiber Laser

As the2μm band is eye-safe area and in the atmosphere of the weak absorptionband, it has very important application in the field of laser radar, environmentalmonitoring, medical diagnostics and treatment, laser ranging, and spectroscopy. What’smore, because the single-frequency2μm laser has irreplaceable advantage in opticalcommunication, precision measurement, precision spectroscopy, coherent radar, laserremote sensing and the nonlinear conversion, it has been the hotspot of current research.Compared to traditional solid-state lasers, the fiber laser system is simpler and smallerand has narrower linewidth, which let it become the main direction of single-frequencylaser.In our research, we sum up the general method to achieve single-frequency outputand analyze the method of the ring cavity with the saturable absorber to achievesingle-frequency output, and theoretical linewidth limit in single longitudinal modeconditions according to the classical rate equation and the energy level relationship.According to the energy level structure of the Tm³⁺doped fiber, we choose theEr³⁺/Yb³⁺co-doped fiber laser as the pump light whose wavelength is1570nm.Respectively, we go both theoretically and experimentally analysis of the Er³⁺/Yb³⁺co-doped fiber laser and have developed the dual FBG linear cavity with the L-bandlaser maximum output power is709mW.Used the1.5μm band coupler in2μm band, when low coupling as the output, thethreshold is35.5mW, and the slope efficiency is8.73%, and when high coupling as theoutput the threshold is243.1mW and the slope efficiency is10.3%.At last we add the saturable absorber which length is7m, and near the threshold（pump power is680mW）, we observe a single longitudinal mode output by Fabry-Perotinterferometer which has output power is5mW to10mW beating. But the modehopping of the fiber laser is serious. At the end we discuss the cause of the modehopping and the orientation of future experiments.