Study On Single - Frequency Yb - Doped Fiber Laser Based On Sagnac Circular Filter

The technology of single-frequency laser has very important applications in gravitational wave detection, high precision spectroscopy, optical fiber sensing, parametric oscillator, coherent beam group, and many other fields. There are several different skills for achieving single-longitudinal mode operation, the most common methods are ultrashort cavity, polarization incoherent, saturated absorption fiber, etc. This paper proposed that combined saturable absorption fiber with polarization incoherent technology. Integrating the Sagnac loop mirror filter with a simple linear cavity, and polarization controllers were introduced to realize single-frequency fiber laser. And we studied the system of1μm single-frequency fiber laser from the theory and experiments. The specific research work and the experimental results are as follows:First of all, the simulations of small signal gain of the ytterbium-doped fiber laser, power distribution in the cavity, and the optimizing length of gain fiber had been demonstrated. The mechanism of Sagnac loop mirror filter for discriminating and selecting longitudinal modes was explained in theory, and the characteristics of the filter was simulated by using Matlab.In order to suppress the spatial hole burning effect of linear cavity which would cause multi-longitudinal modes oscillation, a polarization controller was placed beside the gain fiber, so as to avoid the formation of standing wave. Due to longer length of the resonator, it can narrow the line-width of single frequency laser effectively.Then we realized the output wavelength of1064nm and1083nm single-frequency ytterbium-doped fiber laser with the pump source at975nm. The maximum output power of the single-frequency fiber laser is32mW with the pump power of120mW. The highest slope efficiency of the single longitudinal mode we achieved was33.8%. The power fluctuation was less than2%and the signal-to-noise ratio was around60dB. We used the self-heterodyne method to measure the line-width of the single-frequency fiber laser at1083nm, the measured actual line-width was5kHz.Finally, based on the principle of tuning wavelength, we tuned the temperature of FBG to change the wavelength of the single-frequency laser. At last we got the wavelength-tunalble single-frequency ytterbium-doped fiber laser with the tuning region of1064.06nm-1065.75nm. And the wavelength of output signal would red-shift around0.33nm when increasing the temperature every30centigrades. During the tuning process, the scanning F-P interferometer was used to confirm the single-longitudinal mode operation. When the temperature of system was kept stable, it was the single-frequency operation.