Study On Yb～(3+)-Doped Double-clad Fibers Lasers

The dissertation focuses on the Yb3+-doped double clad fiber laser. The research works in detail are shown as follows:1.With the phase-mask method, we make a fiber Bragg grating in a few-mode fiber whose cut wavelength is greater than 1060nm. The Bragg wavelengths can be simulated by using coupled-mode theory and the results agree on the experiments. Theoretical simulations show that the temperature dependant characteristics of few-mode fiber Bragg gratings are similar to those of single-mode fiber Bragg gratings. That is, each Bragg wavelength shifts as a same rule when the temperature varies and the whole spectrum can maintain its profile. In the Yb3+-doped single or double clad fiber lasers by using few-mode fiber Bragg gratings as wavelength reflectors, the linewidth of the output laser is smaller than 0.1nm, which shows that such a scheme can produce narrow linewidth laser. All of these results show that the few-mode fiber Bragg grating has its advantages in the applications of sensing, optical communication and other aspects.2.Considering the energy level structure and the spectrum characteristics of Yb3+ ion, we deduce the relationships between laser performances of Yb3+-doped double clad fiber laser and parameters of laser cavity from the steady state rate equations with a new model. We numerically analysis the changes of laser performances, such as laser power, with the parameters of laser cavity, including output mirror reflectivity, optimum length of double clad fiber , the cavity loss and concentration values of the fiber. These results are very important when optimizing the structure of a double-clad fiber laser.The variation of the temperature, stress, refractive index and laser efficiency of a high power double clad fiber laser due to the thermal effects is theoretical analyzed by using thermal conduction equations. We calculate the stress fracture limit, critical index limit and melting limit in a high power double clad fiber laser. The results show that the average output power decreases when the temperature is too high. To maintain the pumping efficiency and the output power, some cooling methods should be used when the output power reaches a certain level.3.We study how to improve the pump efficiency in a double clad fiber laser. Several traditional pumping methods are considered and the light propagation...