Research On The Characteristics Of The High-power Diode-Pumped Yb,Na:CaF₂ Laser

All-solid-state lasers based on ytterbium-doped crystals, have attracted great attention recently, because they can be pumped directly by laser diodes (LD), and generate laser pulses at the wavelength of 1 micron with high power, large energy and ultrashort duration. These lasers make it possible to achieve low-cost and compact femtosecond laser sources, which will find important applications in industry and scientific research.In this dissertation,the novel Yb,Na:CaF₂ crystal based on China's independent research and development is studied as the gain material, and issues and characteristics of high power and femtosecond laser operations are investigated, including the cavity design, impacts of the thermal effect, the slope efficiency, and the mode locking dynamics. Finally, the highest continuous-wave (CW) output power of 14.5W under the same pump conditions for Yb-doped fluorides so far is achieved, and the femtosecond CW mode-locked operation is achieved. The main work and innovations in this dissertation are listed as follows:1. The developments of femtosecond laser technology and Yb-doped laser crystals are presented. The characterics and the advantages of the novel Yb,Na:CaF₂ crystal are introduced. In high-power diode-pumped lasers, the absorption saturation, the temperature distribution and the thermal lens effect of the high-power diode-pumped laser medium, are also introduced.2. A high-power and efficient continuous-wave Yb,Na:CaF₂ laser is investigated. A maximum CW output power of 14.5W with a M² factor of 2.0, is achieved for an absorbed pump power of 23W, corresponding to a slope efficiency of 63%. This is the highest CW output power at room temperature for Yb-doped fluorides so far. Besides, a tunable spectral range from 1023nm to 1073nm is achieved.3. A Yb,Na:CaF₂ femtosecond laser is experimentally investigated. A pulse train with a pulse width of 178 fs, an average output power of 460mW, a spectrum centered at 1036nm, and a repetition frequency of 82.4MHz, is obtained for an absorbed pump power of 7.2W.4. The dynamics of mode-locking in the laser concerned with the intra-cavity negative group delay dispersion (GDD) are experimentally investigated. The relationships between the dispersion, the pulse width, the output power, and the operation status, are acquired. By optimizing the intra-cavity negative GDD from 2000fs² to 2700fs² per round trip, a shortest pulse duration of 175fs and a highest average power of 510mw are obtained in the experiments, respectively.5. In a high-power diode-pumped laser, issues that impact on the slope effiencies (defined according to the incident pump power and the absorbed pump power respectively), are investigated, including the absorption saturation, the gain saturation, the changes of the pump spectrum and the thermal lens effect. Methods for correcting the slope effiency are presented, including attenuating the pump power directly to exclude the pump wavelength shift,measuring the absorbed pump power in the referance cavity or directly, simulating for confirmation and so on. The influences of the thermal lens effect on the slope efficiency are also discussed. These studies are significant for correcting and evaluating the slope efficiency of the high-power diode-pumped laser.