| The paper studies the second harmonic generation (SHG) of fiber laser from the construction of fiber laser and nonlinear optic theory. The structure of double-clad fiber and the resonant cavity are introduced, and a common fiber resonates and pump technologies are studied. The rate equation theory of fiber laser is learned, the performance of fiber laser is simulated by the parameter of fiber lengths, pump power etc. The beam quality factor and the key technologies are introduced in high power fiber laser.The coupled wave equations are introduced from the nonlinear polarization, and the SHG related expresses are got. The phase match and the matching angle of nonlinear crystal are researched; the best phase matching is introduced with nonlinear coefficients, the acceptance bandwidth of wavelength, angle and temperature are given. The new quasi-phase is introduced by periodically poled crystal. The very strong and useful software SNLO is studied, and simulating the SHG progresses with different crystals.The characters of common crystals are compared and analyzed, and finally the KTP crystals are selected in extra cavity with high power fiber laser, which has 25W average power, and 18kW peak power and 65ns pulse width with 20 kHz repetition rate. The power of green laser achieved is 2W in the experiment, the conversion efficiency is about 8%, and the structure of fiber laser is learned; the experimental results are analyzed from bandwidth and the beam characters, the results offer valuable information for further research.The damage of KTP crystals in experiments is studied, the progresses of damage are analyzed from microscopic view, it shows that the defects in the crystal will absorb a lot of energy when under strong radiation and form gray-track. In the experiment all the damaged KTP crystals are cracked or catastrophic break in the exit end, the phenomena is analyzed from repetition, pulse width etc. of incident beams. The suggestion is given to avoid the damage of KTP crystals. |
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