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Retracing behaviors of phase-matching angle in optical parametric oscillators were studied theoretically. They implied that two or more sets of signal-idler outputs could be observed at certain phase-matching angles. In other words, four-wavelength or more optical parametric oscillators could be implemented in a nonlinear crystal which showed the retracing behavior. Previous research indicated that a nonlinear crystal showed the retracing behavior only when the pumping wavelength was in a specific wavelength range which was not available. In this thesis, we proposed that in a noncollinear phase- matching configuration, retracing behaviors of phase-matching angle could be observed with the pumping wavelength easily accessed. Particularly, our numerical results showed that four-wavelength optical parametric oscillators could be implemented in a BBO with the pumping wavelength at 266, 355, 532, and 1064 nm, which correspond to the fourth, third, second harmonic of a Nd:YAG laser and itself. Meanwhile, it was observed that over a quite broad signal wavelength range,the phase- matching condition could be simultaneously reached, leading to an optical parametric oscillator of broad-bandwidth signal and idler. This theoretical prediction was already proved in our lab. Optical parametric oscillators of broad-bandwidth were also studied in details.
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