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We overview recent experimental progress in cascading multiple stages of half-harmonic generation of femtosecond frequency combs starting from a 1-μm pump. We have achieved stable operation with efficiencies as high as ~64%, pulses as short as three optical cycles at 4 μm, and output powers as high as 2.6 W at 2 μm. Our recent numerical and analytical studies of nonlinear dynamics and different operation regimes of femtosecond OPOs indicate a path toward achieving even higher efficiencies and shorter pulses. |
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