High-power-seed femtosecond long-wave infrared difference-frequency generators.

We present a high-power-seed difference-frequency generator (DFG) system using two signal pulse trains with the same power level from two optical parametric oscillators (OPOs). Compared to the conventional signal-idler DFG system, our scheme can avoid the efficiency drop of long-wave infrared (LWIR) pulses at longer wavelengths due to the power drop of the OPO at a near-degeneracy wavelength. Moreover, neither the pump nor the seed waves of our DFG would be affected by the atmospheric absorption. Experimentally, femtosecond pulses tunable over 5-20 μm, with up to 62 mW power and 23% quantum conversion efficiency (QCE), are obtained at a repetition rate of ∼82 MHz. Over 20 mW power and nearly 20% QCE can be obtained at a central wavelength of 13 μm. This represents the highest value at such a high repetition rate. Our scheme represents the first, to the best of our knowledge, experimental demonstration of high-repetition-rate DFG systems with the seed and pump waves having the same power level.