High-repetition-rate ultrashort-pulse optical parametric oscillator continuously tunable from 28 to 68 µm

We describe a synchronously pumped femtosecond optical parametric oscillator based on periodically poled ${\mathrm{LiNbO}}_3$ that is broadly tunable in the mid infrared. A transmission window of periodically poled lithium niobate beyond the conventionally accepted infrared absorption edge of $5.4 \mathit{\mu }\mathrm{m}$ has been exploited to produce idler pulses that are tunable across a wavelength range of $4 \mathit{\mu }\mathrm{m}$, with milliwatt-level output powers at wavelengths as long as $6.8 \mathit{\mu }\mathrm{m}$. We also present experimental tuning results that are in good agreement with the theoretical phase matching predicted from published infrared-corrected Sellmeier equations for ${\mathrm{LiNbO}}_3$.