Broadband Mid-IR Light Sources From Difference Frequency Generators Based on a 2-mm-Long Aperiodically-Poled Lithium-Niobate Crystal

We present a difference frequency generator (DFG) based on a 2-mm-long MgO-doped aperiodically-poled lithium-niobate (APPLN) crystal with a linearly-varying poling-period. The DFG was pumped by a femtosecond fiber-laser system with a repetition-rate of 53.4 MHz. It produced broadband mid-infrared light source having an instantaneous-bandwidth covering 1.9–4.6 μm, with an average power of 30 mW. Moreover, by numerical simulations, we revealed that due to the relatively high intensity of the incident signal wave, an APPLN crystal with a length of a few milli-meters was sufficient to produce efficient and broadband nonlinear frequency conversion in a high-repetition-rate, ultra-short-pulsed DFG system.