Broadband mid-infrared coherent light source from fiber-laser-pumped difference frequency generators based on cascaded crystals.

We present difference frequency generators (DFGs) using cascaded PPLN crystals, each with a distinct poling-period, as the parametric gain medium. We show that the phase-matching bandwidth of cascaded crystals is the combination of that of each individual crystal. In the non-phase-matched section of cascaded crystals, there exists periodic backward and forward frequency-conversion processes. Nonetheless, we demonstrate that such a periodic back-conversion process would not compromise the parametric gain bandwidth of cascaded nonlinear crystals. By using two PPLN crystals with poling periods of 31 µm and 29 µm, we experimentally obtained mid-infrared light sources having instantaneous-bandwidth covering 2.8-3.9 µm, which was roughly twice as much as that from a system based on a single crystal. Moreover, our numerical results show that light sources with an instantaneous-bandwidth covering 2.5-5 µm could be obtained by cascading more crystals. This scheme represents a promising technical route to transform conventional DFGs into a device capable of generating spatially-coherent light emission with very broad instantaneous-bandwidth.