Efficient cascaded half-harmonic generation of femtosecond frequency combs centered at 2.09 μm and 4.18 μm from a mode-locked Yb:Fiber laser

We report efficient generation of sub-50-fs pulses at 2.09 μm and 110-fs pulses at 4.18 μm from two cascaded half-harmonic PPLN-based OPOs, which are intrinsically phase-locked to the 250-MHz mode-locked fiber laser pump.