Widely-tunable mid-IR frequency comb source based on difference   frequency generation
  We report on a mid-infrared frequency comb source of unprecedented tunability covering the entire 3-10 {\mu}m molecular fingerprint region. The system is based on difference frequency generation in a GaSe crystal pumped by a 151 MHz Yb:fiber frequency comb. The process was seeded with Raman shifted solitons generated in a highly nonlinear suspended-core fiber with the same source. Average powers up to 1.5 mW were achieved at 4.7 {\mu}m wavelength. 
Axel Ruehl, Alessio Gambetta, Ingmar Hartl, Martin E. Fermann, Kjeld   S. E. Eikema, and Marco Marangoni
physics.optics