Tunable mid-infrared generation using synchronized programmable fiber lasers

Mid-infrared lasers find interesting applications in laser-based countermeasure technologies, remote sensing, maritime/terrestrial awareness and so on. However, the development of laser sources in this spectral region is limited. We present here an alternative solution to the mid-infrared laser which is based on difference-frequency generation (DFG) in a nonlinear crystal pumped by synchronized and tunable near-infrared fiber lasers that are commercially available. This idea is not new and has been explored by other groups, but the latest innovations in near-infrared fiber lasers have enabled the creation of fast-scanning picosecond fiber lasers. One such picosecond system is the synchronized programmable laser from Genia Photonics that can combine two picosecond fiber laser systems in which both output pulses are synchronized at the DFG crystal. The first laser was continuously tunable from 1525 nm to 1600 nm and one million different wavelengths can be scanned within one second. For the second fiber laser, its wavelength was fixed at 1080 nm. In principle, the DFG in a PPLN crystal could produce a tunable mid-infrared source spanning from 3.32 mu m up to 3.7 mu m. Other and wider tuning ranges are possible with different choices of pump wavelengths. For the PPLN crystal used in this work, the DFG phase-matching window for a fixed temperature was 2.6 nm wide and was broad enough for our 25 ps pulse train having a spectral width of 0.25 nm. The quantum efficiency achieved for the DFG was 44% at the maximum power available.