DFG principle
DFG sources are parametric optical frequency converter.
Most commonly, two near-IR lasers are focused into a
non-linear optical crystal to generate the difference
frequency. Periodically-poled lithium niobate (PPLN) is
one of the most efficient crystals used for mid-IR DFG
laser sources.
Over the last 9 years, Difference Frequency Generation
(DFG) sources have matured with the advances in the optical
telecommunication industry. Compact, robust, and spectral
high quality tunable fiber and diode lasers as well as
Periodically Poled Lithium Niobate crystals to convert
near-IR to mid-IR wavelengths. Today, DFG laser sources fit
in one's palm and have enabled record breaking absorption
sensitivities of 2E-7 (2E-11 cm-1 for 100 m sample path
length).