Difference-frequency generation in PPLN at 4.25 mu m: an analysis of sensitivity limits for DFG spectrometers

We report difference-frequency generation (DFG) in periodically poled lithium niobate (PPLN) around 4.25 mu m using a cw Nd:YAG and an injection-locked diode laser. This system provides a narrow linewidth source at 4.25 mu m with near-shot-noise-limited operation. A conversion efficiency close to the theoretical limit is obtained. Detection of CO2 absorption spectra is demonstrated and further improvements and applications to high sensitivity spectroscopy are discussed.