We report on difference-frequency-generation (DFG) in periodically-poled lithium niobate (PPLN) around 4.3 μm, using as pumping sources an injection-locked diode laser, tunable around 852 nm, and a diode-pumped Nd-YAG laser at 1064 nm. Peculiar features of the infrared generated radiation are a linewidth of about 100 kHz, amplitude fluctuations close to the quantum limit and a high conversion efficiency. Details of such a source configuration are given and potential applications to high sensitivity spectroscopy are discussed. With this set-up we were able to observe sub-Doppler lineshapes in the fundamental vibro-rotational band of CO2. A characterization of these derivative profiles is reported.
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