The theory of continuous wave difference frequency generation (DFG) is studied for a mixing of two waves in a nonlinear medium, the absorption effect of the material is included and optimization conditions are studied. Trace concentrations of nitric oxide were observed in a mixture of ambient air using a DFG source with periodically poled lithium niobate (PPLN) as the nonlinear medium. A sensitivity of 300 ppb was achieved. The design and development of a fiber-coupled DFG sensor are reported. This sensor applied to the observation of carbon monoxide
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