Fast sensitive trace gas detection with a portable solid-state mid-infrared laser sensor

The purpose of this work was to construct a portable solid-state gas sensor based on tunable laser difference frequency generation (DFG), and to achieve detection limits of < 10 ppb (parts in 10⁹, by mole fraction) for several atmospheric trace gases. Earlier feasibility tests indicated that this can be done with the use of periodically poled lithium niobate (PPLN). Its quasi-phase-matching properties can be tailored for 2 to 5 μm DFG with near-infrared diode lasers. This, along with large nonlinearity and good optical quality, makes PPLN the ideal mixing material for DFG applications targeted at species like CO, N₂O, CO₂, SO₂, H₂ CO and CH₄. Reported herein are the design, construction, and performance testing of an all-solid-state DFG sensor for carbon monoxide. The sensor employs a 750 mW diode-pumped monolithic Nd:YAG ring laser at 1064.5 nm (signal), and a 100 mW Fabry-Perot diode laser at 865 nm (pump)