Difference frequency generation in periodically poled lithium niobate and its use in the detection of atmospheric methane

Continuous wave mid-infrared radiation at 3.36 μm (power ≈ 18 μW) has been produced by difference frequency generation of the fundamental of a YAG laser at 1064 nm (220 mW) with a fibre pigtailed DFB diode laser operating at 1560 nm (50 mW) in periodically poled lithium niobate. The idler radiation has been used to make real-time measurements of ambient methane present in our laboratory using wavelength modulation spectroscopy via the P(4) multiplet transitions within the ν3 band. We demonstrate a minimum detectable concentration of 47 ppb over a path length of 0.71 m in a 1 s observation time. Measurements of the pressure broadening and shift parameters for these lines are presented for methane in the presence of air, nitrogen and argon.