Methane detection with a narrow-band source at 3.4 μm based on a Nd:YAG pump laser and a combination of stimulated Raman scattering and difference frequency mixing

We report the characterization of a 10-Hz pulsed, narrow-band source that is coincident with a fundamental v₃ rovibrational absorption of methane at 3.43 μm. To generate this midinfrared wavelength, an injection-seeded 1.06-μm Nd:YAG laser is difference frequency mixed with first Stokes light generated in a high-pressure methane cell to result in light at a wavelength of 3.43 μm, that is, the n1 Raman active frequency of methane (~2916.2 cmȒ1). With a modest-energy Nd:YAG laser (200 mJ), a few millijoules of this midinfrared energy can be generated with a pulse width of ~7 ns (FWHM). The methane v₁ frequency can be pressure tuned over 8-32 atm (corresponding to ~13 GHz) and scanned across part of the v₃ P(10) rovibrational level of methane, resulting in a peak measured methane absorption coefficient of 4.2 cm⁻1 atm⁻1.