METHANE DETECTION WITH A NARROW-BAND SOURCE AT 3.4-MU-M 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 t hat is coincident with a fundamental nu(3) rovibrational absorption of methane at 3.43 mu m. To generate this midinfrared wavelength, an inj ection-seeded 1.06-mu m Nd:YAG laser is difference frequency mixed wit h first Stokes light generated in a high-pressure methane cell (1.06 - -> 1.54 mu m) to result in light at a wavelength of 3.43 mu m, that is , the nu(1) Raman active frequency of methane (similar to 2916.2 cm(-1 )). With a modest-energy Nd:YAG laser (200 mJ), a few millijoules of t his midinfrared energy can be generated with a pulse width of similar to 7 ns (FWHM). The methane nu(1) frequency can be pressure tuned over 8-32 atm (corresponding to similar to 13 GHz) and scanned across part of the v(3) P(10) rovibrational level of methane, resulting in a peak measured methane absorption coefficient of 4.2 cm(-1) atm(-1). (C) 19 96 Optical Society of America