Measurement of /sup 13/CH/sub 4///sup 12/CH/sub 4/ ratios in air using diode-pumped 3.3 /spl mu/m difference-frequency generation in PPLN

Methane has a significant effect on the radiative balance of the troposphere and stratosphere because it has a strong absorption at 7.66 /spl mu/m where carbon dioxide and water absorb only weakly. Accurate measurements of the abundance of methane are thus of considerable interest to climatologists. Previously, we demonstrated a spectrometer capable of producing fast, accurate measurements of the methane mixing ratios in natural air samples. This system demonstrated a relative accuracy of less than 10/sup -9/ mol/mol (1 ppb by volume), which is comparable to what can be achieved using non-spectroscopic methods. This system employed bulk periodically poled lithium niobate pumped by a solitary diode laser at 808 nm and a diode pumped monolithic ring Nd:YAG laser at 1064 nm, and a multi-pass absorption cell with 18 m pathlength. We generated approximately 1 /spl mu/W of infrared radiation at 3019 cm/sup -1/ using 21 mW from a grating-tuned extended-cavity diode laser at 806 nm and 382 mW from a diode-laser-pumped Nd:YAG laser at 1064 nm to pump a periodically poled lithium niobate crystal. Because this system is compact, uses all solid-state lasers, and could be made transportable, it has potential applications for field measurements where rapid, nondestructive concentration and isotope ratio measurements are needed. Even though it is not competitive with state-of-the-art nonoptical instruments, this system can measure /sup 13/CH/sub 4///sup 12/CH/sub 4/ ratios at CH/sub 4/ concentrations of only 1.6/spl times/10/sup -6/ mol/mol without gas processing.