Precise measurement of methane in air using diode-pumped 3.4-μm difference-frequency generation in PPLN

Fast, accurate measurement of the methane mixing ratio in natural air samples using a compact solid-state 3.4-μm difference-frequency spectrometer is reported. The spectrometer employed bulk periodically poled lithium niobate (PPLN) pumped by a solitary diode laser at 808 nm and a diode-pumped monolithic ring Nd:YAG laser at 1064 nm, and a 300 cm3 volume multi-pass absorption cell with an 18m path length. The methane mixing ratio was determined by comparing the direct optical absorption measured in the sample with that measured in a reference gas at 100 torr and room temperature. Relative accuracy of better than 1 ppb (parts in 109, by mole fraction) was achieved in measurements of natural air that contained 1700–1900 ppb methane. The typical measurement time for each sample was 60 seconds. The accuracy was limited by residual interference fringes in the multi-pass cell that resulted from scattering. Without the use of reference samples, the relative accuracy was 20 ppb; it was limited by the long-term reproducibility of the spectroscopic baseline, which was affected by drift in the optical alignment coupled to changes in the ambient temperature. This work demonstrates the use of diode-pumped difference-frequency generation (DFG) in PPLN in a high-precision infrared spectrometer. Compact, room-temperature solid-state gas sensors can be built based on this technology, for accurate real-time measurements of trace gases in the 3–5 μm spectroscopic region. PACS: 07.65; 33.00; 42.60; 42.65; 42.80 Precise measurements of the global distribution of trace greenhouse gases such as CH4, CO2, and N2O provide some of the best-known constraints on their global budgets, i.e. sources to, and removal from, the atmosphere. For example, the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) operates a globally distributed network of surface air sampling sites from which more than 7000 air samples are analyzed for CH4 each year [1]. The current measurement technique, gas chromatography (GC), is robust and precise (with a relative precision of ∼ 0.1%), but it is slow, requiring approximately 15 minutes for each measurement. Expansion of the sampling network at the surface into the vertical, a necessary step to better constrain the global CH4 budget [2], would overwhelm the current analysis capacity. Infrared laser spectroscopy is a uniquely effective method for the measurement of trace gas concentrations because it combines high precision, remote sensing capabilities, and fast response. These features can benefit applications in which many gas samples are analyzed or time-dependent changes in gas concentration are monitored. Several instruments have been developed based on lead-salt diode lasers that offer detection sensitivities down to 0.05 ppb (parts in 109, by mole fraction) for several trace species in air at atmospheric or reduced pressure [3–5]. However, lead-salt diode lasers require cooling to liquid nitrogen temperatures, have problems with mode jumps and multi-mode operation, and often require a large monochromator for mode selection, which in many applications are considerable disadvantages. The need for cryogenic cooling can potentially be eliminated in infrared spectrometers based on InAsSb [6] and InGaAsSb [7] semiconductor lasers, which have recently seen considerable development. They hold the promise of potential single-frequency operation with output powers in excess of 1 mW at temperatures that can be reached with Peltier coolers. However, single-frequency, single-spatial-mode lasers are not currently available. An attractive alternative to mid-infrared diode lasers is difference-frequency generation (DFG). Difference-frequency mixing of Ar+ and dye [8], also dye and Ti:Al2O3 [9] lasers was effectively used for high-resolution infrared spectroscopy of stable molecules and short-lived free radicals. Several feasibility tests have recently indicated that DFG-based spectrometers pumped by commercial near-infrared diode lasers can cover most of the spectroscopic fingerprint region from 3 to 18 μm. In particular, typical linewidths of less than 50 MHz and output powers from 0.5 to 30 μW have been reported for diode-pumped DFG sources operating near 3 μm [10], 4 μm [11], 5 μm [12], and 9 μm [13]. In these experiments, spectroscopic measurements of methane and carbon monoxide in natural air have indicated that detector-limited sensitivity can be achieved, corresponding to minimum detectable column densities of less than 10 ppbm(Hz)−1/2. Waveguide DFG sources hold promise for