Compact laser difference-frequency spectrometer for multicomponent trace gas detection

Design, performance characteristics, and application of a room temperature mid-infrared laser spectrometer are reported. This compact instrument is based on differencefrequency mixing of a widely tunable external-cavity diode laser and a diode-pumped monolithic Nd:YAG ring laser in periodically poled lithium niobate (PPLN). The differencefrequency tuning range of 3.98 μm to 4.62 μm was sufficient for detection of several atmospheric trace gases including carbon monoxide (CO), nitrous oxide (N2O), carbon dioxide (CO2), and sulfur dioxide (SO2). Real-time detection of CO, N2O, and CO2 was performed in open air over a path length of 5 to 18 m. The feasibility of DFG spectroscopic measurement of the 13C/12C and 18O/17O/16O isotopic ratios in atmospheric carbon dioxide was also investigated. We report what to our knowledge is the first simultaneous spectroscopic measurement of all three isotopes of oxygen in ambient CO2. PACS: 07:65; 33.00; 42.60; 42.65; 42.80 Application of laser difference-frequency generation (DFG) [1] to high-resolution spectroscopy of methane was first reported by Pine [2]. His experiment demonstrated not only the viability of a new method for generation of tunable midinfrared light, but also its potential benefits to applications such as trace gas detection, chemical analysis, and industrial process monitoring. However, the use of Ar+ and dye lasers as DFG pump sources in the field did not appear feasible because of their large size, fragility, and high power consumption. Simon et al. [3] first obtained a tunable 4.7-μm differencefrequency output by mixing room-temperature diode lasers at 690 nm and 808 nm. Low output power (3 nW) in their experiment was not sufficient for use in high-resolution spectroscopy, but it demonstrated that diode lasers were suitable DFG pump sources. In a later experiment, Simon et al. [4] increased the difference-frequency output power to the microwatt level with the use of cavity-enhanced signal wave, at which point both high-resolution spectroscopy and sensitive trace gas detection appeared feasible. Subsequent tests by Petrov et al. [5] proved the feasibility of fast measurement of methane in ambient air to better than 12 ppb (parts in 109, by mole fraction). From this work it became clear that diode-pumped DFG gas sensors must meet two requirements in order to become an attractive choice for use in field applications. First, each sensor must be versatile enough to detect multiple gas species without interference from water vapor, either in-situ or over a long open path in air. This mandates a substantial tuning range, typically several hundred wavenumbers (Fig. 1), combined with an output power in excess of 10 μW cw. Second, the sensors must have a rugged optical construction in a small package, combined with low power consumption for portability. Goldberg et al. [6] used high-efficiency and wideband quasi-phase-matching properties of bulk periodically poled lithium niobate (PPLN) for 3.3–4.1 μm tunable cw DFG with up to 0.5 mW output power, demonstrating coverage of the major portion of the wavelength region in Fig. 1. Balakrishnan et al. [7] reported the use of two high-power tunable diode lasers as pump sources in a PPLN-based Fig. 1. Mid-infrared wavelength coverage by difference-frequency mixing of a tunable diode laser and a 1064.5 nm Nd:YAG laser. Goldberg et al. [6] demonstrated coverage of the 3.0–5.5 μm range by DFG in bulk periodically poled LiNbO3. MMH is monomethyl-hydrazine, N2H3CH3