Mid-IR difference frequency laser-based sensors for ambient CH4, CO, and N2O monitoring
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A new mid-infrared sensor platform is described, which combines difference frequency generation (DFG)-based tunable laser sources with simple direct absorption spectroscopy. DFG lasers operating in the 3–5 micron window are tuned to access a variety of species in the C–H, N–O, and C–O stretch regions. The sensors are capable of sub-ppb detection of key greenhouse gas species as well as common pollutants and tracer species. Specific examples of sensor data obtained for methane, nitrous oxide, and carbon monoxide are presented, including relevant time series data and associated Allan Variances. The platform provides a cost-effective alternative to other laser-based approaches in some cases, performing at similar or superior levels. Emphasis on achieving key performance metrics driven by World Meteorological Organization guidelines for Global Air Watch program and other applications is highlighted.
KeywordsNitrous Oxide Allan Variance Periodically Pole Lithium Niobate Difference Frequency Generation Periodically Pole Lithium Niobate Crystal
Real-time, ultra-sensitive gas analyzers are required for a wide array of applications, including pollution and greenhouse gas (GHG) monitoring. Commercial instruments based on traditional approaches such as non-dispersed infrared absorption (NDIR) and chemiluminescence are being replaced with new, laser-based analyzers that are capable of providing real-time ppb level detection of key atmospheric species. Most commercially available laser-based analyzers are based on near-infrared telecom lasers, wherein vibrational overtones are measured. However, the detection of chemical species via these bands often requires that elaborate, ultra-sensitive approaches be used to increase overall system performance, including ultra-long path astigmatic Herriott cells , resonant photoacoustic approaches , and optical cavity-based methods such as cavity ring-down spectroscopy (CRDS) and integrated cavity output spectroscopy (ICOS) [3, 4]. Alternatively, monitoring of chemical species via their associated fundamental rovibrational bands in the mid-infrared is employed, and the requisite selectivity and sensitivity for monitoring greenhouse gases, industrial pollutants, and combustion precursors, intermediates, and products have been demonstrated in numerous studies. As the absorption intensity is typically 10–10,000 times stronger in the mid-infrared compared to that of the near-infrared, simple and robust direct absorption spectroscopy can in many cases be used to reliably determine the absolute concentrations or mole fractions of many targets without the need for high finesse optical cavities or other complicated approaches.
We have recently developed a new analyzer platform that is based on difference frequency generation (DFG) mid-infrared lasers, wherein two near-infrared wavelengths are mixed in periodically poled lithium niobate (PPLN) to produce tunable middle infrared light. This approach provides narrow linewidth (~2 MHz), excellent mode quality (TEM00), and high-frequency stability (<10 MHz drift). DFG-based absorption spectrometers have been developed for decades, and in the last decade extremely sensitive instruments have been developed and successfully deployed in numerous embodiments, in some cases achieving sensitivity levels on the order of 10−10/cm/Hz1/2 . Although some research-grade DFG instruments have achieved these impressive performance levels, they have traditionally been based on relatively complex and expensive configurations that would be challenged to compete in the market with other approaches such as CRDS- or ICOS-based instruments, which have largely defined the acceptable price point for commercial analyzers. We have recently developed a relatively low-cost commercial DFG-based analyzer that, although not performing at the highest levels demonstrated for DFG-based instruments, provides a viable option to cavity-enhanced instruments at a competitive price. Moreover, our relatively simple DFG-based analyzer platform is relatively robust and general such that a wide array of species and applications are possible, including potentially open-path systems that are not practically feasible for cavity-enhanced approaches.
In this paper we demonstrate that this relatively simple DFG analyzer platform is capable of monitoring numerous important species with high sensitivity and selectivity, in some cases matching with or exceeding high performance cavity-enhanced approaches. Specifically, the O–H, N–H, and C–H stretching regions between 3 and 4 microns and the C–O and N–O stretching regions between 4 and 4.6 microns are used to measure the three primary greenhouse gases (CH4, CO2, and N2O) as well as carbon monoxide and water vapor. Results from the now commercially available versions as well as pre-market prototypes are shown here, with emphasis on demonstrating the generality, selectivity, precision, and accuracy of the systems for extended periods of time. Extension to other species that are accessible in the spectral regions can be directly inferred from these examples.
2 Instrumentation: mid-IR DFG-based analyzer platform
To tune the mid-IR wavelength, one of the semiconductor lasers is current tuned at a constant temperature, using a pre-programmed waveform generated by the digitizer card. The laser is scanned at a repetition rate of 500 Hz, typically over a 1–2 cm−1 region that includes a few fully resolved absorption lines. The sloping baseline is due to the variation of mid-IR output power as one of the lasers is current tuned and is mathematically removed before normalization of the spectrum to obtain the absorption strength. The zero baseline (i.e., detector offset) is determined at the end of each scan (current ramp) when the laser is turned off. The entire lineshapes in the scanned region are measured, while the gas is maintained at a constant flow rate and pressure is held constant via a simple PID control loop, wherein the built-in vacuum pump speed is throttled. Typically, the pressure is kept low enough such that the absorption lines are primarily Doppler broadened (between 150 and 225 mbar for the various species), simplifying potential linewidth contributions from (moreover, variations in) pressure broadening. The hermetic optical bench houses the PPLN frequency conversion stage, and the combined near-infrared lasers are ported via PM fiber through a small, sealed hole in the optical bench, with no connectors (i.e., spliced after assembly).
The entire optical tub, multipass cell, DFG laser module, and DFG control electronics are housed in an insulated and temperature-controlled thermal enclosure, which is held at slightly elevated temperature (approximately 37–38 °C). A high precision pressure transducer is used to record pressure, and multiple thermistors are placed in the system for monitoring and control. The DFG laser provides approximately 10 Watts of heating inside the enclosure, while a thermoelectric heat exchanger is used to either warm or cool the enclosure as needed. A series of thin-film heaters with simple dead-band controllers are used to heat the outer shell of the thermal enclosure, reducing the temperature differential between the insulated inside and the outside skin of the enclosure. The interior temperature is controlled at the 10 mK level under common laboratory conditions. All temperatures and relevant active variables are recorded and accessible to the user.
Active temperature control one of the near-IR lasers is used to stabilize the frequency of the MIR laser to keep the absorption line center in the same part of the current ramp, effectively removing the slow, long-term drift commonly encountered in near-infrared semiconductor lasers. Stabilizing the laser in this way precludes the need for an external wavemeter and reduces anomalies from baseline variations in the fitted data, which would otherwise occur if the line moved to different parts of the ramp over time. Absorption spectra are fit using a fixed Voigt lineshape profile, which is a safe approximation in this case as the temperature and pressure are held constant. In some cases, target molecules are measured simultaneously with water vapor such that dry mole fractions can be determined as humidity varies. The analyzer includes an internal computer, diaphragm pump, and 4-inlet gas manifold with a calibration menu that allows routine time-stamped calibration of the system at user-defined intervals. The software is windows based, and supports remote log-in functions as well as other common remote connection capabilities via USB or Ethernet ports.
3.1 Methane and water vapor
3.2 Temperature cycling and system drift
The drift apparent in these data are primarily due to the presence of residual optical fringes in the optical train, which in the case of a DFG laser can occur at three wavelengths: the pump, signal, and idler wavelengths in the 1, 1.5, and 3.3 micron regions, respectively, within different parts of the system. In this case, the 1 micron laser is fixed in frequency, and thus any fringes including and upstream of the PPLN crystal tend to be fixed and thus primarily lead to offsets that are easily calibrated. However, tuning of the 1.5 micron laser and the associated translation into the MIR can result in fast as well as slowly varying fringes that can occur throughout the optical system. Such etalons can in some cases be isolated and minimized, yet can be strongly influenced by minute temperature changes inside the optical core.
Thermal control of the lasers as well as optical core comprising the optical bench and multipass cell are key to minimizing drift due to optical fringes, which will typically have less of an effect on the extracted concentrations as long as their period is substantially either narrower or broader than the absorption linewidth. In addition to minimizing the magnitude of the fringes, the etalons should not drift appreciably over time, which means the lasers, associated drive electronics, and physical dimensions of the core must all be as stable as possible. When subjected to relatively rapid external temperature changes as in the above tests, transitory fringing is indeed the primary source of drift in the system as all of these systems reequilibrate to the new temperature, which typically takes tens of minutes. In some cases, temperature appears to correlate well with drift, but in others it appears to be anticorrelated. This phenomenon happens as the phase can change depending on whether riding up or down an associated fringe, and thus typically these drift sources cannot be removed via single or even multiple temperature measurements within the system. This phenomena appears to be prevalent at a significant level in nearly all high precision, narrow linewidth laser-based analyzers, regardless of the technique employed as, at the most basic level, all of these instruments comprise highly sensitive optical interferometers. Given that physical changes at the nm level translate into frequency scanning at the MHz level, temperature changes at the mK level are sufficient to produce a measurable effect on the spectrum when the fidelity required is at the 5,000:1 level or better. Similarly, electronics can in some cases produce measurable effects at nA (or nV) levels of drift. As such, in addition to careful design of the optical components, validation of those components performance, and careful optical alignment, thermal testing of the entire system is presently required to validate analyzer performance for ambient GHG monitoring applications.
3.3 Methane field data: Sutro tower
Field testing of a methane analyzer (Model IRIS 5500) at Sutro Tower in the San Francisco Bay Area was performed working in collaboration with Lawrence Berkeley National Lab, which has been operating tower measurements including collaborative U.S. NOAA flask analyses of methane and other long-lived greenhouse gases. The analyzer in this test was placed inside of an enclosure that enables the system to run in an environmentally uncontrolled shed, wherein it was subjected to significant temperature variation and high moisture, including condensing fog and some rain. The monitoring shed was located at the base of Sutro tower and was relatively open to the environment with partial sheltering from the elements. The enclosure was primarily used to shelter the instrument from rain and fog, while air was circulated through the enclosure for cooling. The sample air was provided from the downstream flow of an existing flask sampling system, which was connected to a draw tube that sampled air from a height of 232 m above ground level. The instrument sample flow rate was fixed at approximately 300 sccm, and the internal cell pressure was maintained at precisely 225 mbar by controlling the small internal diaphragm pump rotational speed. System calibration was achieved using the built-in gas manifold and calibration software, wherein the duration and period of the calibration cycle is set by the user. The instrument was installed in September 2011 and remained in service continuously until approximately mid-December 2011.
3.4 Carbon monoxide
3.5 Nitrous oxide
Three different trace gas analyzers based on the same mid-infrared difference frequency generation laser absorption platform were demonstrated, indicating selectivity, sensitivity, and accuracy levels commensurate with those sought for measuring key greenhouse gas and trace pollutant species. Methane, water vapor, carbon monoxide, and nitrous oxide were measured with instruments configured for operation in the 3.3 micron region for methane, and 4.5–4.6 micron region for CO and N2O. Sensitivity levels in the sub-ppb regime are all clearly indicated in the time series and associated Allan Variances, and low-drift was demonstrated across a temperature range likely to be encountered in most ambient field monitoring stations. Methane measurements were performed for an extended period of time at Sutro tower in the San Francisco Bay Area in an environmentally uncontrolled shed, and autonomous, ppb level performance was demonstrated for an extended period of time over a wide temperature and moisture range. Carbon monoxide was notably measured at 1 ppb levels in 10 s, providing a potentially new cost/performance level for laser-based sensors.
Field testing is presently underway for both N2O and CO instruments, and results to date indicate stable, autonomous operation in the field over a period of months since initial installation. In addition to these specific analyzer configurations, we are presently working on further developing the same core platform to provide instruments with unique multiple species capabilities that are not presently offered by competing approaches. Moreover, the generality and relatively low-cost of the core DFG laser engines used in these systems provide a potential high value proposition for multiple species embodiments compared with competing technologies. Finally, instruments capable of measuring isotopes of key species are also under development, further indicating an exciting future where this platform often exceeds the performance of commercially available optical cavity-enhanced instruments operating in the NIR region.
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