A near-infrared laser dispersion spectrometer with phase modulation for open-path methane sensing.

A laser-based open-path dispersion spectrometer for measuring atmospheric methane has been developed with the goal of achieving a very simple architecture, yet enabling molecular dispersion measurements immune to optical power variation. Well-mature, near-infrared photonics components were retained to demonstrate a compact, cost-effective, and low-power consumption dispersion spectrometer. In particular, measurements immune to received optical power variations are demonstrated despite the use of only phase modulation and are supported by the development of the corresponding physical model. The instrument has been validated under laboratory conditions, finding a precision of 2.6 ppb 100 m for a 2 s measurement, and demonstrated through atmospheric measurements performed continuously over six days with an 86 m path length.

Long-term continuous monitoring of methane emissions at an oil and gas facility using a multi-open-path laser dispersion spectrometer.

A method for methane emissions monitoring at industrial facility level was developed based on a high precision multi-open-path laser dispersion spectrometer combined with Bayesian analysis algorithms using Monte Carlo Markov Chain (MCMC) inference. From the methane path-averaged concentrations spatially distributed over the facility under study, together with the wind vector, the analysis allows detection, localization and quantification of fugitive methane emissions. This paper describes the very first long term (3 months), continuous (24 h/7 days) deployment of this monitoring system at an operational gas processing and distribution facility. The continuous monitoring system, made of the combination of the open-path high-precision (<10 ppb) methane concentration analyser and the data analysis method, was evaluated with controlled releases of methane of about 5 kg/h for short periods of time (30-60 min). Quantification was successful, with actual emission rates lying well within the quoted uncertainty ranges. Source localisation was found to lack accuracy, with biases of 30-50 m in the direction of the line of sight of the spectrometer, due to the short duration of the controlled releases, the limited wind vector diversity, and complications from air flows around buildings not accounted for by the transport model. Using longer-term data from the deployment, the MCMC algorithm led to the identification of unexpected low intensity persistent sources (<1 kg/h) at the site. Localisation of persistent sources was mostly successful at equipment level (within ~20 m) as confirmed by a subsequent survey with an optical gas imaging (OGI) camera. Quantification of these individual sources was challenging owing to their low intensity, but a consistent estimate of the total methane emission from the facility could be derived using two different inference approaches. These results represent a stepping stone in the development of continuous monitoring systems for methane emissions, pivotal in driving greenhouse gas reduction from industrial facilities. The demonstrated continuous monitoring system gives promising performance in early detection of unexpected emissions and quantification of potentially time-varying emissions from an entire facility.

Locating and Quantifying Methane Emissions by Inverse Analysis of Path-Integrated Concentration Data Using a Markov-Chain Monte Carlo Approach.

The action to reduce anthropogenic greenhouse gas emissions is severely constrained by the difficulty of locating sources and quantifying their emission rates. Methane emissions by the energy sector are of particular concern. We report results achieved with a new area monitoring approach using laser dispersion spectroscopy to measure path-averaged concentrations along multiple beams. The method is generally applicable to greenhouse gases, but this work is focused on methane. Nineteen calibrated methane releases in four distinct configurations, including three separate blind trials, were made within a flat test area of 175 m by 175 m. Using a Gaussian plume gas dispersion model, driven by wind velocity data, we calculate the data anticipated for hundreds of automatically proposed candidate source configurations. The Markov-chain Monte Carlo analysis finds source locations and emission rates whose calculated path-averaged concentrations are consistent with those measured and associated uncertainties. This approach found the correct number of sources and located them to be within <9 m in more than 75% of the cases. The relative accuracy of the mass emission rate results was highly correlated to the localization accuracy and better than 30% in 70% of the cases. The discrepancies for mass emission rates were <2 kg/h for 95% of the cases.

Broadband gas phase absorber detection and quantification by chirped laser dispersion spectroscopy at 1.55†µm.

The demonstration and first evaluation of chirped laser dispersion spectroscopy (CLaDS) for quantitative measurements of gas molecules with broad spectral features is reported. The demonstration is conducted on propyne (methyl acetylene) gas, using a widely tunable external cavity near infrared laser, λ ≈ 1.55 µm, whose frequency can be swept at 2.6 MHz/µs. A direct baseband downconversion scheme is implemented to recover molecular dispersion, with a cost-effective 32 GHz radio frequency architecture. Laboratory tests demonstrate in particular the value of laser dispersion spectroscopy for the sensing of turbid media with a large range of variations, owing to a significant immunity of the detection scheme to variations in received optical power. Normalized minimum concentration measurable in the 1.5 ms scan is ∼0.7 ppm.m.√Hz.

Trace gas analysis with laser dispersion spectroscopy.

Trace gas analysis provides a wide range of insights into environmental processes, particularly with regards to global warming and air quality. With the urgent need to identify sources and accurately measure the harmful emissions negatively impacting our planet, Laser Dispersion Spectroscopy (LDS) offers a unique approach. LDS technology measures optical molecular dispersion via a differential phase measurement of light and, operating in the mid-infrared, provides highly sensitive and robust measurements. This enables highly precise, real-time gas measurements even in adverse environmental conditions such as rain, fog, snow or dust. The technology can be used in both extractive and open-path formats, with real-world applications including emissions monitoring on oil and gas sites, measuring the impact of agricultural activities and monitoring carbon capture storage facilities.

Hollow waveguide-miniaturized quantum cascade laser heterodyne spectro-radiometer.

A miniature thermal infrared laser heterodyne spectro-radiometer based on hybrid optical integration is demonstrated. A quantum cascade laser emitting at 953 cm-1 (10.5 µm) is used as the local oscillator. Integration is achieved using hollow waveguides inscribed in a copper substrate, with slot-encapsulated optical components positioned to maintain fundamental hybrid mode coupling. The demonstrator performances are studied in the laboratory and show a noise level within 1.6 times of the ideal case. Atmospheric high-resolution transmittance spectroscopy of carbon dioxide and water vapor in solar occultation is demonstrated. The total column concentrations are derived as well as measurement uncertainties, 399.5 ± 2.2 ppm for CO2 and 1066 ± 62 ppm for H2O. The miniature laser heterodyne spectro-radiometer demonstration opens the prospect for nanosatellite-based high spectral resolution thermal infrared atmospheric sounding.

Hollow waveguide integrated laser spectrometer for 13CO2/12CO2 analysis.

Using hollow waveguide hybrid optical integration, a miniaturized mid-infrared laser absorption spectrometer for 13CO2/12CO2 isotopologue ratio analysis is presented. The laser analyzer described focuses on applications where samples contain a few percent of CO2, such as breath analysis and characterization of geo-carbon fluxes, where miniaturization facilitates deployment. As part of the spectrometer design, hollow waveguide mode coupling and propagation is analyzed to inform the arrangement of the integrated optical system. The encapsulated optical system of the spectrometer occupies a volume of 158 × 60 × 30 mm3 and requires a low sample volume (56 µL) for analysis, while integrating a quantum cascade laser, coupling lens, hollow waveguide cell and optical detector into a single copper alloy substrate. The isotopic analyzer performance is characterized through robust error propagation analysis, from spectral inversion to calibration errors. The analyzer achieves a precision of 0.2‰ in 500 s integration. A stability time greater than 500 s was established to allow two-point calibration. The accuracy achieved is 1.5‰, including a contribution of 0.7‰ from calibrant gases that can be addressed with improved calibration mixtures.

Spectrally resolved thermal emission of atmospheric gases measured by laser heterodyne spectrometry.

The demonstration of thermal infrared quantum cascade laser heterodyne spectrometry to resolve local thermodynamic equilibrium molecular emission lines from earth's atmospheric constituents is presented. The instrument is described, as well as the early steps towards radiometric calibration. Room temperature ethylene emission line measurements carried out in the laboratory are used to validate the instrument. High-resolution (0.02 cm-1) emission lines from atmospheric carbon dioxide and water vapor are then recorded in a zenith-looking configuration and compared to a radiative transfer model and ideal instrument model expectations.

Demonstration and characterization of ultrafast laser-inscribed mid-infrared waveguides in chalcogenide glass IG2.

The first demonstration and characterization of ultrafast laser-inscribed mid-infrared (mid-IR) waveguides in Ge33As12Se55 chalcogenide glass (IG2) is presented. From mode profile and throughput measurements, combined with modelling, the characteristics of the waveguides inscribed in IG2 are studied at 7.8 µm, and compared to those of waveguides inscribed in gallium lanthanum sulfide for reference. Two methods to estimate the local variation of refractive index induced by the inscription process are presented, which indicate a variation of ~0.010 to 0.015 across the inscription parameters investigated. This variation, together with a higher robustness of the material to inscription and large transparency covering the entire mid-IR spectral domain, suggest that IG2 has great potential for integrated optical applications in the mid-IR developed through the ultrafast laser inscription method.

Broadband standoff detection of large molecules by mid-infrared active coherent laser spectrometry.

A widely tunable active coherent laser spectrometer (ACLaS) has been demonstrated for standoff detection of broadband absorbers in the 1280 to 1318 cm-1 spectral region using an external cavity quantum cascade laser as a mid-infrared source. The broad tuning range allows detection and quantification of vapor phase molecules, such as dichloroethane, ethylene glycol dinitrate, and tetrafluoroethane. The level of confidence in molecular mixing ratios retrieved from interfering spectral measurements is assessed in a quantitative manner. A first qualitative demonstration of condensed phase chemical detection on nitroacetanilide has also been conducted. Detection performances of the broadband ACLaS have been placed in the context of explosive detection and compared to that obtained using distributed feedback quantum cascade lasers.

Immediate biofeedback for energy balance via expired breath δ(13)CO2.

Expired breath δ(13)CO2 measured in real time serves as a useful biomarker of altered macronutrient metabolism in response to changes in energy balance. Altered breath δ(13)CO2 is believed to be a result of changes in macronutrient oxidation and the kinetic isotope effect where enzymatic processes discriminate against metabolites naturally enriched with (13)C. Use of breath δ(13)CO2 as a rapid biofeedback of energy balance status will enhance an individual's ability to modify behavior during weight loss efforts. Herein we describe a novel approach for immediate biofeedback for energy deficit using a moderate exercise challenge. Our new mid-infrared isotope ratio-meter for δ(13)CO2 is a step toward miniaturization of a personal device for instant biofeedback for people attempting to lose weight.

Pump and probe spectroscopy with continuous wave quantum cascade lasers.

This paper details infra-red pump and probe studies on nitric oxide conducted with two continuous wave quantum cascade lasers both operating around 5 µm. The pump laser prepares a velocity selected population in a chosen rotational quantum state of the v = 1 level which is subsequently probed using a second laser tuned to a rotational transition within the v = 2 ← v = 1 hot band. The rapid frequency scan of the probe (with respect to the molecular collision rate) in combination with the velocity selective pumping allows observation of marked rapid passage signatures in the transient absorption profiles from the polarized vibrationally excited sample. These coherent transient signals are influenced by the underlying hyperfine structure of the pump and probe transitions, the sample pressure, and the coherent properties of the lasers. Pulsed pump and probe studies show that the transient absorption signals decay within 1 µs at 50 mTorr total pressure, reflecting both the polarization and population dephasing times of the vibrationally excited sample. The experimental observations are supported by simulation based upon solving the optical Bloch equations for a two level system.

Analysis and demonstration of atmospheric methane monitoring by mid-infrared open-path chirped laser dispersion spectroscopy.

Atmospheric methane concentration levels were detected using a custom built laser dispersion spectrometer in a long open-path beam configuration. The instrument is driven by a chirped distributed feedback mid-infrared quantum cascade laser centered at ~1283.46 cm-1 and covers intense rotational-vibrational transitions from the fundamental ν4 band of methane. A full forward model simulating molecular absorption and dispersion profiles, as well as instrumental noise, is demonstrated. The instrument's analytical model is validated and used for quantitative instrumental optimization. The temporal evolution of atmospheric methane mixing ratios is retrieved using a fitting algorithm based on the model. Full error propagation analysis on precision gives a normalized sensitivity of ~3 ppm.m.Hz-0.5 for atmospheric methane.

Waveguide-enhanced 2D-IR spectroscopy in the gas phase.

A method for obtaining high-quality 2D-IR spectra of gas-phase samples is presented. Time-resolved IR absorption spectroscopy techniques, such as 2D-IR spectroscopy, often require that beams are focused into the sample. This limits the exploitable overlapped path length through samples to a few millimeters. To circumvent this limitation, 2D-IR experiments have been performed within a hollow waveguide. This has enabled acquisition of 2D-IR spectra of low-concentration gas-phase samples, with more than an order of magnitude signal enhancement compared with the equivalent experiment in free space. The technique is demonstrated by application to the 2D-IR spectroscopy of iron pentacarbonyl.

Middle infrared active coherent laser spectrometer for standoff detection of chemicals.

Using a quantum cascade laser emitting at 7.85 µm, a middle infrared active coherent laser spectrometer has been developed for the standoff detection of vapor phase chemicals. The first prototype has been tested using diffuse target backscattering at ranges up to ~30 m. Exploiting the continuous frequency tuning of the laser source, spectra of water vapor, methane, nitrous oxide, and hydrogen peroxide were recorded. A forward model of the instrument was used to perform spectral unmixing and retrieve line-of-sight integrated concentrations and their one-sigma uncertainties. Performance was found to be limited by speckle noise originating from topographic targets. For absorbers with large absorption cross sections such as nitrous oxide (>10(-19) cm(2)·molecule(-1)), normalized detection sensitivities range between 14 and 0.3 ppm·m·Hz(-1/2), depending on the efficiency of the speckle reduction scheme implemented.

Intracavity widely-tunable quantum cascade laser spectrometer.

A grating-tuned extended-cavity quantum cascade laser (EC-QCL) operating around 7.6 µm was assembled to provide a tuning range of ~80 cm⁻¹ with output power of up to 30 mW. The EC-QCL output power was shown to be sensitive to the presence of a broadband absorbing gas mixture contained in a 2-cm cell introduced inside the extended laser cavity. In this arrangement, enhanced absorption relative to single path linear absorption was observed. To describe observations, in the QCL rate-equation model was included the effect of intracavity absorption. The model qualitatively reproduced the absorption behavior observed. In addition, it allowed quantitative measurements of mixing ratio of dimethyl carbonate, which was used as a test broadband absorber. A number of alternative data acquisition and reduction methods were identified. As the intracavity absorber modifies the laser threshold current, phase-sensitive detection of the laser threshold current was found to be the most attractive way to determine the mixing ratio of the absorber. The dimethyl carbonate detection limit was estimated to be 1.4 ppmv for 10 second integration. Limitations and possible ways of improvements were also identified.

Coherent transient spectroscopy with continuous wave quantum cascade lasers.

A high power continuous wave quantum cascade laser operating around 1900 cm(-1) has been used to conduct Lamb dip spectroscopy on a low pressure sample of NO. The widths of the Lamb dips indicate that the laser linewidth is 800 ± 60 kHz and the power sufficient to induce significant population transfer of up to 35%. While the Lamb dip signals are symmetric at low laser chirp rates, they become increasingly asymmetric as the chirp rate increases, further confirming the significant degree of population transfer. In addition rapid passage structure on the Lamb dip signal is observed after the weak probe beam is swept through the line center. This structure is sensitive to both the probe chirp rate and the underlying hyperfine structure of the rovibrational transition, and is accurately modeled using the optical Bloch equations.

Atmospheric vertical profiles of O3, N2O, CH4, CCl2F2, and H2O retrieved from external-cavity quantum-cascade laser heterodyne radiometer measurements.

Atmospheric vertical profiles of ozone, nitrous oxide, methane, dichlorodifluoromethane, and water are retrieved from data collected with a widely tunable external-cavity quantum-cascade laser heterodyne radiometer (EC-QC-LHR) covering a spectral range between 1120 and 1238 cm(-1). The instrument was operated in solar occultation mode during a two-month measurement campaign at Rutherford Appleton Laboratory in Oxfordshire, UK, in winter 2010/2011, and ultrahigh-resolution (60 MHz or 0.002 cm(-1)) transmission spectra were recorded for multiple narrow spectral windows (~1 cm(-1) width) specific to each molecule. The ultrahigh spectral resolution of the EC-QC-LHR allows retrieving altitudinal profiles from transmission spectra that contain only few (1-3) significant absorption lines of a target molecule. Profiles are validated by comparing with European Centre for Medium-Range Weather Forecasts operational atmospheric profiles (ozone and water), with other data in the literature (nitrous oxide, methane, dichlorodifluoromethane), and with retrievals from a lower resolution (600 MHz or 0.02 cm(-1)) Fourier transform spectroscopy data that were also recorded during the measurement campaign.

Signal-to-noise ratio in chirped laser dispersion spectroscopy.

Quantitative studies and experimental validation of noise sources occurring in chirped laser dispersion spectroscopy (CLaDS) are reported. Their impact on the signal-to-noise ratio (SNR) achievable with the CLaDS sensing method is analyzed through a noise model supported by experimental results. In particular the model shows that the SNR is optimal for a given value of the laser chirp rate. The experimental studies are conducted with a quantum cascade laser operating at 5.2 µm for the detection of nitric oxide. Optical fringing has been found to be a significant non-random source of noise and an effective reduction method that can improve the SNR is also discussed.

Hollow waveguide photomixing for quantum cascade laser heterodyne spectro-radiometry.

An integrated optic approach, using hollow waveguides, has been evaluated for a compact, rugged, high efficiency heterodyne optical mixing circuit in the middle infrared. The approach has involved the creation of hollow waveguides and alignment features for a beam combiner component in a glass-ceramic substrate. The performance of the integrated beam combiner was tested as part of a full laser heterodyne spectro-radiometer in which a quantum cascade laser local oscillator emitting at 9.7 µm was mixed with incoherent radiation. The performance has been evaluated with both cryogenically-cooled and peltier-cooled photomixers demonstrating consistent detection limits of two and five times the shot noise limit, respectively. The hollow waveguide mixer has also shown advantages in temporal stability, laser spatial mode cleansing, and reduced sensitivity to optical feedback.