Precise methane absorption measurements in the 1.64 μm spectral region for the MERLIN mission

Non-impact effects in the absorption spectra of HCl diluted in CO2, air, and He: Measurements and predictions

Non-impact effects in the absorption spectra of HCl in various collision-partners are investigated both experimentally and theoretically. Fourier transform spectra of HCl broadened by CO2, air, and He have been recorded in the 2-0 band region at room temperature and for a wide pressure range, from 1 to up to 11.5 bars. Comparisons between measurements and calculations using Voigt profiles show strong super-Lorentzian absorptions in the troughs between successive lines in the P and R branches for HCl in CO2. A weaker effect is observed for HCl in air, while for HCl in He, Lorentzian wings are in very good agreement with measurements. In addition, the line intensities retrieved by fitting the Voigt profile on the measured spectra decrease with the density of the perturber. This perturber-density dependence decreases with the rotational quantum number. For HCl in CO2, the decrease in the retrieved line intensity can reach 2.5% per amagat for the first rotational quantum numbers. This number is about 0.8% per amagat for HCl in air, while for HCl in He, no density dependence of the retrieved line intensity is observed. Requantized classical molecular dynamics simulations have been performed for HCl-CO2 and HCl-He in order to simulate the absorption spectra for various perturber-density conditions. The density dependence of the intensities retrieved from the simulated spectra and the predicted super-Lorentzian behavior in the troughs between lines are in good agreement with experimental determinations for both HCl-CO2 and HCl-He. Our analysis shows that these effects are due to incomplete or ongoing collisions, which govern the dipole auto-correlation function at very short times. The effects of these ongoing collisions strongly depend on the details of the intermolecular potential: they are negligible for HCl-He but significant for HCl-CO2 for which a line-shape model beyond the impact approximation will be needed to correctly model the absorption spectra from the center to the far wings.

Effect of Non-Markovian Collisions on Measured Integrated Line Shapes of CO

Using cavity ring-down spectroscopy to probe R-branch transitions of CO in N_{2}, we show that the spectral core of the line shapes associated with the first few rotational quantum numbers, J, can be accurately modeled using a sophisticated line profile, provided that a pressure-dependent line area is introduced. This correction vanishes as J increases and is always negligible in CO-He mixtures. The results are supported by molecular dynamics simulations attributing the effect to non-Markovian behavior of collisions at short times. This work has large implications because corrections must be considered for accurate determinations of integrated line intensities, and for spectroscopic databases and radiative transfer codes used for climate predictions and remote sensing.

Monitoring methane emissions from oil and gas operations‡

The atmospheric concentration of methane has more than doubled since the start of the Industrial Revolution. Methane is the second-most-abundant greenhouse gas created by human activities and a major driver of climate change. This APS-Optica report provides a technical assessment of the current state of monitoring U.S. methane emissions from oil and gas operations, which accounts for roughly 30% of U.S. anthropogenic methane emissions. The report identifies current technological and policy gaps and makes recommendations for the federal government in three key areas: methane emissions detection, reliable and systematized data and models to support mitigation measures, and effective regulation.

Partial Amplification of Octave-Spanning Supercontinuum in the Spectral Region of 1.5–2.2 μm

Octave-spanning supercontinuum conversion in three different rare-earth doped fiber amplifiers have been investigated. Using an erbium amplifier, it turned out to increase the output power to 445 mW with a spectral width of 1250 nm. For a thulium amplifier, an average output power of 390 mW and a spectral width of 569 nm was obtained. Additionally, for holmium, the average output power was 724 mW with a spectral width of 450 nm. For all cases, the output pulses envelope did not exceed 0.72 ns.

Assessment of the precision, bias and numerical correlation of fitted parameters obtained by multi-spectrum fits of the Hartmann-Tran line profile to simulated absorption spectra

Although the Voigt profile has long been used to analyze absorption spectra, the quest for increased precision, accuracy and generality drives the application of advanced models of atomic and molecular line shapes. To this end, the Hartmann-Tran profile is now recommended as a standard for high-resolution spectroscopy because it parameterizes relevant higher-order physical effects, is computationally efficient, and reduces to other widely used profiles as limiting cases. This work explores the uncertainty with which line shape parameters can be obtained from constrained multi-spectrum fits of spectra simulated with this standard profile, varying uncertainty levels in the spectrum detuning and absorption axes, and spanning a range of sampling density, pressure, and line shape parameter values. The analysis focuses on how noise-limited measurement precision of frequency detuning and absorption drive statistical uncertainties in fitted parameters and numerical correlations between these quantities. Also, we quantify the degree of equivalence between the full Hartmann-Tran profile and those derived from it in terms of fitted peak areas and line shape parameters. Finally, we introduce a new open-source software package named Multi-spectrum Analysis Tool for Spectroscopy (MATS), which allows users to fit the HTP and its derived profiles to experimental or simulated absorption spectra to explore the limits of the HTP under actual experimental or user-defined conditions.

Impact of Meteorological Uncertainties in the Methane Retrieval Ground Segment of the MERLIN Lidar Mission

MERLIN (MEthane Remote sensing LIdar missioN) is a Franco-German space mission designed to provide weighted columns of atmospheric methane through an inversion of the lidar signal using a priori information on the atmospheric state. Uncertainties about the meteorological parameters of the observed scene used in the ground segment contribute to the error budget on the retrieved methane column. With the LIDSIM (LIDar SIMulator) data simulator and the PROLID (PROcessor LIDar) inversion processor developed for MERLIN, we perform an impact experiment using ECMWF (European Centre for Medium Weather Range Forecast) ensemble forecast data. In addition, we estimate the standard deviation of the error in the methane column due to the meteorological uncertainties to be about 0.6 ppb. In addition, we innovate by discussing the impact of interpolations both in time and space, focusing on vertical extrapolations under the topography by using state-of-the-art methods to determine from the scatter between these methods the range in which the actual profile should be. We conclude that, in areas where the topography variations exceed 10 m over 10 km, an additional random error of 0.1 ppb is due to our lack of knowledge of the adjustment of atmospheric profiles to terrain. Finally, we point out that further work needs to be performed on temporal interpolation. Indeed, the 3 h time interpolation of atmospheric tides can create regional biases of up to 2 ppm (which is a major problem for models trying to identify methane sinks and sources).

Comb coherence-transfer and cavity ring-down saturation spectroscopy around 1.65 μm: kHz-accurate frequencies of transitions in the 2ν3 band of 12CH4

Comb Coherence Transfer (CCT) uses a feed-forward frequency correction to transfer the optical phase of a frequency comb to the beam of a free-running diode laser. This allows the amplification of a selected comb tooth by 50 dB while adding agile and accurate frequency tuning. In the present work, SI-traceable frequency calibration and comb tooth narrowing down to 20 kHz is additionally provided by comb frequency locking to an ultrastable optical frequency reference distributed from Paris to Grenoble through the RENATER optical fiber network [Lisdat et al., Nat. Commun., 2016, 7, 12443]. We apply this CCT broadly tunable source for saturated cavity ring-down spectroscopy of ro-vibrational R0 to R10 multiplets in the 2ν₃ band of ¹²CH₄ (from 6015 to 6115 cm^-1). Indeed, efficient cavity injection with large intra-cavity power build-up induces saturation of the ro-vibrational transitions at low pressure and Doppler-free Lamb dips are observed with high signal/noise. kHz-accurate transition frequencies are derived improving by three orders of magnitude previous values from spectra in the Doppler regime, which are strongly affected by line blending. While previous saturation spectroscopy investigations addressed specific 2ν₃ multiplets (R6 or R9), the CCT approach allowed for a rapid coverage of the entire R0-R10 series. Measured transition frequencies are compared with experimental and theoretical line lists available in the literature.

Optically Switched Dual-Wavelength Cavity Ring-Down Spectrometer for High-Precision Isotope Ratio Measurements of Methane δD in the Near Infrared

We report a spectrometer employing optically switched dual-wavelength cavity ring-down spectroscopy (OSDW-CRDS) for high-precision measurements of methane isotope ratios. A waveguide optical switch rapidly alternated between two wavelengths to detect absorption by two isotopologues using near-infrared CRDS. This approach alleviated common-mode noise that originated primarily from temperature and frequency fluctuations. We demonstrated the measurement of δD in natural abundance methane to a precision of 2.3 ‰, despite the lack of active temperature or frequency stabilization of the cavity. The ability of alternating OSDW-CRDS to improve the isotope precision in the absence of cavity stabilization were measured by comparing the Allan deviation with that obtained when frequency-stabilizing the cavity length. The system can be extended to a wide variety of applications such as isotope analysis of other species, kinetic isotope effects, ortho-para ratio measurements, and isomer abundance measurements. Furthermore, our technique can be extended to multiple isotope analysis or two species involved in kinetics studies through the use of multiport or high-speed optical switches, respectively.

Ultra-short wavelength operation of thulium-doped fiber amplifiers and lasers

We fabricate and characterize a germanium/thulium (Ge/Tm) co-doped silica fiber in order to enhance the gain at the short wavelength edge of the thulium emission band (i.e. 1620-1660 nm). The Ge/Tm doped fiber shows an intrinsic blue-shifted absorption/emission cross-section compared to aluminum/thulium (Al/Tm) co-doped fiber, which greatly improves the short wavelength amplification and has enabled us to further extend the shortest wavelength of emission towards 1600 nm. Using this glass fiber composition, we have demonstrated both a silica-based thulium doped fiber amplifier (TDFA) in the 1628-1655 nm waveband and a tunable thulium-doped fiber laser (TDFL) capable of accessing the telecom U-band wavelength region. These results represent by far the shortest amplifier/laser wavelengths reported to-date from TDFAs/TDFLs.

Optical Energy Variability Induced by Speckle: The Cases of MERLIN and CHARM-F IPDA Lidar

In the context of the FrenchGerman space lidar mission MERLIN (MEthane Remote LIdar missioN) dedicated to the determination of the atmospheric methane content, an end-to-end mission simulator is being developed. In order to check whether the instrument design meets the performance requirements, simulations have to count all the sources of noise on the measurements like the optical energy variability induced by speckle. Speckle is due to interference as the lidar beam is quasi monochromatic. Speckle contribution to the error budget has to be estimated but also simulated. In this paper, the speckle theory is revisited and applied to MERLIN lidar and also to the DLR (Deutsches Zentrum für Luft und Raumfahrt) demonstrator lidar CHARM-F. Results show: on the signal path, speckle noise depends mainly on the size of the illuminated area on ground; on the solar flux, speckle is fully negligible both because of the pixel size and the optical filter spectral width; on the energy monitoring path a decorrelation mechanism is needed to reduce speckle noise on averaged data. Speckle noises for MERLIN and CHARM-F can be simulated by Gaussian noises with only one random draw by shot separately for energy monitoring and signal paths.

Twenty-Five-Fold Reduction in Measurement Uncertainty for a Molecular Line Intensity

To accurately attribute sources and sinks of molecules like CO_{2}, remote sensing missions require line intensities (S) with relative uncertainties u_{r}(S)<0.1%. However, discrepancies in S of ≈1% are common when comparing different experiments, thus limiting their potential impact. Here we report a cavity ring-down spectroscopy multi-instrument comparison which revealed that the hardware used to digitize analog ring-down signals caused variability in spectral integrals which yield S. Our refined approach improved measurement accuracy 25-fold, resulting in u_{r}(S)=0.06%.

Prediction of line shape parameters and their temperature dependences for CO2-N2 using molecular dynamics simulations

We show in this paper that requantized classical molecular dynamics simulations (rCMDSs) are capable of predicting various refined spectral-shape parameters of absorption lines of CO₂ broadened by N₂ with high precision. Combining CMDSs and a requantization procedure, we computed the auto-correlation function of the CO₂ dipole moment responsible for the absorption transition. Its Fourier-Laplace transform directly yields the spectrum. Calculations were made for two temperatures, 200 and 296 K, at 1 atm and for a large range of Doppler widths, from the near-Doppler to the collision-dominant regimes. For each temperature and each line, the spectra calculated for various Doppler widths were simultaneously fit with the Hartmann-Tran (HT) profile. This refined profile, which takes into account the effects of the speed dependent collisional line broadening, the Dicke narrowing, and the collisional line mixing, was recommended as a reference model to be used in high-resolution spectroscopy (instead of the simplified Voigt model). The HT parameters retrieved from the rCMDS-calculated spectra were then directly compared with those deduced from high-precision measurements [J. S. Wilzewski et al., J. Quant. Spectrosc. Radiat. Transfer 206, 296-305 (2018)]. The results show a very good agreement, even for those parameters whose influence on the spectra is very small. Good agreement is also obtained between measured and predicted temperature dependences of these parameters. This demonstrates that rCMDS is an excellent tool, highly competitive with respect to high quality measurements for precise line-shape studies.

Large regional shortwave forcing by anthropogenic methane informed by Jovian observations

Recently, it was recognized that widely used calculations of methane radiative forcing systematically underestimated its global value by 15% by omitting its shortwave effects. We show that shortwave forcing by methane can be accurately calculated despite considerable uncertainty and large gaps in its shortwave spectroscopy. We demonstrate that the forcing is insensitive, even when confronted with much more complete methane absorption spectra extending to violet light wavelengths derived from observations of methane-rich Jovian planets. We undertake the first spatially resolved global calculations of this forcing and find that it is dependent on bright surface features and clouds. Localized annual mean forcing from preindustrial to present-day methane increases approaches +0.25 W/m², 10 times the global annualized shortwave forcing and 43% of the total direct CH₄ forcing. Shortwave forcing by anthropogenic methane is sufficiently large and accurate to warrant its inclusion in historical analyses, projections, and mitigation strategies for climate change.

Temperature-scanning saturation cavity ring-down spectrometry for Doppler-free spectroscopy

Saturation cavity ring-down spectroscopy (SCRDS) is a powerful Doppler-free spectroscopy means for measuring absolute frequencies of transitions at the ultra-low uncertainties. We report in this paper a simple way to implement it by temperature scanning the cavity length, which circumvents the need for a complex optical cavity-length stabilization system based upon a piezoelectric actuator (PZT). To demonstrate this approach, the absolute frequencies of the two transitions, R6F1 of the 2v₃ and Q9A1 of the 2v₂ + v₃ bands, of ¹²CH₄, are determined to be 182 185 269.362(20) MHz and 182 187 617.543(39) MHz. The accuracy of measurements is improved by about 3-4 orders of magnitude when compared to those obtained with conventional spectroscopic methods.

Feed-forward coherent link from a comb to a diode laser: Application to widely tunable cavity ring-down spectroscopy

We apply a feed-forward frequency control scheme to establish a phase-coherent link from an optical frequency comb to a distributed feedback (DFB) diode laser: This allows us to exploit the full laser tuning range (up to 1 THz) with the linewidth and frequency accuracy of the comb modes. The approach relies on the combination of an RF single-sideband modulator (SSM) and of an electro-optical SSM, providing a correction bandwidth in excess of 10 MHz and a comb-referenced RF-driven agile tuning over several GHz. As a demonstration, we obtain a 0.3 THz cavity ring-down scan of the low-pressure methane absorption spectrum. The spectral resolution is 100 kHz, limited by the self-referenced comb, starting from a DFB diode linewidth of 3 MHz. To illustrate the spectral resolution, we obtain saturation dips for the 2ν₃ R(6) methane multiplet at μbar pressure. Repeated measurements of the Lamb-dip positions provide a statistical uncertainty in the kHz range.

Methane optical density measurements with an integrated path differential absorption lidar from an airborne platform

We report on an airborne demonstration of atmospheric methane (CH₄) measurements with an Integrated Path Differential Absorption (IPDA) lidar using an optical parametric amplifier (OPA) and optical parametric oscillator (OPO) laser transmitter and sensitive avalanche photodiode detector. The lidar measures the atmospheric CH₄ absorption at multiple, discrete wavelengths near 1650.96 nm. The instrument was deployed in the fall of 2015, aboard NASA's DC-8 airborne laboratory along with an in-situ spectrometer and measured CH₄ over a wide range of surfaces and atmospheric conditions from altitudes of 2 km to 13 km. In this paper, we will show the results from our flights, compare the performance of the two laser transmitters, and identify areas of improvement for the lidar.

Ground-based, integrated path differential absorption LIDAR measurement of CO2, CH4, and H2O near 1.6 μm

A ground-based, integrated path, differential absorption light detection and ranging (IPDA LIDAR) system is described and characterized for a series of nighttime studies of CO₂, CH₄, and H₂O. The transmitter is based on an actively stabilized, continuous-wave, single-frequency external-cavity diode laser (ECDL) operating from 1.60 to 1.65 μm. The fixed frequency output of the ECDL is microwave sideband tuned using an electro-optical phase modulator driven by an arbitrary waveform generator and filtered using a confocal cavity to generate a sequence of 123 frequencies separated by 300 MHz. The scan sequence of single sideband frequencies of 600 ns duration covers a 37 GHz region at a spectral scan rate of 10 kHz (100 μs per scan). Simultaneously, an eye-safe backscatter LIDAR system at 1.064 μm is used to monitor the atmospheric boundary layer. IPDA LIDAR measurements of the CO₂ and CH₄ dry air mixing ratios are presented in comparison with those from a commercial cavity ring-down (CRD) instrument. Differences between the IPDA LIDAR and CRD concentrations in several cases appear to be well correlated with the atmospheric aerosol structure from the backscatter LIDAR measurements. IPDA LIDAR dry air mixing ratios of CO₂ and CH₄ are determined with fit uncertainties of 2.8 μmol/mol (ppm) for CO₂ and 22 nmol/mol (ppb) for CH₄ over 30 s measurement periods. For longer averaging times (up to 1200 s), improvements in these detection limits by up to 3-fold are estimated from Allan variance analyses. Two sources of systematic error are identified and methods to remove them are discussed, including speckle interference from wavelength decorrelation and the seed power dependence of amplified spontaneous emission. Accuracies in the dry air retrievals of CO₂ and CH₄ in a 30 s measurement period are estimated at 4 μmol/mol (1% of ambient levels) and 50 nmol/mol (3%), respectively.

Coherent cavity-enhanced dual-comb spectroscopy

Dual-comb spectroscopy allows for the rapid, multiplexed acquisition of high-resolution spectra without the need for moving parts or low-resolution dispersive optics. This method of broadband spectroscopy is most often accomplished via tight phase locking of two mode-locked lasers or via sophisticated signal processing algorithms, and therefore, long integration times of phase coherent signals are difficult to achieve. Here we demonstrate an alternative approach to dual-comb spectroscopy using two phase modulator combs originating from a single continuous-wave laser capable of > 2 hours of coherent real-time averaging. The dual combs were generated by driving the phase modulators with step-recovery diodes where each comb consisted of > 250 teeth with 203 MHz spacing and spanned > 50 GHz region in the near-infrared. The step-recovery diodes are passive devices that provide low-phase-noise harmonics for efficient coupling into an enhancement cavity at picowatt optical powers. With this approach, we demonstrate the sensitivity to simultaneously monitor ambient levels of CO2, CO, HDO, and H2O in a single spectral region at a maximum acquisition rate of 150 kHz. Robust, compact, low-cost and widely tunable dual-comb systems could enable a network of distributed multiplexed optical sensors.