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Quantifying sector-resolved methane fluxes in complex emissions environments is challenging yet necessary for inventory validations. We separate energy and agriculture sector methane using a dynamic linear model of methane, ethane, and ammonia mixing ratios measured at a Northern Colorado site from November 2021 to January 2022. Combining observations with spatially resolved inventories and inverse methods, energy and agriculture methane fluxes are constrained across a ~850 km area. Optimized energy sector fluxes were 22% lower than the inventory despite a ~360% increase in regional energy production since the inventory was constructed, suggesting a regional decline in emissions factors. In contrast, optimized agriculture fluxes were 3× larger than the inventory; we demonstrate this discrepancy is consistent with the spatial distribution of agricultural sources. These results highlight the utility of sector-apportioned methane observations for multi-sector inventory optimization in complex environments, which may prove valuable for national and global quantification of sector-resolved methane fluxes.

Determination of trace gas emissions from sources is critical for understanding and regulating air quality and climate change. Here, we demonstrate a method for rapid quantification of the emission rate of multiple gases from simple and complex sources using a mass-balance approach with a spatially scannable open-path sensor – in this case, an open-path dual-comb spectrometer. The open-path spectrometer measures the total column density of gases between the spectrometer and a retroreflector mounted on an unmanned aerial vehicle (UAV). By measuring slant columns at multiple UAV altitudes downwind of a source (or sink), the total emission rate can be rapidly determined without the need for an atmospheric dispersion model. Here, we demonstrate this technique using controlled releases of CH4 and C2H2. We show an emission rate determination to within 50 % of the known flux with a single 10-minute flight and within 10 % of the known flux after 10 flights. Furthermore, we estimate a detection limit for CH4 emissions to be 0.03 g CH4/s. This detection limit is approximately the same as the emissions from 25 head of beef cattle and is less than the average emissions from a small oil field pneumatic controller. Other gases including CO2, NH3, HDO, ethane, formaldehyde (HCHO), CO, and N2O can be measured by simply changing the dual-comb spectrometer.

We present an open-path mid-infrared dual-comb spectrometer (DCS) capable of precise measurement of the stable water isotopologues H216O and HD16O. This system ran in a remote configuration at a rural test site for 3.75 months with 60 % uptime and achieved a precision of 216O and HD16O (δD) in 1000 seconds. Here, we compare the δD values from the DCS to those from the National Ecological Observatory Network (NEON) isotopologue point sensor network. Over the multi-month campaign, the mean difference between the DCS δD values and the NEON δD values from a similar ecosystem is

We demonstrate the application of mode-locked mid-infrared dual frequency comb spectroscopy for combustion analysis. With two settings of the same dual-comb system, the measurement spans 1500 cm−1 from 2.8 to 5 µm with 0.0067 cm−1 (200 MHz) point spacing, or almost a quarter-million discrete comb modes. Using this broadband spectrometer, we quantify the pyrolysis and smoldering combustion of wood samples. Specifically, we measure 20-second time-resolved mole fractions of CH4, H2O, two isotopologues of CO2 (12C16O2, 13C16O2), two isotopologues of CO (12C16O, 13C16O), ethane, formaldehyde, methanol, formic acid, as well as gas temperature directly above radiatively heated wood samples. The combination of the fine spectral resolution and the broad bandwidth of the dual-comb spectrometer allows for precise separation of absorption signatures of individual molecules from the congested spectra.

Spectrally resolved photoacoustic imaging is promising for label-free imaging in optically scattering materials. However, this technique often requires acquisition of a separate image at each wavelength of interest. This reduces imaging speeds and causes errors if the sample changes in time between images acquired at different wavelengths. We demonstrate a solution to this problem by using dual-comb spectroscopy for photoacoustic measurements. This approach enables a photoacoustic measurement at thousands of wavelengths simultaneously. In this technique, two optical-frequency combs are interfered on a sample and the resulting pressure wave is measured with an ultrasound transducer. This acoustic signal is processed in the frequency-domain to obtain an optical absorption spectrum. For a proof-of-concept demonstration, we measure photoacoustic signals from polymer films. The absorption spectra obtained from these measurements agree with those measured using a spectrophotometer. Improving the signal-to-noise ratio of the dual-comb photoacoustic spectrometer could enable high-speed spectrally resolved photoacoustic imaging., Spectrally resolved photoacoustic images often require the acquisition of data for each wavelength separately. Here, the authors use dual frequency-comb spectroscopy for photoacoustic measurements, enabling spectrally resolved measurements without the need to scan the illumination wavelength.

Using an open-path mid-infrared dual-comb spectrometer, we measure the δD of atmospheric water vapor at a remote site with

Multi-month methane and ethane measurements in the Northern Colorado Front Range using open-path dual-comb spectroscopy are reported. Conditional probability densities of measurements stratified by composition ratio distinguishes between wellhead and non-wellhead emissions.

We demonstrate a programmable optical frequency comb that emits coherent pulses with user-specified time and phase at sub-10 attosecond accuracy while maintaining the underlying referenced stability characteristic of combs.

Dual-comb spectroscopy measures greenhouse gas concentrations over kilometers of open air with high precision. However, the accuracy of these outdoor spectra is challenging to disentangle from the absorption model and the fluctuating, heterogenous concentrations over these paths. Relative to greenhouse gases, O2 concentrations are well-known and evenly mixed throughout the atmosphere. Assuming a constant O2 background, we can use O2 concentration measurements to evaluate the consistency of open-path dual-comb spectroscopy with laboratory-derived absorption models. To this end, we construct a dual-comb spectrometer spanning 1240 nm to 1700nm, which measures O2 absorption features in addition to CO2 and CH4. O2 concentration measurements across a 560 m round-trip outdoor path reach 0.1% precision in 10 minutes. Over seven days of shifting meteorology and spectrometer conditions, the measured O2 has -0.07% mean bias, and 90% of the measurements are within 0.4% of the expected hemisphere-average concentration. The excursions of up to 0.4% seem to track outdoor temperature and humidity, suggesting that accuracy may be limited by the O2 absorption model or by water interference. This simultaneous O2, CO2, and CH4 spectrometer will be useful for measuring accurate CO2 mole fractions over vertical or many-kilometer open-air paths, where the air density varies.

Opto-optical loss modulation (OOM) for stabilization of the carrier-envelope offset (CEO) frequency of a femtosecond all-fiber laser is performed using a collinear geometry. Amplitude-modulated 1064 nm light is fiber coupled into an end-pumped semiconductor saturable absorber mirror (SESAM)-mode-locked all-polarization-maintaining erbium fiber femtosecond laser, where it optically modulates the loss of the SESAM resulting in modulation of the CEO frequency. A noise rejection bandwidth of 150 kHz is achieved when OOM and optical gain modulation are combined in a hybrid analog/digital loop. Collinear OOM provides a simple, all-fiber, high-bandwidth method for improving the CEO frequency stability of SESAM mode-locked fiber lasers.

We combine mid-infrared dual-comb spectroscopy with attenuated total reflectance measurements to provide in-situ monitoring of a chemical reaction. The mid-infrared dual-comb spectrometer measures quantitative absorption cross-sections of mesityl oxide and diacetone alcohol in the region of 3300–2700 cm−1 (3.0–3.7 μm). Based on these spectra, the system is then used to monitor the hydration of mesityl oxide to diacetone alcohol from which we extract reaction kinetics values. This work shows the potential of dual comb systems to measure and monitor reactions in the condensed phase.

Frequency-comb-based optical two-way time and frequency transfer (O-TWTFT) can support future ultra-precise clock networks over free-space links. However, demonstrations have thus far only operated at one corner of a complex parameter space, which balances received optical power, timing performance, and update rate. Here, we analyze the performance of O-TWTFT across this parameter space, with a specific focus on extending the link distance at constant launch power and telescope aperture. We perform experiments across a three-node network spanning 28 km of turbulent air, and successfully demonstrate a more than 2000\ifmmode\times\else\texttimes\fi{} reduction in the required received optical power corresponding to a potential 45\ifmmode\times\else\texttimes\fi{} increase in distance. This distance increase does come with an associated reduction in timing precision, with the uncertainty increasing from 60 as to 20 fs at 10 s averaging times. However, this level of precision is still more than adequate for most applications. In addition, it comes with a reduction in sampling rate, which potentially limits this approach to static links. Interestingly, because this system optimization does not require any hardware modifications, O-TWTFT links could be dynamically tuned to support future long-distance optical clock networks over a range of conditions and applications.

A versatile method of dual-comb spectroscopy is presented. Digital control of the comb frequencies enables dynamically adaptable spectrometer resolution to efficiently match a sample’s feature width, optimizing the system for new applications.

Simultaneous retrieval of H216O and H218 concentrations at an agricultural site is demonstrated using near-infrared open-path dual-comb spectroscopy (DCS). Initial retrievals of the isotopologue ratio δ18O have a statistical precision of 5‰ at one hour. Work of the US Government and not subject to copyright.

Dual-comb laser absorption spectroscopy is demonstrated over a 28 km open path over an urban environment. Path averaged concentrations of CO2, H2O and CH4 are retrieved despite significant turbulence induced link loss.

Open-path dual-comb spectroscopy measures multispecies livestock emissions, enabling future field-scale ecological studies., Advances in spectroscopy have the potential to improve our understanding of agricultural processes and associated trace gas emissions. We implement field-deployed, open-path dual-comb spectroscopy (DCS) for precise multispecies emissions estimation from livestock. With broad atmospheric dual-comb spectra, we interrogate upwind and downwind paths from pens containing approximately 300 head of cattle, providing time-resolved concentration enhancements and fluxes of CH4, NH3, CO2, and H2O. The methane fluxes determined from DCS data and fluxes obtained with a colocated closed-path cavity ring-down spectroscopy gas analyzer agree to within 6%. The NH3 concentration retrievals have sensitivity of 10 parts per billion and yield corresponding NH3 fluxes with a statistical precision of 8% and low systematic uncertainty. Open-path DCS offers accurate multispecies agricultural gas flux quantification without external calibration and is easily extended to larger agricultural systems where point-sampling-based approaches are insufficient, presenting opportunities for field-scale biogeochemical studies and ecological monitoring.

We measure speed-dependent Voigt lineshape parameters with temperature-dependence exponents for several hundred spectroscopic features of pure water spanning 6801–7188 cm(−1). The parameters are extracted from broad bandwidth, high-resolution dual frequency comb absorption spectra with multispectrum fitting techniques. The data encompass 25 spectra ranging from 296 K to 1305 K and 1 to 17 Torr of pure water vapor. We present the extracted parameters, compare them to published data, and present speed-dependence, self-shift, and self-broadening temperature-dependent parameters for the first time. Lineshape data is extracted using a quadratic speed-dependent Voigt profile and a single self-broadening power law temperature-dependence exponent over the entire temperature range. The results represent an important step toward a new high-temperature database using advanced lineshape profiles.

We report argon-broadened water vapor transition parameters and their temperature dependence based on measured spectra spanning 6801–7188 cm(−1) from a broad-bandwidth, high-resolution dual frequency comb spectrometer. The 25 collected spectra of 2% water vapor in argon ranged from 296 K to 1305 K with total pressure spanning 100 Torr to 600 Torr. A multispectrum fitting routine was used in conjunction with a quadratic speed-dependent Voigt profile to extract broadening and shift parameters, and a power-law temperature-dependence exponent for both. The measurements represent the first broad bandwidth, argon-broadened water vapor absorption study, and are an important step toward a foreign-gas-perturbed, high-temperature database developed using advanced lineshape profiles.

We demonstrate optical two-way time-frequency transfer (O-TWTFT) across a three-node free-space network. The network spans 28 km of outdoor air and relays time and frequency from a master node, through an intermediate fielded node, to an end node. Despite strong atmospheric turbulence along the close-to-ground free-space links and large ambient temperature swings at the fielded node, the network transfers time with relative timing offsets below 5 fs. Additionally, the network transfers frequency with instability below 10−18 at averaging times greater than 200 s—i.e. below the level needed to compare state-of-the-art optical atomic clocks.

The output of a laser frequency comb is composed of 100,000+ perfectly spaced, discrete wavelength elements or comb teeth, that act as a massively parallel set of single frequency (CW) lasers with highly stable, well-known frequencies. In dual-comb spectroscopy, two such frequency combs are interfered on a single detector yielding absorption information for each individual comb tooth. This approach combines the strengths of both cw laser spectroscopy and broadband spectroscopy providing high spectral resolution and broad optical bandwidths, all with a single-mode, high-brightness laser beam and a simple, single photodetector, detection scheme. Here I will touch on the application of this system for open-path measurements of atmospheric trace gases (CH4, CO2, CO, NH3, water, ethane, and N2O) and volatile organic compounds (acetone, isopropanol, propane) with field applications targeting industrial oil and gas monitoring and agriculture.

A major design goal for femtosecond fiber lasers is to increase the output power but not at the cost of increasing the noise level or narrowing the bandwidth. Here, we perform a computational study to optimize the cavity design of a femtosecond fiber laser that is passively modelocked with a semiconductor saturable absorbing mirror (SESAM). We use dynamical methods that are more than a thousand times faster than standard evolutionary methods. We show that we can obtain higher pulse energies and hence higher output powers by simultaneously increasing the output coupling ratio, the gain, and the anomalous group delay dispersion. We can obtain output pulses that are from 5 to 15 times the energy of the pulse in the current experimental design with no penalty in the noise level or bandwidth.

We use frequency comb-based optical two-way time-frequency transfer (O-TWTFT) to measure the optical frequency ratio of state-of-the-art ytterbium and strontium optical atomic clocks separated by a 1.5 km open-air link. Our free-space measurement is compared to a simultaneous measurement acquired via a noise-cancelled fiber link. Despite non-stationary, ps-level time-of-flight variations in the free-space link, ratio measurements obtained from the two links, averaged over 30.5 hours across six days, agree to $6\times10^{-19}$, showing that O-TWTFT can support free-space atomic clock comparisons below the $10^{-18}$ level.

This manuscript describes the design of a robust, mid-infrared dual-comb spectrometer operating in the 3.1-µm to 4-µm spectral window for future field applications. The design represents an improvement in system size, power consumption, and robustness relative to previous work while also providing a high spectral signal-to-noise ratio. We demonstrate a system quality factor of 2×106 and 30 hours of continuous operation over a 120-meter outdoor air path.

Open-path measurements of atmospheric gas species over km-scale path lengths are well suited to quantify emissions from sources like oil and gas, forest fires, and industry. is a relatively new technique that combines high-resolution and broad spectral coverage with no instrument lineshape and near perfect frequency calibration. These features have enabled open-path DCS to provide accurate measurements of multiple trace gas species simultaneously in the near-infrared across path lengths ranging from 100 m to several km. However, in order to reach the sensitivity necessary to detect many atmospheric trace constituents, including volatile organic compounds (VOCs), operation in the mid-infrared (or UV/Vis) is required.Here, we show a mid-infrared open-path dual comb spectrometer operating in the 3-4 and 4.5-5 μm spectral regions. We have used this spectrometer to measure methane, ethane, and propane (arising primarily from oil and gas activity) across a 1-km-long path in Boulder, CO for 1 week with an ethane sensitivity of ∼0.1 ppb for a 2-minute time resolution. In addition, we show quantitative measurements of intentionally released acetone and isopropanol with a 1-σ sensitivity of 5.7 ppm·m and 2.4 ppm·m, respectively. In the 4.5-5 μm region, we have used this system to detect N2O, CO, and O3. Finally, we have developed a second-generation instrument in the 3-4 μm region that is more compact and has improved stability. This system was recently deployed in a van at an active oil and gas drilling operation. We present preliminary measurements of methane, ethane, and higher hydrocarbons from this deployment as well as initial efforts at emissions quantification.

Optical two-way time-frequency transfer, based on the exchange of frequency comb pulses, can support femtosecond-level time transfer and 10-18-level frequency comparison. Here, we present recent results with a focus on the impact of atmospheric turbulence.

Open-path dual-comb spectroscopy is used to quantify CO2, CH4, H2O and NH3 concentrations adjacent to a cattle feedlot. In all cases except NH3, the data is compared against closed-path cavity ring-down spectroscopy.

We demonstrate optical frequency comparison of the 171Yb, 27Al+ and 87Sr atomic clocks with measurement uncertainties below 1 part in 1017, and discuss how phase-coherent and synchronous clock comparisons can be used to improve measurement stability.

We measure optical timing jitter due to atmospheric turbulence using frequency comb lasers. Retrieving the refractive index structure parameter, Cn2, from this data, we find windspeed- dependent agreement to Cn2 values derived from in situ instruments.

Dual-comb photoacoustic spectroscopy (DCPAS) is a new spectroscopy technique that measures photoacoustic signals in thousands of wavelength bands simultaneously. We report the first demonstration of this technique and show DCPAS spectra of polymer films. Work of the United States government, not subject to copyright.

Open-path mid-infrared dual-comb spectroscopy is performed over a 2 km- long link with bandwidth spanning the atmospheric transmission window near 4.7 pm. We quantify mixing ratios for CO, CO2, N2O, H2O, and O3. Work of the US Government and not subject to copyright.

We use a broadband mid-IR dual-comb spectrometer operating between 2.8 and 5 microns to measure time-resolved mole fractions of 12 gaseous species and isotopologues in the pyrolysis products above a heated Eucalyptus wood sample.

We characterize the impact of turbulence on optical pulse timing jitter over a uniform, near-ground path and compare the results with theory. Use of frequency combs enabled phase- continuous measurements down to 200-fs over multiple hours.

We deploy a mid-infrared dual-comb spectrometer in a van for field measurements of volatile organic compounds (VOCs). We also use a similar, laboratory system to measure ambient VOCs over a 2km round-trip path. Work of the US Government and not subject to copyright.

Open-path dual-comb spectroscopy in the 4.5-4.9 pm region is performed over a 300 m- long link to quantify mixing ratios for CO, CO2, N2O, H2O, and O3.